Basic patient monitoring during anesthesia

INTRODUCTION — Monitoring is an essential component of’ anesthesia care. Anesthesia clinicians must monitor patient physiologic variables and anesthesia equipment during all types of anesthesia, as anesthesia and surgery can cause rapid changes in vital functions. Patient and equipment monitoring is used to titrate administration of anesthetic medication, to detect physiologic perturbations and allow intervention before the patient suffers harm, and to detect and correct equipment malfunction.

This topic will discuss the basic patient monitors used during anesthesia. Monitoring neuromuscular blockade, transesophageal echocardiography (TEE), monitoring to prevent awareness during anesthesia, neuromonitoring during anesthesia and surgery, and pulmonary artery catheter monitoring are discussed separately.

●(See "Monitoring neuromuscular blockade".)

●(See "Transesophageal echocardiography: Indications, complications, and normal views".)

●(See "Pulmonary artery catheterization: Indications, contraindications, and complications in adults".)

●(See "Accidental awareness during general anesthesia", section on 'Monitoring'.)

●(See "Neuromonitoring in surgery and anesthesia".)

STANDARDS FOR MONITORING DURING ANESTHESIA — The term "standard ASA monitors" is often used to refer to the basic physiologic monitors recommended by the American Society of Anesthesiologists [1]. Standard ASA monitors applied to the patient include a pulse oximeter, electrocardiography (ECG), noninvasive blood pressure device, and a temperature monitor. In addition, the ASA monitoring standards include measurement of end-tidal carbon dioxide (ETCO₂), inspired oxygen concentration, and the use of low oxygen concentration and ventilator disconnect alarms. Quantitative monitoring of the volume of expired gas is strongly encouraged by the ASA. According to the ASA standards for monitoring, continual monitoring is defined as measurement repeated regularly and frequently in steady, rapid succession (eg, automatic noninvasive blood pressure measurement), whereas continuous monitoring is that which is prolonged, without interruption (eg, ECG monitoring).

The standards for monitoring during anesthesia created by international anesthesia organizations are shown in a table (table 1) [2-6]. All of these standards state that the most important monitor is the presence of an anesthesia clinician throughout anesthesia, and include statements that a blood pressure monitor, pulse oximeter, and ETCO₂ monitor should be used during anesthesia.

MONITORING MODALITIES

Clinician monitoring — Clinical monitoring using visual inspection, auscultation, and palpation is a primary determinant of patient safety. Changes in clinical signs may be subtle, and often precede abnormalities in parameters measured by monitoring devices. Monitoring devices do not replace clinical observation; rather, they amplify and quantify clinical information (table 2).

Respiratory system monitoring

Oxygenation

Cyanosis — Clinical assessment of hypoxemia judged by perioral cyanosis is known to be notoriously unreliable. Many factors such as natural skin pigment, room lighting, interobserver variation, and hemoglobin concentration can affect detection of cyanosis. Approximately 5 g/dL of unoxygenated hemoglobin in the capillaries generates the dark blue color appreciated clinically as cyanosis [7]. However, this threshold may occur at varying levels of arterial oxygen saturation and arterial hemoglobin content, as shown in a figure (figure 1).

Pulse oximetry — Pulse oximetry is a quantitative method of assessing oxygenation that should be used when possible during all anesthetics, and is part of the World Health Organization preoperative safe surgery checklist [8]. If pulse oximetry cannot be used during induction of anesthesia (eg, in small children, with uncooperative adults), the monitor should be attached as soon as consciousness is lost and the reason for the delay should be recorded in the anesthesia record. Importantly, the variable pitch pulse tone and the low threshold alarm should be audible to the anesthesia clinician.

The principles of pulse oximetry, equipment, accuracy, and sources of error are discussed separately (table 3). (See "Pulse oximetry", section on 'Probes'.)

In the operating room, pulse oximeters are commonly placed on the finger or earlobe in adults, and on the foot/ankle or wrist/palm in infants, to shine light through tissue and detect it on the other side. Newer forehead probes use reflectance technology to measure back scattered rather than transmitted light. The response time to changes in oxygen saturation is faster with forehead and ear probes than with finger probes. (See "Pulse oximetry", section on 'Probes'.)

Most pulse oximeters display a plethysmographic (pleth) waveform (figure 2). The practical purpose of displaying the pleth waveform is to verify good probe placement and function. In commercially available pulse oximeters, the waveform is only processed to determine heart rate. Most pulse oximeter monitors display an indicator of the quality of the signal; a good signal indicates that the oximeter is working.

The pleth waveform resembles an intraarterial wave form, and advanced analysis may similarly provide additional clinical information. One specialized monitor incorporates an algorithm to measure changes in the pulse volume continuously, to determine the plethysmographic variability index (PVI). The PVI is a measure of the dynamic changes in the perfusion index over respiratory cycles, with the goal of predicting fluid responsiveness [9]. (See "Novel tools for hemodynamic monitoring in critically ill patients with shock", section on 'Pulse contour analysis (fluid responsiveness)' and "Intraoperative fluid management", section on 'Goal-directed fluid therapy'.)

