Anesthesia machines: Prevention, diagnosis, and management of malfunctions

INTRODUCTION — This topic will discuss strategies to prevent, diagnose, and manage the most common and/or severe misuses and malfunctions of the anesthesia machine and its integrated monitoring systems.

A separate topic describes preparing anesthesia machines for use during the COVID-19 pandemic and repurposing anesthesia machines as intensive care ventilators. (See "COVID-19: Intensive care ventilation with anesthesia machines".)

Other hazards involving anesthesia and other equipment in the operating room (eg, electrical injuries, burns) are discussed in separate topics. (See "Patient safety in the operating room" and "Fire safety in the operating room".)

GENERAL PRINCIPLES — Anesthetic care is provided using an anesthesia machine and integrated monitors (ie, the anesthesia workstation (figure 1 and figure 2 and figure 3 and figure 4 and figure 5)) that include systems for:

●Blending a precision mixture of respiratory and anesthetic gases (anesthesia machine and vaporizers)

●Dispensing those gases to the patient (anesthesia breathing system)

●Delivering positive pressure ventilation (anesthesia ventilator)

●Removing waste gases (scavenger system)

●Monitoring respiratory and machine function (integrated system monitors)

Misuse or malfunction of the anesthesia machine and its integrated monitors has been cited as the cause of 1 to 2 percent of closed claim lawsuits involving anesthesiology personnel, with equipment misuse being three to five times more frequent than equipment failure [1,2].

STANDARDIZED ANESTHESIA MACHINE CHECKOUT

Anesthesia machine checkout procedures: General principles — Investigators have noted that meticulous adherence to published guidelines for a pre-use anesthesia machine and equipment checkout would prevent most critical incidents and lawsuits due to anesthesia workstation misuse or failure [1-4]. Without a formal checkout procedure, anesthesia providers are not likely to detect all machine problems. A prospective study published in 1984 noted that fewer than 5 percent of a sample of attendees at a professional anesthesia meeting could find all five faults intentionally created in an anesthesia machine (the average number of faults detected was 2.2) [5]. In a 2007 follow-up study, attendees found an average of 3.1 of five intentionally created faults [6]. Notably, formal checklists were not used by the participants in these studies.

A number of national and international organizations have created pre-use checklists for the anesthesia workstation. The American Society of Anesthesiologists (ASA) Anesthesia Machine Checkout Recommendations are used in the United States (table 1) [7]. Similar anesthesia workstation checkout recommendations have been published in other countries (eg, the Association of Anaesthetists of Great Britain and Ireland [AAGBI] and the Australian and New Zealand College of Anaesthetists [ANZCA]) [8,9]. The ASA recommendations are unique since they specify that some checks can be routinely accomplished by support personnel (eg, anesthesia technicians) to ensure compliance and lessen clinician workload.

Since no single checkout procedure is applicable to all machines, the ASA and other professional society recommendations are meant to serve as general guidelines and should be modified as necessary for the anesthesia delivery system(s) available in a local practice as well as the patient population. For example, when caring for infants under 10 kg, it is especially important to expand the breathing system hoses prior to performing the compliance compensation portion of a self-check [10].

With advances in integrated computer technology, many new anesthesia workstations perform an automated pre-use checkout when powered on. Electronic prompts on the workstation screen communicate additional steps that the anesthesia provider must perform to complete the non-automated portions of the checkout procedure. In one prospective study, the proportion of cases with failure to perform all necessary pre-induction steps decreased from 10 to 6.4 percent before and after implementation of a computerized pre-induction anesthesia checklist [11].

American Society of Anesthesiologists Anesthesia Machine Checkout — In the United States, the ASA Anesthesia Machine Checkout Recommendations are most commonly used (table 1). These guidelines note 15 specific checks that are to be performed daily, and eight of these items are completed before beginning anesthetic care for each individual patient [7].

ANESTHESIA WORKSTATION ALARMS — Alarms for the most common and/or serious anesthesia machine problems, possible causes, and suggested clinician responses are shown in the tables:

●Electrical power loss alarm (table 2)

●Oxygen (O₂) supply pressure alarm (table 3)

●Apnea alarm overview (table 4)

●Apnea-pressure alarm [12] (table 5 and algorithm 1)

●Apnea-flow alarm (table 6)

●Apnea-carbon dioxide (CO₂) alarm (table 7)

●High airway pressure alarm (table 8 and algorithm 2)

●Continuous airway pressure alarm (table 9)

●Negative airway pressure alarm (table 10)

●Low inspired oxygen alarm (table 11)

●High inspired CO₂ alarm (table 12 and algorithm 3)

●Low inspired anesthetic agent alarm (table 13)

These alarms are generated by integrated monitors within the anesthesia workstation. They may fail to alert the clinician to a critical situation if the alarm is disabled, the sound volume is set too low, or the threshold settings for the monitored parameter are not appropriate (which may occur if settings are not changed for each individual patient). (See 'Malfunction of integrated anesthesia monitors' below.)

MALFUNCTION OF ANESTHESIA MACHINE COMPONENTS — Anesthesia workstations are well-designed machines that rarely malfunction. Typically, they are used daily for 10 to 20 years.

Leaks and obstructions are the most common causes of problems with anesthesia delivery equipment [4]. Most problems with machine function are preventable, and the root cause is more likely to be improper use than frank equipment failure [1-4,13].

Loss of electrical supply — The electrical power loss alarm warns that the system is running on battery (table 2).

●Potential problems – Anesthesia workstations require electrical power for monitors and alarms, screen displays, and ventilator controls. In addition, newer anesthesia machines may have electrically powered ventilators, compressed gas flowmeters, and vaporizers for administration of volatile anesthetics. Thus, the functionality of anesthesia workstations is severely compromised if electrical power fails, although all have integrated backup batteries that allow continued operation for a short period of time (typically, 30 minutes to approximately one hour).

●Prevention – Alternating current (AC) and battery power should be confirmed during the pre-use checkout (item #3) (table 1) (see 'American Society of Anesthesiologists Anesthesia Machine Checkout' above). The power source indicators on the anesthesia workstation show the presence of both AC and backup battery power.

●Diagnosis – The first obvious sign of power failure may be an electrical power loss alarm, or even complete workstation shutdown once the batteries are no longer providing power (table 2).

●Response – All anesthesia workstations are able to support manual ventilation with oxygen in the event of total electrical failure. This should be initiated while the root cause of the problem is sought. In some workstations the normal flowmeter controls and displays are used for this; in other machines, backup mechanical flowmeter controls and displays must be used.

Troubleshooting strategies for loss of electrical are described in the table (table 2). Malfunctions of electrical supply include local power loss due to a blown fuse, tripped circuit breaker, or tripped ground fault circuit interrupter (GFCI) outlet (see "Patient safety in the operating room", section on 'Potential physical injuries'). In one case known to the authors, the hospital maintenance crew shut off electrical supply to the operating room without verifying that the room was not in use.

More widespread power loss can also occur throughout a health care facility or the surrounding community. In the United States, anesthesia workstations are only used in facilities with a backup generator.

Compressed gas supply malfunction — Compressed gas supply malfunction should trigger alarm messages for oxygen supply pressure and low inspired oxygen (table 3 and table 11).

●Potential problems – Anesthesia workstations require a source of compressed oxygen, and other compressed respiratory gases are also typically available (eg, air, nitrous oxide [N₂O]). These compressed gases are dispensed via pipelines and hoses from central facility sources. Oxygen and other gases may also be dispensed from cylinders attached to the anesthesia workstation, if necessary.

•Pipeline malfunctions include:

-Inadequate gas line pressure

-Wrong gas in the pipeline [14]

-Particulate contamination

-Water contamination (eg, in compressed air)

•Hose malfunctions include:

-Disconnection

-Connection to the wrong gas source

-Leak in the hose

-Obstruction of the hose

•Cylinder malfunctions include:

-Empty cylinder

-Wrong gas in the cylinder [15]

-Attachment of the cylinder to the wrong yoke

•Malfunctions in the high-pressure sections of the anesthesia workstation (between the compressed gas inlets and the flowmeters) include:

-Leak

-Cross connection to the wrong flowmeter

-Obstruction

●Prevention – Verify that the piped gas pressures are ≥45 and not >55 pounds per square inch gauge (psig) (equivalent to 310 to 380 kilopascals [kPa]), and that pressure is adequate on the spare oxygen cylinder gauge mounted on the anesthesia machine (table 1). (See 'American Society of Anesthesiologists Anesthesia Machine Checkout' above.)

Wrong gas administration due to a misconnection of pipeline tubing or a cylinder that was filled with the wrong gas would be difficult to detect. Notably, this situation is very rare, but potentially devastating. This is another reason to calibrate the oxygen analyzer as part of the pre-use checkout of the anesthesia machine, thereby ensuring that the oxygen analyzer reads 21 percent in-room air and >90 percent when the breathing system is flushed with oxygen (table 1). The oxygen analyzer is the only device that can definitively tell the anesthesiologist what percent oxygen is being delivered to the patient. (See 'Standardized anesthesia machine checkout' above and 'Oxygen analyzer' below.)

●Diagnosis – There is only an alarm for low oxygen supply pressure. If the supply pressure of any gas is low, the flowmeter reading for that gas will be lower than the expected setting, or even zero. Gauges on the anesthesia workstation show the pressures of all compressed gases. Pipelines should nominally supply gases at 45 to 55 psig (310 to 380 kPa). A full oxygen cylinder has a pressure of 2000 psig (13,790 kPa) and contains 625 L of oxygen; however, cylinder gauges show the correct pressure only if the cylinder is open.

