Assist Control Ventilation

During assist control ventilation (ACV), the ventilator delivers a mandatory breath every time the patient initiates an inspiration. A backup respiratory rate is set to guarantee that the patient always receives a minimal number of breaths, even in the absence of spontaneous inspiratory activity. Mandatory breaths can be delivered with either volume or pressure control. During ACV, the inspiratory time is preset and invariable.

Pressure Support Ventilation

Pressure support ventilation (PSV) assists each inspiratory attempt by the patient with a pressure-limited breath, thus partitioning the work of breathing between the patient and ventilator. The patient maintains partial control of TV and respiratory rate; the operator allows the patient to perform more or less work by modifying the level of inspiratory pressure. PSV differs from ACV in the lack of a backup rate and in the fact that, during PSV, inspirations have variable durations and are terminated when inspiratory flow decreases below a predetermined threshold value.

Synchronized Intermittent Mandatory Ventilation

Synchronized intermittent mandatory ventilation (SIMV) assists with a mandatory breath only an adjustable fraction of patient’s inspiratory attempts. Unlike ACV, additional inspirations are either unassisted or partially assisted with PSV. During SIMV, higher mandatory rates are used for patients who require higher levels of ventilatory assistance and are progressively decreased during the weaning process, which allows the patient to accomplish more unsupported breaths.

Proportional Assist Ventilation

Proportional assist ventilation (PAV) is characterized by the delivery of a variable airway pressure that is continuously adjusted throughout each breath to match the patient’s inspiratory effort. The patient’s effort is estimated with the use of continuous measurement of inspired flow and volume in relation to respiratory system compliance and resistance. The clinical use of PAV is now facilitated by the incorporation of a new method to frequently measure respiratory mechanics variables at the bedside.

Airway Pressure-Release Ventilation and Biphasic Positive Airway Pressure

Airway pressure-release ventilation (APRV) is a mode of ventilatory support in which the patient breathes spontaneously at a high level of continuous airway pressure, with periodic releases to a low positive end expiratory pressure (PEEP). CO ₂ exchange is partly accomplished by the patient’s activity and partly by exhalations during pressure releases. The volume exhaled during releases depends on the patient’s mechanics and on the difference between the high pressure and the PEEP. The release time is typically maintained lower than 1.5 seconds, and the PEEP is usually very low or zero. Biphasic positive airway pressure (BiPAP), also known as Bi-Level ventilation, is a variant of APRV in which a non-negligible PEEP is applied during releases, which are of longer duration. During BiPAP, a patient’s inspiratory activity also occurs at PEEP.

High-Frequency Oscillatory Ventilation

High-frequency oscillatory ventilation (HFOV) is a mode of ventilatory support in which small TVs are delivered at a very high rate, in the range of 3 to 15 Hz. During HFOV, gas runs continuously through the ventilator tubing and is oscillated by a piston placed within the circuit. The oscillations are thus transmitted to the patient’s lungs, producing cyclic, rapid inflations and deflations. The clinician adjusts the amplitude of the oscillations, their frequency, and the continuous gas flow rate to modulate CO ₂ exchange. Arterial oxygenation is proportional to mean airway pressure, which is regulated by a valve placed on the exhaust port of the circuit. The main advantage of HFOV is that it allows the delivery of TVs, which, although not negligible, are still lower than with any other modes of ventilation, thus minimizing alveolar overdistension.

Lung Protective Strategies

The scope of mechanical ventilation has recently shifted from pure life support to protecting patients from ventilator-induced lung injury (VILI). VILI is a form of pulmonary damage that is primarily caused by excessive alveolar stress due to high TV ventilation and by elevated inspiratory pressures. The presence of atelectasis also promotes VILI, likely through the imposition of high stress by collapsed or unstable airspaces. Patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) seem to be at particularly high risk of lung damage. The clinical relevance of VILI was demonstrated by a large RCT performed by the ARDSnet investigators showing that ventilation with small TV improves outcomes of ALI compared with larger TV. Additionally, low TV ventilation decreases 2-year mortality in ALI, as suggested by a recent prospective cohort study. It is also likely that lung protective strategies may ameliorate other long-term outcomes, such as the pronounced disability typically affecting ALI survivors. Although the use of low TV will result in impaired CO ₂ clearance in many patients, lung protection should take precedence over the goal of normalizing arterial P co ₂ .