The utility of the PVI for predicting intraoperative fluid responsiveness is unclear. A meta-analysis of 10 small studies that compared PVI with cardiac output or stroke volume based measures of fluid responsiveness found that PVI was reasonably accurate at predicting fluid responsiveness in mechanically ventilated adults [10]. PVI is not reliable in patients with atrial fibrillation or arrhythmias, and is affected by changes in peripheral perfusion (eg, use of vasoconstrictors, or cold extremities).

Respiratory variation in pulse volume changes with ordinary pulse oximeters are rarely visible, and are a crude and late indicator of massive hypovolemia.

Inspired oxygen analyzer — During every general anesthetic employing an anesthesia machine, an oxygen analyzer should be used to ensure that a hypoxic mixture of gases is not delivered. The analyzer is calibrated by exposure to air (21 percent oxygen [O₂]) and 100 percent O₂, and should read 21 percent before each anesthetic when open to room air. O₂ analyzers usually have a low-level alarm that is automatically activated by turning on the anesthesia machine. The sensor should be placed into the inspiratory or expiratory limb of the breathing circuit and not into the fresh gas flow, to ensure measurement of the concentration actually delivered, rather than the dialed concentration.

Most modern anesthesia gas analyzers incorporate methods allowing simultaneous measurement of concentrations of at least O₂, carbon dioxide (CO₂), and the inhalation agent. Non-dispersive infrared spectroscopy is usually employed, and gas analyzers that measure oxygen utilize ancillary technologies such as paramagnetic or fuel-cell sensors in conjunction with the infrared sensor. (See "Accidental awareness during general anesthesia", section on 'End-tidal anesthetic concentration'.)

Ventilation — Adequacy of ventilation should be continuously monitored in all patients during anesthesia, including observation of clinical signs such as chest excursion, auscultation of breath sounds (using a precordial or esophageal stethoscope), and movement of the reservoir bag. Patients who are breathing spontaneously should be observed for signs of airway obstruction, including a tracheal tug, paradoxical chest movement, snoring, or upper airway sounds.

During anesthesia without sedation (ie, regional or local anesthesia without sedation), clinical observation may be adequate for monitoring ventilation.

Capnography — Most anesthesia machines are capable of providing an exhaled capnograph, which is a graph that shows the respiratory rate and the concentration of carbon dioxide over time, as well as the end-tidal carbon dioxide concentration (ETCO₂). Capnography should be used to assess the adequacy of ventilation for all patients who undergo sedation or general anesthesia, if possible. Capnography should also be used to confirm correct placement of an endotracheal tube or supraglottic airway.

●The principles of operation of exhaled carbon dioxide monitors are discussed separately. (See "Carbon dioxide monitoring (capnography)", section on 'Principles of operation' and "Carbon dioxide monitoring (capnography)", section on 'CO2 Waveform'.)

Most CO₂ analyzers used in the operating room sample gas via a sidestream mechanism. This means that the CO₂ sensor is part of the anesthesia machine, and gas is aspirated through sampling tubing connected to the patient breathing circuit. Sample tubing may be six or more feet long, and there may be a several second delay before appearance of the CO₂ trace after a breath.

●The phases of the normal capnogram are shown in a figure (figure 3). The continuous wave form allows a visual breath to breath assessment of the patient’s airway and ventilation. Changes of the shape of the capnogram can be the result of patient factors (eg, small or large airway obstruction, intrinsic lung disease, or ventilatory effort), equipment malfunction (eg, breathing circuit leak), or surgical manipulation. Examples of normal and abnormal capnograms are shown in a figure (figure 4).

In general, when square wave capnography is preserved during mechanical ventilation,

•A sudden drop in ETCO₂ suggests a sudden drop in lung perfusion caused by either an obstruction to blood flow through the lungs (thrombus, air, or fat) or a reduction in cardiac output.

•Steady increase in ETCO₂ suggests hypoventilation or very rarely malignant hyperthermia if associated with rise in temperature.

●Correlation with PaCO₂ – The ETCO₂ is a non-invasive estimate of the CO₂ concentration or partial pressure in the arterial blood (PaCO₂) [11]. This is based on the assumption that end-tidal gas is primarily alveolar gas and alveolar CO₂ tension (PACO₂) is virtually identical to pulmonary end-capillary tension, which in turn is identical to PaCO₂. The PACO₂-PETCO₂ gradient in a healthy patient with normal lungs is less than 5 mmHg and represents dilution of alveolar gas with CO₂-free gas from non-perfused alveoli (alveolar dead space) [12]. The difference is increased if there is a mismatch between ventilation and perfusion (V/Q) of the lungs, as seen in patients with lung disease, pulmonary emboli, low-cardiac output states, and hypotension [11], but may also be variably increased because of advanced age, and with surgical positioning [13,14]. For this reason, PaCO₂ should be measured when close control of ventilation is necessary (eg, patients with increased intracranial pressure).