An inspired oxygen alarm (low or high) may indicate wrong gas administration due to misconnection anywhere in the gas supply chain (eg, wrong gas in cylinder, wrong gas in the pipeline, misconnected hose). Wrong gas administration should be suspected whenever steady-state gas concentrations in the breathing circuit are dramatically different than those predicted from compressed gas flowmeter settings.

●Response – In the event of low oxygen supply pressure during a case, open the oxygen cylinder, conserve anesthesia machine oxygen supply (turn off auxiliary flowmeter if in use, lower fresh gas oxygen flow, turn off pneumatically driven ventilator), and obtain additional backup cylinders.

In the event of a wrong gas coming from the oxygen pipeline, it is critically important to disconnect the anesthesia machine from the oxygen pipeline source and open the backup cylinder. If the backup cylinder is opened while the pipeline source is still connected the problem will persist since gases are preferentially drawn from the pipelines instead of the cylinders (ie, cylinder pressures are reduced to ≤45 psig by a regulator inside the anesthesia machine). If low oxygen supply pressure is a persistent problem, disconnect the patient from the anesthesia machine and initiate positive pressure ventilation with a self-inflating manual ventilation device attached to a fresh oxygen cylinder (see 'Backup equipment for oxygenation and ventilation' below). Do not use the auxiliary oxygen flowmeter on the anesthesia workstation since it has the same oxygen source as the anesthesia breathing circuit.

Compressed gas flowmeter malfunction — There are no specific alarms for the compressed gas flowmeters (ie, the fresh gas flowmeters). Compressed gas flowmeter malfunction may trigger the apnea-pressure alarm and the low inspired oxygen alarm (table 5 and table 11).

●Potential problems – While many new workstations have electronic flowmeters, some new and most older anesthesia machines have glass flowmeters (figure 3). Glass flowmeters may be more prone to malfunction due to physical issues (eg, contamination, leaks, cracks) compared with electronic flowmeters.

•Miscalibration – All flowmeters are factory calibrated for a specific gas based on that gas's density, viscosity, and specific heat; thus, any flowmeter gives incorrect readings for other gases. Glass flowmeters consist of a tube and a float that are calibrated as a matched set, and register incorrect readings if mismatched. Also, dirt, oil, or water contamination and static electricity buildup can cause incorrect readings.

•Leak – Flowmeters may become cracked, broken, or loosened, resulting in a leak in that flowmeter and any upstream flowmeters. For this reason, oxygen flowmeters are always positioned downstream, so that oxygen will not be lost unless the leak is in the oxygen flowmeter itself (figure 6).

●Prevention – Flowmeter malfunctions are very rare with the glass and electronic flowmeters installed on newer anesthesia machines. For this reason, checks of flowmeter function have been removed from the standard American Society of Anesthesiologists (ASA) Anesthesia Machine Checkout Recommendations [7].

●Diagnosis – A flowmeter leak is easily detected with a pre-use negative pressure leak test. In this test, with all compressed gas flows set to zero, a suction bulb is attached to the anesthesia machine common gas outlet and repeatedly squeezed until it collapses (picture 1). The bulb will not stay collapsed if there is a leak in any low-pressure components internal to the anesthesia machine, which includes the flowmeters. A leak in the circle breathing system may alert the clinician to a flowmeter leak (table 5), while unexpected gas concentrations in the breathing circuit may alert the clinician to a flowmeter leak or a calibration problem (table 11).

However, this suction bulb leak test will not detect all leaks that can occur within the anesthesia machine. Depending on machine design, certain types of leaks can occur in components that are encased within the interior of the machine and are inaccessible to clinicians [16]. It is also important to check for a leak by pressurizing the breathing system, as explained below. (See 'Breathing system leak' below.)

●Response – If water is seen in the glass flowmeter, cracks in glass flowmeters are noted during the anesthesia machine checkout, or inaccurate flowmeter readings are suspected then the machine should be taken out of service to correct the problem (table 1).

During a case, if the pressure in the breathing circuit cannot be maintained or the gas concentrations in the breathing circuit cannot be controlled, initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder until a new anesthesia machine is available. (See 'Backup equipment for oxygenation and ventilation' below.)

Proportioning system or pressure sensor shut-off (fail-safe) valve malfunction — Proportioning system or pressure sensor shut-off valve (also called fail-safe valve) malfunction could trigger a low inspired oxygen alarm (table 11).

●Potential problems – Each anesthesia workstation has components to prevent administration of a hypoxic gas mixture. A proportioning system (figure 3) prevents the user from setting a hypoxic mixture of oxygen and nitrous oxide. A separate pressure sensor shut-off mechanism automatically turns off the flow of N₂O if the oxygen supply pressure is lost. Rare mechanical failures may occur in these components that would allow administration of a hypoxic gas mixture.

●Prevention – To test the proportioning system function during the pre-use machine checkout, attempt to create a hypoxic mixture by turning up the N₂O flow while the oxygen flowmeter is off. A functioning proportioning system will either keep the N₂O flow off or automatically turn on the oxygen flow, depending on the manufacturer of the workstation.

To test the function of the pressure sensor shut-off mechanism during the pre-use machine checkout, close the backup oxygen cylinder and disconnect the oxygen pipeline connector while all gases (eg, oxygen, air, N₂O) are flowing. A functioning mechanism will automatically decrease or turn off the flow of N₂O, and in some workstations will also turn off the flow of air, in synchrony with the decrease in oxygen flow.

Failure or malfunction of these internal components is rare. For this reason, tests of proportioning system and pressure sensor shut-off valve function have been removed from the ASA Anesthesia Machine Checkout Recommendations (table 1).

●Diagnosis – Proportioning system or pressure sensor shut-off valve malfunction is suspected if a lower than expected fraction of inspired oxygen (FiO2) is noted during the pre-use machine checkout.

During a case, FiO₂ is predictably less than the set FiO₂ whenever the fresh gas flow is less than the patient's minute volume, because of rebreathing. At very low fresh gas flows, this can result in hypoxic inspired oxygen levels within the breathing system, regardless of functioning proportioning and pressure sensor shut-off systems [17]. Therefore, the fresh gas flow should be increased as the first step if the FiO2 is lower than the set value, rather than assuming that there may be a proportioning system or fail-safe valve malfunction.

●Response – If a malfunctioning fail-safe valve or proportioning system is detected, the machine should be taken out of service to correct the problem.

These components are quiescent safety devices that are only active in specific situations. They do not affect the normal function of the anesthesia workstation. If only oxygen is administered, the workstation can be used for the remainder of the case in the unlikely event that such a malfunction is detected after beginning anesthetic administration.

Vaporizer malfunction — Vaporizer malfunction may trigger the apnea-pressure alarm and the low inspired anesthetic agent alarm (table 5 and table 13).

●Potential problems – Vaporizer malfunctions may occur due to human errors during refilling or installation. Although newer anesthesia workstations have automated leak check procedures, these do not check for a vaporizer leak or a leak at the vaporizer manifold unless that vaporizer control is dialed to the "on" position.

Newer vaporizers have features to prevent errors. All have keyed filling systems that discourage refilling the vaporizer with the wrong agent; many have filling systems containing a one-way valve that does not leak when left open. Injector vaporizers, such as all those for desflurane and those that inject liquid anesthetics into the fresh gas flow stream, are unlikely to leak. Some vaporizers alarm when empty.

Common user errors include:

•Failing to refill the vaporizer

•Refilling the vaporizer with the wrong agent [18]

•Failing to close the filler port, resulting in a leak in the breathing system (see 'Breathing system leak' below)

Another user error is failure to turn on the vaporizer (eg, after transport of an unconscious intubated patient into the operating room), resulting in awareness with recall of the surgical procedure. There is no standard alarm warning of absence of a volatile anesthetic agent [19].

Vaporizers are installed on the anesthesia workstation, typically by the anesthesia provider, technician, or assistant. Installation errors may occur due to:

•Improper seating of a compatible vaporizer on the manifold of the anesthesia machine (eg, without an O-ring or askew)

•Installation of a vaporizer that is incompatible with the anesthesia machine

•Installation of a vaporizer in a way that defeats the interlock mechanism that normally prevents turning on more than one vaporizer at a time (eg, if two vaporizers are mounted on a manifold that normally accommodates three vaporizers, with a gap in the middle)

True vaporizer malfunctions include:

•Output of very high concentrations due to tipping or mechanical impact [20]

•Minor miscalibration due to pumping effect or fresh gas flow effects

•Leaks

•Electrical failure or electronic control failure [21]

•Mechanical failure of vaporizer interlock mechanism

●Prevention – A crucial pre-use step in the anesthesia machine checkout is to ensure that all vaporizers are filled, closed, and not leaking. Also, the leak check, either during the anesthesia workstation automated checkout or using a suction bulb, requires checking for a leak with each vaporizer dialed on (one at a time). Furthermore, alarm settings for high and low inspired anesthetic agent concentrations can be set (table 13).

