Page 144 - Clinical Application of Mechanical Ventilation

110 Chapter 4

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Airway Pressure (cm H 2 O) 25
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Figure 4-10 (A) Pressure tracing of a volume-controlled mechanical breath; (B) Pressure
tracing of a pressure-controlled mechanical breath; note the prolonged inspiratory pressure
plateau in pressure-controlled ventilation.

maintained for as long as the patient maintains a spontaneous inspiratory flow.
In PCV, however, the pressure plateau is maintained for a preset inspiratory
time (Figure 4-10).
PCV is usually indicated for patients with severe ARDS who require extremely
high peak inspiratory pressures during mechanical ventilation in a volume-controlled
mode. As a result of these high airway pressures, incidence of barotraumas is more
likely (Gurevitch et al., 1986).
The advantage of switching these patients from the conventional volume-
controlled ventilation to pressure control is that a lower peak inspiratory pressure
can be used and maintained while providing oxygenation (PaO ) and ventilation
2
(PaCO ) (Lain, 1989). Being able to decrease the PIP significantly reduces the risk
2
of barotrauma for these patients.
Table 4-9 summarizes the major characteristics of the pressure-controlled
ventilation mode.

TABLE 4-9 Characteristics of the Pressure-Controlled Ventilation Mode

Characteristic Description

Type of breath Only mandatory breaths are available to the patient in the
pressure-controlled mode.

Triggering mechanism The mandatory breaths in the pressure-controlled mode are
time-triggered by a preset frequency (may be patient-triggered for
additional breaths).

Cycling mechanism The mandatory breaths are time-cycled by a preset inspiratory time.
Notes: The peak inspiratory pressure is controlled by the preset pressure limit. As with any pressure-limited ventilator, the tidal volume will vary
directly with lung compliance and inversely with airway resistance. It may be necessary to invert the I:E ratio beyond 1:2 to maintain oxygenation.
I:E ratios as high as 4:1 have been reported with successful outcome.
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