Modes of mechanical ventilation

INTRODUCTION — Numerous decisions need to be made once it is determined that a patient requires mechanical ventilation, including the mode of mechanical ventilation. The mode refers to the method of inspiratory support. Its selection is generally based on clinician familiarity and institutional preferences since there is a paucity of evidence indicating that mode selection affects clinical outcome.

Common modes of mechanical ventilation are described in this topic review (table 1). Other aspects of initiating mechanical ventilation are discussed separately. (See "Overview of initiating invasive mechanical ventilation in adults in the intensive care unit".)

GENERAL PRINCIPLES — When considering the modes of mechanical ventilation, there are two main categorizations of modes to help conceptualize the physiology of the support of respiration for the patient (table 1).

●First, we consider volume-controlled versus pressure-controlled breath strategy, which describes the patterns of ventilator controls and the physiology of each breath delivered by the ventilator.

●Second, we consider modes of ventilation that describe either the ventilator-patient interaction or the pattern of breaths over time; the latter has become a well-recognized terminology for a mode that describes a unique set of ventilator settings with specific clinical outcomes data (these can be based on either volume- or pressure-controlled breath strategies).

Each ventilator-supported breath can be considered through the key mechanics within the inspiratory and expiratory cycle. Each breath evolves through three major physiologic timepoints that describe the interaction between the ventilator and the patient: the trigger, target, and cycle (figure 1). The trigger describes the ventilator setting that initiates inspiration. The target is the setting that is not exceeded during inspiration. And the cycle is the setting that switches the breath from inspiration to expiration. In most ventilator modes, the trigger is either time, which is extrapolated from a set respiratory rate (ie, ventilator-triggered), or patient-triggered. However, depending on the breath strategy, the target and cycle can vary.

The two breath strategies, volume-controlled ventilation and pressure-controlled ventilation, are discussed below (see 'Volume-controlled ventilation' below and 'Pressure-controlled ventilation' below). Pressure-controlled ventilation was compared with volume-controlled ventilation in a randomized trial and several observational studies [1-3]:

●There were no statistically significant differences in mortality, oxygenation, or work of breathing between the two ventilation strategies.

●Pressure-controlled ventilation was associated with lower peak airway pressures, a more homogeneous gas distribution (less regional alveolar overdistension), improved patient-ventilator synchrony, and earlier liberation from mechanical ventilation than volume-controlled ventilation.

●Volume-controlled ventilation can guarantee a consistent tidal volume, thereby ensuring a minimum minute ventilation as well as a maximum per-breath volume, unlike pressure-controlled ventilation.

Choosing a mode is discussed separately. (See "Overview of initiating invasive mechanical ventilation in adults in the intensive care unit", section on 'Selecting an initial mode'.)

VOLUME-CONTROLLED VENTILATION — Volume-controlled ventilation (also sometimes referred to as volume-limited or volume-cycled ventilation) requires the clinician to set the flow rate, flow pattern, tidal volume, respiratory rate, applied positive end-expiratory pressure (PEEP; also known as extrinsic PEEP), and fraction of inspired oxygen. Inspiration ends once the target tidal volume has been achieved. In other words, the target is flow, the cycle is volume, and the trigger can be either ventilator or patient depending on the ventilator mode.

The inspiratory time and inspiratory to expiratory (I:E) ratio are determined by the inspiratory flow rate, in the context of the set respiratory rate. With a set respiratory rate, increasing the inspiratory flow rate will decrease inspiratory time, increase expiratory time, and decrease the I:E ratio.

Airway pressures (peak, plateau, and mean) depend on both the ventilator settings and patient-related variables (eg, compliance, airway resistance). High airway pressures may be a consequence of large tidal volumes, a high peak flow, poor compliance (eg, acute respiratory distress syndrome, minimal sedation, thoracic wall restriction), or increased airway resistance (figure 2 and waveform 1 and figure 3).

