Mechanical ventilation during anesthesia in adults

INTRODUCTION — Mechanical ventilation is used during general anesthesia for patients with endotracheal tubes or supraglottic airways in place. This topic will discuss the modes of ventilation, ventilator settings, and lung protective ventilation during anesthesia. The deleterious effects of mechanical ventilation are discussed in detail separately. (See "Ventilator-induced lung injury" and "Clinical and physiologic complications of mechanical ventilation: Overview".)

Strategies for mechanical ventilation for specific patient populations and surgical procedures are also discussed separately.

●(See "Anesthesia for laparoscopic and abdominal robotic surgery in adults", section on 'Mechanical ventilation'.)

●(See "Anesthesia for the patient with obesity", section on 'Ventilation management'.)

●(See "Anesthesia for patients with chronic obstructive pulmonary disease", section on 'Mechanical ventilation'.)

●(See "One lung ventilation: General principles", section on 'Lung-protective ventilation strategies'.)

MODES OF INTRAOPERATIVE MECHANICAL VENTILATION — Ventilators on anesthesia work stations increasingly allow modes of ventilation with most, but not all, of the capabilities of ventilators used in the intensive care unit (ICU). Terminology for the modes of ventilation is not standardized, and comparison between the technologies available in the ICU and in the operating room can be confusing. The modes of assisted and controlled ventilation that are available with most anesthesia machines are discussed here. Modes of ventilation in the intensive care unit are discussed separately. (See "Modes of mechanical ventilation".)

Volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) are the basic modes of controlled mechanical ventilation used during general anesthesia. Pressure support and pressure control with volume guarantee (PV-VG) are also available on newer anesthesia machines.

Volume-controlled ventilation — VCV is also called volume-limited, or volume-cycled ventilation. At a minimum, the clinician sets the tidal volume and respiratory rate (and thus the minute ventilation), and the ventilator delivers the tidal volume at a constant flow rate. Advantages of VCV are that the set minute ventilation is essentially guaranteed (unless peak pressures exceed the set limit), and the fact that VCV is a commonly used technique, familiar to clinicians. However, VCV is associated with higher peak pressure for a given inspired volume, compared with PCV. Barotrauma is possible, and gas distribution in the lung may be uneven, particularly in patients with lung disease.

The parameters that may be set by the clinician during VCV include the following:

●Tidal volume (TV)

●Respiratory rate (RR)

●Inspiratory time (TI) to expiratory time (TE) ratio (I:E ratio)

●Positive end-expiratory pressure (PEEP)

●Pressure limit (Pmax)

Compliance of the breathing circuit and fresh gas flows can affect the delivered tidal volume during VCV, though the degree of variation is of little clinical significance in most adults. Some anesthesia ventilators can adjust for fresh gas flows and breathing circuit compliance [1]; these features may be particularly important for neonates and infants, in whom small changes in their already small tidal volumes can cause over or under ventilation.

Pressure-controlled ventilation — For PCV, also called pressure-limited ventilation, the clinician sets the inspiratory pressure, inspiratory time, and respiratory rate. The ventilator maintains constant pressure during the inspiratory time, while flow decelerates. PCV is associated with lower peak airway pressure for the same inspired volume, and in some patient populations, more homogeneous gas distribution, less alveolar overdistention, and lower risk of barotrauma, compared with VCV. (See "Modes of mechanical ventilation", section on 'Volume-limited versus pressure-limited'.)

An important limitation of PCV, particularly during surgery, is that tidal volume varies with changes in lung compliance. Compliance can change because of insufflation during laparoscopy, placement of retractors or surgical packs, changes in positioning, or the degree of muscle relaxation. Therefore, the use of PCV in the operating room may require frequent adjustment of ventilator settings to avoid over- or under-ventilation.

The parameters that may be set by the clinician during PCV include the following:

●Peak inspiratory pressure (PIP)

●RR

●Inspiratory time or I:E ratio

●PEEP

●Pmax

Pressure control with volume guarantee — PCV-VG combines the advantages of VCV and PCV and is our preferred mode of ventilation. PCV-VG is available only on newer anesthesia machines. The set tidal volume is delivered at the lowest required inspiratory pressure, adjusted breath by breath, with the decelerating flow pattern used for PCV. PCV-VG may be useful for patients with low compliance (eg, severe obesity, underlying lung disease such as emphysema or acute respiratory distress syndrome, neonates, and infants) and during surgery in which lung compliance is likely to vary (eg, laparoscopy or thoracoscopy). Parameters that may be set by the clinician using PCV-VG include the following:

●TV

●RR

●I:E ratio

●PEEP

●Pmax

Pressure support — Pressure support ventilation (PSV) is a mode of assisted ventilation used to augment the patient’s breaths. It may be used to reduce the work of breathing related to airway device resistance, to counteract respiratory depressant effects of anesthetic medications, and to support ventilation during emergence from anesthesia. PSV is often used with a supraglottic airway (SGA) in place. Peak pressures should be kept between 15 and 20 cm H₂O to avoid leakage around the SGA and gastric insufflation. (See "Supraglottic devices (including laryngeal mask airways) for airway management for anesthesia in adults", section on 'Choice of mode of ventilation'.)

During PSV, the ventilator delivers a preset pressure above the set level of PEEP to augment a patient initiated breath. Inspiratory flow decreases during the breath, and inspiration ends when the inspiratory flow rate reaches a preset value, usually a decline of 25 percent. Tidal volume varies with lung compliance and airway resistance.

With most anesthesia machines, a minimum respiratory rate can be set during pressure support ventilation. The ventilator will deliver pressure control breaths when the spontaneous respiratory rate is below the set minimum. Parameters that may be set by the clinician include the following:

●Level of pressure support

●Minimum RR

●PEEP

●Pmax

●Pressure or flow trigger

Synchronized intermittent mandatory ventilation modes — Synchronized intermittent mandatory ventilation (SIMV) is a mode of ventilation used for spontaneously breathing patients as they recover from muscle relaxation. SIMV provides assured rates and tidal volumes, with ventilator breaths synchronized with patient breaths. SIMV may be used with VCV or PCV, with or without pressure support (figure 1).

