Page 145 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 111

AIRWAY PRESSURE RELEASE VENTILATION (APRV)

Airway pressure release ventilation (APRV) has two CPAP or pressure levels—high
airway pressure release
ventilation (APRV): A mode of pressure (P high or P INSP ) and low pressure (P or PEEP), and the patient is allowed
low
ventilation in which the spontane- to breathe spontaneously without restriction. When the high pressure (P ) level is
ous breaths are at an elevated high
baseline (i.e., CPAP). This elevated dropped or “released” to the low pressure (P ) level, it simulates a mechanical exhala-
low
baseline is periodically “released” tion. Likewise, when the low pressure (P ) level is raised to the high pressure (P )
to facilitate expiration. low high
level, it simulates an inspiratory mechanical breath. In APRV, the patient spends most
of the time at the high pressure level with less than 1.5 sec at the low pressure level.
To provide APRV, the ventilator must have a high flow CPAP circuit that has
been modified with the addition of a release valve. When the release valve opens, the
CPAP pressure is vented and the circuit pressure decreases to zero or a lower CPAP
level. Figure 4-11 shows the airway pressure release during CPAP mode.
A mandatory inspiration begins with time-triggered closing of the release valve.
The airway pressure rapidly increases to the baseline CPAP pressure and is main-
tained for the duration of inspiration (for as long as the release valve remains closed).
The mandatory inspiration ends with time-triggered opening of the release valve,
which allows the circuit pressure to decrease as the patient exhales. What is unique
about this mode is that the patient is allowed to breathe spontaneously at the high
The tidal volume during or low pressure levels. Since APRV mode is pressure-limited, for a given pressure
APRV is determined by the
pressure gradient between gradient (P 2P ), the patient’s tidal volume will vary directly with changes in
CPAP and final pressure fol- high low
lowing pressure release. lung compliance and inversely with changes in airway resistance. For this reason, the
exhaled tidal volume should be closely monitored to prevent hyperinflation.
Patient-ventilator dyssynchrony may result when pressure release (from P high to
P ) occurs during spontaneous inspiration, or when pressure increase (from P
low
low
to P high ) occurs during spontaneous expiration. The timing of pressure release and

40
35
Airway Pressure (cm H 2 O) 20 CPAP Level Release Pressure
30
25

15
10
5
0

Expiration Inspiration © Cengage Learning 2014
Figure 4-11 Airway pressure release ventilation (APRV) at a CPAP level of 10 cm H 2 O and
pressure release to 0 cm H 2 O with a net release pressure gradient of 10 (10–0) cm H 2 O. During
APRV, the expiratory phase occurs when the airway pressure is released from 10 to 0 cm H 2 O. On
inspiration, the CPAP level is maintained at 10 cm H 2 O. Since the pressure release time period is
rather short, an inversed I:E ratio is usually observed. With APRV, the patient’s tidal volume will vary
directly with changes in lung compliance and inversely with changes in airway resistance.

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