In patients with obstructive lung disease and an upsloping ETCO₂ plateau, inspiration may occur before the true end of expiration, and result in a falsely low ETCO₂ (in addition to predisposing to dynamic hyperinflation) (figure 5). Reducing the respiratory rate (or briefly stopping the ventilator) should allow full exhalation and an accurate ETCO₂ reading.

Measurement of pulmonary mechanics — Most newer anesthesia machines provide the capability to continuously monitor inspiratory and expiratory volumes, pressures, and flow and to display pressure volume loops and flow volume loops. The use of these parameters to optimize ventilator settings, and to monitor changes in pulmonary mechanics are discussed separately. (See "Mechanical ventilation during anesthesia in adults", section on 'Monitoring pulmonary mechanics'.)

●Airway pressure – Airway pressure should ideally be measured in the trachea. However, for practical reasons pressure is measured at the anesthesia machine with pressure transducers. The most commonly used transducer is the inexpensive, piezo-resistive transducer that relies on a pressure sensing diaphragm whose resistance changes when it is deformed in response to a differential pressure. The pressure transducers must be zeroed at the beginning of each day as part of the pre-use checkout. Inaccuracies or even failures may occur when humidity condenses in the pressure transducer tubing. Because the transducers are hidden in the machine, a leak in the breathing system because of a cracked transducer may not be obvious to the clinician [15].

●Gas flow – The two main methods for measurement of flow on contemporary anesthesia workstations are hot wire anemometers and variable orifice sensors.

•Hot wire anemometers – Gas flows past and cools a thin wire that is heated to maintain a constant temperature. The heating current required is therefore related to gas flow. Changes in gas composition (density) affect the accuracy of this design of flowmeters.

•Variable orifice flow sensor/differential pressure – The pressure difference across a variable size orifice is used to infer gas flow.

●Respiratory volume – The anesthesia machine software computes the respiratory volume by integrating the flow with respect to time (flow = volume/time).

Disconnection alarms — Most anesthesia machines automatically set alarm limits for changes in respiratory rate and pressure that serve as disconnect alarms. An audible signal occurs if the limits are exceeded. Alarms can be changed manually according to clinical circumstances.

Circulatory system monitoring — Adequacy of circulatory function is assessed by both clinical observation and physiologic monitors. Assessment of skin color and temperature, the quality of a palpable pulse, and heart tones via an esophageal or precordial stethoscope are valuable clinical parameters that must be supplemented by measurement of blood pressure, heart rate, electrocardiography (ECG), and in some cases, advanced cardiovascular monitoring. Urine output may also be used to assess organ perfusion.

Adequate lighting should be maintained in the operating room to allow observation of exposed body parts.

Blood pressure — Blood pressure (BP) and heart rate should be measured at least every five minutes during anesthesia, and more frequently as indicated clinically. In practice, heart rate is continuously monitored by ECG and the pulse oximeter, and by the arterial pressure trace if invasive BP monitoring is.

It is important to establish an awake BP measurement prior to induction of anesthesia. However, BPs taken in the preoperative holding area and in the operating room just prior to anesthesia are often higher than those that should be considered the patient’s baseline [16]. When possible, baseline BP should be established by asking the patient or reviewing prior medical records, particularly for patients with hypertension. (See "Preoperative evaluation for anesthesia for noncardiac surgery", section on 'Anesthesia directed physical examination'.)

Noninvasive blood pressure monitoring — The standard device for intraoperative noninvasive BP monitoring is the automated office BP monitor. A sphygmomanometer should be available for situations in which an oscillometric reading cannot be obtained.

●Technology – Oscillometric BP devices operate by sensing the magnitude of oscillations in pressure caused by the blood flow. The cuff is inflated by a pump high enough to occlude arterial flow to the limb, and then released gradually, typically at approximately 2 to 4 mmHg per second [17]. A sensor in the device detects changes in oscillations, or amplitude of pressure pulses, created by the blood flow as the occlusion is released. Very faint blood flow oscillations begin to be detected as the pressure in the cuff coincides with systolic blood pressure (SBP). As pressure is released, the amplitude of pulsatile oscillations increases to a maximum that corresponds with mean arterial pressure (MAP), and ultimately levels off at a pressure that indicates diastolic pressure. Oscillometric BP devices are set to "time out" (ie, not display a reading) at 120 seconds if the measurement cannot be made.