PRESSURE-CONTROLLED VENTILATION — Pressure-controlled ventilation (also sometimes referred to as pressure-limited or pressure-cycled ventilation) requires the clinician to set the inspiratory pressure level, inspiratory to expiratory (I:E) ratio, respiratory rate, applied positive end-expiratory pressure (PEEP), and fraction of inspired oxygen [4,5]. Inspiration ends after delivery of the set inspiratory pressure over the set inspiratory time (governed by the I:E ratio). In other words, the target is pressure, the cycle is time, and the trigger can be either ventilator or patient depending on the ventilator mode.

The tidal volume is variable during pressure-controlled ventilation. It is related to the set inspiratory pressure level, the compliance of the patient's lungs and chest wall, and the resistance of the patient's airway and ventilator tubing. Specifically, larger tidal volumes generally correlate with factors including a high set inspiratory pressure level, normal compliance, little airway resistance, or little resistance from the ventilator tubing.

Importantly, the physiology of inspiratory airway pressure is constant during pressure-controlled ventilation. It is equal to the sum of the set inspiratory pressure level and the applied PEEP. As an example, a patient with a set inspiratory pressure level of 20 cm H₂O and an applied PEEP of 10 cm H₂O will have a peak airway pressure of 30 cm H₂O. Notably, the plateau pressure during inspiration is also equal to the peak inspiratory pressure.

COMMON VENTILATOR MODES — Both volume-controlled and pressure-controlled breath strategies can be delivered via several ventilator modes, including continuous mandatory ventilation (CMV), assist control (AC), intermittent mandatory ventilation (IMV), and synchronized IMV (SIMV) (table 1). However, some nomenclature for ventilator modes describe patterns of ventilator mode as well as the individual breath strategy, such as adaptive pressure control (APC), airway pressure release ventilation (APRV), and pressure support ventilation (PSV). The following sections will present the ventilator mode in the context of individual breath mechanics, breath-to-breath variations, as well as patient-ventilator interactions.

Continuous mandatory ventilation — CMV describes a mode of ventilation where each breath is mandated by the ventilator. In other words, only the ventilator can trigger a set breath cycle while the patient cannot. The patient does not receive additional minute ventilation above that set on the ventilator. CMV can be used in the setting of pharmacologic paralysis, heavy sedation, coma, or lack of incentive to increase the minute ventilation in cases where the set minute ventilation meets or exceeds physiologic need. CMV does not require any patient work.

During CMV, the minute ventilation is determined entirely by the set respiratory rate and tidal volume. In the volume-controlled breath strategy, the tidal volume is set and guaranteed by the ventilator. In the pressure-controlled breath strategy, the tidal volume is dependent on the set inspiratory pressure level and inspiratory time.

Assist control — AC is similar to CMV in that each ventilator breath is set by specific parameters so that the support is consistent from breath to breath. However, AC allows both ventilator-triggered and patient-triggered supported breaths.

During AC, the clinician determines the minimal minute ventilation by setting the respiratory rate and parameters that guide the tidal volume per breath. Again, the tidal volume is set in the volume-controlled breath strategy and is extrapolated from the inspiratory pressure and time in the pressure-controlled breath strategy. Then, the patient can increase the maximum (or actual) minute ventilation by triggering additional breaths. Each patient-initiated breath receives the set tidal volume or inspiratory pressure/time from the ventilator; in other words, each breath is fully supported by the ventilator regardless of triggering mechanism.

Consider the following example in volume-controlled AC ventilation. If the clinician sets the respiratory rate to 20 breaths per minute and the tidal volume to 500 mL, the lowest possible minute ventilation is 10 L per minute (20 breaths per minute x 500 mL per breath). If the patient triggers an additional 5 breaths beyond the preset 20 breaths, the ventilator will deliver 500 mL for each additional breath and the minute ventilation will be 12.5 L per minute (25 breaths per minute x 500 mL per breath).

Adaptive pressure control — APC has become a mainstay of care for mechanically ventilated patients in modern intensive care units with advanced mechanical ventilators. Ventilator-specific nomenclature for APC includes pressure regulated volume control (PRVC), autoFlow, volume control plus (VC+), and adaptive pressure ventilation (APV).

APC is a ventilator strategy describing both the breath strategy as well as breath-to-breath variation in the ventilator's support (figure 4). APC allows the clinician to set the desired tidal volume and respiratory rate. Then the ventilator applies inspiratory pressure to attain the set tidal volume. The applied inspiratory pressure is determined by the change in pressure required by the previous breath to attain the set tidal volume.