Similar to PSV, SIMV may be used to reduce the work of breathing, counter the respiratory depressant effects of anesthetic medications, and to support ventilation while weaning from controlled ventilation at the end of surgery.

Parameters that may be set by the clinician during SIMV with VCV or PCV plus PSV include the following:

●Tidal volume (TV) for SIMV-VCV or peak inspiratory (PIP) for SIMV-PCV

●Minimal RR

●Inspiratory time (TI) to expiratory time (TE) ratio (I:E ratio)

●PEEP

●Pmax

●Level of pressure support, if applicable

●Pressure or flow trigger

CHOICE OF MODE OF VENTILATION — Choice of the mode of ventilation should be based on patient factors, the surgical procedure, and available technology. For many healthy patients who undergo routine surgery, any mode of ventilation may be used to provide effective and safe intraoperative ventilation [2]. Pressure-controlled ventilation (PCV) may be preferred in some clinical scenarios, as follows:

●PCV or pressure control with volume guarantee (PCV-VG) may be preferred when a supraglottic airway is in place, and for patients in whom high inspiratory pressures may be dangerous (eg, emphysema, neonates, and infants).

●PCV-VG may be preferred to maintain minute ventilation in patients with high or changing intraabdominal pressure (eg, severe obesity, during laparoscopy or thoracoscopy, pregnancy).

FRACTION OF INSPIRED OXYGEN — Supplemental oxygen (ie, >21 percent) is routinely administered before, during, and immediately after anesthesia, to prevent desaturation during airway management, and to compensate for the impairment of gas exchange associated with the residual effects of anesthetic and analgesic drugs. However, a high fraction of inspired oxygen (FiO₂) may have deleterious effects, including absorption atelectasis [3], airway and parenchymal lung injury, and extrapulmonary toxicity [4]. (See "Adverse effects of supplemental oxygen".)

The optimal FiO₂ that should be routinely used during anesthesia is a controversial issue, with conflicting data. Given a lack of definitive studies, the total amount of oxygen exposure during anesthesia should be limited and FiO₂ of 0.3 to 0.5 should provide adequate oxygenation with a margin of safety for most patients. Recruitment maneuvers and positive end-expiratory pressure (PEEP) should be used in preference over higher FiO₂, with the goal of oxygen saturation of ≥94 percent with FiO₂ of <40 percent, depending on the baseline. If higher FiO₂ is required to maintain oxygenation, alveolar recruitment maneuvers should be performed and PEEP levels optimized to improve dynamic compliance of the respiratory system. (See 'Driving pressure' below.)

FiO2 and postoperative complications — Studies of the effects of high FiO₂ on postoperative complications are conflicting. A large US database study found associations between high intraoperative FiO₂ and kidney, cardiac, and pulmonary complications, though causality could not be proven with this type of study [5].

●Postoperative pulmonary complications

•In one randomized trial of 250 patients who underwent abdominal surgery with lung protective ventilation, the incidence of postoperative pulmonary complications (PPCs) was similar in patients who received FiO₂ of 30 versus 80 percent, though the severity of PPCs was reduced by a low FiO₂ [6]. Consistent with these results, in a post hoc analysis of a prospective single center alternating cohort trial including nearly 5000 surgical patients who received 80 percent or 30 percent FiO₂ during surgery, the incidence of PPCs and oxygenation in the post-anesthesia care unit (assessed by SpO₂:FiO₂) was similar between groups [7].

•In contrast, in one large, single center retrospective database study including 73,922 cases, median FiO₂ was associated with a dose-dependent increase in major respiratory complications and mortality, across a range of low to high FiO₂ (median 0.31 to 0.79) [8]. In this study, 4.1 percent of patients developed a major respiratory complication within seven days of surgery. The odds ratio for high versus low FiO₂ was 1.99 (95 percent confidence interval 1.72 to 2.31) for respiratory complications. Unmeasured confounding effects due to the observational and retrospective nature of this second trial could explain the difference in results between these two studies.

●Cardiovascular complications – Hyperoxia can reduce coronary blood flow and may decrease cardiac output and increase systemic vascular resistance in some patient populations. (See "Adverse effects of supplemental oxygen", section on 'Cardiovascular effects'.)

The clinical relevance of these effects is unclear. Whereas an observational study found an association between high FiO₂ and postoperative myocardial injury after noncardiac surgery (MINS), randomized trials have not found that increased FiO₂ results in adverse cardiac events.

•One purported mechanism for myocardial injury with hyperoxia is the generation of reactive oxygen species. This was tested in a randomized trial including 600 patients with significant cardiovascular risk factors who underwent elective or emergency surgery with general anesthesia lasting one hour or more [9]. Patients were randomly assigned in a 2 x 2 fashion to receive FiO₂ of 80 percent versus 30 percent, and antioxidants (vitamin C plus N-acetylcysteine) versus placebo. Myocardial injury within three days of surgery (assessed by troponins), mortality at 30 days, and myocardial infarction were similar among groups.

•Similar findings were reported in a post-hoc analysis of an alternating intervention controlled trial in patients who underwent colorectal surgery and received FiO₂ of 80 percent or 30 percent [10]. The incidence of a composite cardiovascular outcome of myocardial injury in the first three days, in hospital nonfatal cardiac arrest, and 30 day mortality was similar in the two groups.

•In contrast, in a post-hoc analysis of an international observational study of over 6500 patients who underwent various types of surgery (the VISION study), each 0.10 increase in median FiO₂ was associated with an adjusted increase in the odds for MINS (odds ratio [OR] 1.17, 95% CI 1.12–1.23; P <.0001) [11]. There was no association between FiO₂ and pulmonary complications or mortality. Conclusions from this study are limited by the possibility of residual confounding, and the fact that the association between FiO₂ and myocardial injury was limited to only three of the six study sites.

●Long-term mortality

•A secondary analysis of data from a large randomized trial that compared surgical site infections in 1400 patients who were randomly assigned to 80 versus 30 percent FiO₂ during surgery (the PROXI trial), long-term mortality was increased in the subgroup of patients with cancer who received higher FiO₂ (hazard ratio 1.45, 95% CI 1.10-1.90) [12]. Other measured outcomes (acute coronary syndrome, myocardial infarction) were similar in the two treatment groups, though the study was likely underpowered to assess such outcomes.