●Cuff size and placement – The proper cuff size should be used for the most accurate blood pressure measurement. The length of the cuff bladder should be 80 percent, and the width 46 percent of the circumference of the upper arm. (See "Blood pressure measurement in the diagnosis and management of hypertension in adults", section on 'Cuff size'.)

In most patients, this means that the cuff covers two-thirds of the distance between the elbow and shoulder. The cuff should be placed on the bare skin of the upper arm, snugly applied with the mid-bladder (sometimes marked) over the brachial artery.

When necessary, the blood pressure cuff can be placed on the forearm, ankle, calf, or thigh. The cuff size should be chosen as it would be for the arm (ie, related to the circumference of the limb). An ankle cuff should be placed as far distally as possible, with the mid-bladder just behind the medial malleolus. For measurement at the calf or thigh, the mid-bladder should be placed posteriorly.

●Accuracy – Systolic and diastolic blood pressures with oscillometric devices are calculated according to proprietary algorithms that vary by manufacturer, hence their accuracy depends on the algorithm used. Oscillometric methods tend to overestimate SBP and underestimate diastolic blood pressure (DBP), whereas they more accurately estimate mean arterial pressure (since this corresponds to a maximum amplitude of pulsatile oscillations) [18,19].

Although the accuracy of non-invasive blood pressure (NIBP) measurement is excellent over a wide range of blood pressures, its accuracy with very low or very high blood pressure remains questionable, compared with auscultatory and invasive arterial BP measurements [20]. As examples, oscillometric automated measurements are consistently higher than ausculatory measurements in trauma patients with systolic blood pressure less than 110 mmHg [21]. An analysis of 27,022 simultaneously measured invasive arterial and non-invasive BP (NIBP) pairs reported that in hypotensive states, the NIBP significantly overestimated the systolic blood pressure, and this difference increased as patients became more hypotensive [22]. Mean arterial blood pressures showed better agreement.

Aside from errors intrinsic to the technology, the most common source of error with the use of non-invasive blood pressure measurement is an inappropriate cuff size. Cuffs that are too large produce erroneously low oscillometric readings, and cuffs that are too small produce higher readings. (See "Blood pressure measurement in the diagnosis and management of hypertension in adults", section on 'Cuff size'.)

Other factors that may lead to errors or prevent measurement with a noninvasive blood pressure cuff include the following:

•Any motion such as shivering, tremors, seizures, or arm flexion

•Severe hypotension

•Arrhythmias such as atrial fibrillation or frequent premature beats

•Air leak, kink, or alteration of the cuff or tubing

•Rapidly repeated cuff inflations, which can cause venous congestion [23]

Blood pressure measured at other sites may not correlate with measurements in the upper arm. Studies of comparative measurements in the arm and ankle or calf have reported conflicting and widely variable results, and have been performed in heterogeneous patient populations [24-27]. If the cuff must be placed on the leg, when possible a baseline measurement in the arm should be performed first for comparison.

Systolic, diastolic, and mean blood pressure measurements with the cuff on the forearm tend to be higher than measurements in the upper arm [28,29]. (See "Anesthesia for the patient with obesity", section on 'Special equipment needs'.)

Blood pressure measurement should be corrected for patient position, particularly if the patient is placed in steep head up, steep head down, or sitting positions. Hydrostatic effects cause an increase in measurements in dependent limbs, and a decrease in measurements in elevated limbs. This difference may have important clinical implications. As an example, a patient in the sitting position with a normal mean arterial pressure measured in the arm may have unacceptably low mean arterial pressure in the brain. (See "Patient positioning for surgery and anesthesia in adults", section on 'Physiologic effects of sitting position'.)

●Continuous noninvasive blood pressure monitoring – Several devices are available for continuous noninvasive blood pressure monitoring. Examples include the Clearsight and CNAP systems, which use volume clamp technology with a small finger cuff, and the T-line system that uses arterial tonometry at the radial artery. These devices should not routinely replace intraarterial pressure monitoring for patients who require continuous blood pressure monitoring. Their utility in routine practice has not been defined [30].

•Accuracy of these devices compared with invasive arterial pressure monitoring in the perioperative period does not meet standards set by the Association for the Advancement of Medical Instrumentation [31].

•Accuracy of these devices may degrade during hypotension and conditions that affect peripheral perfusion [32].

•Accuracy of other hemodynamic parameters reported by these devices (eg, cardiac output, stroke volume) is insufficient to guide goal directed fluid therapy [33].

Invasive blood pressure monitoring — Direct or invasive blood pressure monitoring may be used for anesthesia for high risk patients and/or high risk surgical procedures. Indications, techniques for catheterization, interpretation of the arterial waveform, accuracy, and sources of error are discussed separately. (See "Intra-arterial catheterization for invasive monitoring: Indications, insertion techniques, and interpretation".)

