Alternative modes of mechanical ventilation: A review for the hospitalist
ABSTRACTNewer ventilators can be set to modes other than the pressure-control and volume-control modes of older machines. In this paper, the authors review several of these alternative modes (adaptive pressure control, adaptive support ventilation, proportional assist ventilation, airway pressure-release ventilation, biphasic positive airway pressure, and high-frequency oscillatory ventilation), explaining how they work and contrasting their theoretical benefits and the actual evidence of benefit.
KEY POINTS
- The alternative modes of ventilation were developed to prevent lung injury and asynchrony, promote better oxygenation and faster weaning, and be easier to use. However, evidence of their benefit is scant.
- Until now, we have lacked a standard nomenclature for mechanical ventilation, leading to confusion.
- Regardless of the mode used, the goals are to avoid lung injury, keep the patient comfortable, and wean the patient from mechanical ventilation as soon as possible.
AIRWAY PRESSURE-RELEASE VENTILATION AND BIPHASIC POSITIVE AIRWAY PRESSURE
Airway pressure-release ventilation (APRV) was described in 1987 by Stock et al30 as a mode for delivering ventilation in acute lung injury while avoiding high airway pressures. APRV combines high constant positive airway pressure (improving oxygenation and promoting alveolar recruitment) with intermittent releases (causing exhalation).
Other names for biphasic positive airway pressure
Other names for biphasic positive airway pressure are:
- BiLevel (Puritan Bennett)
- BIPAP (Dräger Europe)
- Bi Vent (Siemens)
- BiPhasic (Avea, Cardinal Health, Inc, Dublin, OH)
- PCV+ (Dräger Medical)
- DuoPAP (Hamilton).
Caution—name confusion. In North America, BiPAP (Respironics, Murrysville, PA) and BiLevel are used to refer to noninvasive modes of ventilation.
APRV has no other name.
What do these modes do?
These modes deliver pressure-controlled, time-triggered, and time-cycled breaths using a set-point targeting scheme (Table 2). This means that the ventilator maintains a constant pressure (set point) even in the face of spontaneous breaths.
Caution—source of confusion. The term continuous positive airway pressure (CPAP) is often used to describe this mode. However, CPAP is pressure that is applied continuously at the same level; the patient generates all the work to maintain ventilation (“pressure-controlled continuous spontaneous ventilation” in the current nomenclature). In APRV, the airway pressure is intermittently released and reapplied, generating a tidal volume that supports ventilation. In other words, this is a pressure-controlled breath with a very prolonged inspiratory time and a short expiratory time in which spontaneous ventilation is possible at any point (“pressure-controlled intermittent mandatory ventilation” in the current nomenclature).
How these modes are set in the ventilator may also be a source of confusion. To describe the time spent in high and low airway pressures, we use the terms Thigh and Tlow, respectively. By convention, the difference between APRV and biphasic mode is the duration of Tlow (< 1.5 sec for APRV).
Similarly, Phigh and Plow are used to describe the high and low airway pressure. To better understand this concept, you can create the same mode in conventional pressure-control ventilation by thinking of the Thigh as the inspiratory time, the Tlow as the expiratory time, the Phigh as inspiratory pressure, and the Plow as PEEP.
Hence, APRV is an extreme form of inverse ratio ventilation, with an inspiration-to-expiration ratio of 4:1. This means a patient spends most of the time in Phigh and Thigh, and exhalations are short (Tlow and Plow). In contrast, the biphasic mode uses conventional inspiration-expiration ratios (Figure 4).
As with any form of pressure control, the tidal volume is generated by airway pressure rising above baseline (ie, the end-expiratory value). Hence, to ensure an increase in minute ventilation, the mandatory breath rate must be increased (ie, decreasing Thigh, Tlow, or both) or the tidal volume must be increased (ie, increasing the difference between Phigh and Plow). This means that in APRV the Tlow has to happen more often (by increasing the number of breaths) or be more prolonged (allowing more air to exhale). Because unrestricted spontaneous breaths are permitted at any point of the cycle, the patient contributes to the total minute ventilation (usually 10%–40%).
In APRV and biphasic mode, the operator’s set time and pressure in inspiration and expiration will be delivered regardless of the patient’s breathing efforts—the patient’s spontaneous breath does not trigger a mechanical breath. Some ventilators have automatic adjustments to improve the trigger synchrony.
Ventilator settings in APRV and biphasic mode
These modes require the setting of two pressure levels (Phigh and Plow) and two time durations (Thigh and Tlow). One can add pressure support or automatic tube compensation to assist spontaneous breaths. The difference in Tlow generates differences in the Thigh:Tlow ratio: APRV has a short Tlow (an inspiration-expiration ratio of 4:1). Biphasic mode has a conventional inspiration-expiration ratio of 1:1 to 1:4.
Clinical applications
APRV is used in acute lung injury and ARDS. This mode should be used with caution or not at all in patients with obstructive lung disease or inappropriately increased respiratory drive.32–35
Biphasic mode is intended for both ventilation and weaning. In a patient who has poor respiratory effort or who is paralyzed, biphasic is identical to pressure-control/continuous mandatory ventilation.
Theoretical benefits of APRV and biphasic mode
Multiple benefits have been ascribed to these modes. In theory, APRV will maximize and maintain alveolar recruitment, improve oxygenation, lower inflation pressures, and decrease overinflation. Both APRV and biphasic, by preserving spontaneous breathing, will improve ventilation-perfusion matching and gas diffusion, improve the hemodynamic profile (less need for vasopressors, higher cardiac output, reduced ventricular workload, improved organ perfusion), and improve synchrony (decrease the work of breathing and the need for sedation).
Evidence of benefit of APRV and biphasic mode
APRV and biphasic are different modes. However studies evaluating their effects are combined. This is in part the result of the nomenclature confusion and different practice in different countries.36
Physiologic benefits. In studies, spontaneous breaths contributed to 10% to 40% of minute ventilation,37,38 improved ventilation of dependent areas of the lung, improved ventilation-perfusion match and recruitment,39 and improved hemodynamic profile.40
Patient comfort. These modes are thought to decrease the need for analgesia and sedation,38 but a recent trial showed no difference with pressure-controlled intermittent mandatory ventilation.41 Patient ventilator synchrony and comfort have not been studied.32,42
Outcomes. In small trials, these modes made no difference in terms of deaths, but they may decrease the length of mechanical ventilation.38,41,43,44
APRV and biphasic mode: Bottom line
Maintaining spontaneous breathing while on mechanical ventilation has hemodynamic and ventilatory benefits.
APRV and biphasic mode are not the same thing. APRV’s main goal is to maximize mean airway pressure and, hence, lung recruitment, whereas the main goal of the biphasic mode is synchrony.
There is a plethora of ventilator settings and questions related to physiologic effects.33,34,36
Although these modes are widely used in some centers, there is no evidence yet that they are superior to conventional volume- or pressure-control ventilation with low tidal volume for ARDS and acute lung injury. There is no conclusive evidence that these modes improve synchrony, time to weaning, or patient comfort.
