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.
What does proportional assist ventilation do?
This mode delivers pressure-controlled breaths with a servo control scheme (Table 2).
To better understand PAV, we can compare it with PSV. With PSV, the pressure applied by the ventilator rises to a preset level that is held constant (a set-point scheme) until a cycling criterion (a percent of the maximum inspiratory flow value) is reached. The inspiratory flow and tidal volume are the result of the patient’s inspiratory effort, the level of pressure applied, and the respiratory system mechanics.
In PAV, as in PSV, all breaths are spontaneous (Table 1). The patient controls the timing and size of the breath. There are no preset pressure, flow, or volume goals, but safety limits on the volume and pressure delivered can be set.
Ventilator settings in proportional assist ventilation
Ventilator settings in PAV are:
- Airway type (endotracheal tube, tracheostomy)
- Airway size (inner diameter)
- Percentage of work supported (assist range 5%–95%)
- Tidal volume limit
- Pressure limit
- Expiratory sensitivity (normally, as inspiration ends, flow should stop; this parameter tells the ventilator at what flow to end inspiration).
Caution when assessing the literature. Earlier ventilator versions, ie, Dräger and Manitoba (University of Manitoba, Winnipeg, MB, Canada), which are not available in the United States, required the repeated calculation of the respiratory system mechanics and the manual setting of flow and volume assists (amplification factors) independently. To overcome this limitation, new software automatically adjusts the flow and volume amplification to support the loads imposed by the automatically measured values of resistance and elastance (inverse of compliance) of the respiratory system.21 This software is included in the model (Puritan Bennett) available in the United States.
Clinical applications of proportional assist ventilation
The PAV mode is indicated for maximizing ventilator patient synchrony for assisted spontaneous ventilation.
PAV is contraindicated in patients with respiratory depression (bradypnea) or large air leaks (eg, bronchopleural fistulas). It should be used with caution in patients with severe hyperinflation, in which the patient may still be exhaling but the ventilator doesn’t recognize it. Another group in which PAV should be used with caution is those with high ventilatory drives, in which the ventilator overestimates respiratory system mechanics. This situation can lead to overassistance due to the “runaway phenomenon,” in which the ventilator continues to provide support even if the patient has stopped inspiration.22
Theoretical benefits of proportional assist ventilation
In theory, PAV should reduce the work of breathing, improve synchrony, automatically adapt to changing patient lung mechanics and effort, decrease the need for ventilator intervention and manipulation, decrease the need for sedation, and improve sleep.
Evidence of benefit of proportional assist ventilation
Physiologic benefits. PAV reduces the work of breathing better than PSV,21 even in the face of changing respiratory mechanics or increased respiratory demand (hypercapnia).23–25 The hemodynamic profile is similar to that in PSV. Tidal volumes are variable; however, in recent reports the tidal volumes were within the lung-protective range (6–8 mL/kg, plateau pressure < 30 cm H20).26,27
Comfort. PAV entails less patient effort and discomfort that PSV does.23,25 PAV significantly reduces asynchrony,27 which in turn may favorably affect sleep in critically ill patients. 28
Outcomes. The probability of spontaneous breathing without assistance was significantly better in critically ill patients ventilated with PAV than with PSV. No trial has reported the effect of PAV on deaths.27,29
Proportional assist ventilation: Bottom line
Extensive basic research has been done with PAV in different forms of respiratory failure, such as obstructive lung disease, acute respiratory distress syndrome (ARDS), and chronic respiratory failure. It fulfills its main goal, which is to improve patient-ventilator synchrony. Clinical experience with PAV in the United States is limited, as it was only recently approved.