In practice, in patients for whom an arterial catheter is planned, it may be beneficial to place the catheter and to start monitoring prior to rather than after induction of anesthesia. In one randomized trial including 242 patients who underwent noncardiac surgery, invasive blood pressure monitoring during induction reduced the incidence of hypotension (mean arterial pressure <65 mmHg) in the first 15 minutes after induction, compared with intermittent noninvasive blood pressure monitoring [34].

We place a non-invasive blood pressure cuff for patients in whom we use intraarterial monitoring to compare measurements and to serve as a backup should technical problems occur. We measure a noninvasive pressure after the arterial catheter is placed and the transducer is zeroed and leveled, expecting mean pressures to be similar, and make adjustments as necessary. We then set the noninvasive cuff to cycle at 30 minute intervals.

Electrocardiogram — The ECG should be monitored continuously during anesthesia. It is a reliable monitor for heart rate, rhythm, and the cardiac conduction system; the ability of the standard three or five lead ECG to detect intraoperative cardiac ischemia is limited. Abnormalities of the ECG may also provide evidence of electrolyte abnormalities. The components of the ECG and the basics of interpretation are discussed separately. (See "ECG tutorial: Electrical components of the ECG" and "ECG tutorial: Basic principles of ECG analysis".)

ECG leads — The standard ten electrode, twelve lead ECG that is used in other clinical settings is usually impractical in the operating room. Instead, three or five leads are usually applied. Whenever possible, a five lead system should be used to display two channels and improve the sensitivity for detection of ischemia.

●Five electrode system – The five electrode system includes four extremity (ie, right arm, left arm, right leg, left leg) and one precordial electrode, and allows monitoring of seven leads (I, II, III, aVR, aVL, aVF, and a single precordial lead). The desired precordial lead may be selected by placing the precordial electrode in any position from V1 to V6. (See "ECG tutorial: Electrical components of the ECG", section on 'Precordial leads'.)

For continuous ECG monitoring in the operating room (and in other parts of the hospital), the limb leads are typically placed on the torso. The arm leads are placed in the infraclavicular fossae close to the shoulders, and the left leg lead is placed below the rib cage in the anterior axillary line. The right leg electrode is a ground that can be placed anywhere and is often placed on the right chest or abdomen.

Most monitors allow selection and display of multiple leads simultaneously. We usually select leads II and V5.

●Three electrode system – The three electrode system allows monitoring along one bipolar lead between two electrodes while the third serves as a ground. Electrodes are placed in the infraclavicular fossae on left and right sides, and the left leg electrode is placed on the left side of the abdomen below the ribcage. Three leads may be individually selected with monitor controls. The three lead system can be used to monitor heart rate and to detect the existence of a p wave or presence of ventricular fibrillation, but cannot diagnose more complicated arrhythmias or conduction system abnormalities, for which a true V1 lead is required (eg, to distinguish between right and left bundle branch blocks).

The three lead system may be modified to approximate standard precordial leads by changing the standard electrode position. Such modification may be necessary if a five lead system is unavailable, or cannot be used because of the site of surgery, though the five lead system is the standard of care for ischemia monitoring [35]. A three electrode ECG only allows for identification of ischemia and myocardial infarction in myocardial regions represented in the selected leads, and many perioperative ischemic events will be missed when using three electrode recordings.

Three modified leads can be used to monitor for anterior ischemia. For each of these leads, limb lead I is selected on the monitor, and the electrodes are placed as follows:

•CS5 – The right arm (RA) electrode is placed under the right clavicle, left arm (LA) electrode is placed in the V5 position, and the left leg (LL) electrode is placed anywhere on the left side.

•CM5 – The RA electrode is placed over the manubrium, the LA electrode is placed in the V5 position, and the LL electrode can be placed anywhere on the left.

•CC5 – The RA electrode is placed in the mid-chest at the right anterior axillary line, the LA electrode is placed at the V5 locations, and the LL electrode is placed anywhere on the left.

•CB5 – The RA electrode is placed over the center of the right scapula, the LA electrode is placed in the V5 position, and the LL electrode is placed anywhere on the right side. CB5 can be used to monitor for dysrhythmias, since it provides an obvious p wave.

The modified chest lead 1 (MCL1) provides p wave and QRS morphology adequate to detect some dysrhythmias. The LA electrode is placed below the left clavicle, the LL electrode is placed in the V2R to V3R location (ie, V2 and V3 on the right chest), and the RA electrode is placed anywhere on the right side. Limb lead III is selected on the monitor.

Ischemia detection — For optimal detection of ischemia, more than one lead should be monitored, including an accurately placed precordial electrode. The changes visible on the ECG monitor may be subtle and difficult to quantify. Suspected ischemia should be confirmed with printout of a paper copy of the ECG trace, which is possible with most anesthesia monitors. Detailed perioperative ECG analysis should always be performed on ECG printouts because monitors do not provide grids for adequate analysis. In high risk patients, a baseline ECG strip should be printed prior to induction of anesthesia for later comparison if necessary.