Lung protective strategies in ALI may also include the use of higher PEEP to prevent atelectasis-related injury. In three RCTs, the survival rate was not different between groups treated with higher versus lower PEEP. However, a recent meta-analysis suggested that high PEEP may improve the outcomes of patients who have worse oxygenation. In the absence of better evidence, clinicians should continue to prioritize minimization of lung overdistention in their choice of ventilator settings. In ALI patients who seem to favorably respond to PEEP without untoward effects, maintenance of higher PEEP is probably not harmful based on the existing evidence ( Table 21-1 ).

Use of Partial Ventilatory Support

The main goal of mechanical ventilation is to support CO ₂ excretion, which can be accomplished either by having the ventilator substituting for the patient’s inspiratory muscles (total ventilatory support) or by letting the patient and the ventilator share the effort of breathing (partial support). Although no RCT has suggested a superiority of either strategy, it is currently accepted that partial support is more desirable. In fact, total ventilatory support invariably requires deep sedation and often muscle relaxants. It is now recognized that minimization of sedatives is beneficial. This is based on results of RCTs in which protocols to decrease sedation improved clinical outcomes compared with standard management. Additionally, complete suppression of inspiratory activity has been shown to be associated with diaphragm atrophy in animal models and in human subjects receiving ventilatory support for longer than 18 hours. Such atrophy is likely a key factor in delaying liberation from the ventilator.

PSV has been in circulation for many years and is probably one of the simplest ways to provide partial ventilatory support. However, its use is still relatively limited as shown by a large prospective cohort study and is mainly relegated to the weaning process in patients who do not have severe oxygenation impairment. However, PSV can be used more broadly: in an observational prospective study, PSV was tolerated by a majority of patients with ALI.

SIMV was an early form of partial ventilatory support and is still widely used both for weaning and as a primary mode of ventilation for patients who require high-level support. However, the advantages of SIMV over other modes are unclear and not demonstrated. The rationale for using SIMV is to alternate spontaneous inspirations with mechanical breaths during which the patient’s respiratory muscles are allowed to rest. However, it has been demonstrated that this rationale is largely flawed because patient unloading is less efficient during SIMV than during PSV.

APRV, BiPAP, and PAV are newer modalities of partial ventilatory support. Because of its features, PAV provides a level of support that is adjustable and always proportional to a patient’s inspiratory drive and mechanical load, adapting to short-term changes in clinical conditions.

Liberation from the Ventilator

It is widely recognized that early liberation from mechanical ventilation is a very desirable target because it decreases the rate of complications and the costs of medical care. A large research effort has been made in evaluating strategies for ventilator weaning, but studies have failed to clearly identify an ideal mode for this purpose. It is still unclear whether progressive resumption of spontaneous breathing with the use of PSV offers any advantages over daily performance of spontaneous breathing trials. Two RCTs performed in difficult-to-wean patients provided discordant answers to this question, which was likely due to methodologic differences. However, the results of both studies suggested that SIMV was associated with delayed liberation from the ventilator compared with PSV and with spontaneous breathing trials.

Studies have demonstrated that the process of liberation from the ventilator is shortened by the use of protocols that identify and liberate patients who are able to tolerate a spontaneous breathing trial. A more recent clinical trial evaluated a care pathway that combined daily sedation interruptions with spontaneous breathing trials in eligible patients. Compared with conventional management, the test strategy improved outcomes, including survival rates, in the absence of significant complications. Although the results of these studies may not be translatable to all intensive care unit settings and patient populations, adherence to clinical pathways is probably more important than the choice of mode of ventilation used in the process.

Patient–Ventilator Interaction

A considerable amount of research effort has been dedicated to improving the interaction between the patient and the ventilator, with the goal of optimizing patient comfort and decreasing sedation requirements. ACV is often suboptimal in this aspect. In fact, during volume-controlled ACV, the patient may accomplish undesired work of breathing when the ventilator does not match the patient’s flow and volume demands. This is due to the fact that a patient’s inspiratory effort does not cease after triggering the ventilator but continues throughout the mandatory breath. This problem is particularly relevant during a lung protective strategy, as suggested by the detection of high work of breathing in ALI patients undergoing ventilation with a TV of 5 to 6 mL/kg. It is a common observation that these settings can lead to discomfort, although retrospective analysis of existing RCTs has not proved that TV limitation results in an increased need for sedation. Additionally, during ACV the inspiratory time is invariable and may not match a patient’s inspiratory time, which often results in patient–ventilator asynchrony, causing discomfort or hyperinflation.