•In contrast, analysis of mortality data from two randomized trials including over 900 surgical patients, long-term mortality was similar in patients who received 80 versus 30 percent FiO₂ during surgery [13]. Similarly, a post hoc analysis of approximately 3000 patients who received 80 versus 30 percent FiO₂ during colorectal surgery, mortality at a median of three years was similar in the two groups [14].

Oxygen supplementation and surgical site infection — The perioperative use of high FiO₂ to prevent surgical site infection (SSI) is controversial. Taken together, the existing literature suggests that perioperative use of high FiO₂ does not reduce SSI, and we do not target a high FiO₂ to prevent SSI.

Although WHO and CDC have recommended high perioperative FiO₂ [15,16], these recommendations and the meta-analyses used to support them have been widely criticized [17-19]. Some of the studies included in the meta-analyses have been retracted for compromised data integrity.

The effects of high perioperative FiO₂ on SSI were evaluated in a large multicenter trial (i-PROVE O₂) of over 700 patients who underwent abdominal surgery with intraoperative lung protective ventilation (ie, low tidal volume, individualized PEEP, recruitment maneuvers) [20]. Patients were randomly assigned to receive 80 percent or 30 percent FiO₂ intraoperatively and for three hours postoperatively, along with postoperative continuous positive airway pressure (CPAP) as needed. The rates of SSI at 7 and 30 postoperative days, atelectasis, and myocardial ischemia were similar between groups.

A large, single center prospective study including 5749 patients who had major intestinal surgery with 30 or 80 percent inspired oxygen concentration reported no differences in deep tissue surgical site infection, healing related wound complications, or mortality [21].

A 2018 meta-analysis of 26 randomized trials including approximately 14,700 cases found a lower risk of SSI in patients who received high intraoperative FiO₂ (relative risk [RR] 0.81, 95% CI 0.70-0.94) [22]. However, meta-analysis of only studies with a low risk of bias found no benefit from high FiO₂. Similarly, a 2019 meta-analysis of 20 randomized trials found similar risk of SSI with high versus low FiO₂ (odds ratio 0.89, 95% CI 0.73-1.08) [18].

Supplementing oxygen beyond the level required for normoxemia is not without adverse effects. (See 'Fraction of inspired oxygen' above.)

Postoperative nausea and vomiting — Studies of the effect of high FiO₂ on reducing the risk of postoperative nausea and vomiting have reported mixed results [23-26]. High FiO₂ probably has no effect on postoperative nausea [27] and other interventions for prophylaxis are of much greater value [28]. (See "Postoperative nausea and vomiting", section on 'Prevention'.)

GOAL END-TIDAL CARBON DIOXIDE — The goal for intraoperative ventilation should be an end-tidal carbon dioxide (ETCO₂) of approximately 40 mmHg unless therapeutic hyperventilation is indicated.

In most cases, ETCO₂ is measured (in exhaled gas), rather than PaCO₂ (with blood gases). ETCO₂ generally correlates well with PaCO₂, but a number of factors (eg, age, lung disease, surgical positioning) can cause a significant, variable discrepancy [29,30]. It is not uncommon to find a difference of 8 to 10 mmHg between ETCO₂ and PaCO₂. 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). Reducing the respiratory rate (or briefly stopping the ventilator) should allow full exhalation and an accurate ETCO₂ reading (figure 2).

Mild intraoperative hypocapnia has historically been the aim during anesthesia. However, hypocapnia (ie, PaCO₂ 25 to 30 mmHg) induced by hyperventilation and the associated alkalosis may have deleterious physiologic effects, including decreased cerebral blood flow and oxygen delivery [31,32], increases in lung microvascular permeability, bronchoconstriction and decreased lung compliance, increase in shunt, and a leftward shift of the oxyhemoglobin dissociation curve [33].

In contrast, mild hypercapnia (ie, PaCO₂ 50 mmHg or ETCO₂ 40 to 45 mmHg) may increase tissue perfusion and oxygenation, due to an increase in cardiac output and vasodilation, and a rightward shift of the oxyhemoglobin dissociation curve [34-37]. In addition, the lower respiratory rates required for permissive hypercapnia decrease lung strain and stress. Hypercapnia should be used cautiously in patients with cardiac disease, because of the associated increase in cardiac output.

RESPIRATORY RATE — Respiratory rate for controlled ventilation should be set to achieve minute ventilation that maintains ETCO₂ at approximately 40 mmHg. Respiratory rate is often set at 8 to 12 breaths per minute, and adjusted thereafter based on ETCO₂. The use of low tidal volumes as part of lung protective ventilation may require a high respiratory rate to adequately eliminate CO₂, especially during CO₂ insufflation for laparoscopy. During high respiratory rates (>20 breaths/min), and at lower respiratory rates for patients with limited expiratory flow (eg, patients with chronic obstructive pulmonary disease [COPD]), inspiration can occur before full exhalation (ie, breath stacking). Dynamic hyperinflation and auto-positive end-expiratory pressure (PEEP) can occur, and can lead to hypoxemia, hyperinflation, and barotrauma. (See "Anesthesia for patients with chronic obstructive pulmonary disease", section on 'Mechanical ventilation'.)

End-expiratory flow may be monitored when respiratory rates are high. As long as expiratory flow tracing displayed on the anesthesia machine returns to zero, auto-PEEP should not occur [38].

INSPIRATORY TO EXPIRATORY RATIO — The inspiratory:expiratory (I:E) ratio during mechanical ventilation is usually set at approximately 1:2, similar to the ratio during normal spontaneous ventilation. The I:E ratio may be reduced (ie, increasing the inspiratory time) to improve ventilation of lung regions that fill more slowly (ie, atelectatic lung regions) or to reduce peak pressure during volume-controlled ventilation (VCV). The author usually sets the I:E ratio at 1:1 which provides a 'balanced stress to time product' [39].

MONITORING PULMONARY MECHANICS — Anesthesia machines display a number of parameters (eg, flow, pressure, volume) that allow assessment of ventilation, either directly or by providing a means for calculating variables. Such parameters are displayed in both numeric and graphic form.

Most newer anesthesia machines allow display of pressure volume loops and flow volume loops. These graphs can be used to optimize ventilator settings, and to monitor changes in compliance, and extrinsic and intrinsic airway obstruction. Some machines display both baseline and real time curves for comparison.