As such, the individual breath mechanics of APC are similar to pressure-controlled ventilation in that the target is pressure and the cycle is time. As in pressure-controlled ventilation, the inspiratory flow is variable in APC and changes with patient effort and lung mechanics. Variable flow may be more comfortable to the patient. For any given tidal volume and lung compliance in a patient, plateau pressure can be thought to be the same with AC and APC. In this way, APC is essentially a hybrid mode of mechanical ventilation that successfully addresses concerns about potentially excessive airway pressures resulting from volume-controlled ventilation and inadequate tidal volumes being delivered with pressure-controlled ventilation [6].

LESS COMMONLY USED MODES

Adaptive support ventilation — Adaptive support ventilation (ASV) is a variation of adaptive pressure control in which respiratory mechanics dictate adjustments to the respiratory rate and inspiratory pressure that are necessary to achieve a desired minute ventilation:

●Patients who are unable to trigger the ventilator are given pressure-control breaths. (See 'Pressure-controlled ventilation' above.)

●Patients who are able to trigger the ventilator are given pressure support for the triggered breaths, supplemented with pressure-control breaths as needed to achieve the desired respiratory rate. (See 'Pressure support ventilation' below and 'Pressure-controlled ventilation' above.)

The basis for the adjustments is an equation that determines the respiratory rate that minimizes the work of inspiration at a given minute ventilation. This equation relies on an expiratory time constant, which can be obtained from the expiratory limb of the flow volume loop on a breath by breath basis [7,8]. Patients who have a long expiratory time constant (eg, chronic obstructive pulmonary disease) receive a higher tidal volume and a lower respiratory rate when ventilated by ASV than patients with stiff lungs (eg, acute respiratory distress syndrome [ARDS]) or chest wall stiffness (eg, kyphoscoliosis, morbid obesity, neuromuscular disorder) who expire quickly [9,10]. The effect of ASV has not been demonstrated to be superior to other modes of mechanical ventilation on important clinical outcomes, although time to weaning initiation and weaning duration were shown to be somewhat shorter with ASV in one randomized trial [11].

Intermittent mandatory ventilation — Intermittent mandatory ventilation (IMV) is similar to assist control (AC) in two ways: the clinician determines the minimal minute ventilation (by setting the respiratory rate and individual-supported breath mechanics) and the patient is able to increase the minute ventilation. However, IMV differs from AC in the way that the minute ventilation is increased. Specifically, patients increase the minute ventilation by spontaneous breathing rather than patient-triggered ventilator-supported breaths. Minute ventilation may be augmented by pressure support for spontaneous breaths. (See 'Pressure support ventilation' below.)

IMV may be used in either the volume-controlled or pressure-controlled breath strategy. In the volume-controlled strategy, the clinician sets the tidal volume to guide individual-supported breaths. While in the pressure-controlled strategy, the clinician sets the inspiratory pressure and time.

Consider the following example. If the clinician sets the respiratory rate to 10 breaths per minute and the tidal volume to 500 mL per breath, the lowest possible minute ventilation is 5 L per minute (10 breaths per minute times 500 mL per breath). If the patient initiates an additional 5 breaths beyond the preset 10 breaths, the tidal volume for each additional breath will be whatever size the patient is able to generate and the total minute ventilation will be some amount greater than 5 L per minute. The precise minute ventilation depends on the size of the tidal volume generated by each spontaneous breath, which may be influenced by factors that may include patient strength or thoracic compliance.

Synchronized intermittent mandatory ventilation — Synchronized IMV (SIMV) is a variation of IMV where the ventilator breaths are synchronized with patient inspiratory effort [12,13]. SIMV differs in that it allows pressure support to patient-triggered breaths in addition to the set mandatory ventilator-supported breaths. Thus, the clinician must set ventilator parameters for both ventilator-supported and pressure support breaths in SIMV.

The ventilator-supported breaths in SIMV may be set by either the volume- or pressure-controlled strategy. But additional pressure support breaths, if any, are set using principles of pressure-controlled ventilation. (See 'Pressure-controlled ventilation' above.)

