Assisted Pressure-Controlled Ventilation
Assisted PCV is similar to PSV in that the breaths can be triggered by the patient (physicians set the sensitivity and triggering mechanism). The difference is that cycling off is determined by time instead of flow. Another obvious difference is that assisted PCV possesses the same backup mechanism available with traditional assisted flow-controlled breaths: breaths can be triggered by patient effort or by elapsed expiratory time, whichever occurs first.
Compared with assisted VCV at equivalent tidal volumes, assisted PCV decreases peak airway pressures. Although irrelevant during invasive ventilation, this characteristic is relevant during noninvasive ventilation, minimizing leaks through the mask80,148 and avoiding uncomfortably high pressures in the upper airways.46,149 When comparing the level of patient assistance during invasive ventilation, the characteristic decelerating flow (on demand) provided by PCV seems to reduce workload more efficiently than most flow-controlled, volume-cycled breaths.114,140 The reduction in inspiratory muscle load is especially prominent during moderate to high tidal volume (8 to 10 mL/kg) ventilation or use of a low inspiratory flow during VCV (<0.7 L/s). It is important to stress, however, that with use of low tidal volume (≤6 mL/kg), this assumption is not always valid.25,113,125 Because physicians have to set low values of PSET during pressure-controlled breaths (to keep tidal volume in a safe range), peak inspiratory flow rates are commonly at lower during PCV than during flow-controlled VCV. To circumvent this problem, it is tempting to use higher levels of PSET, guaranteeing a high peak flow at the beginning of a breath, but keeping inspiratory time short enough so that a high tidal volume is not delivered.39 This maneuver, however, must be used under close monitoring, or combined with some inspiratory pause so as to avoid double triggering and “breath stacking.”
The algorithm configured for assisted PCV also may carry some advantages over PSV. First, in patients with unstable respiratory drive, the backup rate works as a safety and stabilizing mechanism, avoiding central apneas and sleep fragmentation, especially in patients with cardiac failure or in those receiving heavy sedation.150 Second, although PSV is popular during noninvasive ventilation, serious concerns have been raised about potential dyssynchrony at end inspiration. During PSV, transition from inspiration to expiration is triggered by flow (i.e., when inspiratory flow decays below a certain threshold). Hence the presence of mask leaks or severe airway obstruction can render this mechanism inefficient—both conditions cause a relatively high inspiratory flow at end inspiration. Leaks simply mislead the inspiratory flow regulator of the ventilator, which cannot detect that inspiratory flow is not being delivered to patient. Conversely, severe obstruction changes the shape of inspiratory flow (see Fig. 9-7), making it less decelerating and pushing end-inspiratory flows closer to peak flow. With both conditions, the default expiratory triggering threshold for PSV (usually 25% of peak inspiratory flow) may never be reached, causing excessive prolongation of inspiratory time and patient overinflation.39,151–153 Under such conditions, pressure-controlled, time-cycled ventilation could be convenient: The intensivist needs only to adjust inspiratory time to match the patient’s spontaneous inspiratory time, usually 0.6 to 1.2 seconds.39,151,152 By changing from PSV to PCV, the intensivist necessarily decreases the freedom of ventilation pattern (restraining, for instance, the possibility of a spontaneous sigh and potentially increasing discomfort) but also avoids dangerous prolongation of inspiratory time. This is a difficult balance that has to be judged at the bedside.
During assisted PCV, especially under high flow demands, a fast “attack rate” to peak flow (at the initial phase) of the breath is desirable and decreases the work of breathing.37 Thus, when changing from totally controlled (time-triggered) PCV to assisted PCV, it is advisable to increase the speed of pressurization (i.e., to decrease the rise time or to increase the inspiratory pressure slope).