●Diagnosis – Vaporizer misuses and malfunctions are more likely to cause output of a volatile anesthetic that is too low rather than too high. Low inspired anesthetic agent concentration may result in awareness, as well as signs of "light" anesthetic depth (eg, patient movement, tachycardia, hypertension, sweating) and/or a low inspired anesthetic agent alarm (table 13) (see "Accidental awareness during general anesthesia", section on 'End-tidal anesthetic concentration'). A leaking or open vaporizer will create a breathing circuit leak that occurs as soon as the vaporizer is turned on (table 5).

●Response – The problem may be resolved if an empty or loosely capped vaporizer is found and fixed, or the vaporizer can be properly remounted. Vaporizers are easily removable so that a defective vaporizer can be removed from service for repairs. In such cases, anesthetic care can be continued with a replacement vaporizer, or a different vaporizer already mounted on the workstation, or with administration of intravenous agents.

If a large leak does not resolve after vaporizer removal, discontinue use of the anesthesia workstation and initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder. (See 'Backup equipment for oxygenation and ventilation' below.)

Circle breathing system malfunction — In the United States, the majority of general anesthetics are delivered with a circle breathing system (figure 4 and figure 2 and figure 5). Both leaks and obstructions may occur in a circle system.

Breathing system leak — A large breathing system leak should trigger an apnea alarm: apnea-pressure, apnea-flow, and/or apnea-carbon dioxide (CO₂) alarm (table 4 and table 5 and table 6 and table 7). The algorithm provides a systematic response to an apnea-pressure alarm (algorithm 1).

●Potential problems – A large breathing system leak (or a disconnection) is a major malfunction that prevents positive pressure ventilation and anesthetic gas delivery [12]. Small and moderate leaks can be overcome by increasing fresh gas flow, but can cause anesthetic gas pollution of the operating room. Leaks can occur in many breathing system locations including:

•Cracks in disposable tubing

•Loose tubing connection sites

•Loose or cracked gas analyzer sample line

•Poorly seated oxygen sensor

•Cracked or poorly seated CO₂ absorbent container [22]

•Cracked or loose unidirectional valve housing

•Hole in the reservoir bag

•Incompetent adjustable pressure-limiting (APL or "pop-off") valve

•Leaks or disconnects internal to the anesthesia machine (eg, flowmeters, tubing, vaporizers)

●Prevention – Many anesthesia machines test for a circuit leak during the self-test. A quick manual circuit leak test may be performed by occluding the Y-piece and pressurizing the breathing system by pressing the oxygen flush with the reservoir bag attached. However, this circuit leak test is relatively insensitive to small leaks (eg, <100 mL/minute with a generated pressure of 30 cmH₂O) and does not detect internal leaks in some anesthesia machines.

Thus, in anesthesia machines that do not perform an automated leak test, we instead employ a more sensitive method to test for leaks. This involves removing the reservoir bag and occluding the circuit at both the Y-piece and the site of the reservoir bag connection, then pressurizing the breathing system by slowly turning up the oxygen flow. With this test, the leak rate is the oxygen flow rate at the point where the circuit pressure remains steady at 30 cmH₂O. This method also tests for internal leaks in the anesthesia machine that are downstream of the flow meters, including those caused by an anesthetic vaporizer with a dial that is turned on (see 'Vaporizer malfunction' above). A leak >50 mL/min at 30 cmH₂O should prompt a search for the source of the leak.

Notably, newer machines have few visible connections to the fresh gas outlet, ventilator, scavenging system, or airway flow and pressure analyzers. Thus, there is a low risk of misconnection, which was historically a relatively common source of gas delivery system malfunction [23]. These newer anesthesia machines use sensitive methods during the automated pre-use checkout to measure any breathing system leak and the compliance of the system. While the specific method depends on the make and model of the anesthesia workstation, in most cases the breathing circuit is pressurized with the reservoir bag in place, and the machine adjusts the flow of gas into the breathing circuit to maintain a stable circuit pressure as measured by a sensitive electronic gauge. However, as noted above, these automated machine pre-use checks will not detect a leak in the vaporizer or its manifold unless repeated with each vaporizer turned on. (See 'Vaporizer malfunction' above.)

●Diagnosis – A large leak manifests as an empty reservoir bag or ventilator bellows (with inability to ventilate due to leakage of respiratory gases), as well as sounding of apnea alarms (table 6 and table 7 and table 4 and table 5). During anesthetic administration, new onset breathing system leaks are most commonly due to an imperfect seal or disconnection around the patient's airway device (eg, facemask, laryngeal mask airway [LMA], or endotracheal tube [ETT]). After rapidly ruling out this most common cause, the potential problems listed above must be rapidly checked (eg, hole in the breathing system tubing, disconnection of the tubing, open or leaking vaporizer).

●Response – Switch to manual ventilation, fill the breathing circuit with oxygen flush or high fresh gas flows, look for a breathing circuit disconnection, and briefly disconnect the patient from the breathing circuit to perform a circuit leak test, as described above. If the cause is not immediately found and fixed, discontinue use of the anesthesia workstation and initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder. (See 'Backup equipment for oxygenation and ventilation' below.)

Fresh gas decoupled breathing systems, found on modern Dräger (ie, Draeger) anesthesia machines (figure 5 and figure 7), do not pressurize the absorbent canister or reservoir bag during mechanical ventilation. If a large leak occurs in the canister or the reservoir bag is missing, the patient can be mechanically ventilated, but room air will be entrained [22,24].

Breathing system obstruction — Breathing system obstruction is a rare but severe malfunction (table 4 and table 5 and table 6 and table 7).

●Potential problems – Obstruction may occur due to user error, malfunction of a unidirectional valve, defects in the breathing system hose or CO₂ absorbent canister.

●Prevention – A crucial step in the pre-use anesthesia machine checkout is to verify that gas flows properly through the breathing system during both inspiration and exhalation (table 1). This is accomplished by connecting a second reservoir bag at the patient Y-piece connection site to serve as a "test lung," and then ensuring that breaths can be delivered both manually and with the anesthesia machine ventilator without evidence of obstruction manifesting as inability to inflate the test lung, high airway pressure, or slow emptying of the test lung.

●Diagnosis – Obstruction manifests as inability to manually ventilate due to obstruction to air flow (or a non-moving ventilator bellows), as well as sounding of apnea alarms (table 6 and table 7 and table 4 and table 5), and sometimes high or continuous airway pressure alarms (table 8 and table 9), depending on the site of the obstruction.

Diagnosis must rule out obstruction due to patient-related issues, which are more common. Examples include bronchospasm, endobronchial intubation, or a problem with the ETT such as kinking or mucous plugging.

●Response – Check for user errors that can obstruct the breathing system (eg, an occlusive plug that was left on the inspiratory port during the pre-use checkout procedure (picture 2), CO₂ absorbent that was placed in the absorbent container without unwrapping the product, disposable tubing wrapping getting caught at a connection site, backward insertion of a unidirectional positive end-expiratory pressure [PEEP] valve into the breathing circuit). Also, check for visual evidence of a unidirectional valve that might be stuck in the closed position or a manufacturing defect affecting patency of the disposable tubing. During long cases, water can accumulate in breathing circuit hoses or the absorbent canister [25], causing low-grade obstruction or inadvertent PEEP.

If the cause is not immediately found and fixed, discontinue use of the anesthesia workstation and initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder. (See 'Backup equipment for oxygenation and ventilation' below.)

Carbon dioxide absorbent exhaustion or toxicity — Carbon dioxide (CO₂) absorption is used in all circle breathing systems (figure 4 and figure 2 and figure 5) to prevent hypercapnia due to rebreathing of CO₂ in previously exhaled gas [26]. These absorbents all contain calcium hydroxide, which ultimately reacts with the CO₂ to yield calcium carbonate. Older types of absorbents (eg, Baralyme, soda lime) contain strong bases (eg, sodium hydroxide, potassium hydroxide, barium hydroxide), which accelerate this process by reacting with CO₂ to form a carbonate. These strong bases can also react with volatile anesthetics to form toxic substances.

Newer types of absorbents (eg, Amsorb, LoFloSorb, SpiraLith) do not contain such strong bases and thus do not react with volatile anesthetic agents.

●Potential problems

•Exhaustion – Failure to change the absorbent when it is exhausted leads to CO₂ rebreathing and hypercarbia. Absorbents become exhausted more quickly if fresh gas flow rates are low [27]. Absorbents become less alkaline as they absorb carbon dioxide, and an incorporated colorimetric pH indicator changes color to purple, signaling that the absorbent is exhausted. Notably, soda lime reverts to white during periods of non-use even though it remains exhausted. An alternative to using the indicator to guide when to change the absorbent is to change the absorbent when the inspired CO₂ concentration reaches around 5 mmHg or 0.5 percent or 0.5 KPa, depending on the units being used [28]. While a slow increase in inspired CO₂ concentration usually indicates exhausted absorbent, there are other causes of high inspired CO₂, as detailed in the table (table 12 and algorithm 3) [26].

A risk associated with changing an absorbent canister during use is that an unintended leak can be introduced if the canister is cracked [22]. A temporary method to overcome increasing levels of inspired CO₂ due to exhausted absorbent is to increase the fresh gas flow rate until the absorbent can be exchanged.

•Reaction with volatile anesthetic agents – Barium hydroxide lime and soda lime absorbents contain strong bases that react with volatile anesthetic agents, particularly when the absorbent becomes desiccated. Absorbent desiccation occurs when fresh gas is left flowing into the breathing system during periods of non-use, especially if the reservoir bag is not attached. All volatile anesthetics can break down to produce carbon monoxide (desflurane > isoflurane > sevoflurane) and heat. Enough heat has been produced by the reaction of sevoflurane with desiccated barium hydroxide lime to cause combustion within the absorbent canister [29,30] (see "Fire safety in the operating room", section on 'Other potential ignition sources'). Because of these concerns, Baralyme has been removed from the United States market.