Most intraoperative ischemia is manifested by ST depression [36,37]. ECG criteria for diagnosing ischemia are discussed separately. (See "ECG tutorial: Myocardial ischemia and infarction".)

●Lead combinations – Lead II is usually monitored for rhythm detection, since the p wave is typically obvious and upright in lead II, and for detection of inferior ischemic changes. In addition, a lateral precordial lead should be monitored. In one study, high risk patients who underwent noncardiac surgery were monitored for ST segment changes with combinations of ECG leads, compared with continuous simultaneous 12 lead ECG monitoring [36]. Sensitivity for a single lead was highest for V5 (75 percent). The combination of lead II with V5 increased sensitivity for ischemia detection to 80 percent, and the sensitivity of three lead combinations was highest (96 percent) for leads II, V4, and V5. In another study, lead V4 was the most sensitive lead when monitored in isolation for detecting prolonged postoperative ischemia and infarction [37].

●ST segment analysis – Most anesthesia monitors now allow real-time ST segment analysis with trending. The default setting for ST segment analysis is usually for the device to measure deviation of the ST segment from isoelectric at 60 or 80 msec after the J point on the ECG trace (ie, J+60). Among the precordial leads (V3 to V5), monitoring the lead with the most isoelectric ST segment on the preoperative ECG may increase the utility of ST segment analysis [37].

The measurement point, and the J Point, may be adjusted manually to improve accuracy of ST segment analysis for patients with abnormal ECGs. In patients without isoelectric ST segments, the alarm limits should be configured to account for the patient’s baseline deviation. A number of conditions may affect the ST segment and reduce the utility of ST segment analysis for ischemia detection, such as left bundle branch block, Wolff-Parkinson-White syndrome, hypokalemia, digitalis, and left ventricular hypertrophy.

Sources of ECG artifact — A number of factors can affect the ECG trace in the operating room. Some may be modified to improve the ECG trace, whereas others cannot. They include the following:

●Electrodes may lose contact with the skin because of inadequate skin preparation prior to placement, tension on the lead wire, or the surgical skin preparation solution, irrigant, or blood seeping under the lead.

●The ECG baseline can wander because of shivering, tremor, respiration, or body movement associated with surgery. Baseline wandering may also be the result of poor electrode contact or electrode placement over bony prominences. Selecting a monitoring filter, rather than a diagnostic filter, on the ECG monitor may reduce wandering. (See 'Monitoring modes' below.)

●The ECG trace may be obliterated during electrosurgery. Such interference can be minimized by placing the grounding pad on the leg, when appropriate. Safety issues related to electrosurgery are discussed separately. (See "Overview of electrosurgery", section on 'Improving safety'.)

●The ECG trace may be affected by electrical interference from devices in the operating room. When possible, plugging devices into outlets away from the anesthesia machine or on separate electrical circuits may resolve this type of interference.

●The amplitude of the ECG trace may be reduced in patients with obesity, as increased thoracic wall thickness may attenuate transmission.

Monitoring modes — Most ECG monitors allow the clinician to select among at least two monitoring modes that set filters for different signal frequencies to reduce electrical interference, in addition to pacemaker detection mode.

The frequency bandwidths for the monitor and diagnostic modes vary by manufacturer, and are different for adults and neonates.

●Monitor mode – Monitor mode is particularly useful for rhythm monitoring, in which noise can be distracting and ST segment interpretation is relatively less important. High and low pass filters are set for a bandwidth between 0.5 and 40 Hz for adults. A disadvantage of this mode is that pacemaker spikes are sometimes filtered out and not visible on the monitor. This mode may help reduce baseline wandering.

●Diagnostic mode – Diagnostic mode is useful for ST segment analysis. The bandwidth for adults is typically 0.05 to 100 Hz.

●Pacemaker detection – A variety of both hardware and software methods may be used by monitor manufacturers to filter noise and enhance the pacemaker spike [38].

Other monitors of circulation — Clinical use of advanced cardiovascular monitoring is discussed separately, in the UpToDate topics on clinical scenarios in which they are frequently used. (See "Anesthesia for cardiac surgery: General principles", section on 'Monitoring' and "Anesthesia for open abdominal aortic surgery", section on 'Monitoring'.)

Ultrasound may be used to detect pulses that are not readily palpable, such as in patients with obesity, pediatric patients, or patients in shock. Continuous ultrasound monitoring for peripheral pulses is not widely available.

●Central venous catheters – Central venous pressure (CVP) monitoring is no longer routinely used to guide intraoperative fluid replacement. CVP is an inaccurate surrogate for cardiac preload, and does not detect or predict impending pulmonary edema indicative of hypervolemia. (See "Intraoperative fluid management", section on 'Traditional static parameters'.)

