Page 393 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 359

0.8
V (L)

2 4 6 8 10 12
V (L/m) 60

2 4 6 8 10 12
P (cm H 2 O) 40

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2 4 6 8 10 12
Time (sec)
Figure 11-37 Changes to the volume-, flow-, and pressure-time waveforms demon-
strate the effect of an air leak when PEEP is used. A reduced circuit pressure (due to air leak)
is sufficient to drop the sensitivity level below the PEEP level, causing autotriggering and fast
mechanical frequency.

This may lead to autotriggering and go unnoticed unless respiratory frequencies
reach extremely high levels (.25 to 30/min).

PRESSURE-VOLUME LOOP (PVL) AND FLOW-VOLUME

LOOP (FVL)

Pressure-Volume Loop (PVL)

Another option offered by graphics software is the pressure-volume loop (PVL)
(Figure 11-38) The
double-headed arrow shows presented in Figure 11-38 for an assist breath during constant flow ventilation.
the flow-resistive pressure or The P can be read from the x-axis and volume from the y-axis. The double-
transairway pressure (P TA ). AO
P TA 5 P AO 2 P ALV throughout headed arrow indicates that the flow-resistive pressure (P ) is the difference
TA
the pressure-volume loop. between P and P (dashed line). P remains the same throughout inspira-
[Note: end-inspiratory P AO 5 AO ALV TA
PIP, end-inspiratory P ALV 5 peak tion, as it does for the step ascending ramp pressure waveform once the initial
P ALV or plateau pressure. Refer peak flow is reached and maintained during constant flow ventilation. The PIP
to Figure 11-4 for review.]
of 30 cm H O (dashed line) and peak P ALV of 25 cm H O are labeled. It is as-
2
2
sumed that the C is unchanged in this example since P ALV rises linearly with
LT
increase in volume.

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