Pressure volume loops — Pressure-volume loops are most useful as an indication of compliance. Pressure is plotted on the horizontal axis, and volume is plotted on the vertical axis. During positive pressure ventilation, breaths progress in a counterclockwise direction, with inspiration curving upward, and expiration curving downward (figure 3). Thus, the upper right point on the graph represents peak inspiratory pressure and tidal volume. The lower left point of the loop appears at zero volume, and the set level of positive end-expiratory pressure (PEEP).

A line drawn from the zero point through the point of end inspiration represents compliance. With good compliance, this line forms an angle of ≤45 degrees with the vertical axis. If compliance decreases during mechanical ventilation, the line becomes more horizontal and the curve is rotated to the right. This shift will appear differently during pressure-controlled ventilation (PCV) compared with volume-controlled ventilation (VCV) (figure 4). Compliance may change intraoperatively (and be reflected in the pressure volume loops) because of changes in position, surgical manipulation, intraabdominal bleeding, carbon dioxide insufflation for laparoscopy, and development of lung disease (eg, pneumonia, pulmonary edema). A mainstem intubation may be indicated by a higher peak airway pressure and a more horizontal pressure volume loop than expected.

Pressure volume loops may indicate that excessive pressure or volume is being used during positive pressure ventilation. A "bird beak" pattern (figure 5) suggests that a portion of the breath is increasing inspiratory pressure without achieving an appreciable increase in volume, which may increase the risk of barotrauma. The pressure (during PCV), or volume (during VCV) should be reduced to optimize ventilation.

Flow volume loops — The flow-volume loop is most useful as an indication of resistance. Volume is plotted on the horizontal axis and flow is plotted on the vertical axis. On anesthesia machines, flow volume loops may be displayed in reverse orientation from conventional spirometry, with inspiration above the horizontal line and expiration below, and with volume increasing from left to right on the horizontal axis (figure 6). When displayed this way, breaths progress in a clockwise direction, and the shape of the inspiratory curve depends on the mode of ventilation (eg, pressure control or volume control). Flow increases as inspiration begins, and decreases to zero as the breath ends. Tidal volume is represented by the point on the volume axis where the flow curve crosses zero.

The shape of the exhalation portion of the flow volume loop is determined by elastic recoil of the lung and chest wall, and airway resistance. With increased resistance (eg, obstructive lung disease, kinked or obstructed endotracheal tube), the exhalation curve may take on a flattened, curved shape, with lower peak expiratory flow and a gradual return to baseline (figure 7). Normalization of the flow volume curve can be used to monitor treatment.

Monitoring driving pressure — Driving pressure (DP or ΔP) is increasingly recognized as the most important variable for minimizing dynamic strain during mechanical ventilation, and can be easily monitored in the operating room. DP is defined as the difference between plateau pressure (P_pl; ie, peak inspiratory pressure in the absence of respiratory effort, as in paralyzed patients) and PEEP. Most newer anesthesia ventilators deliver a 10 percent inspiratory pause, which allows plateau pressure to be calculated and displayed. In clinical practice, DP is measured as the difference between the airway plateau pressure (PPLAT) minus PEEP, which equals VT scaled to the compliance of the respiratory system (CRS; ΔP = V_T/C_RS).

DP represents the ratio of tidal volume (V_t) to respiratory system compliance (C_RS) and it therefore estimates the ratio of V_t to aerated volume (ie, dynamic strain). The relevant formulas are as follows:

ΔP = P_pl – PEEP

C_RS = V_t /P_pl – PEEP = V_t/ΔP

ΔP = V_t/C_RS

The use of DP during lung protective ventilation is discussed below. (See 'Driving pressure' below.)

LUNG PROTECTIVE VENTILATION DURING ANESTHESIA — We suggest the use of lung protective ventilation or "nonharmful ventilation" for all patients who receive mechanical ventilation during anesthesia [39]. Lung protective ventilation (ie, low tidal volumes, use of positive end-expiratory pressure [PEEP]) is beneficial for patients with acute respiratory distress syndrome (ARDS) and acute lung injury and has become the standard of care in the intensive care unit. (See "Acute respiratory distress syndrome: Ventilator management strategies for adults".)

Lung protective ventilation may also be beneficial for patients without lung injury, including those who undergo intraoperative ventilation.

Goals for lung protective ventilation — The goals for lung protective ventilation during anesthesia are to reduce alveolar overdistention and cyclic atelectasis (also called atelectrauma), which are the principal initiators of ventilator associated lung injury. Atelectasis can be prevented or reversed with PEEP or recruitment maneuvers, and alveolar overdistention can be avoided with low tidal volumes and reduced plateau and/or driving pressures (DPs). (See "Ventilator-induced lung injury", section on 'Mechanisms'.)

Pulmonary effects of general anesthesia — General anesthesia causes a reduction in functional residual capacity (FRC), and results in atelectasis in up to 90 percent of patients [40]. In addition, absorption atelectasis occurs during general anesthesia with high inspired oxygen concentrations. Mechanical ventilation in the face of atelectasis cyclically deforms lung parenchyma, produces alveolar strain, and predisposes the lung to volutrauma, barotrauma, and biotrauma. (See "Ventilator-induced lung injury", section on 'Mechanisms' and "Ventilator-induced lung injury".)

The pulmonary effects of general anesthesia are usually well tolerated by healthy patients during short anesthetics [41]. However, the risk of postoperative pulmonary complications (PPCs) may be increased by patient characteristics, the surgical procedure, and the anesthetic technique (table 1). Particularly vulnerable patients may benefit from specific management during anesthesia. (See "Evaluation of perioperative pulmonary risk".)

Our approach — The ventilatory strategy during anesthesia should be individualized, based on patient factors (eg, diseased or injured lung, other comorbidities) and the type of surgery. The goal is to apply the least harmful energy to the lung, defined as mechanical power. This may be a better predictor of lung injury than any single, isolated parameter like tidal volume, driving pressure or respiratory rate. Thus, protective mechanical ventilation seems to require that individual components (including respiratory rate) be set at safe levels and that their summed contribution expressed as mechanical power not be excessive [42-44]. Calculating mechanical power is complex, and its role in adjusting mechanical ventilation in the operating room has yet to be determined.