In addition, sevoflurane can break down in desiccated soda lime to produce Compound A, which is nephrotoxic in rats. However, no clinically significant renal toxicity has been observed in humans (see "Maintenance of general anesthesia: Overview", section on 'Sevoflurane'). Regardless, the sevoflurane package insert (US Food and Drug Administration [FDA] mandated) states that fresh gas flows of at least 2 liters per minute must be used for cases more than two hours duration to limit exposure to Compound A. In our practice, sevoflurane is administered at low flow with absorbents (eg, Amsorb, LoFloSorb, SpiraLith) that do not contain strong bases.

Because of these potential reactions, less reactive absorbents are used in many institutions.

•Dust – Absorbents can release dust into the breathing system that can become deposited on valves causing them to fail over time [31].

●Prevention

•Exhaustion – The color of the absorbent should be checked during the pre-use checkout and at the end of each case. If it is violet, purple, or blue, rather than white, then the absorbent should be replaced before the case (table 1) unless it is in a quick-connect canister. Some anesthesia workstations have quick-connect absorbent canisters designed to be exchanged during an anesthesia case so that it is safe to continue use of the partially exhausted absorbent temporarily, until inspired CO₂ reaches some threshold value (typically 5 mmHg).

•Reaction with volatile anesthetic agents – Since there is no easy way to identify desiccation of CO₂ absorbent, the canister should be changed if the anesthesia provider or technician notices that compressed gas flow (eg, oxygen, air, or N₂O flow) was left on during a period when that anesthesia workstation was not in use. Low alkalinity absorbents may change color when desiccated but are still safe to use [32].

●Diagnosis

•Exhaustion – High inspired CO₂ will be evident on the capnometer, an anesthesia monitoring standard that may be integrated into the anesthesia workstation. If properly set, a high inspired CO₂ alarm alerts the clinician regarding exhausted CO₂ absorbent (table 12). The diagnosis is confirmed if the problem disappears after increasing the fresh gas flow.

•Reaction with volatile anesthetic agents – There is no easy way to test for the presence of carbon monoxide or Compound A in the breathing circuit, but there may be other signs of anesthetic breakdown such as low agent concentration in the breathing system, high temperature of the absorbent canister, or color change of the absorbent at the opposite end of where it typically changes color [32]. Although the patient exposed to high levels of carbon monoxide will have high carboxyhemoglobin levels measured by co-oximetry, the signs of carbon monoxide poisoning in an anesthetized patient are typically too subtle to detect.

●Response

•Exhaustion – If absorbent exhaustion is suspected, increase the fresh gas flow rate immediately to reduce rebreathing until the inhaled CO₂ concentration drops below 5 mmHg, and increase minute ventilation as needed to reduce exhaled CO_2. If inspired CO₂ is not rapidly reduced by these measures, the problem is CO₂ rebreathing from another source, such as an incompetent one-way valve.

•Reaction with volatile anesthetic agents – If anesthetic agent breakdown by desiccated absorbent is suspected, change the absorbent immediately (while ventilating with a self-inflating manual ventilation device and an auxiliary oxygen cylinder while the breathing circuit is opened to rapidly change the absorbent). Then administer 100% oxygen to the patient and obtain a venous or arterial blood gas with co-oximetry to check for carboxyhemoglobin.

Valve malfunctions

Adjustable pressure-limiting valve — The adjustable pressure-limiting (APL) valve, also known as the "pop-off" valve, is the interface between the breathing system and the scavenger system during spontaneous or manual ventilation (figure 4 and figure 2 and figure 5). This valve allows the anesthesia provider to control how much gas exits the breathing system, which is one of the factors determining the gas volume and pressure within the reservoir bag during this mode of ventilation.

●Potential problems – Although the APL valve rarely malfunctions, it can remain open when its control knob is closed [33] (resulting in a massive circuit leak) or fail to open after closure (resulting in excessive continuous airway pressure). APL valve settings may not be linear, so the dial position does not necessarily set a dependable airway pressure. Thus, continuous monitoring of airway pressure is necessary [34].

●Prevention – The APL valve is checked during the pre-use checkout by pressurizing the breathing system (which tests that the APL valve closes and does not leak), and then opening the APL valve and observing that the breathing system pressure falls to zero (which tests that the APL valve opens fully).

●Diagnosis – An incompetent (leaking) APL valve should be suspected if there is a large circuit leak with an inability to generate positive pressure by squeezing the reservoir bag, particularly if other possible sites of disconnection have been ruled out. It can be confirmed by filling the bag with the oxygen flush and observing a rush of gas into the scavenger system when the bag is squeezed. Notably, the APL valve is included in the breathing circuit only during the spontaneous/manual ventilation mode.

An obstructed APL valve should be suspected if inspiratory and expiratory pressures build up in the breathing circuit when the APL valve is open.

●Response – If the APL valve is stuck in either the open or closed position, ventilation is not possible in the spontaneous/manual ventilation mode. However, ventilation should be possible by switching to a controlled ventilation mode.

A reasonable alternative is to discontinue use of the anesthesia workstation and initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder, and administer intravenous anesthetic agents. (See 'Backup equipment for oxygenation and ventilation' below and "Maintenance of general anesthesia: Overview", section on 'Total intravenous anesthesia'.)

Unidirectional (inspiratory and expiratory) valves — Inspiratory and expiratory one-way valves (figure 4 and figure 2 and figure 5) prevent rebreathing of previously exhaled CO₂.

●Potential problems – If a unidirectional valve is incompetent, it may not close completely, which results in the patient inhaling CO₂. In particular, expiratory valve incompetence may result in a high concentration of inspired CO₂ because of the large volume of exhaled gas that is contained in the tubing and reservoir bag on the expiratory side of the breathing system (algorithm 3) [28].

●Prevention – Presence of the unidirectional valves should be confirmed during the pre-use check. Proper function of unidirectional valves may not be detected during a pre-use check since subtle valve incompetence is difficult to recognize with visual inspection. However, unidirectional valve incompetence can be detected using the three-step modified pressure decline method [35], which is not a part of the ASA Anesthesia Machine Checkout Recommendations (table 1). This method may not be used daily during a pre-use check, but the procedure should be performed at regular intervals by a trained technician.

The test must be performed after routine system leak checks. Step 1 tests for inspiratory valve occlusion: With fresh gas flows set to zero and the APL valve closed, a second reservoir bag is attached to the inspiratory hose mount and the circuit is pressurized to 30 cm H₂0 with the oxygen flush. If the second reservoir bag does not inflate, then the inspiratory valve is occluded. Step 2 tests for expiratory valve incompetence: If the primary reservoir bag on its normal mount starts deflating over 20 seconds (ie, faster than it deflated during the routine breathing circuit pressure test), then the expiratory valve is incompetent. Step 3 tests for inspiratory valve incompetence: The APL valve is opened. If the secondary reservoir bag starts deflating over 20 seconds, then the inspiratory valve is incompetent [35].

●Diagnosis – Manifestations of unidirectional valve incompetence include an increasing inspiratory CO₂ level that does not decrease when the fresh gas flow is turned up (table 12), as well as delayed return or failure of the capnogram to zero before onset of the next inspiration, as shown in the figure (figure 8) (see "Basic patient monitoring during anesthesia", section on 'Capnography'). Some anesthesia machines also have flowmeters in the inspiratory and/or expiratory limb of the breathing circuit that generate an alarm if flow in the wrong direction is detected.

Unidirectional valves can also become stuck in the closed position, resulting in obvious total occlusion of the breathing system. (See 'Breathing system obstruction' above.)

●Response – If valve incompetence is detected during use, minute ventilation can be increased to compensate for CO₂ rebreathing. If the amount of rebreathing is too large to be safely tolerated by the patient, the anesthesia machine should be taken out of service (table 1). In such cases, initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder until a replacement anesthesia machine is available. (See 'Backup equipment for oxygenation and ventilation' below.)

Positive end-expiratory pressure valve — In modern anesthesia machines, electronically-controlled positive end-expiratory pressure (PEEP) valves are integrated into the anesthesia ventilator (eg, ventilator-controlled PEEP/maximum pressure valve in the figure (figure 2) and PEEP/Pmax valves and PEEP/APL valve in the figures (figure 5 and figure 7 and figure 9)). In older machines built prior to 2000, mechanical PEEP valves were built into the breathing system, or even added to the breathing system on an as needed basis.

●Potential problems – The primary hazard of all types of PEEP valves is that PEEP may be unknowingly enabled if the ventilator settings remain unchanged after a previous case, rather than being reset for the current patient.

Additional hazards for removable mechanical valves that were inserted into the expiratory limb of the breathing system include:

•Erroneous insertion into the inspiratory limb of the breathing circuit, resulting in no PEEP.

•Backward insertion into either the inspiratory or expiratory limb of the breathing circuit, causing complete obstruction of flow if the valve only allows unidirectional flow.

•Failure to recognize that PEEP is not registering even though the patient is receiving PEEP. This can occur when the airway pressure gauge senses pressure downstream of the PEEP valve (eg, in the CO₂ absorbent canister or within the expiratory tubing mount).