Activation of Exhalation Valve during Pressure-Controlled Ventilation: Airway Pressure Release Ventilation
Opening of the Exhalation Valve
In theory, any pressure-controlled breath should limit airway pressures to PSET independent of patient demand, dyssynchrony, premature expiratory efforts, or cough. In practice, however, hardware and software limitations of most ventilators preclude such ideal configuration. Usually, ventilators can overcome an increase in inspiratory-flow demands efficiently during PCV (caused by leaks or strong inspiratory efforts) by boosting flow through the demand valve. Airway pressures are raised quickly back to PSET. Additionally, during the period in which the demand valve is still open, any sudden increment in alveolar pressure (cough) can be counterbalanced promptly by a sudden decrease in demand flow, attenuating the potential raise in airway pressures. Once the demand valve is already closed, however—because alveolar pressures have equalized airway pressures—the ventilator is no longer able to control airway pressures. Because most ventilators do not actively control the exhalation valve during inspiration (they just close it tightly until the end of inspiratory time), any unexpected increment in airway pressures can only be counterbalanced by total closure of the demand valve, but this is already at its minimum position. The end result is a steep increment in airway pressures, as in flow-controlled breaths.
Physicians have to keep this limitation of PCV in mind because it may be responsible for undesirable increments in airway pressures above PSET in most ventilators. This situation has been observed during continuous tracheal gas insufflation,154 during cough, and during strong efforts in assisted PCV breaths. In the latter circumstance, marked elevations of airway pressures can be found even in the absence of active expiratory efforts. This is so because the patient effort can increase effective driving pressures in early inspiration, increasing tidal volume and, consequently, the elastic recoil pressure at end inspiration (when diaphragmatic contraction already has ceased). An inactive exhalation valve could not release this extra tidal volume before the beginning of next exhalation, thus increasing airway pressures.
To overcome such limitations, some modern ventilators have incorporated improved hardware in their exhalation system that allows the simultaneous control of both valves (the inspiratory demand-flow valve and the exhalation valve) during the inspiratory phase of PCV. The end result is a smoother control of airway pressures during cough or during continuous tracheal gas insufflation.155 Whenever available, such active control of the exhalation valve enhances ventilator operation.
The opening of the exhalation valve and its active control during inspiration are the technological basis for incorporation of APRV in microprocessed ventilators.
Airway Pressure Release Ventilation and Its Variants
APRV is a form of partial ventilatory support intended originally to offload a portion of the work required to ventilate during a primary crisis of oxygenation.143,144 In its original conception, APRV elevated mean airway pressure by maintaining a moderately high level of CPAP, delivered through a specially designed high-flow CPAP system linked to a release valve operated by a time controller. The system was design to work independently of any commercial ventilator. Spontaneous breaths were planned to occur around this pressure baseline. Periodically, the airway was depressurized rapidly during one of the patient’s deflation cycles, exhausting waste gas from the expiratory reserve before replacing it with fresh gas as CPAP rebuilt to the baseline level. Total time for both phases of the release cycle generally was brief, ideally occupying only a single breath of the patient’s rhythm. The release pattern was repeated at a clinician-selected frequency. As predicated by the mathematics of pressure-preset ventilation,33 ventilator support (achieved by the machine) was necessarily a function of the number of release cycles, the magnitude of the pressure drop during release, the duration of the release period, and the impedance to inflation and deflation. Synchronization between the patient’s own exhalation and airway depressurization also affects the effectiveness of ventilation. Ineffectual inspiratory efforts during the deflation phase could impair the released volume. Despite such concerns, the original system was not designed to be synchronous.
Subsequently, different names were used for similar systems and arrangements, biphasic positive airway pressure, intermittent mandatory pressure-release ventilation, or simply PCV+ (this latter mostly used in the United States). Despite the similarity, the term bilevel airway pressure, commercially known as biphasic continuous positive airway pressure, represents something different, being simply a combination of pressure-supported ventilation and CPAP; this product is designed for noninvasive use. The basic difference among these systems is the duration of each phase, although all the systems can be approximated to periodic alternation between two CPAP levels according to a time controller. A longer period at the lower CPAP level, enough to allow one or more spontaneous breaths, is characteristic of biphasic positive airway pressure ventilation. This definition, however, is not consistent in the literature, with considerable ambiguity in the use of terms.21 Commercial ventilator branding further add to confusion. The specific transition from low to high CPAP levels differs among these variants, whether it is synchronized or not.
Adjustment of all these features is now possible in modern ventilators, and this does not represent any important conceptual modification. Accordingly, we believe that the confusing proliferation of names should be simplified under a single acronym: APRV.