SIMV (or IMV) can be used to titrate the level of ventilatory support over a wide range of ratios of ventilator-supported or patient-effort breathing (figure 5) [14]. Ventilatory support can range from full support (set respiratory rate is high enough that the patient does not over breathe) to no ventilatory support (set respiratory rate is zero). If the patient is not over breathing the ventilator, then SIMV is akin to AC or continuous mandatory ventilation. Whereas if the set rate is zero, it is akin to pressure support ventilation (PSV). (See 'Pressure support ventilation' below.)

The level of support may need to be modified if hemodynamic consequences of positive pressure ventilation develop. (See "Clinical and physiologic complications of mechanical ventilation: Overview", section on 'Hypotension'.)

SIMV and AC are the most frequently used forms of volume-limited mechanical ventilation. Possible advantages of SIMV compared to AC include better patient-ventilator synchrony, better preservation of respiratory muscle function, lower mean airway pressures, and greater control over the level of support [15]. In addition, auto-positive end-expiratory pressure may be less likely with SIMV. In contrast, AC may be better suited for critically ill patients who require a constant tidal volume or full or near-maximal ventilator support.

Airway pressure release ventilation — During airway pressure release ventilation (APRV), a high continuous positive airway pressure (CPAP; P high) is delivered for a long duration (T high) and then falls to a lower pressure (P low) for a shorter duration (T low) (figure 6 and figure 7).

The transition from P high to P low deflates the lungs and eliminates carbon dioxide. Conversely, the transition from P low to P high inflates the lungs. Alveolar recruitment is maximized by the high CPAP [16,17].

The difference between P high and P low is the driving pressure. Larger differences are associated with greater inflation and deflation, while smaller differences are associated with smaller inflation and deflation. The exact size of the tidal volume is related to both the driving pressure and the compliance.

T high and T low determine the frequency of inflations and deflations. As an example, a patient whose T high is set to 5.4 seconds and whose T low is set to 0.6 seconds has an inflation-deflation cycle lasting 6 seconds. This allows 10 inflations and deflations to be completed each minute. Most adherents believe adjusting the Flow-Time waveform should be done in order to optimize settings. Ideally, expiratory time is adjusted to cut off the expiratory flow during a release at about 75 percent of peak expiratory flow rate.

Spontaneous breathing is possible at both P high and P low, although most spontaneous breathing occurs at P high because the time spent at P low is brief. This is a novel feature that distinguishes APRV from other types of inverse ratio ventilation (IRV).

Efficacy — APRV has not been shown to improve mortality. However, it may benefit alternative clinical parameters compared to other modes of ventilation. In one trial, 30 patients being mechanically ventilated because of trauma were randomly assigned to receive APRV alone or pressure-limited ventilation for 72 hours followed by APRV [18]. The APRV alone group had a shorter duration of mechanical ventilation, a shorter intensive care unit (ICU) stay, and required less sedation and pharmacologic paralysis. Mortality did not differ between groups. In another trial of 148 patients with ARDS, APRV was associated with improved oxygenation and respiratory system compliance, decreased plateau pressure, more ventilator free days, and decreased ICU stay. However, there was an imbalance in randomization evident in the trial [19]. In contrast to these potential associated physiologic benefits of APRV, in one randomized trial, APRV use has been associated with the generation of large tidal volumes, often exceeding 12 mL/kg of ideal body weight [20], which might be expected to contribute to lung injury. Therefore, the optimal implementation of this modality remains the subject of debate [21].

Numerous observational studies suggest that APRV may decrease the peak airway pressure, improve alveolar recruitment, increase ventilation of the dependent lung zones and improve oxygenation [22-27]. However, such findings have not been universal. In one clinical trial, 58 patients being mechanically ventilated for acute lung injury were randomly assigned to receive either APRV or SIMV plus PSV [28]. There were no differences in physiologic or clinical outcomes.

APRV is well tolerated hemodynamically. Much of the hemodynamic improvement is probably related to lower airway pressures, although spontaneous breathing also confers some hemodynamic benefit.