●Prevention – During the pre-use checkout, verify that the airway pressure declines to zero during exhalation. If not, check for the cause of inadvertent PEEP.

●Diagnosis – Unexpected PEEP is indicated on the airway pressure gauge, and fluctuations in end expiratory pressure may indicate a sticking or malfunctioning valve [31]. A continuous airway pressure alarm may be triggered for high PEEP (table 9).

If PEEP is enabled due to settings used in a previous case, this typically results in a stable end expiratory airway pressure over multiple breaths. Conversely, an obstruction in the breathing system exhaust (at the APL valve, ventilator relief valve, or scavenger) results in a gradually increasing level of PEEP proportional to the fresh gas flow rate into the breathing circuit. Complete obstruction to flow in the expiratory limb of the breathing circuit results in a rapid increase in PEEP up to the level of the peak inspiratory pressure.

●Response — In the event of very high PEEP levels (>20 cmH₂O), the patient should be rapidly disconnected from the breathing system to prevent barotrauma and cardiovascular collapse. The patient can then be reattached for two breaths to attempt to find the source of the obstruction. If the source is not quickly found and fixed, then positive pressure ventilation is initiated with a self-inflating manual ventilation device and an auxiliary oxygen cylinder until a replacement anesthesia machine is available. (See 'Backup equipment for oxygenation and ventilation' below.)

Ventilator malfunction — Anesthesia ventilators and breathing systems facilitate some rebreathing of previously exhaled gas (after removal of CO₂), unlike intensive care unit (ICU) ventilators that deliver fresh gas during every breath.

Modern anesthesia ventilators are controlled by embedded computer processors that use sensor feedback from the breathing system to provide many of the same modes of ventilation used in the ICU. (See "Mechanical ventilation during anesthesia in adults".)

Anesthesia ventilator and breathing system technology has changed dramatically since 2000, and various manufacturers have taken adopted radically different designs (figure 5):

●Anesthesia machines manufactured by GE Healthcare (formerly Datex-Ohmeda), as well as many older anesthesia machines, have a traditional circle breathing system and a ventilator that powers a bellows (figure 4 and figure 10). In these machines, the ventilator delivers drive gas during the inspiratory phase, which compresses the bellows containing respiratory gases. During the expiratory phase, the ventilator releases drive gas into the room, thereby allowing the bellows to re-expand with exhaled gas from the patient. The bellows expands until full and the pressure within exceeds the pressure outside by more than 3 cmH₂O, whereupon excess gas exits the breathing circuit via the ventilator relief valve. Thus, the bellows ventilator exerts a minimal obligatory PEEP of 3 cm H₂O.

●Anesthesia machines manufactured by Dräger (ie, Draeger) anesthesia workstations have a nontraditional circle breathing system with a ventilator that is driven by a piston or a turbine, rather than by drive gas outside of a bellows (figure 2 and figure 5 and figure 7). These ventilators are electrically powered and do not consume fresh gas; therefore, they continue to function in the event of oxygen supply pressure loss (see 'Compressed gas supply malfunction' above). Excess gas bypasses the APL valve and exits the breathing circuit into the scavenger through a single very low-pressure (<1 cmH₂O) exhaust valve. Thus, the ventilator exerts no obligatory PEEP.

●Anesthesia machines manufactured by Getinge (formerly MAQUET) such as the Flow-i anesthesia workstations have a volume reflector in place of a bellows. During inspiration, drive gas enters the back-end of the reflector which compresses gas toward the patient so that there is minimal mixing of patient gases with drive gases within the reflector. On exhalation, drive gas and excess patient gas exit out the back-end of the reflector into the scavenger through a single very low-pressure exhaust valve. Thus, the ventilator exerts no obligatory PEEP.

During mechanical ventilation, the ventilator relief valve (not the APL valve) is the interface between the breathing system and the scavenger system. Examples are the ventilator exhaust valve and ventilator-controlled exhaust valves in the figures (figure 2 and figure 4), or the exhalation valve, exhaust valve, and PEEP/APL valve (figure 5) (see 'Waste gas disposal (scavenger) system malfunction' below). Ventilator relief valves, unlike APL valves, do not open during the inspiratory phase of ventilation. Thus, fresh gas flows continuously into a traditional circle breathing circuit, thereby contributing to the patient’s total inspired tidal volume, and changes in the fresh gas flow can significantly alter delivered tidal volume. Modern anesthesia machines use one of two approaches to prevent changes in delivered tidal volume when fresh gas flow is changed:

●Dräger Fabius, Tiro, and Apollo workstations use modified "fresh gas decoupled" circle breathing systems where fresh gas enters the breathing circuit upstream of the location of the ventilator supply of inspiratory flow. Thus, fresh gas flow is isolated from inspired tidal volume (figure 5). On machines with a fresh gas decoupled breathing circuit, the oxygen flush can be activated without affecting delivered tidal volume.

●All GE Healthcare anesthesia workstations, as well as the Getinge Flow-i and Dräger Persius anesthesia workstations use fresh gas compensation to adjust the volume of drive gas or turbine flow that is delivered (figure 7 and figure 10 and figure 9). Older GE workstations adjust drive gas over sequential breaths to maintain a set inspired tidal volume. The Perseus, Flow-i, and newer GE ventilators instantaneously adjust ventilator gas flow based on measured fresh gas flow. Of the anesthesia machines with fresh gas flow compensation, only the Perseus compensates during oxygen flush. In the others, activating the oxygen flush during ventilator inspiration dramatically increases the delivered tidal volume.

●Potential problems – Anesthesia ventilators are relatively complex electromechanical devices that are generally reliable but occasionally fail. There are over 150 reports from 2007 through 2016 in the FDA Manufacturer and User Facility Device Experience (MAUDE) database involving the anesthesia ventilator. In many of these cases the ventilator suddenly stopped working, and in some cases the screen suddenly went blank. Cell phones and ultrahigh frequency radio communication devices may affect electronically controlled anesthesia ventilator output and displays.

•Hypoventilation or apnea (table 6 and table 7 and table 4 and table 5):

-Sudden cessation of ventilator activity, with or without loss of the integrated display screen and alarm function [36]

-Loss of patient ventilation due to valve failure while the ventilator continues to cycle [37]

-User error by temporarily turning off the ventilator and forgetting to turn it back on

-Inadequate inspiratory pressure due to user error in setting the high pressure limit

-Leakage of patient gas from a hole in the ventilator bellows, improperly seated bellows, break in the bellows housing, or malfunctioning ventilator pressure relief valve causing a leak

-Addition of PEEP, which may decrease the delivered tidal volume in ventilators without automatic adjustment of peak airway pressure

•Hyperventilation or high airway pressure (table 8):

-User error in activating the oxygen flush during the inspiratory phase of ventilation

-Fresh gas flows that are too high during mechanical ventilation (this is possible only in older anesthesia machines that do not have either a fresh gas compensated ventilator or a fresh gas decoupled breathing system)

-Malfunctioning ventilator relief valve causing obstruction

-Obstruction in the scavenger system

•Dilution of anesthetic gas (table 13):

-Entrainment of room air into the breathing system by a piston ventilator, turbine ventilator, or a ventilator with a descending bellows

-User error in refilling the breathing circuit after a disconnection using the oxygen flush.

-Leakage of drive gas into an improperly seated bellows or one with a hole

●Prevention – During the pre-use checkout, verify that the ventilator is able to ventilate a test lung or second reservoir bag attached to the patient connector (Y-piece). Measured inhaled and exhaled volumes should be approximately equal to the set tidal volume on the ventilator. If not, check ventilator connections and function, and check inspiratory and expiratory valve function. For example, an incompetent inspiratory valve will cause a lower than expected exhaled tidal volume. The last step in the pre-use check is to confirm that ventilator settings are appropriate for the patient.

Habits that decrease the chance of forgetting to resume ventilation after a temporary period of apnea are employed to prevent user error. One technique is to disconnect the breathing circuit rather than turning off the ventilator, leaving all apnea alarms active and all ventilator settings intact. Another technique is to leave one's hand on the ventilator control during the period of apnea or to hold one's own breath. Some anesthesia workstations have a control to temporarily suspend ventilation for a fixed duration (typically one minute). It is never wise to disable apnea alarms while ventilation is briefly suspended. The ventilator bellows should be refilled by temporarily turning up the fresh gas flow rather than by pressing the oxygen flush.

Checklists are useful to ensure that ventilation is resumed after a more prolonged period of apnea (eg, during weaning from cardiopulmonary bypass [CPB]). (See "Weaning from cardiopulmonary bypass", section on 'Checklist'.)

●Diagnosis – Usually, problems with mechanical ventilation are resolved by switching to manual ventilation (ie, moving the selector switch from "mechanical" to "spontaneous/manual" ventilation). This maneuver replaces the bellows, piston, or turbine and the ventilator relief valve with the reservoir bag and its APL valve. Thus, leaks and obstructions in the former are removed from the system. If the problem is not resolved by switching to manual ventilation, then the problem is probably not in the ventilator components of the system.

●Response – If the ventilator suddenly stops working, immediately switch to manual ventilation. If the problem does not resolve rapidly, initiate positive pressure ventilation with a self-inflating manual ventilation device and an auxiliary oxygen cylinder, and administer intravenous anesthetic agents (see 'Backup equipment for oxygenation and ventilation' below and "Maintenance of general anesthesia: Overview", section on 'Total intravenous anesthesia'). With the patient disconnected, the anesthesia machine can be powered off and restarted; such a "reset" may resolve the problem. If not, replacement of the anesthesia machine is the best alternative.

