Page 361 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 327

120

V (L/min) 1 Area 1 = Area 2 2

1 2 3 4 5 6 7

P (cm H 2 O)

80
PIP
PIP P TA 40 cm H O
P TA 2
20 cm H O
2
P ALV P ALV
1 2 3 4 5 6 7 © Cengage Learning 2014
Time (sec)
Figure 11-12 Flow- and pressure-time waveforms showing the effects of lung-thorax com-
pliance (C LT ) on P TA , P ALV , and expiratory flow. Decrease of C LT causes an increase of P ALV (plateau
pressure) and peak expiratory flow, but no changes in P TA .

exponential rise in circuit pressure with increments in flow. Conversely, an expo-
nential decay in pressure is observed when flow is reduced (Dennsion et al., 1989).
Thus, the area depicting the P gradient (P 2 P ALV ) has been more than doubled
AO
TA
in the second pressure waveform. Note, however, that the remarkable change in P
AO
and PIP did not affect the peak P ALV because neither the C nor the lung volume
LT
was changed. Only the rise time to peak P ALV was affected because T was reduced
I
by half. In this example, resistance was increased by an increase in flow. An increase
in airway resistance from various lung pathologies causing bronchoconstriction and
obstruction will also increase the area depicting the P and P gradient without a
TA
AO
change in flow, but P ALV will not be affected. Airway obstruction and bronchocon-
striction will affect waveforms in other ways, which will be explained during the
discussion of using expiratory waveforms as a diagnostic tool, later in this chapter.

Compliance and Alveolar Pressure

(Figure 11-12) Decrease Figure 11-12 demonstrates a similar comparison between flow and pressure wave-
in lung-thorax compliance
causes an increase in P ALV and forms. PIP has substantially increased in the second example as it did in Figure 11-11.
PIP. Transairway pressure (P TA ) Given no change in flow or V to explain the increase in PIP, an end-inspiratory pause
is not affected by changes in T
compliance. needs to be set to create the peak alveolar pressure (peak P ALV ), or plateau pressure for
analysis. Since P remains the same for each waveform, neither the flow nor the
TA

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