Despite uncertainty regarding the beneficial clinical effects of intraoperative lung protective strategies in general and specific components in particular, we suggest the use of low tidal volumes (ie, 6 to 8 mL/kg ideal body weight [IBW]) (calculator 1), and initial PEEP of 5 cm H₂O (8 to 10 cm H₂O during laparoscopy), with adjustments based on DP. For patients in steep head down position for surgery, PEEP values of 10 to 15 cm H₂O may be required to prevent atelectasis and can be applied without the risk of increased lung stress or other harmful side effects [45]. If DP at these settings is >15 cm H₂O, we increase PEEP in an attempt to recruit alveoli and improve compliance (algorithm 1). (See 'Monitoring driving pressure' above and 'Driving pressure' below.)

If DP remains above 15 to 18 cm H₂O after optimizing PEEP, we suggest performing recruitment maneuvers, and if necessary, further decreasing tidal volume (V_t) below 6 mL/kg IBW. (See 'Recruitment maneuvers' below.)

The respiratory rate is set to achieve an ETCO₂ of approximately 40 mmHg. We perform recruitment maneuvers only when indicated to improve oxygenation (eg, pediatric patients under school age, patients with obesity, during open abdominal surgery, and before, during, and after insufflation for laparoscopy and thoracoscopy), and in specific circumstances (eg, after disconnect from the ventilator for suctioning).

Components of intraoperative lung protective ventilation — Most, though not all, studies have reported a lower incidence of PPCs with relatively low tidal volumes (ie, 6 to 8 mL/kg IBW) and PEEP ≥5 cm H₂O during anesthesia, compared with more traditional ventilation using higher tidal volumes (ie, 10 to 12 mL/kg ) without PEEP [46-56]. (See "Overview of initiating invasive mechanical ventilation in adults in the intensive care unit", section on 'Tidal volume' and "Positive end-expiratory pressure (PEEP)", section on 'Intraoperative patients'.)

Three tools are available for protective ventilation which have been adapted from the intensive care unit (ICU):

●Low tidal volumes or DP to reduce stress and strain

●PEEP to keep the lung open and

●Recruitment maneuvers to reopen collapsed alveoli as indicated

The protective effect of each of these protective maneuvers varies among patients and clinical scenarios, and their effect may not last into the postoperative period [57].

The relative importance of low tidal volumes, reduced inspiratory pressures, PEEP, and recruitment maneuvers is unclear, as these strategies are often bundled together in studies of lung protective ventilation. In addition, management of ventilation immediately prior to, during, and after extubation and emergence may have important effects on postoperative respiratory complications, and are not generally analyzed in these studies.

In a meta-analysis of 63 randomized trials that evaluated various components of lung protective ventilation in patients who underwent noncardiac surgery with mechanical ventilation, low tidal volume with PEEP reduced the risk of pulmonary complications, atelectasis, and postoperative mechanical ventilation, and recruitment maneuvers reduced atelectasis [58]. However, there was insufficient evidence to support specific levels of PEEP or tidal volume. There was no effect of lung protective ventilation on mortality.

Tidal volume — Low, or better termed "physiologic," tidal volumes have long been considered to be the major determinant of lung protective ventilation. However, studies of the isolated effect of low tidal volumes on postoperative respiratory outcomes have shown mixed results, and many are small, retrospective, and/or include heterogeneous patient populations and surgical procedures. The best evidence suggests that low tidal volume may account for a relatively small portion of beneficial effects of lung protective ventilation.

●A 2018 meta-analysis of 19 small randomized trials that compared low tidal volume (6 to 8 mL/kg IBW) with high tidal volume (≥10 mL/kg IBW) ventilation during various surgeries in patients without lung disease reported an uncertain effect on 28 day mortality (relative risk [RR] 0.8, 95% CI 0.42-1.53), but a reduction in postoperative pneumonia (RR 0.45, 95% CI 0.25-0.82) and the need for postoperative invasive ventilation (RR 0.33, 95% CI 0.14-0.77) with the use of low tidal volumes [55]. The overall quality of the evidence was rated as moderate.

Tidal volumes of 6 to 10 mL/kg may be safe and reasonable for many patients.

●In a randomized trial of approximately 1200 surgical patients who were ventilated with 6 mL/kg versus 10 mL/kg tidal volumes, all with PEEP at 5 cm H₂O, composite outcome of pulmonary complications within 7 days was similar in the two groups [59].

●Similarly, in a two-by-two crossover cluster trial including over 2800 patients who underwent orthopedic surgery and were ventilated with tidal volumes of 6 versus 10 mL/kg and PEEP at 5 versus 8 cm, clinical outcomes were similar with all ventilatory settings [60].

Tidal volume should be based on ideal body weight, which is calculated based on the patient's height and therefore more accurately reflects lung size than using actual body weight [61].

DP in conjunction with low tidal volumes has emerged as the one variable that predicts pulmonary complications in mechanically ventilated surgical patients [62] (see 'Driving pressure' below). Similarly, maintaining an open lung may be more important than the selection of an absolute tidal volume. (See 'The open lung approach (OLA)' below.)

During low tidal volume ventilation, PEEP with or without recruitment maneuvers should be used to compensate for development of atelectasis, especially for patients who undergo abdominal surgery or laparoscopy.

Plateau pressure — Low inspiratory pressure, specifically low plateau pressures, are associated with less alveolar distention, and may protect against PPCs. This was demonstrated by a study which reviewed electronic anesthesia and medical records of approximately 70,000 patients who required general anesthesia with endotracheal intubation [50]. Intraoperative lung protective ventilation was associated with a decreased risk of major respiratory complications; PEEP of 5 cm H₂O and plateau pressure ≤16 cm H₂O were identified as protective strategies, while low tidal volume was not.

Most newer anesthesia ventilators deliver a 10 percent inspiratory pause, which allows plateau pressure to be calculated and displayed.

Driving pressure — Among ventilator variables (eg, V_t, PEEP, plateau pressure, DP), DP may best predict survival in mechanically ventilated patients with ARDS. (See "Acute respiratory distress syndrome: Ventilator management strategies for adults", section on 'Efficacy and harm'.)

