Page 397 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 363

800

1 2
600

Volume (mL) 400 Point of Upper Inflection

200 P ALV
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10 20 30 40 50 60
Pressure (cm H O)
2
Figure 11-42 The point of upper inflection (Ipu) shows a reducing lung-thorax compliance
due to hyperdistention of the alveoli. The tidal volume may be reduced until the Ipu (or duckbill)
disappears.

Upper Inflection Point on PVL and Adjustment of V T

Figure 11-42 presents a PVL where an upper inflection point (Ipu) existed accord-
(Figure 11-42) In the ing to early research on this subject. In this example, the P has been plotted
presence of Ipu on the ALV
pressure-volume loop, the as described for Figure 11-37. C changed later during V delivery because of
LT
T
tidal volume should be re- hyperdistention of the alveoli. The reduction in C late in the inspiratory cycle was
duced until the Ipu (duckbill) LT
disappears. called the Ipu (Dambrosio et al., 1997). In this example, slope 1 for P ALV (dashed
line) is normal, and slope 2 (dashed line) shows a decrease in C and the point of
LT
upper inflection. The appearance of the upper shape of the P curve indicating the
AO
presence of an Ipu was known as the duckbill PVL. When a duckbill occurred, the
(Figure 11-43) On a flow- V could be reduced until the duckbill vanished (Roupe et al., 1995). Presently, like
volume loop, the inspiratory T
flow is above the horizontal the Ipi, research is not supporting clear or accurate indications of the Ipu, and it,
axis, whereas the expiratory too, is presented only as a historical reference to issues related to ventilator-patient
flow is below.
management of ARDS patients that are in contention and need further study.
Effects of Airway Status on Flow-Volume
(Figure 11-43) Positive
response to bronchodilator Loop (FVL)
therapy improves the expira-
tory flow as airflow resistance
is reduced. Inspiratory flow Figure 11-43 shows another waveform option, the flow-volume loop (FVL). FVLs
is unchanged because it is
determined by the settings on show flow measurements on the y-axis, and volume measurement on the x-axis. In-
the ventilator. spiratory flow is above the x-axis and expiratory flow is below. This example shows
two superimposed waveforms during constant flow ventilation at 40 L/min and
results of pre- and postbronchodilator therapy. Following bronchodilator therapy,
airflow resistance is typically reduced (Garner, 2002).

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