Page 389 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 355

10 cm H O), and as a result the peak expiratory flow pattern develops more
2
slowly and is rounded, and T is prolonged as demonstrated by the expiratory
E
flow and pressure patterns.
Decreased Lung-Thorax Compliance (C )
LT
When airway resistance remains constant, a decreased C (e.g., ARDS) will
LT
(Figure 11-34) The solid increase the peak expiratory flow rate and shorten the expiratory time. In
expiratory flow curve shows
that in conditions of low C LT , Figure 11-34, the normal expiratory flow curve (dashed) is compared to the
the expiratory flow is gener- expiratory flow curve in conditions of low C (solid lines). The expiratory flow
ally higher and expiratory LT
time is shorter. in conditions of low C is generally higher and expiratory time is shorter (solid
LT
line). A shorter expiratory time correlates with faster lung-emptying in condi-
tions of low C .
LT
In air trapping, the The pressure waveform (Figure 11-34) shows that the PIP (solid line) would
expired volume is less than
the inspired tidal volume increase as C decreased. Note that since the flow rate is the same and air-
LT
for one or more breaths. The way resistance is unaffected, the initial flow-resistive pressure (arrow) has not
trapped air comes out with a
subsequent mandatory breath changed. Thus, the increase in PIP must be the result of a rise in peak P ALV
resulting in a much larger (plateau pressure). The expiratory portion of the pressure waveform (solid line)
expired tidal volume.
generated in the circuit on average is higher under conditions of low C and
LT

80
V (L/min)

P (cm H 2 O) 280

40

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Time (sec)
Figure 11-34 The effects of normal compliance (dotted lines) and decreased compliance
(solid lines) on the expiratory flow and pressure-time waveforms. When the compliance is de-
creased, a higher expiratory flow, a shorter expiratory time, and a higher peak inspiratory pressure
are noted.

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