Waste gas disposal (scavenger) system malfunction — The primary purpose of the waste anesthetic gas disposal (WAGD) system (commonly termed the scavenger system) is to prevent excessive positive or negative pressure at the outlet of the exhaust valves (ie, downstream of the APL valve and ventilator relief valve) (figure 11). The secondary purpose is to prevent venting of waste anesthetic gases into the operating room; rather, these gases are evacuated from the building.

●Potential problems – The scavenger system can malfunction due to:

•Inadequate or absent suction flow (causing waste gas to leak into the room, resulting in room pollution, and, in some makes of anesthesia workstations, additional PEEP in the breathing circuit).

•Passive waste gas evacuation (eg, waste gas passed through activated charcoal) used with an open scavenger interface.

•Exhausted activated charcoal (when used for waste gas capture).

•Excessive suction flow (causing background noise and wasting suction).

•High pressure at the breathing circuit exhaust valve (ie, APL valve or ventilator relief valve) due to [38]:

-Inadequate scavenger suction flow if the outflow safety valve is obstructed or

-Scavenger obstruction proximal to the outflow safety valve.

This situation typically causes gradually increasing breathing circuit PEEP and peak airway pressures, with the increase being more rapid with high fresh gas flow rates.

•High vacuum suction at the breathing circuit exhaust valve (ie, APL valve or ventilator relief valve) due to a blocked inflow safety valve. This situation typically sucks gas out of the breathing circuit, creating negative airway pressure. High vacuum suction at the breathing circuit exhaust valve (ie, APL valve or ventilator relief valve) can paradoxically cause the valve to become stuck closed, which increases breathing circuit PEEP and peak airway pressures.

•Misconnections to the breathing system (causing breathing system obstruction).

●Prevention – Scavenger system function should be checked daily to ensure that the suction flow setting is adequate and the positive and negative pressure relief mechanisms are functioning properly.

•With a closed scavenger system (figure 12), the scavenger reservoir bag is usually totally deflated, indicating that the vacuum is connected and the negative-pressure relief valve is open. During the pre-use check, the outflow safety valve can be checked by opening the APL valve, obstructing the Y-piece, and pressing the oxygen flush; this will cause gas to rapidly pass out of the breathing circuit and into the scavenger system. With this test, the scavenger reservoir bag should fill but not continue to inflate, which indicates that the scavenger system is connected to the breathing circuit and that the positive-pressure relief valve is opening.

•With an open scavenger system, the vacuum flow should be adjusted so that the integrated flowmeter float is between the two setting lines. During the pre-use check, scavenger openness can be checked by opening the APL valve, obstructing the Y-Piece, and pressing the oxygen flush; the breathing circuit reservoir bag should fill but not inflate.

●Diagnosis – Malfunction of the scavenger system may be indicated by negative pressure in the breathing system manifesting as an airway pressure less than zero, a collapsed reservoir bag or bellows, and/or a negative airway pressure alarm (table 10). Conversely, obstruction of the scavenger system outflow and outflow safety valve may result in a hyper-inflated reservoir bag and continuous airway pressure on the airway pressure gauge, as well as a continuous airway pressure alarm (table 9). (See 'Airway pressure' below.)

●Response – With a negative airway pressure alarm, immediately disconnect the patient from the breathing system and ventilate with a self-inflating manual ventilation device and an auxiliary oxygen cylinder until the source of the negative pressure is found and fixed (eg, scavenger malfunction, gastric suction misplaced into the airway, suctioning during bronchoscopy) (table 10).

Likewise, with a continuous airway pressure alarm, immediately disconnect the patient from the breathing system and ventilate with a self-inflating manual ventilation device and an auxiliary oxygen cylinder until the source of the obstruction is found and fixed (eg, scavenger system obstruction or malfunction, or obstruction of the APL valve, ventilator relief valve, or expiratory valve) (table 9).

Leakage from the scavenger system is difficult to detect, but there may be a smell of anesthetic agents in the room. Look for evidence that the scavenger system is not connected properly or that the waste anesthetic gas vacuum is disconnected or turned too low. Conversely, a loud background hissing from the scavenger system indicates that the vacuum is turned too high.

Mapleson breathing system malfunction — In the United States, Mapleson breathing systems are used much less often than circle breathing systems during anesthesia, but are more commonly employed for transport of small children and neonates. In other countries, a modified Mapleson D system (the Bain system) is often used during anesthesia. Anesthesia providers using different types of Mapleson circuits must be specifically trained in their use because different circuit configurations, ventilation modes, fresh gas flow rates, and minute ventilation all interact to affect the partial pressure of arterial CO₂ (PaCO₂) level [39,40]. In general, high fresh gas flow rates must be used to prevent hypercapnia.

Modified Mapleson D (Bain) circuit — In a Mapleson D circuit, fresh gas enters near the patient connector and the APL valve is located at the end of the corrugated tube that the patient breaths in and out through, near the reservoir bag, or at the end of the reservoir bag itself. The Bain modification has the fresh gas tube running inside the corrugated tube (a coaxial arrangement) (picture 3).

●Potential problems – A particular hazard of the Bain circuit is that the inner fresh gas tube can become detached from its connection at either end, develop a leak, or become obstructed. These problems with the inner tube of the circuit may prevent fresh gas from flowing out of the distal end of the system, which results in hypercapnia because the outer corrugated tube becomes dead space.

●Prevention – To confirm the integrity of the inner tube, it should be occluded during the pre-use check by placing a finger over the distal end while oxygen is set at a low flow. If the tube is properly connected and intact, the oxygen flowmeter float will slowly fall due to the increasing backpressure.

●Diagnosis – Capnometry is important when using any Mapleson circuit to adjust the end-tidal CO₂ by titrating the amount of CO₂ rebreathing. For accuracy, the capnometer sample should be taken as close to the patient's airway as possible (for instance, distal to the elbow connector or from within the endotracheal tube) to minimize fresh gas dilution of exhaled gas. Some degree of rebreathing is expected, which results in a distinctive capnogram, as shown in the figure (figure 13). However, hypercapnia is typically prevented by increasing the fresh gas flow rate, increasing minute ventilation, and decreasing the inspiratory to expiratory ratio.

In some patients, it may not be possible to achieve high enough fresh gas flow rates. Examples include large patients, those with high metabolic rates (eg, fever, malignant hyperthermia), or if there is malfunction preventing delivery of fresh gas flow to the distal end of the circuit (eg, disconnected fresh gas line, leak in fresh gas line).

●Response – If end-tidal CO₂ cannot be adequately controlled by increasing the fresh gas flow rate, then the patient should be ventilated with another breathing system that includes valves that prevent CO₂ rebreathing (eg, a circle breathing system or a self-inflating manual ventilation device and an auxiliary oxygen cylinder. (See 'Circle breathing system malfunction' above and 'Backup equipment for oxygenation and ventilation' below.)

Mapleson E (T-piece) circuit

●Potential problems – A particular hazard of the T-piece is that the expiratory port can become occluded simply by placing a hand over it to inflate the lungs. This may lead to pulmonary overinflation and barotrauma because:

•Gas will keep flowing from a flowmeter or anesthesia machine fresh gas outlet until the outlet pressure is equal to the supply pressure (nominally 50 psig, which equals 3515 cmH₂O) (picture 4).

•There is no pressure-buffering effect of a reservoir bag in a T-piece, unlike other Mapleson circuits or circle system.

•The clinician cannot accurately judge the airway pressure with tactile sensation of the hand exposed to the expiratory port.

There are 11 reports of this type of overinflation and/or barotrauma injury in the ASA Closed Claims database from 1990 through 2011; each of these incidents occurred with misuse of a Mapleson E (T-piece) circuit or supplemental oxygen delivery outside the operating room [2].

●Prevention – The key to preventing overinflation or barotrauma by this mechanism is to recognize that a flowmeter or anesthesia machine directly connected to the lungs will pressurize to thousands of cmH₂O. To prevent patient injury, there must always be an outlet for that pressure, as provided by the open end of a T-piece or a compliant reservoir bag or a safety exhaust valve.

●Diagnosis – Injuries of this type result in pneumothorax, bilateral tension pneumothorax, pneumomediastinum, cardiac tamponade, and cardiorespiratory collapse.

●Response – Prompt diagnosis and venting of the chest cavity (eg, needle decompression, chest tubes) is potentially life-saving. (See "Intraoperative management of shock in adults", section on 'Tension pneumothorax or hemothorax' and "Intraoperative management of shock in adults", section on 'Pericardial tamponade'.)

MALFUNCTION OF INTEGRATED ANESTHESIA MONITORS — Anesthesia workstations have integrated monitors that may malfunction. Each monitor displays selected parameters on an integrated screen and has one or more alarms to alert the clinician to a potential problem. (See 'Anesthesia workstation alarms' above.)

Oxygen analyzer — All anesthesia workstations must have an oxygen analyzer either in the inspiratory limb of the breathing system, or in the multigas analyzer. These devices monitor the fraction of inspired oxygen (FiO₂). Some utilize an electrochemical (polarographic or galvanic) sensor cell, and the FiO₂ reading appears as a continuously updated digital display on the screen of the anesthesia workstation. A low inspired oxygen alarm occurs if FiO₂ falls below the lower set limit (table 11). (See "Mechanical ventilation during anesthesia in adults", section on 'Fraction of inspired oxygen'.)