Indications — There are no universally accepted indications for APRV. At present, there are no large, definitive clinical trials that have demonstrated benefit to the use of APRV when compared with conventional modes. Use of APRV and its related modes (intermittent mandatory airway pressure release ventilation [IMPRV] and biphasic intermittent PAP) has been best described in patients who have ARDS (see 'Related modes' below). In theory, APRV may recruit alveoli and improve oxygenation.

Contraindications — APRV and its related modes are infrequently used in patients with severe obstructive airways disease or a high ventilatory requirement because hyperinflation, high alveolar pressure, and pulmonary barotrauma may result (see "Diagnosis, management, and prevention of pulmonary barotrauma during invasive mechanical ventilation in adults"). APRV should be used with caution in patients with higher risks for overall higher intrathoracic pressure, such as pulmonary vascular diseases and right heart failure [29].

Related modes — IMPRV and biphasic intermittent PAP (herein called biphasic ventilation) are similar to APRV. Specifically, they allow spontaneous breathing and have cyclic inflations and deflations due to transitions between P high and P low.

●During IMPRV, the cyclic inflations and deflations are synchronized to occur after every few spontaneous breaths [30].

●The principal difference between biphasic ventilation and APRV is that T low is longer during biphasic ventilation, allowing more spontaneous breaths to occur at P low (figure 8) [31,32]. Another difference is that IRV is more often performed using APRV than biphasic ventilation. Biphasic ventilation is also referred to as Bi-Vent, BiLevel, BiPhasic, and DuoPAP ventilation. It is different than bilevel PAP, a common type of noninvasive positive pressure ventilation. (See "Noninvasive ventilation in adults with acute respiratory failure: Benefits and contraindications".)

RARELY USED OR INVESTIGATIONAL MODES

Neurally adjusted ventilatory assist ventilation — Neurally adjusted ventilatory assist ventilation (NAVA) is an investigational ventilatory mode in which the electrical discharge from the diaphragm (ie, diaphragmatic excitation; EA_di) is used to trigger a mechanical breath [33]. When a deflection in the EA_di signal greater than the set threshold (typically 0.5 microvolts) is detected by a catheter embedded in a gastric tube, a mechanical breath is delivered. The degree of assist varies with the amplitude of the detected EA_di and an assist level set by the clinician such that there is breath-to-breath variation in the tidal volume. The set assist level is determined by a short empirical adjustment period where the assist level is increased to detect a comfortable and consistent tidal volume for the patient and an EA_di signal that remains flat.

Neural-ventilator coupling (ie, time between a spontaneous breath and the delivery of a mechanical breath) is faster with NAVA than with conventional modes of mechanical ventilation. Thus, NAVA has the potential to improve patient-ventilator synchrony (eg, in patients with chronic obstructive pulmonary disease). However, although small uncontrolled trials have suggested improved patient-ventilator asynchrony with NAVA, none have shown improvement in clinically important outcomes, such as death or ventilator-free survival [34-36].

The success of NAVA depends upon an intact ventilatory drive (ie, patient has to be spontaneously breathing) and is not a plausible mode of ventilation in patients who have blunted or no respiratory drive (eg, hypoventilation due to heavy sedation or cervical spinal cord injury).

Inverse ratio ventilation — Inverse ratio ventilation (IRV) is not a mode of mechanical ventilation, but rather a strategy employed during volume-limited or pressure-limited mechanical ventilation. In this mode, the ratio of inspiration to expiration features a prolonged inspiratory time compared to what is typically normal (normal being approximately 1:2 to 1:3), increasing the mean airway pressure and potentially improving oxygenation. For example, in IRV, the inspiratory to expiratory (I:E) ratio could be set at 1:1.5, 1:1, or 2:1. A trial of IRV may be warranted when a patient is severely hypoxemic despite optimal positive end-expiratory pressure (PEEP) and fraction of inspired oxygen.

IRV has never been shown to improve important clinical outcomes, such as mortality, duration of mechanical ventilation, or duration of intensive care unit stay. The preponderance of evidence suggests that IRV improves oxygenation, although the evidence is weak and characterized by low quality, conflicting studies [37-48]. The following studies are illustrative of the data that exist:

●In an observational study of 31 patients undergoing pressure control ventilation, initiation of IRV was followed by a significant increase in the mean airway pressure and the arterial oxygen tension (PaO₂; from 69 to 80 mmHg), despite reduction of the PEEP [46].

