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.
ADAPTIVE PRESSURE CONTROL
Other names for adaptive pressure control
- Pressure Regulated Volume Control (Maquet Servo-i, Rastatt, Germany)
- AutoFlow (Dräger Medical AG, Lübeck, Germany)
- Adaptive Pressure Ventilation (Hamilton Galileo, Hamilton Medical AG, Bonaduz, Switzerland)
- Volume Control+ (Puritan Bennett, Tyco Healthcare; Mansfield, MA)
- Volume Targeted Pressure Control, Pressure Controlled Volume Guaranteed (Engström, General Electric, Madison, WI).
What does adaptive pressure control do?
The APC mode delivers pressure-controlled breaths with an adaptive targeting scheme (Table 2).
In pressure-control ventilation, tidal volumes depend on the lung’s physiologic mechanics (compliance and resistance) and patient effort (Figure 1). Therefore, the tidal volume varies with changes in lung physiology (ie, larger or smaller tidal volumes than targeted).
To overcome this effect, a machine in APC mode adjusts the inspiratory pressure to deliver the set minimal target tidal volume. If tidal volume increases, the machine decreases the inspiratory pressure, and if tidal volume decreases, the machine increases the inspiratory pressure. However, if the patient effort is large enough, the tidal volume will increase in spite of decreasing the inspiratory pressure (Figure 2). The adjustments to the inspiratory pressure occur after the tidal volume is off-target in a number of breaths.
Common sources of confusion with adaptive pressure control
First, APC is not a volume-control mode. In volume control, the tidal volume does not change; in APC the tidal volume can increase or decrease, and the ventilator will adjust the inflation pressure to achieve the target volume. Thus, APC guarantees an average minimum tidal volume but not a maximum tidal volume.
Second, a characteristic of pressure control (and hence, APC) is that the flow of gas varies to maintain constant airway pressure (ie, maintain the set inspiratory pressure). This characteristic allows a patient who generates an inspiratory effort to receive flow as demanded, which is likely more comfortable. This is essentially different from volume control, in which flow is set by the operator and hence is fixed. Thus, if the patient effort is strong enough (Figure 1), this leads to what is called flow asynchrony, in which the patient does not get the flow asked for in a breath.
Ventilator settings in adaptive pressure control
Ventilator settings in APC are:
- Tidal volume
- Time spent in inspiration (inspiratory time)
- Frequency
- Fraction of inspired oxygen (Fio2)
- Positive end-expiratory pressure (PEEP).
Some ventilators also require setting the speed to reach the peak pressure (also known as slope percent or inspiratory rise time).
Clinical applications of adaptive pressure control
This mode is designed to maintain a consistent tidal volume during pressure-control ventilation and to promote inspiratory flow synchrony. It is a means of automatically reducing ventilatory support (ie, weaning) as the patient’s inspiratory effort becomes stronger, as in awakening from anesthesia.
APC may not be ideal for patients who have an inappropriately increased respiratory drive (eg, in severe metabolic acidosis), since the inspiratory pressure will decrease to maintain the targeted average tidal volume, inappropriately shifting the work of breathing onto the patient.
Theoretical benefits of adaptive pressure control
APC guarantees a minimum average tidal volume (unless the pressure alarm threshold is set too low, so that the target tidal volume is not delivered). Other theoretical benefits are flow synchrony, less ventilator manipulation by the operator, and automatic weaning of ventilator support.
Evidence of benefit of adaptive pressure control
Physiologic benefits. This mode has lower peak inspiratory pressures than does volume-control ventilation,3,4 which is often reported as a positive finding. However, in volume-control mode (the usual comparator), the peak inspiratory pressure is a manifestation of both resistance and compliance. Hence, peak inspiratory pressure is expected to be higher but does not reflect actual lung-distending pressures. It is the plateau pressure, a manifestation of lung compliance, that is related to lung injury.
Patient comfort. APC may increase the work of breathing when using low tidal volume ventilation and when there is increased respiratory effort (drive).5 Interestingly, APC was less comfortable than pressure support ventilation in a small trial.6
Outcomes have not been studied.7
Adaptive pressure control: Bottom line
APC is widely available and widely used, sometimes unknowingly (eg, if the operator thinks it is volume control). It is relatively easy to use and to set; however, evidence of its benefit is scant.