In patients without preexisting lung disease in the operating room, high DP above 18 cm H₂O may be associated with the development of PPCs. DP is linearly related to dynamic strain, which can result from volutrauma, barotrauma, or inflammation, and is a key mechanism for ventilator-induced lung injury. Whereas there is insufficient evidence to suggest a specific target value for DP during intraoperative ventilation, 15 cm H₂O can probably be considered a safe upper limit. (See "Ventilator-induced lung injury", section on 'Mechanisms'.)

Strain is a measure of a change in the dimension of a structure from its original dimension. Lung strain during mechanical ventilation consists of a static component and a dynamic component, both of which are reflected in DP. Static strain results from application of a level of PEEP that increases end-expiratory volume beyond functional residual capacity. Dynamic strain is that which results from tidal ventilation. Dynamic strain is heterogeneous throughout the lungs and is considered the most pertinent mechanism of injury. DP relates V_t to respiratory system compliance (C_RS), which is presumed to be a measure of aerated lung volume. A given V_t will result in higher DP and greater dynamic strain in patients with low compliance.

An increase in PEEP beyond a given value can result in differing effects on lung mechanics, including DP, depending on the degree of non-aerated lung. The degree of aerated lung is variably reduced during general anesthesia. An increase in PEEP may recruit available alveoli, improve compliance, and reduce DP for a given V_t. This may effectively decrease the severity of atelectasis, improve oxygenation, and reduce the incidence of clinically significant PPCs after open upper abdominal surgery [63]. Conversely, an increase in PEEP in unrecruitable lung causes an increase in DP, and suggests potential for overdistention. A small study of the effects of different levels of PEEP on respiratory mechanics found that the relationships between DP and strain, and between C_RS and lung volumes, respectively, held true only if overdistention was avoided (ie, aerated lung volume did not exceed predicted FRC) [64]. Aerated lung volume exceeded predicted FRC in 35 percent of patients who received PEEP at 2 cm H₂O and in 55 percent of patients on a PEEP of 7 cm H₂O. (See 'Positive end-expiratory pressure' below.)

In a meta-analysis of 17 randomized controlled trials involving various mechanical ventilation strategies during surgery, DP and changes in PEEP that resulted in increased DP were associated with an increase in PPCs [62]. Similarly, in a large registry study including over 69,000 patients who underwent general anesthesia with mechanical ventilation, high DP (>12.5 cm H₂O) and high plateau pressure (>16 cm H₂O) were associated with an increase in PPCs [50]. These clinical observations also correlate with findings in animal research [65]. However, no trial has randomly assigned patients to management based on DP versus usual treatment, so definitive conclusions cannot be drawn.

●A meta-analysis described above found that strategies with varying PEEP and tidal volume dramatically reduced pulmonary complications [58], suggesting an extremely potent synergy between the two interventions.

●In a retrospective database study of over 87,000 patients ≥60 years of age who were living at home prior to elective noncardiac surgery, maintaining lower driving pressure during intraoperative ventilation was associated with greater likelihood of discharge to home [66].

Positive end-expiratory pressure — PEEP prevents alveolar collapse and can maintain but not restore end-expiratory lung volume recruited during inspiration [67]. However, PEEP may be deleterious in some patients (ie, hypovolemia, increased intracranial pressure), and the optimal level of PEEP varies among patients. Low levels of PEEP (≤5 cm H₂O) may not reliably reverse atelectasis or improve arterial oxygenation during anesthesia [68], unless preceded by a recruitment maneuver [69]. (See "Positive end-expiratory pressure (PEEP)", section on 'Intraoperative patients' and "Positive end-expiratory pressure (PEEP)", section on 'Contraindications'.)

Individualized PEEP — PEEP should be individualized based on patient and surgical factors, in an attempt to avoid both alveolar overdistention and atelectasis [70]. As examples, for healthy, normal weight patients who undergo low risk surgery, it is reasonable to start ventilation with PEEP at 5 cm H₂O. On the other hand, for patients who undergo open abdominal surgery or laparoscopy, for patients with obesity, or for surgery performed in Trendelenburg position, higher levels of PEEP make sense. (See "Anesthesia for the patient with obesity", section on 'Ventilation management' and "Anesthesia for laparoscopic and abdominal robotic surgery in adults", section on 'Mechanical ventilation'.)

Objective methods for optimizing PEEP after an initial set value have been used and studied, but none have been shown to reduce PPCs. One option that can easily be used in the operating room is to adjust PEEP to the level that results in the lowest driving pressure for the selected tidal volume, as a measure of optimal lung compliance ("PEEP-step maneuver") [71]. This approach aims at two major risk factors for PPCs, that is, minimizing driving pressure to limit stress and optimizing PEEP to reduce atelectasis [63]. Such an approach is shown in an algorithm (algorithm 1). Another option, and one that is used in more commonly in the intensive care unit, is to place an esophageal balloon to allow calculation of transpulmonary pressure, and then adjust PEEP to optimize. The use of an esophageal balloon to calculate transpulmonary pressure is discussed separately. (See "Positive end-expiratory pressure (PEEP)", section on 'Tools for titrating applied PEEP'.)

Electrical impedance tomography (EIT) is a noninvasive, dynamic imaging tool that can be used to assess lung parenchymal collapse or hyperinflation at various levels of PEEP. EIT is not generally available in the operating room, and has primarily been used as a research tool [72-74].

The results of randomized trials of optimal levels and intraoperative adjustment of PEEP are mixed. Examples include the following:

●The iPROVE trial was a multicenter four way randomized trial of individualized versus standard PEEP with or without continuous positive airway pressure (CPAP) in approximately 1000 patients at moderate or high risk of PPCs who underwent abdominal surgery [75]. For individualized PEEP, after a recruitment maneuver, PEEP was titrated based on the level that achieved the highest dynamic compliance; mean individualized PEEP was 10 cm H₂O. For standard PEEP, patients received fixed PEEP at 5 cm H₂O without a recruitment maneuver. The risks of pulmonary and systemic complications were similar among all groups.

