Page 104 - Clinical Application of Mechanical Ventilation

70 Chapter 3

P high
15

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5 R P low
0
s s

Figure 3-16 A scalar presentation of airway pressure release ventilation (APRV) mode.
A scalar presentation of airway pressure release ventilation (APRV) mode.
P high. Upper (higher) airway pressure (15 cm H 2 O)
P low. Lower airway pressure (PEEP of 5 cm H 2 O as shown)
R. Pressure release 10 cm H 2 O (15 cm H 2 O - 5 cm H 2 O)
S. Beginning of spontaneous breaths at P high

Airway Pressure Release Ventilation

Airway pressure release ventilation (APRV) is a form of continuous positive air-
way pressure (CPAP) with two distinct pressure levels. APRV maintains spon-
taneous breathing throughout the entire ventilatory cycle at both pressure levels
(Figure 3-16). APRV is a time-triggered, pressure-limited, and time-cycled mode
which allows spontaneous breathing.
The clinician sets the high and low pressures, and the inspiratory times at
each pressure level. Typically, the higher pressure is initially set above the lower
inflection point of the lung’s pressure volume curve, close to what the mean
airway pressure would be during pressure-controlled ventilation (Myers, 2007).
The higher pressure keeps the alveoli inflated and enhances recruitment. The
time interval at the higher pressure (T high ) is longer than the time spent at the
lower pressure (T ). Release of the pressure from the higher to the lower pres-
low
sure setting helps to facilitate removal of CO (Myers, 2007). Time triggering is
2
established using set time intervals for T high and T low . Additionally, most ventila-
tors allow patient triggering of a breath (pressure or flow). Some manufacturers
also permit the application of pressure support during the spontaneous portion
at the higher CPAP level.

OUTPUT WAVEFORMS

Output waveforms are graphical representations of the control or phase variables
in relation to time. Output waveforms are typically presented in the order of pres-
sure, volume, and flow. The ventilator determines the shape of the control vari-
able, whereas the other two depend on the patient’s compliance and resistance.
Convention dictates that flow values above the horizontal axis are inspiratory,
whereas flow below the horizontal axis is expiratory. This corresponds to pressure

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