Page 372 - Clinical Application of Mechanical Ventilation

338 Chapter 11

V (L/min) 120

2 4 6 8 10 12 14

P (cm H 2 O) 2120

40
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2 4 6 8 10 12 14
Time (sec)
Figure 11-20 Effects of changing compliance and airflow resistance on the flow and
delivered volume during PCV. The pressure waveforms remain unchanged during PCV. Flow
waveforms change in response to the fluctuating compliance and airflow resistance characteris-
tics. Since the area under each flow waveform reflects the delivered volume, the volume changes
accordingly.

Assist Breaths during Pressure-Controlled

During PCV, the pressure Ventilation
level, rate, and I:E ratio
are preset. Flow and V T are
dependent on the pressure Figure 11-20 is a theoretical example of a patient who is air-hungry, not relaxed, has
level, patient effort, and lung/
airway characteristics. variable flow demands, and is triggering all the breaths during PCV. By definition,
PCV should sustain the pressure level set regardless of variable flow demand and
lung characteristics. In this example the flow pattern fluctuates with each breath,
so the V would also be fluctuating. The PIP is set at 20 cm H O, and is sustained
2
T
(dashed line) by the ventilator. For the pressure level to be sustained, the ventila-
During PCV, the flow
wave patterns may fluctuate tor would have to vary the flow to maintain the pressure set. Historically, pressure
based on the patient’s respira- control has always offered assisted breaths, but it has not offered such flexibility in
tory requirement. The V T is
dependent on the pressure flow delivery, and such variable flow demands would likely cause dyssynchrony and
level, patient effort, and lung/
airway characteristics. necessitate an increase in the pressure level set to meet the patient’s flow and vol-
ume demands. Pressure support ventilation, discussed later, demonstrates a greater
capacity for accommodating variable flow demands while maintaining the pressure
set on the ventilator.
Inverse Ratio Pressure-Controlled
inverse ratio pressure-
controlled ventilation (IRPCV): Ventilation (IRPCV)
A mode of ventilation in which the
peak inspiratory pressure is preset
and the I time is longer than the Figure 11-21 demonstrates inverse ratio pressure-controlled ventilation
E time.
(IRPCV) set at 20 cm H O with 10 cm H O of PEEP for a combined pressure (PIP)
2
2

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