●Another multicenter trial (PROVHILO) randomly assigned 900 abdominal surgery patients at risk for postoperative pulmonary complication to receive either high PEEP (12 cm H₂O) with recruitment maneuvers(RM) or low PEEP (≤2 cm H₂O) without RM, at a constant tidal volume of 8 mL/kg IBW [47]. There were no differences at five days in PPCs (39 versus 40 percent), length of hospital stay, or mortality. However, patients in the high PEEP group had a higher rate of hypotension (46 versus 36 percent) that required vasopressors and fluid resuscitation (62 versus 52 percent). A major flaw of this trial was the use of a fixed level of PEEP at 12 cm H₂O, which is unusually high and potentially detrimental in patients who do not have ARDS; this could have mitigated any potential clinical benefit of PEEP.

●A meta-analysis of patient level data from the iPROVE, PROVHILO, and PROBESE trials (total 3837 patients), intraoperative use of high PEEP with recruitment maneuvers did not reduce the incidence of pulmonary complications within the first week after surgery [76]. High PEEP reduced intraoperative oxygen desaturation, but increased the risk of hypotension. The iPROVE and PROVHILO trials are described above; PROBESE is described separately. (See "Anesthesia for the patient with obesity", section on 'Ventilation management'.)

●In a PROBESE substudy of 162 patients who underwent abdominal surgery and who were evaluated with electrical impedance tomography (EIT), PEEP at 12 versus 4 cmH₂ resulted in slightly increased ventilation in the dorsal lung regions, without a meaningful clinical benefit [77].

●In contrast, in the IMPROVE trial, 400 patients at moderate risk for pulmonary complications who underwent major abdominal surgery were randomly assigned to a nonprotective strategy (high tidal volume; 10 to 12 mL/kg, without PEEP) or a lung protective strategy (low tidal volume [6 to 8 mL/kg IBW]) with PEEP [6 to 8 cm H₂O]) [46]. At seven days, the use of lung protective ventilation was associated with a reduction in adverse pulmonary events (28 versus 11 percent), need for mechanical ventilation (17 versus 5 percent), and shorter hospital length of stay (mean difference 2.5 days) but was not associated with a mortality benefit. The relative contributions of PEEP and tidal volume to the results of this study are unclear.

The beneficial effects of PEEP may depend on the surgical procedure. A review of approximately 11,000 electronic anesthesia records found that PEEP ≥5 cm H₂O was associated with a lower risk of pulmonary complications in patients who underwent major abdominal surgery, but not in patients who underwent craniotomy [78]. Similarly, a crossover trial in patients having orthopedic surgery found that postoperative hypoxemia, pulmonary complications, and duration of hospitalization were similar in patients who had intraoperative ventilation with tidal volumes of 6 versus 10 mL/kg and PEEP levels of 5 versus 8 cm H₂O [60]. However, it is still not clear whether decremental or incremental protocols are equivalent in setting the optimal PEEP.

Recruitment maneuvers — A recruitment maneuver (RM) is a brief application of a high level of continuous positive airway pressure with the goal of reversing atelectasis and improve oxygenation. The aim of lung recruitment should be to achieve oxygen saturation ≥90 percent with a fraction of inspired oxygen (FiO₂) ≤0.6.

During a recruitment maneuver, inspiratory pressures of 30 cm H₂O are required to re-expand half of the anesthesia-induced atelectatic lung, but peak inspiratory pressures of up to 40 cm H₂O may be needed to fully reverse anesthesia-induced collapse of healthy lungs, and even higher pressures may be required if the patient is severely obese [79]. The duration of the recruitment maneuver should generally be at least seven to eight seconds [80].

Recruitment maneuvers can be performed manually, by squeezing and holding pressure on the breathing bag, or by using the ventilator. Ventilator driven RMs are preferred, since switching back from a manual breath hold to the ventilator leads to loss of the recruited lung areas [70]. For a ventilator driven RM, tidal volume is increased in a stepwise fashion using a low respiratory rate (during volume-controlled ventilation [VCV]), or by sequentially increasing the PEEP level while maintaining DP until an PIP of at least 40 cm H₂O (during pressure-controlled ventilation [PCV]) [39,81].

Recruitment maneuvers can reduce preload and cause hypotension [82], and there is no evidence to support routine use of these techniques. Recruitment breaths should be used after disconnection from the ventilator (eg, for tracheal suction), and as indicated to improve oxygenation, and should be followed by application of, or increase in, PEEP [83]. Recruitment maneuvers may improve oxygenation before insufflation [84] and compliance after desufflation during laparoscopy [85], including during laparoscopic bariatric surgery [86,87].

Recruitment maneuvers may be especially beneficial for young children and patients with severe obesity [88]. Anesthesia related reduction in functional residual capacity is more pronounced in children than adults [89], and they are therefore more prone to atelectasis. Studies using magnetic resonance imaging [69] and measurements of pulmonary mechanics [90] in anesthetized children have reported reversal of atelectasis, and prevention of airway closure, when recruitment maneuvers were performed with and without PEEP. The same applies to patients with obesity.

The open lung approach (OLA) — Open lung ventilation refers to an approach whereby the lung is maximally recruited, usually through application of higher PEEP, recruitment maneuvers, and subsequent efforts to minimize de-recruitment. In theory increased lung volume after recruitment will result in less tidal overinflation and potentially improved outcome. However, whether an open lung reduces PPCs, and whether an OLA is necessary to achieve an open lung have been unclear. There is no standard definition of open lung volume, and no easy way to confirm an open lung in the operating room. An open lung can be confirmed with imaging, or in theory by using a predefined physiological response. Comparative studies evaluating an OLA have provided mixed results, possibly because they did not confirm whether an open lung was reached, with or without use of an OLA.

One retrospective study suggested that an intraoperative OLA did not necessarily achieve an open lung, and that conventional ventilation achieved an open lung in many patients [91]. Patients who were found to have an open lung with or without an OLA had a lower risk of developing severe PPCs. This study used data from the previously published iPROVE and iPROVE-O₂ trials, which involved patients at high risk of PPCs who underwent abdominal surgery.

●Arterial blood gases (ABGs) were used as a measure of atelectasis to define the OLC. The open lung was defined as a ratio of partial pressure of arterial oxygen to the fraction of inspired oxygen (PaO₂/FIO₂) ≥400 mmHg on ABGs 60 min after intubation and at the end of surgery before extubation. A non-open lung condition was defined as PaO₂/FiO₂<400 mmHg on at least one of those ABGs.