●A crossover trial randomly assigned 16 patients with acute respiratory distress syndrome (ARDS) to receive IRV or no IRV [38]. IRV increased the mean airway pressure, but the improvement in the PaO₂ did not reach statistical significance (93 versus 86 mmHg).

IRV generally requires heavy sedation or neuromuscular paralysis because the inverse I:E ratio is unnatural and uncomfortable. It is usually well tolerated hemodynamically [43,49].

Types — IRV can be performed during pressure-limited ventilation (PL-IRV) or volume-limited ventilation (VL-IRV). Neither is clearly superior to the other. In a multicenter, randomized trial that compared PL-IRV to VL-IRV in patients with ARDS, the type of IRV did not affect mortality [50].

Pressure-limited — During PL-IRV, IRV is initiated by increasing the I:E ratio until the inspiratory time exceeds the expiratory time. The primary advantage of PC-IRV is the ability to guarantee that a maximal plateau airway pressure will not be exceeded. This may limit the risk of pulmonary barotrauma or ventilator-associated lung injury. In addition, many clinicians believe that clinically significant auto-PEEP is less likely with PC-IRV than VC-IRV, although this is unproven [51]. (See "Ventilator-induced lung injury" and "Diagnosis, management, and prevention of pulmonary barotrauma during invasive mechanical ventilation in adults".)

Volume-limited — During volume-limited ventilation, IRV can be initiated using a ramp wave (decelerating flow) or a square wave (constant flow) flow pattern:

●With the ramp wave, the peak inspiratory flow rate is initially set at least four times higher than the minute ventilation and then slowly decreased until the inspiratory time exceeds the expiratory time

●With the square wave, an end-inspiratory pause is added (0.2 sec works well) and then slowly lengthened until the inspiratory time exceeds the expiratory time

Risks — The shorter expiratory time during IRV increases the risk of auto-PEEP and its adverse sequelae (eg, pulmonary barotrauma, hypotension) (figure 9) [46]. IRV also appears to increase the risk of pulmonary barotrauma independent of auto-PEEP. In a study of 14 patients undergoing mechanical ventilation with PC-IRV, the incidence of pneumothorax was 29 percent despite the lack of measurable auto-PEEP [52].

High frequency ventilation — High frequency mechanical ventilation employs very high respiratory rates and small tidal volumes. It is described in detail separately. (See "High-frequency ventilation in adults".)

PRESSURE SUPPORT VENTILATION — Pressure support ventilation (PSV) is a mode of ventilation that delivers a set inspiratory pressure until the inspiratory flow decreases to a predetermined percentage of its peak value (usually 25 percent (figure 10) [53]).

For PSV, the clinician sets the pressure support level (inspiratory pressure level), applied positive end-expiratory pressure (PEEP), and fraction of inspired oxygen. The patient must trigger each breath because there is no set respiratory rate. The tidal volume, respiratory rate, and minute ventilation are dependent on multiple factors, including the ventilator settings and patient-related variables (eg, compliance, sedation). In general, a high pressure support level correlates with a larger tidal volume. As such, in each supported breath, the trigger is the patient, the target is pressure, and the cycle is the patient.

The work of breathing is inversely proportional to PSV level, provided that inspiratory flow is sufficient to meet patient demand [53,54]. In other words, increasing the level of pressure support decreases the work of breathing. The work of breathing is also inversely proportional to the inspiratory flow rate. Increasing the inspiratory flow rate shortens the time until the maximal airway pressures are achieved, which decreases the work of breathing [55].

Potential uses — PSV is often used for weaning from mechanical ventilation because it tends to be a comfortable mode, giving the patient greater control over the inspiratory flow rate and respiratory rate. However, clinical studies have failed to show that PSV improves weaning. (See "Initial weaning strategy in mechanically ventilated adults", section on 'Choosing a weaning method'.)