Central venous catheters may be inserted for secure intravascular access, for administration of concentrated vasopressors, and/or to allow placement of a pulmonary artery catheter.

●Pulmonary artery catheters – In selected patients, a pulmonary artery catheter (PAC) may be inserted to provide dynamic information regarding pulmonary artery pressure (PAP), cardiac output (CO), and mixed venous oxygen saturation (SVO₂). Routine use of pulmonary artery catheters has fallen out of favor, because of lack of evidence for mortality benefit in surgical patients. (See "Anesthesia for cardiac surgery: General principles", section on 'Intravascular cardiac monitors'.)

Techniques for insertion, indications and contraindications, and interpretation of data provided by PACs are discussed in detail separately. (See "Pulmonary artery catheterization: Indications, contraindications, and complications in adults" and "Pulmonary artery catheters: Insertion technique in adults" and "Pulmonary artery catheterization: Interpretation of hemodynamic values and waveforms in adults".)

●Intraoperative transesophageal echocardiography (TEE) – TEE involves insertion of a miniaturized ultrasound probe into the esophagus to visualize the heart. Compared with transthoracic echocardiography, TEE provides superior views of posterior cardiac structures, and facilitates continuous monitoring. Intraoperative TEE is discussed in detail separately. (See "Intraoperative transesophageal echocardiography for noncardiac surgery" and "Intraoperative rescue transesophageal echocardiography (TEE)".)

●Other hemodynamic (CO) monitors – A number of noninvasive devices are available for intraoperative CO monitoring, including Doppler-based technologies, pulse contour (arterial waveform) analysis, and thoracic impedance devices.

Doppler-based techniques appear to achieve accuracy similar to thermodilution pulmonary artery catheters. Pulse contour (arterial waveform) analysis and thoracic impedance devices have not been studied as rigorously, but the majority of studies suggest that they are less accurate [39].

These devices provide real time estimates of cardiac output, stroke volume, and systolic flow time. Trends in these variables, and respiratory variation in stroke volume and pulse pressure, can be used to guide intraoperative fluid therapy. (See "Intraoperative fluid management", section on 'Dynamic parameters to assess volume responsiveness'.)

•Transesophageal Doppler – Transesophageal Doppler (TED) measures blood-flow velocity in the descending thoracic aorta using a flexible probe which contains a Doppler transducer in its tip. (See "Novel tools for hemodynamic monitoring in critically ill patients with shock", section on 'Aortic Doppler'.)

TED requires training, and results depend on user expertise and accurate probe positioning. The position of the probe may require frequent adjustment, particularly if patient position is changed during surgery. The device can only be used in sedated or anesthetized patients, and is generally not continued into the post anesthesia care unit. TED may be less accurate in some patients. As an example, the calculations assume that approximately 70 percent of cardiac output enters the descending aorta, which may not be accurate in hypovolemic patients whose blood flow is redirected to the cerebral circulation.

•Pulse contour (arterial pulse waveform) analyzers – These devices estimate cardiac output by analyzing the shape of the waveform from an intraarterial catheter. (See "Novel tools for hemodynamic monitoring in critically ill patients with shock", section on 'Arterial waveform-based devices'.)

•Thoracic impedance monitors – These devices are not widely used in the operating room. They use the principle that as the amount of blood in the thorax varies with each heartbeat, it causes a corresponding change in the electrical conductance of the thorax. (See "Novel tools for hemodynamic monitoring in critically ill patients with shock", section on 'Thoracic electrical bioimpedance or bioreactance'.)

•Plethysmographic waveform – The plethysmographic (pleth) waveform displayed by a pulse oximeter is analyzed by some monitors to predict fluid responsiveness. (See 'Pulse oximetry' above.)

Temperature monitoring — We agree with the American Society of Anesthesiologists’ standards for monitoring temperature, which state that every patient should have temperature monitored when clinically significant changes in body temperature are intended, anticipated, or suspected [40]. In practice, this means that temperature should be monitored for most patients who have general anesthesia lasting more than 30 minutes, or major surgery with neuraxial anesthesia [41]. Patient temperature should be monitored to detect changes (usually hypothermia), to guide thermal management, and to detect malignant hyperthermia. Core temperature should be monitored intraoperatively whenever possible; the optimal monitoring site depends on the surgical procedure and type of anesthesia [42]. (See "Perioperative temperature management".)

Temperature monitoring sites — Core temperature is monitored at highly perfused sites where temperature is homogeneous and high compared with other sites. Core temperature is monitored with nasopharyngeal (with probe inserted 10 to 20 cm), distal esophageal, tympanic membrane, or pulmonary artery probes [43]. Nasopharyngeal or esophageal probes are used routinely during general anesthesia.