PSV is characterized by a high level of adaptability to patient demands. However, in certain conditions the mechanical breath may not finish exactly at the end of a patient’s inspiratory time, causing asynchronies, hyperinflation, and discomfort. In newer ventilators, the flow threshold that ends inspiration is adjustable, which allows the inspiratory duration to be prolonged or shortened to better match the patient’s timing. Another frequently encountered problem with PSV is overassistance, which occurs when inspiratory pressure is too high. This may result in excessive TV and hypocapnia, thus causing central apnea episodes. In fact, PSV is associated with more apneas and sleep disruptions than ACV, probably because of the fact that the latter mode has fixed TV and a backup rate. Ventilator settings may be important contributors in the genesis of sleep deprivation and disruption in critically ill patients. Because of its algorithm, PAV improves the matching between neural and machine inspiratory times, which should translate into improved patient comfort and better tolerance of the ventilator. In a RCT, PAV was tolerated by more patients and decreased the incidence of patient–ventilator asynchronies, in comparison with PSV. In addition, PAV seems to result in less sleep fragmentation than PSV.

Use of Alternative Modes

APRV and BiPAP are used in many centers for patients with severe hypoxemia because they allow maintenance of alveolar recruitment and oxygenation while avoiding alveolar overdistention, possibly decreasing VILI. In fact, APRV has been shown to achieve similar or better gas exchange at lower peak inspiratory pressures compared with other modes of ventilation. Another advantage of APRV and BiPAP is that the presence of spontaneous breathing has been shown to improve gas exchange. This effect seems to be related to improved diaphragmatic activity causing alveolar recruitment in the dorsobasal regions of the lungs. Additional benefits of APRV that are related to spontaneous breathing are improvements in hemodynamics, renal function, and visceral perfusion. The ability to allow unsupported breathing renders APRV and BiPAP useful in limiting sedative doses in patients who require high-level ventilatory support. APRV was associated with decreased sedation needs and earlier liberation from ventilation in two RCTs: one performed in patients recovering from cardiac surgery and one in patients with ALI and trauma. However, extrapolation of the results of the latter study is hindered by the fact that the control group was receiving muscle relaxants, a rare practice in modern days. Although APRV and BiPAP have gained popularity, further research should clarify whether they have outcome advantages over modes that are routinely used. In the meantime, APRV and BiPAP should be considered only in patients who need high airway pressures to maintain gas exchange. Care should be taken to assure that TVs and peak alveolar distention are compatible with a lung protective strategy. Because of the short release time, APRV should be avoided in patients with chronic obstructive pulmonary disease (COPD) or asthma because of the risk of air trapping.

HFOV is also used in patients with severe, refractory hypoxemia, with the rationale of providing high mean airway pressures while minimizing alveolar distention and, possibly, VILI. HFOV has been extensively studied in the pediatric population, and large RCTs have been performed in newborns. In the adult population, two small RCTs found no significant effects of HFOV on outcomes of patients with ARDS compared with conventional mechanical ventilation. In one of these studies, a trend toward improved survival rates was detected with HFOV, although this study was underpowered to detect survival differences. It is likely that HFOV may be beneficial when used in the setting of an open lung strategy. To provide support to this approach, a multinational trial on the use of HFOV versus conventional ventilation in patients with severe ALI is currently being conducted. Until such evidence becomes available, HFOV should be used as a rescue therapy in select patients who cannot achieve acceptable oxygenation while undergoing other modes of ventilation.

Management of Obstructive Lung Disease

The ventilatory management of patients with asthma and COPD is supported by a large number of physiologic studies, but few outcome trials are available. In these patients, the general goal of ventilation is to avoid hyperinflation and intrinsic PEEP. For this purpose, permissive hypercapnia is routinely practiced, but its use is only supported by an observational study on patients with status asthmaticus. However, the consensus is that the adoption of this strategy has contributed to improved survival rates in these patients. Although once considered contraindicated, PEEP is commonly used to decrease the inspiratory threshold load of intrinsic PEEP.

Noninvasive ventilation (NIV) is currently considered a standard treatment in COPD exacerbation. This is based on strong clinical evidence from RCTs that demonstrated improved outcomes and decreased rates of intubation from its early use. A systematic review of existing RCTs suggested that NIV might also be beneficial in other forms of hypoxemic respiratory failure, although the studies had conflicting results due to population heterogeneity. Therefore NIV cannot be recommended for routine use in non-COPD patients with acute respiratory failure but should only be considered in select cases.