●All patients received lung protective ventilation using tidal volume of 8 mL/kg, and plateau pressure ≤25 cmH₂O. The OLA consisted of individualized PEEP based on the highest respiratory system compliance (Cdyn level after a RM. Additional RM and PEEP titration trials were performed every 40 minutes if Cdyn decreased ≥10 percent and hemoglobin oxygen saturation (SpO₂) decreased to ≤96 percent five minutes after reducing the FIO₂ to room air. The OLA was compared with conventional ventilation consisting of fixed PEEP at 5 cmH₂O, and no recruitment maneuvers.

●An open lung was achieved in 55 percent of patients who had OLA ventilation, and in 33 percent of patients with conventional ventilation. An open lung was associated with reduced risk of severe PPCs at seven days (4.4 versus 7.9 percent, odds ratio 0.58 [95% CI 0.34-0.99]).

Further study is required to verify these retrospective findings, assess components of the OLA, and validate endpoints for the open lung.

EMERGENCE AND EXTUBATION — Management of oxygenation and ventilation during emergence and extubation from anesthesia are discussed separately. (See "Extubation following anesthesia", section on 'Awake extubation'.)

NONINVASIVE VENTILATION — Noninvasive ventilation may be used in the perioperative period to prevent or to treat respiratory failure after tracheal extubation, and can reduce the need for tracheal intubation and mechanical ventilation. High risk patients who may be candidates for prophylactic noninvasive ventilation include patients with severe obesity and/or obstructive sleep apnea or pulmonary disease, particularly after cardiac, thoracic, or upper abdominal surgery.

Two commonly used modes of noninvasive ventilation include continuous positive airway pressure (CPAP) and noninvasive positive pressure ventilation (nPPV). The use of noninvasive ventilation is discussed separately. (See "Noninvasive ventilation in adults with acute respiratory failure: Benefits and contraindications" and "Noninvasive ventilation in adults with acute respiratory failure: Practical aspects of initiation".)

CRITICALLY ILL PATIENTS — Anesthesia ventilators do not offer the full range of options available on ventilators in the intensive care unit (ICU). Patients in respiratory failure in the ICU who require surgery may require transport to the operating room and intraoperative ventilation with an intensive care ventilator. In general, patients who require >30 cm PEEP or minute ventilation >15 L/min may not tolerate the transition to an anesthesia ventilator. When possible, surgery in the ICU may be considered for such patients.

SUMMARY AND RECOMMENDATIONS

●Modes of ventilation – The basic modes of controlled ventilation for anesthesia are volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV). Pressure control with volume guarantee (PC-VG) is available on newer anesthesia machines. (See 'Modes of intraoperative mechanical ventilation' above.)

•With VCV, the tidal volume and respiratory rate set by the clinician determine the minute ventilation. The tidal volume is essentially guaranteed, but higher inspiratory pressures are required for a given inspired volume, compared with PCV. (See 'Volume-controlled ventilation' above.)

•For PCV, the clinician sets the inspiratory pressure, inspiratory time, and respiratory rate. PCV is associated with lower peak airway pressures than VCV for a given inspired volume, but tidal volume and minute ventilation can vary with changes in compliance that may occur during surgery (eg, insufflation during laparoscopy, placement of retractors or surgical packs, changes in position, and the degree of muscle relaxation). (See 'Pressure-controlled ventilation' above.)

•PCV-VG is the preferred mode of controlled ventilation where it is available. The set tidal volume is delivered at the lowest required inspiratory pressure, adjusted breath by breath as necessary based on changes in compliance. PCV-VG is particularly useful for patients in whom compliance is expected to change repeatedly during surgery (eg, during laparoscopy or thoracoscopy). (See 'Pressure control with volume guarantee' above.)

●Augmenting spontaneous ventilation

•Pressure support – Pressure support may be used to augment spontaneous ventilation, and is often used with a supraglottic airway in place, or to support ventilation during emergence from anesthesia. The ventilator delivers a preset pressure above the set level of positive end-expiratory pressure (PEEP), to augment a patient initiated breath. (See 'Pressure support' above.)

•Synchronized intermittent ventilation (SIMV) – SIMV is also used to augment spontaneous ventilation with ensured tidal volume and rate, and can be used with either volume control or pressure control, with or without pressure support. (See 'Synchronized intermittent mandatory ventilation modes' above.)

●Supplemental oxygen – Supplemental oxygen is routinely administered before, during, and after anesthesia. Administration of high fraction of inspired oxygen (FiO₂) may prevent desaturation during airway management, and can compensate for the impairment of gas exchange associated with anesthesia. However, a high FiO₂ may cause absorption atelectasis. The administration of high fraction of inspired oxygen with the goal of preventing surgical infections is controversial. An oxygen saturation (SpO₂) of ≥95 percent with FiO₂ of <60 percent is generally considered appropriate, depending on the baseline level of oxygenation. (See 'Fraction of inspired oxygen' above.)

●Goal end-tidal carbon dioxide (ETCO₂) – A goal for intraoperative ventilation should be an ETCO₂ of approximately 40 mmHg unless therapeutic hyperventilation is indicated. (See 'Goal end-tidal carbon dioxide' above.)

●Lung protective ventilation

•Primary goals for intraoperative ventilation are to provide non-harmful ventilation, to open the lung, and keep it open into the postoperative period.

•We suggest the use of lung protective ventilation for all patients who receive mechanical ventilation during anesthesia (Grade 2C). For most patients, we suggest the used of physiologic tidal volumes (ie, 6 to 8 mL/kg ideal body weight [IBW]), initial PEEP of 5 cm H₂O (10 cm H₂O during laparoscopy), driving pressure (DP) ≤15 cm H₂O, and plateau pressures ≤16 cm H₂O, during anesthesia (algorithm 1). The benefits of individual components of lung protective ventilation have not been determined. (See 'Lung protective ventilation during anesthesia' above.)

•We perform recruitment maneuvers only when indicated in specific patient populations, to improve oxygenation (eg, patients with obesity, pediatric patients, during open abdominal surgery, and before, during, and after insufflation for laparoscopy and thoracoscopy), and in specific circumstances (eg, after disconnect from the ventilator for suctioning). (See 'Recruitment maneuvers' above.)