Some newer anesthesia workstations have an integrated multigas analyzer that samples gas from the airway and analyzes both inhaled and exhaled oxygen concentrations, as well as the concentrations of other respiratory gases (eg, carbon dioxide [CO₂], nitrous oxide [N₂O], volatile anesthetic agents). Typically, these multigas analyzers use a paramagnetic oxygen sensor that has a faster response time than electrochemical ones.

●Potential problems

•Malfunctions of electrochemical oxygen analyzers may be caused by inaccurate calibration, an exhausted sensor, or poor electrical connections (typically between the sensor and monitor).

•Malfunction of a paramagnetic oxygen analyzer may be caused by inaccurate calibration, dilution of respiratory gases due to entrainment of room air through a leak in the sampling line, or obstruction or disconnection of the sampling line.

●Prevention – Oxygen analyzer sensor cells must be calibrated at least once per day during the pre-use checkout. Calibration is accomplished by exposing the sensor to room air after the monitor has warmed up, and ensuring that the FiO₂ reading is at 21 percent. An important follow-up step is to then expose the sensor to 100 percent oxygen to ensure that the FiO₂ reading approximates 100 percent. If the reading is <90 percent, the sensor cell should be replaced before the next anesthetic administration. All sensor cells must be replaced at intervals for proper function. Although they do not require frequent recalibration, the oxygen channel of a multigas analyzer should also be checked daily with both room air and 100 percent oxygen. If flushing with 100 percent oxygen does not result in a monitor reading of approximately 100 percent, a wrong gas situation should be strongly suspected and another analyzer should be used to determine whether the oxygen flowmeter and/or oxygen flush are actually dispensing oxygen. (See 'Compressed gas supply malfunction' above.)

●Diagnosis – During anesthetic administration, the FiO₂ may fall below the lower set limit due to causes such as compressed gas supply problems in the pipeline or cylinder (see 'Compressed gas supply malfunction' above), flowmeter malfunction (see 'Compressed gas flowmeter malfunction' above), proportioning system or fail-safe valve malfunction (see 'Proportioning system or pressure sensor shut-off (fail-safe) valve malfunction' above), or air entrainment into the breathing system (see 'Circle breathing system malfunction' above). At low fresh gas flow rates, the FiO₂ is less than the oxygen concentration set on the flowmeters (termed "delivered oxygen concentration" [FdO₂]) because of rebreathing of exhaled gas that has a lower oxygen concentration than fresh gas. Because of this, the FiO₂ may decrease below 21 percent over time [17].

●Response – Immediately increase the oxygen flow rate and decrease the flow rate of other gases. If FiO₂ does not increase, check for possible wrong compressed gas administration (see 'Compressed gas supply malfunction' above); if confirmed, initiate positive pressure ventilation with a self-inflating manual ventilation device and a fresh oxygen cylinder (see 'Backup equipment for oxygenation and ventilation' below). Do not use the auxiliary flowmeter on the anesthesia workstation since it has the same oxygen source as the breathing circuit.

Capnometer and respiratory gas analyzers — A capnometer measures the partial pressure of CO₂ in gas that is continuously sampled from the airway and displays both the nadir during inspiration and the peak (end-tidal) CO₂ values. The capnogram appears as a continuously updated waveform whose shape contains information not necessarily apparent in the digital values (figure 14 and figure 15). Alarms for apnea (table 7), high inspired CO₂ (table 12), and high and low end-tidal CO₂ are integrated into the anesthesia workstation. (See "Basic patient monitoring during anesthesia", section on 'Capnography'.)

Some newer anesthesia workstations have multigas analyzers that measure CO₂, N₂O, and halogenated anesthetic agent concentrations using infrared spectroscopy, as well as oxygen concentration using a paramagnetic sensor (see 'Oxygen analyzer' above). These multigas analyzers actually measure the partial pressure of each gas (mmHg or KPa), and calculate gas concentrations (%) using the current barometric pressure measured by an integrated barometer. Inspired and exhaled digital values for each gas are displayed, as determined from the inspiratory and expiratory timing of the capnogram.

●Potential problems – Similar to O₂ analyzers, inaccurate readings for end-tidal CO₂ may be caused by dilution of respiratory gases due to entrainment of room air via a leak in the sampling line. Obstruction or disconnection of the sampling line causes the apnea-CO₂ alarm to sound (table 7).

The following factors affect end-tidal CO₂ readings:

•Although end-tidal CO₂ generally correlates well with the partial pressure of arterial CO₂ (PaCO₂), a number of factors (eg, age, lung disease, surgical positioning) affect the capnogram and can cause a significant and variable discrepancy (figure 16). Thus, differences of 8 to 10 mmHg between end-tidal CO₂ and PaCO₂ may occur.

•CO₂ and multigas analyzers are typically self-zeroing, which causes loss of the visual display of the waveform at regular intervals.

•CO₂ and multigas analyzers typically sample gases from the airway (termed sidestream analyzers). Disconnection of the sample line will cause a leak in the breathing circuit and analyzer readings of room air. Partial disconnection of the sample line will cause low measured CO₂ and anesthetic values due to room air entrainment during spontaneous ventilation; during positive pressure ventilation the leak is phasic with the respiratory cycle, which can result in a "steeple sign capnogram", as shown in the figure (figure 17).

•Since CO₂ and multigas analyzers draw respiratory gas through a sampling tube into an analyzer chamber, a delay in the readout occurs due to transit time of gas through the sampling tube. Also, some gas mixing occurs during its transit through the sampling tube causing waveforms that are somewhat smoothed. Particularly when respiratory rate is rapid (ie, >40 breaths per minute), inspired CO₂ values may be artificially high and end-tidal CO₂ values may be artificially low.

•Sidestream CO₂ analyzers incorporate a water trap to protect the sensor optics from liquids. This trap introduces a small false peak (ie, an artifact) during positive pressure ventilation in the inspiratory section of the displayed capnogram, which may become more pronounced when peak airway pressures are high (waveform 1). Since the appearance of the artifact is similar to the appearance of a capnogram of a patient who is "overbreathing" the set ventilator parameters, the clinician may be prompted to unnecessarily administer a neuromuscular blocking agent (NMBA) to stop patient effort [41].

●Prevention – Basic function of the capnometer can be confirmed during the pre-use check by blowing into the sample line. Capnometers and multigas analyzers require periodic recalibration by a service technician who samples calibration gas mixtures of known concentrations.

●Diagnosis – A false apnea-CO₂ alarm occurs when the capnometer sample line becomes disconnected (table 7), but the clinician should confirm that ventilation is actually appropriate by observing respiration, auscultating the chest, and thoroughly checking all respiratory monitors and the pulse oximetry reading.

Obstruction of the sample line or a full water trap will typically trigger an obstruction alarm and the analyzer will stop working until the obstruction is resolved. In this case, the capnogram tracing disappears and the analyzer displays a message alerting the clinician to change the sample line and/or water trap.

●Response – In many cases, replacement of the capnometer sample line or sensor with the extra equipment stocked in the anesthesia workstation will resolve problems with a false alarm. Critically important responses to true alarms for apnea (table 7) or high inspired CO₂ (table 12) are described in the tables.

Airway pressure and flow measurements

Airway pressure — Airway pressure is visualized on a mechanical gauge, and an electronically sensed pressure waveform is often shown on the screen of the anesthesia workstation; derived peak, nadir (ie, peak end-expiratory pressure [PEEP]), and often mean airway and pause airway pressures are often displayed as well. Alarms for high airway pressure (table 8), continuous airway pressure (table 9), and negative airway pressure (table 10) are standard.

●Potential problems

•Mechanical airway pressure gauges can be inaccurately calibrated, which is usually apparent if the needle on the gauge does not read zero when the breathing system is open to ambient pressure.

•An electronic pressure sensor connected to the breathing system by a flexible tube can become obstructed or disconnected.

•Airway pressure is usually measured at a permanent site within the breathing system of the anesthesia machine (eg, at the patient side of the unidirectional inspiratory valve or the unidirectional expiratory valve, or within the CO₂ absorbent canister). These locations are not ideal due to distance from the patient's airway. Thus, measurements may not accurately indicate the patient's airway pressure if the breathing system becomes partially or totally obstructed. Although airway pressure is most accurately measured at the closest connector to the airway (eg, at the Y-piece connection site of the breathing system to the endotracheal tube [ETT] or laryngeal mask airway [LMA]), disconnection of the breathing circuit is a risk with this site.

●Prevention – The zero value of airway pressure is easily checked by opening the breathing circuit to atmosphere at the patient connector. Basic function of airway pressure gauges is confirmed when the breathing circuit is pressurized for a leak test. The pressure readings will not change if the sensor is nonfunctional or if the sample line is disconnected or broken.

●Diagnosis – Although airway pressure sensor accuracy is not routinely checked by the user, most newer anesthesia workstations have multiple pressure sensors, with alerts regarding the need for service if sensor measurements are not in agreement.

●Response – Because so many alarms and ventilator settings are dependent on a functioning airway pressure sensor, a case should not be started if a broken airway pressure sensor is suspected.