PSV is frequently combined with synchronized intermittent mandatory ventilation (SIMV) (see 'Synchronized intermittent mandatory ventilation' above). The ventilator delivers the set respiratory rate using SIMV, but patient-initiated breaths beyond the set respiratory rate are delivered using PSV. The purpose of adding PSV for patient-initiated breaths is to overcome the resistance of the endotracheal tube and ventilator circuit. The necessary level of pressure support is unknown and generally estimated. Resistance of the endotracheal tube is related to the tube diameter and inspiratory flow rate [56]. With small endotracheal tubes (eg, <7 mm), a pressure support level ≥10 cm H₂O may be needed to overcome the resistance [57,58]. Levels of pressure support higher than that required to overcome resistance will augment tidal volume.

Disadvantages — PSV is poorly suited to provide full or near-full ventilatory support. The following characteristics of PSV are disadvantages in that setting:

●Each breath must be initiated by the patient. Central apnea may occur if the respiratory drive is depressed due to sedatives, critical illness, or hypocapnia due to excessive ventilation [59].

●An adequate minute ventilation cannot be guaranteed because tidal volume and respiratory rate are variable.

●Ventilator asynchrony can occur when PSV is employed for full ventilatory support, potentially prolonging the duration of mechanical ventilation [60,61].

●PSV is associated with poorer sleep than assist control. Specifically, there is greater sleep fragmentation, less stage 1 and 2 non-rapid eye movement (NREM) sleep, more wakefulness during the first part of the night, and less stage 3 and 4 NREM sleep during the second part of the night [62].

●Relatively high levels of pressure support (eg, >20 cm H₂O) are required during full ventilatory support to prevent alveolar collapse (which can lead to cyclic atelectasis and ventilator-associated lung injury) and to attain a stable breathing pattern [63,64]. Such high levels of pressure support are not as comfortable as moderate levels (eg, 10 to 15 cm H₂O) [65]. (See "Ventilator-induced lung injury".)

While PSV is poorly suited to provide full or nearly full ventilatory support in general, it is not generally used for patients with increased airway resistance (eg, chronic obstructive pulmonary disease or asthma exacerbation). Minute ventilation is more likely to be insufficient when airway resistance is high, which may be related to decreased airflow causing inspiration to be terminated after a smaller than optimal tidal volume has been delivered [66,67]. In addition, PSV does little to decrease auto-PEEP (also known as intrinsic PEEP), which can increase patient work and worsen respiratory muscle fatigue [68]. Choosing a higher percentage of the peak inspiratory flow as the trigger to end inspiration may improve auto-PEEP slightly [69].

Tube compensation — Many ventilators can be set to a mode called automatic tube compensation. This mode is a type of PSV that applies a sufficient level of positive pressure to overcome the work of breathing imposed by the endotracheal tube, which can vary from breath to breath. Automatic tube compensation is often used for a spontaneous breathing trial. Patients who undergo a spontaneous breathing trial with automatic tube compensation are more likely to successfully tolerate their trial than those who receive continuous positive airway pressure alone [70]. In addition, many ventilators have the option of combining automatic tube compensation with other modes, so that resistance of the endotracheal tube has no impact on ventilation.

CONTINUOUS POSITIVE AIRWAY PRESSURE — Continuous positive airway pressure (CPAP) refers to the delivery of a continuous level of positive airway pressure. It is functionally similar to positive end-expiratory pressure. The ventilator does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all breaths.

CPAP is sometimes used for weaning from mechanical ventilation in cases where the clinician desires less ventilator support than pressure support ventilation. In noninvasive ventilator support scenarios, it is commonly used in the management of sleep-related breathing disorders, cardiogenic pulmonary edema, and obesity hypoventilation syndrome. (See "Titration of positive airway pressure therapy for adults with obstructive sleep apnea" and "Noninvasive positive airway pressure therapy for the obesity hypoventilation syndrome" and "Noninvasive ventilation in adults with acute respiratory failure: Benefits and contraindications", section on 'Acute cardiogenic pulmonary edema (ACPE)'.)

