Chapter 11

Controlled mechanical ventilation (CMV) is traditionally provided via an artificial airway to completely unload a patient’s work of breathing and assure adequate gas exchange during the acute phase of respiratory insufficiency, until the underlying respiratory function has resolved.1 The criteria used to determine when to terminate mechanical ventilation are essentially based on the clinical, and often, subjective assessment of the intensive care physician or on standardized weaning methods.2,3 The actual process of weaning the patient from CMV is carried out by allowing spontaneous breathing attempts with a T piece or continuous positive airway pressure (CPAP) or by gradually reducing mechanical assistance.4,5 Not surprisingly, gradual reduction of partial ventilator support benefits only patients who have difficulty in sustaining unassisted breathing.4 Although introduced as weaning techniques, partial support modes have become standard methods of providing primary mechanical ventilatory support in critically ill patients.

Airway pressure release ventilation (APRV)6 ventilates by time-cycled switching between two pressure levels in a high-flow (or demand-valve) CPAP circuit, permitting unrestricted spontaneous breathing in any phase of the mechanical ventilator cycle (Fig. 11-1). The degree of ventilator support with APRV is determined by the duration of the two CPAP levels and the mechanically delivered tidal volume (VT).6,7 VT depends mainly on respiratory compliance and the difference between the CPAP levels. By design, changes in ventilatory demand do not alter the level of mechanical support during APRV. When spontaneous breathing is absent, APRV is not different from conventional pressure-controlled, time-cycled mechanical ventilation.6,7

###### Figure 11-1

Airway pressure release ventilation ventilates by time-cycled switching between a high and low continuous positive airway pressure (CPAP) level in the circuit. Consequently, unrestricted spontaneous breathing is permitted in any phase of the mechanical ventilator cycle. Change between the two CPAP levels results in a change in functional residual capacity (ΔFRC), which equals the mechanical delivered tidal volume (VT). VT depends mainly on respiratory compliance and resistance and the airway pressure difference (ΔPaw) between the CPAP levels. Setting the time for the low (Tlow) and the high (Thigh) CPAP enables the adjustment of ventilator rate.

Synonyms used for APRV are biphasic positive airway pressure7 (BIPAP) and bilevel airway pressure (Bilevel). Biphasic positive airway pressure is identical to APRV except that no restriction is imposed on the duration of the low-CPAP level (release pressure).6,7 Based on the initial description, APRV keeps the duration of the low-CPAP level (release time) at 1.5 seconds or less.

### Ventilation Distributions

Radiologic studies demonstrate that ventilation is distributed differently during pure spontaneous breathing and CMV.8 During spontaneous breathing, the posterior muscular sections of the diaphragm move more than the anterior tendon plate.8 Consequently, when patients are ...

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