Page 262 - Clinical Application of Mechanical Ventilation

228 Chapter 8

TABLE 8-12 I Time % and I:E Ratio Equivalent

I Time % I:E Ratio

14.3% 1:6

16.7% 1:5
20% 1:4

25% 1:3
33.3% 1:2

50% 1:1

60% 1.5:1
66.7% 2:1

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phase. The initial peak flow at the very beginning of the inspiratory phase should
help to overcome the airway resistance and parenchymal elastance, and the remain-
ing peak flow throughout the inspiratory phase should enhance gas distribution in
the lungs. Adjustment of the flow pattern may be made after stabilization of the pa-
tient. Note that the constant flow pattern is the only flow pattern in which the peak
flow rate equals the mean flow rate. All other flow patterns will produce a mean flow
rate that is less than the peak flow.
With its increasing flow throughout the respiratory cycle, the accelerating wave-
form may improve the distribution of ventilation in patients with partial airway
obstruction. The decelerating flow pattern typically produces a high initial inspi-
ratory pressure and the decrease in flow may help improve distribution of tidal
volume and gas exchange (Campbell et al., 2002; Waugh et al., 2007). For patients
with COPD, the decelerating flow may reduce the peak inspiratory pressure, mean
airway pressures, physiologic deadspace, and PaCO (Yang et al., 2002). The sine
2

Peak Flow

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Constant Accelerating Decelerating Sine

Figure 8-1 Normal flow tracing of four different flow patterns: constant, accelerating,
decelerating, and sine.

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