In modern ventilators, provided that the ventilator works with an active (open) exhalation valve during inspiration, APRV can be perfectly imitated by PCV breaths with a single difference. During APRV, if the expiratory time is set long enough to allow the patient to complete one or more spontaneous breaths during the low-pressure phase, the spontaneous breath will be assisted by an efficient CPAP system (equivalent to PSV set at zero driving pressure). Conversely, during the exhalation phase of PCV, the spontaneous effort cannot be well assisted in most ventilators, because the bias-flow systems responsible for PEEP maintenance are not designed to support strong efforts. The end result may be some drop in PEEP unless sensitivity is adjusted to allow triggered inspirations (this latter, however, will result in an assisted PCV breath, which will necessarily cause an increase in the I:E ratio).
During the inspiratory phase, the opening of the exhalation valve during PCV assures that both systems perform identically, allowing spontaneous breaths to occur around highly sustained airway pressures. When spontaneous breaths are abolished by paralysis, APRV is not different from PCV.
It is important to remember that spontaneous cycles and ventilator cycles are completely uncoupled in the most frequent usage of APRV. This means that spontaneous breathing can occur at any phase of the ventilator cycle without any synchronization mechanism. Switching between the two CPAP levels obeys a timed-cycled mechanism, and patient inspiratory work is mechanically supported only when it coincides with the restoration of the high CPAP phase. In view of such inherent dyssynchrony, more sophisticated APRV algorithms have been introduced in recent years, although without a clear rationale or clinical benefit. These include triggered transitions between low and high CPAP levels and use of PSV during one or both phases, on top of the corresponding baseline pressures.
Regarding the synchronized transition, it is important to remember that whenever commercial ventilators allow the patient to trigger the CPAP transition, usually during a time-window that spans the last 25% of the preset low-pressure phase, the actual time intervals at high and low pressures (equivalent to I:E ratio) may vary according to the respiratory drive of the patient.156 Furthermore, even when some synchronization with CPAP transitions is attempted, most spontaneous breaths during APRV are still nonsynchronous because of the long duration of both phases. Under such conditions, the real advantage of synchronized systems is questionable. From a workload perspective, APRV is an inefficient way of assisting the patient, imposing a higher workload and greater discomfort as compared with equivalent levels of PSV or assisted PCV.65,139,157,158 The benefit, if any, of providing some synchronization has to be balanced against the loss of control over TI/TTOT. Synchronization would only make sense if applied to most spontaneous efforts, meaning that APRV should be set at higher frequencies and shorter inspiratory times, as with assisted PCV. Another potential disadvantage of synchronization is the creation of high driving pressures during this upward transition, caused by the coupling of muscle effort with the positive-pressure wave from the ventilator. The high inspiratory volume generated, however, will not be evident when looking at the next exhalation, because the release will occur against a higher PEEP level. To compute this hidden, large, inspired volume, one should carefully look at the next downward transition, a few breaths ahead, when most of the insufflated volume will be finally released.
Whenever APRV is applied in its common configuration, with long time intervals set for both phases, the intermittent mechanical assistance (applied only when the spontaneous effort coincides with transition from low to high pressures) shares some similarities with intermittent mandatory ventilation. As with synchronized intermittent mandatory ventilation, the presence of alternating levels of assistance probably impairs smooth accommodation of patient drive,159 potentially causing some distress in patients when sedation is removed. Therefore, it is still an open question as to whether APRV will prove helpful during weaning or for patients with extreme weakness, very high workloads, or conditions in which release cycles are relatively inefficient in achieving ventilation (e.g., severe airflow obstruction).
For the sake of simplicity, whenever the patient gets too distressed during APRV and sedation is contraindicated, it is preferable to change to PSV or assisted PCV, assisting the patient in a more predictable way. The combination of PSV with APRV introduces unnecessary complexity, making little sense in terms of physiologic benefit or as an attempt to minimize driving pressures and ventilator-induced lung injury.
By extending the higher CPAP level further, APRV gets closer to inverse-ratio PCV. The shorter the time at low CPAP levels, the lower is the ventilator contribution to minute ventilation,156 and the higher is the PaCO2. On the other hand, PaO2 tends to increase because intrinsic PEEP increases. The full consequences of the duty-cycle settings on oxygenation are discussed in the section Intrinsic Positive End-Expiratory Pressure versus Extrinsic Positive End-Expiratory Pressure.
