Alternative Modes of Ventilation

This chapter describes alternative methods of ventilatory support when conventional mechanical ventilation is not sufficient, or may not be necessary. Included are rescue modes of ventilation (i.e., high frequency oscillatory ventilation and airway pressure release ventilation) and noninvasive modes of ventilation (i.e., continuous positive airway pressure, bilevel positive airway pressure, and pressure support ventilation).

I. Rescue Modes of Ventilation

A small percentage (10–15%) of patients with acute respiratory distress syndrome (ARDS) develop hypoxemia that is refractory to O₂ therapy and conventional mechanical ventilation (CMV) (1). The following methods of ventilation can be beneficial in these patients.

A. High Frequency Oscillation

High frequency oscillatory ventilation (HFOV) uses high-frequency, low volume oscillations like the ones shown in Figure 20.1. These oscillations create a high mean airway pressure, which improves gas exchange by opening collapsed alveoli (alveolar recruitment) and preventing further alveolar collapse. The small tidal volumes (typically 1–2 mL/kg) limit the risk of alveolar injury from overdistension (volutrauma) (2).

FIGURE 20.1 Airway pressure oscillations during high frequency oscillatory ventilation (HFOV) with a superimposed lung inflation during conventional mechanical ventilation (CMV). Dotted line represents mean airway pressure. From Reference 3.

1. Ventilator Settings

HFOV requires a specialized ventilator (Sensormedics 3100B, Viasys Healthcare, Yorba Linda, CA) that allows the following adjustments: (a) the frequency and amplitude of the oscillations, (b) the mean airway pressure, (c) the bias flow rate (similar to an inspiratory flow rate), and (d) the inspiratory time (time of the bias flow).

The frequency range for oscillations is 4–7 Hz (oscillations/sec). The frequency selected is determined by the arterial pH (which is a reflection of the CO₂ burden). Lower frequencies have higher pulse amplitudes, and are more effective for CO₂ removal, which reduces the risk of respiratory acidosis.
The initial pulse amplitude is set at 70–90 cm H₂O.
The mean airway pressure is usually set slightly above the end-inspiratory alveolar pressure during CMV (see Chapter 19, Figures 19.1 and 19.2) (3).
The bias flow rate is usually set at 40 L/min.

2. Advantages

Clinical trials comparing HFOV with CMV have shown 16–24% increase in the PaO₂/FIO₂ ratio associated with HFOV (2). However, there is no documented survival benefit with HFOV (3,4).

3. Disadvantages

A special ventilator is needed, along with trained personnel to operate the device.
Cardiac output is often decreased during HFOV because of the high mean airway (intrathoracic) pressures (3).

B. Airway Pressure Release Ventilation

Airway pressure release ventilation (APRV) is a modified form of continuous positive airway pressure (CPAP), and involves prolonged periods of spontaneous breathing with high-level CPAP, interrupted by brief periods of pressure release to atmospheric pressure. This is demonstrated in the middle panel of Figure 20.2. The high CPAP level improves arterial oxygenation by opening collapsed alveoli (alveolar recruitment), and the pressure release is designed to facilitate CO₂ removal (5). The increase in arterial oxygenation occurs gradually, over 24 hours (6).

1. Ventilator Settings

APRV is available in most modern critical care ventilators. The variables that must be selected when initiating APRV include the high and low airway pressures, and the time spent at each pressure level. Suggested settings are as follows (3):

The high airway pressure should be equivalent to the end-inspiratory alveolar pressure during CMV (see Chapter 19, Figures 19.1 and 19.2).
The low airway pressure is set to zero.
The time spent at the high airway pressure is usually 85–90% of the total cycle time. Recommended times are 4–6 seconds for the high pressure level, and 0.6 to 0.8 seconds for the low pressure level.

FIGURE 20.2 Related modes of pressure-regulated spontaneous ventilation. CPAP = continuous positive airway pressure, APRV = airway pressure release ventilation, BiPAP = bilevel positive airway pressure, IPAP = inspiratory positive airway pressure, EPAP = expiratory positive airway pressure, P_mean = mean airway pressure, I = inspiration, E = expiration. See text for explanation.

2. Advantages