Alternative temperature monitoring sites may be required during some procedures (eg, oral or esophageal surgery), for patients who have regional anesthesia, or during general anesthesia with a face mask or supraglottic airway for airway management. Outside the United States, a double-sensor thermometer is applied to the forehead. Several studies have reported that this type of monitor correlates well with other methods of core temperature measurement during anesthesia [44,45].

Axillary, bladder, and rectal temperature may reasonably estimate core temperature, though each site is subject to possible artifact.

●Axillary temperature is most accurate with the probe positioned over the axillary artery, and with the arm at the patient's side [46]. In one study, a novel wireless temperature-monitoring device taped to shaved skin over the axillary artery closely approximated simultaneously-measured core temperature during general anesthesia for noncardiac surgery, with mean difference (esophageal minus axillary) of 0.14ºC ± 0.26ºC [47].

●Bladder temperature correlates with rectal temperature, and accuracy is affected by urine flow [48].

●Rectal temperature may accurately reflect core temperature [43], but changes slowly during rapid changes in body temperature, and may not increase rapidly during malignant hyperthermia crises [49]. Rectal temperature is rarely monitored because of the risk of rectal perforation with the probe.

Equipment — Most of the temperature probes used in anesthesia for nasopharyngeal, esophageal, and axillary sites are thermistors, which are semiconductors whose electrical resistance decreases with temperature. They are usually coated with a smooth plastic layer to facilitate atraumatic insertion, and are disposable. Temperature probes embedded in Foley catheters are used to monitor bladder temperature.

Tympanic membrane probes are soft-tipped thermocouple devices that are inserted into the auditory canal to rest against the tympanic membrane. These devices are distinct from and much more accurate than tympanic membrane infrared scanners, and provide real time continuous measurement of core body temperature [50].

The temporal artery and tympanic infrared scanners that are increasingly used in recovery areas are not sufficiently accurate for monitoring during anesthesia [51,52].

Temperature measurement during cardiopulmonary bypass is discussed separately. (See "Management of cardiopulmonary bypass", section on 'Temperature'.)

Other monitors — Intraoperative use of cerebral oximetry, transcranial Doppler, and jugular venous bulb monitoring are discussed separately. (See "Anesthesia for aortic surgery with hypothermia and elective circulatory arrest in adult patients", section on 'Cerebral oximetry' and "Anesthesia for patients with acute traumatic brain injury", section on 'Monitoring'.)

SUMMARY AND RECOMMENDATIONS

●Standard monitors – The standard monitors applied to the patient during anesthesia include a pulse oximeter, electrocardiography, noninvasive blood pressure device, and a temperature monitor. Standard equipment monitors include measurement of end-tidal carbon dioxide (ETCO₂), inspired oxygen concentration, and the use of low oxygen concentration and ventilator disconnect alarms. (See 'Standards for monitoring during anesthesia' above.)

Clinical monitoring using visual inspection, auscultation, and palpation is a primary determinant of patient safety.

●Respiratory monitors

•Pulse oximeters provide an objective measure of oxygenation and display a plethysmographic waveform that is processed to determine heart rate. An inspired oxygen analyzer should be used during every anesthetic in which an anesthesia machine is used. (See 'Pulse oximetry' above and 'Inspired oxygen analyzer' above.)

•Ventilation during anesthesia is assessed clinically (ie, by observation of chest excursion, auscultation of breath sounds, movement of the reservoir bag), and by capnography. Capnography can be used to detect clinical changes (eg, small or large airway obstruction, intrinsic lung disease, or ventilatory effort) and/or equipment malfunction (eg, breathing circuit leak, or kinked endotracheal tube). (See 'Capnography' above.)

●Cardiovascular monitoring

•The standard device for intraoperative noninvasive blood pressure monitoring is the automated office blood pressure monitor. The most common source of measurement error is the use of an incorrectly sized cuff. The length of the cuff bladder should be 80 percent, and the width 46 percent of the circumference of the upper arm. Direct or invasive blood pressure monitoring may be used for anesthesia for high-risk patients and/or high-risk surgical procedures. (See 'Blood pressure' above.)

•A five lead electrocardiogram is preferred during anesthesia. Leads II and V5 are usually monitored simultaneously to optimize detection of both rhythm and ischemia. (See 'Electrocardiogram' above.)

•Pulmonary artery catheters, transesophageal echocardiography (TEE), and other noninvasive hemodynamic monitors may be used to monitor cardiovascular function in select patients. (See 'Other monitors of circulation' above.)

●Temperature monitoring – Patient temperature should be monitored during every anesthetic. Core temperature should be monitored whenever possible, using nasopharyngeal, distal esophageal, or less commonly tympanic membrane or pulmonary artery probes. When using an axillary temperature probe, the probe should be placed over the axillary artery with the arm kept at the patient’s side for the most accurate measurement. (See 'Temperature monitoring sites' above.)