Comparison of APRV associated with spontaneous breaths versus APRV without spontaneous efforts (identical to conventional PCV), when both modes are set at equivalent end-expiratory and end-inspiratory airway pressures, showed that the former achieved better oxygenation,147 higher cardiac output,147,160 higher renal blood flow,161 higher lung aeration, and higher functional residual capacity.145 Whereas the hemodynamic benefits seem to be related to lower intrathoracic pressures generated during spontaneous efforts,146 the benefits in lung function seem to be mostly related to higher transpulmonary pressures.145,162 When equivalent transpulmonary pressures are accomplished by controlled PCV without any patient effort (with PSET adjusted at higher levels, matching the transpulmonary pressures observed during APRV), the oxygenation benefits are no longer observed.162
It is also important to stress that equivalent physiologic benefits can be achieved during simple CPAP application (matched to the high-pressure phase of APRV), in fact, with better outcomes in terms of oxygenation.163 These findings demonstrate that most of the benefits attributed to APRV relate to the replacement of mechanically applied driving pressures by patient-generated driving pressures.
When considering the overall implications on hemodynamics, lung injury, and muscle function, APRV still poses a well-defined set of potential problems. Central vascular congestion and exacerbated pulmonary edema are natural consequences of lower intrathoracic pressures and increased cardiac workload. Interstitial pressures are expected to be lower in this setting, potentially increasing transcapillary pressures and interstitial edema.26 Thus, in patients with cardiac failure, APRV should be used with caution. Conversely, to unload inspiratory muscles significantly, the high-pressure CPAP in APRV first must be raised to a considerable degree, obscuring the major advantage of APRV, which is the use of lower airway pressures. Moreover, inspiratory muscles will be disadvantaged by the resulting hyperinflation, promoting a sense of dyspnea that APRV is designed to relieve.
Finally, the intrinsic design of APRV raises important concerns about VILI. Because relaxation of inspiratory muscles is common during end-exhalation, even during spontaneous effort, we should not expect any significant advantage of APRV in terms cyclic lung collapse and its related damage.164–167 Also, because inspiratory transpulmonary pressures are unpredictable, probably higher than during passive PCV breaths (supposing a situation where PSET matches the high CPAP level of APRV) we should not expect much improvements in terms of overdistension.18,43,168–170 Conceptually, the same principles of protective ventilation should be respected during APRV: the need to reduce effective driving pressures, whatever their source of generation, and the need to avoid cyclic collapse and tidal recruitment. The practical implications are that the releasing pressures of APRV should be titrated carefully (as during any PEEP titration), and, similarly, driving pressures should be estimated carefully and minimized (as during any attempt to reduce plateau pressures during controlled ventilation). Unfortunately, the general use of APRV rarely follows such protective principles.21
In conclusion, the important message provided by studies of APRV is that the preservation of spontaneous efforts is an important aspect of mechanical ventilation that should be employed prudently. Often, the simple use of assisted PCV, instead of fully controlled PCV, is the required change to promote profound benefits in hemodynamics, lung recruitment and oxygenation.
In recent years, attempts have been made to combine the desirable features of both flow-controlled, volume-cycled and pressure-controlled, time-cycled ventilation. These modes include pressure-regulated volume-control, volume-assured pressure-support ventilation,24 and autoflow among others; they guarantee any tidal volume compatible with the physician-set upper pressure limit. Some modes accomplish a dual target over a few breaths, whereas others, like volume-assured pressure-support ventilation, guarantee a dual target within the same breath. Despite limited research, the original rationale for theses modes was the desire to combine the benefits of pressure and flow controllers while avoiding their problems.
A major advantage of such modes is their use in patients in whom some level of spontaneous ventilation is desirable, but in whom some control over transpulmonary pressures is necessary. Provided that the patient is not extremely agitated, a volume-cycled target promotes a decrease in inspiratory airway pressures in proportion to the magnitude of patient effort, still allowing the patient to choose inspiratory flow pattern. Such feedback limits transpulmonary pressures and, theoretically, would be a convenient strategy to avoid ventilator-induced lung injury. Also, such feedback usually provides a smooth transition from ventilator-imposed driving pressures to patient-generated driving pressures. The drawback of such strategy is the increase in the work of breathing, especially when the volume target is set below 6 mL/kg.125,127