BILEVEL POSITIVE AIRWAY PRESSURE — Bilevel positive airway pressure (BPAP) is a mode used during noninvasive positive pressure ventilation. It delivers a preset inspiratory PAP (IPAP) and expiratory PAP (EPAP). The tidal volume correlates with the difference between the IPAP and the EPAP. As an example, the tidal volume is greater using an IPAP of 15 cm H₂O and an EPAP of 5 cm H₂O (difference of 10 cm H₂O), than an IPAP of 10 cm H₂O and an EPAP of 5 cm H₂O (difference of 5 cm H₂O). Most BPAP devices also permit a backup respiratory rate to be set.

The term "BiPAP" is often used incorrectly to refer to the BPAP mode. BiPAP is the name of a portable ventilator manufactured by Respironics Corporation; it is just one of many ventilators that can deliver BPAP. BPAP is discussed in more detail separately. (See "Noninvasive positive airway pressure therapy for the obesity hypoventilation syndrome", section on 'Bilevel positive airway pressure' and "Noninvasive ventilation in adults with acute respiratory failure: Practical aspects of initiation", section on 'Bilevel noninvasive ventilation' and "Noninvasive ventilation in adults with acute respiratory failure: Benefits and contraindications".)

SUMMARY AND RECOMMENDATIONS

●Definition – The mode of mechanical ventilation refers to the method of inspiratory support. (See 'Introduction' above.)

●Volume- and pressure-controlled breath strategies – During volume-controlled ventilation (figure 2), inspiration ends after delivery of the set tidal volume. During pressure-controlled ventilation, inspiration ends after delivery of the set inspiratory time (figure 3). Each has unique advantages and disadvantages. (See 'Volume-controlled ventilation' above and 'Pressure-controlled ventilation' above.)

●Modes – Modes of ventilation describe ventilator-patient interactions, breath-to-breath variations, or specific modes that incorporate unique ventilator settings.

•Assist control (AC) ventilation – AC is a ventilatory mode in which each breath is supported by the ventilator by clinician-determined settings but can be either triggered by the patient or the ventilator in the absence of patient effort. The ventilator-supported breaths may be either volume- or pressure-controlled breath strategies.

•Adaptive pressure control (APC) ventilation – APC is a ventilatory mode in which respiratory mechanics dictate adjustments to the respiratory rate and inspiratory pressure that are necessary to achieve a desired minute ventilation. The breath strategy is based on pressure control mechanics.

•Synchronized intermittent mandatory ventilation (SIMV) – SIMV is a blended mode that includes both full ventilator-supported breaths and pressure support of patient-triggered breaths in addition to the set respiratory rate. The full ventilator-supported breaths may utilize either volume- or pressure-controlled breath strategies.

•Airway pressure release ventilation (APRV) – APRV cycles between a high continuous positive airway pressure (CPAP; P high) and a low CPAP (P low) (figure 6 and figure 7). Spontaneous breathing can occur during APRV. Variants include intermittent mandatory airway pressure release ventilation and biphasic intermittent PAP. (See 'Airway pressure release ventilation' above.)

•Rare modes – Rarely used modes include the following:

-Neurally adjusted ventilatory assist ventilation (NAVA) – NAVA is an investigational ventilatory mode in which the electrical discharge from the diaphragm (ie, diaphragmatic excitation) is used to trigger a mechanical breath.

-Inverse ratio ventilation (IRV) – IRV is not a mode of mechanical ventilation but rather a strategy employed during volume-limited or pressure-limited mechanical ventilation (figure 9). The inspiratory time exceeds the expiratory time during IRV (the inspiratory:expiratory ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation. (See 'Inverse ratio ventilation' above.)

●Pressure support ventilation (PSV) – PSV is neither volume-limited nor pressure-limited. Once a breath is triggered by the patient, inspiratory pressure is delivered until the inspiratory flow decreases to a predetermined percentage of its peak value (figure 10). (See 'Pressure support ventilation' above.)

●PAP – CPAP refers to the delivery of a continuous level of PAP. (See 'Continuous positive airway pressure' above.)

Bilevel PAP is a mode used during noninvasive positive pressure ventilation. It delivers a preset inspiratory PAP and an expiratory PAP. (See 'Bilevel positive airway pressure' above.)