Airway flow — All anesthesia workstations have a device to measure exhaled tidal volume. Older workstations typically incorporated respirometers that did not measure instantaneous flow. New workstations have an airway flowmeter that does measure expiratory flow (typically within the expiratory limb of the breathing circuit). Derived values that are displayed include respiratory rate, tidal volume, and minute volume. Some workstations automatically adjust the volume dispensed from the ventilator bellows during inspiration based on measured tidal volumes (ie, fresh gas compensation).

Some workstations measure both inspired and exhaled flows, which is necessary for displaying pressure and flow volume loops. (See "Mechanical ventilation during anesthesia in adults", section on 'Pressure volume loops' and "Mechanical ventilation during anesthesia in adults", section on 'Flow volume loops'.)

Some anesthesia machines have flowmeters that can detect flow direction, which is useful to diagnose unidirectional valve malfunction.

●Potential problems

•Differential pressure sensor flowmeters can malfunction if the flow channel or tubing becomes contaminated with liquid. The erroneous values can lead to hyper- or hypoventilation of the patient during pressure-control volume guaranteed ventilation [42].

•Hotwire anemometers stop functioning if the wire filament burns out [43].

•Breathing circuit leaks can occur if the sensor housing breaks or cracks, or if the differential pressure sensor lines become disconnected.

•Airway flowmeters are only accurate to ±15 percent because their readings are influenced by the composition and humidity of the respiratory gases. Anesthesia workstations with an integrated multigas analyzer can automatically correct flowmeter sensor readings. However, if there is no integrated analyzer, the user typically must indicate which specific gas is being used (eg, desflurane, N₂O) to allow compensation for gas effects on flowmeter measurements.

•Airway flow is most accurately measured at the airway opening, similar to airway pressure (see 'Airway pressure' above). Tidal volumes derived from flows measured in the inspiratory and expiratory limbs are overestimated by the amount of gas that is compressed in the corrugated breathing tubes during positive pressure ventilation [44]. Some newer anesthesia workstations apply corrections using measurements of tubing compliance obtained during the automated pre-use checkout. If the tubing compliance changes after the checkout (eg, by stretching out the tubing or adding additional tubing), then the derived tidal volumes will be overestimated [10].

•Very small tidal volumes (eg, those used for neonates) may trigger a false apnea alarm. The clinician must confirm that ventilation is actually appropriate by observing respiration, auscultating the chest, and thoroughly checking all respiratory monitors, including CO₂, airway pressure, and pulse oximetry readings. However, disabling the apnea-flow alarm may be appropriate in this instance.

●Prevention – Basic function of the airway flowmeter(s) is confirmed when the breathing circuit is tested for gas flow during inspiration and exhalation. The breathing circuit pressure test checks for leaks in the flowmeter housing or connections. To maximize flowmeter accuracy, the user should extend breathing circuit tubing before performing any pre-use automated checkout that includes compliance testing. For some machines, it is necessary to ensure appropriate preset gas compensation settings by entering the anesthetic gas that will be used when performing the pre-use machine checkout.

●Diagnosis – Flowmeter accuracy is not routinely checked by the user, and measured tidal volumes may be different from ventilator set tidal volumes. In general, measured tidal volumes are less accurate than set tidal volumes with a piston ventilator (assuming that breathing system compliance and leak volume has not changed since the automated pre-use checkout). Also, in general, measured tidal volumes are more accurate than tidal volumes indicated on a bellows housing.

●Response – Since many alarms and ventilator settings are dependent on a functioning airway flowmeter, an anesthetic should not be started if a broken airway flowmeter is suspected. If a hot wire anemometer filament burns out during an anesthetic, the sensor should be replaced immediately after the case ends.

BACKUP EQUIPMENT FOR OXYGENATION AND VENTILATION — If malfunction of the anesthesia machine occurs during anesthetic administration and the problem is not resolved immediately, positive pressure ventilation is initiated with a self-inflating manual ventilation device and an auxiliary oxygen cylinder. A total intravenous anesthesia (TIVA) technique may be employed until the problem is resolved or a replacement machine is available. (See "Maintenance of general anesthesia: Overview", section on 'Total intravenous anesthesia'.)

Missing backup equipment for oxygenation and ventilation may result in inability to oxygenate and ventilate a patient if anesthesia machine malfunction occurs. Thus, the American Society of Anesthesiologists (ASA) Anesthesia Machine Checkout Recommendations stipulate that a full auxiliary backup oxygen "E" cylinder and a self-inflating manual ventilation device should always be available on the anesthesia workstation for emergency use. This is a critical part of the pre-use anesthesia machine checkout procedure (item #1) (table 1) [7]. If needed, additional oxygen "E" cylinders are typically available in several nearby locations in the operating room suite because they are often used for brief periods during patient transport within the hospital.

Failure to oxygenate and/or ventilate is a major cause of anesthesia morbidity and mortality [1-4]. When a critical anesthesia workstation malfunction results in an inability to ventilate the patient, fixing the root cause may distract the clinician for several minutes. For this reason, the anesthesia provider should rapidly employ backup equipment if the patient cannot be ventilated, while a second anesthesia provider or technician is called to find and fix the cause of the malfunction and/or bring a second anesthesia machine to serve as a replacement.

The ASA recommendations also specify that airway suction equipment (which is attached to the side of the anesthesia machine in many institutions) should be checked by the anesthesia provider before a procedure begins (item #2) (table 1), to ensure that it is sufficient to clear the patient's airway. Missing components of the suction apparatus, system leaks, occluded tubing, insufficient tubing length, and incorrectly set vacuum settings are common causes of suction malfunction.

SUMMARY AND RECOMMENDATIONS

●General principles – Anesthetic care is provided using an anesthesia machine and integrated monitors (ie, the anesthesia workstation) that include systems for (see 'General principles' above):

•Blending a precision mixture of gases (anesthesia machine, respiratory gases, anesthetic gases and vaporizers)

•Dispensing those gases to the patient (anesthesia breathing system)

•Delivering positive pressure ventilation (anesthesia ventilator)

•Removing waste gases (scavenger system)

•Monitoring respiratory and machine function (integrated system monitors)

●Anesthesia machine checkout – The American Society of Anesthesiologists (ASA) Anesthesia Machine Checkout (table 1), or similar recommendations should be performed daily, and selected items should be checked before administration of each anesthetic to prevent anesthesia workstation misuse or failure. (See 'Standardized anesthesia machine checkout' above.)

●Anesthesia machine alarms – Alarms for the most common and/or serious anesthesia machine problems, possible causes, and suggested clinician responses are shown in the tables (see 'Anesthesia workstation alarms' above):

•Electrical power loss alarm (table 2)

•Oxygen (O₂) supply pressure alarm (table 3)

•Apnea alarm (table 4)

•Apnea-pressure alarm (table 5)

•Apnea-flow alarm (table 6)

•Apnea carbon dioxide (CO₂) alarm (table 7)

•High airway pressure alarm (table 8)

•Continuous airway pressure alarm (table 9)

•Negative airway pressure alarm (table 10)

•Low inspired oxygen alarm (table 11)

•High inspired CO₂ alarm (table 12)

•Low inspired anesthetic agent alarm (if indicated) (table 13)

●Prevention, diagnosis, and troubleshooting problems with anesthesia machines – Strategies for avoiding misuses and managing malfunctions of anesthesia machine components include:

•Loss of electrical supply (see 'Loss of electrical supply' above)

•Compressed gas supply malfunction (see 'Compressed gas supply malfunction' above)

•Compressed gas flowmeter malfunction (see 'Compressed gas flowmeter malfunction' above)

•Proportioning system or fail-safe valve malfunction (see 'Proportioning system or pressure sensor shut-off (fail-safe) valve malfunction' above)

•Vaporizer malfunction (see 'Vaporizer malfunction' above)

•Circle breathing system leak or obstruction (see 'Breathing system leak' above and 'Breathing system obstruction' above)

•CO₂ absorbent exhaustion or toxicity (see 'Carbon dioxide absorbent exhaustion or toxicity' above)

•Valve malfunctions:

-Adjustable pressure-limiting (APL) valve (see 'Adjustable pressure-limiting valve' above)

-Unidirectional valves (see 'Unidirectional (inspiratory and expiratory) valves' above)

-Positive end-expiratory pressure (PEEP) valve (see 'Positive end-expiratory pressure valve' above)

•Ventilator malfunction (see 'Ventilator malfunction' above)

•Waste gas disposal (scavenger) system malfunction (see 'Waste gas disposal (scavenger) system malfunction' above)

•Mapleson breathing system malfunction (see 'Mapleson breathing system malfunction' above):

-Modified Mapleson D (Bain) circuit (see 'Modified Mapleson D (Bain) circuit' above)

-Mapleson E (T-piece) circuit (see 'Mapleson E (T-piece) circuit' above)

●Prevention, diagnosis, and troubleshooting problems with anesthesia monitors – Strategies for avoiding misuses and managing malfunctions of anesthesia monitors integrated into the anesthesia workstation include:

•Oxygen analyzer (see 'Oxygen analyzer' above)

•Capnometer and respiratory gas analyzers (see 'Capnometer and respiratory gas analyzers' above)

•Airway pressure and flow measurements:

-Airway pressure (see 'Airway pressure' above)

-Airway flow (see 'Airway flow' above)

●Ensuring availability of backup equipment – Missing backup equipment for oxygenation and ventilation (eg, full auxiliary oxygen cylinder and self-inflating manual ventilation device) may result in inability to oxygenate and ventilate a patient if malfunction of the anesthesia machine occurs. (See 'Backup equipment for oxygenation and ventilation' above.)