Page 370 - Clinical Application of Mechanical Ventilation

336 Chapter 11

of the P ALV is developed in the second half of inspiration (P ALV 5 25% 3 40 cm
H O 5 10 cm H O, or P ALV 5 0.125 L/0.0125 L/cm H O 5 10 cm H O). Thus,
2
2
2
2
any point along the P ALV curve can be predicted and plotted by calculating the
volume delivered per time provided that total C is known. This calculation for
LT
DRFW also applies to CFW.
In the second example (Figure 11-17), peak flow is maintained at 60 L/min,
but V is doubled [1/2(60 L/min]/60s/min 3 2s 5 1 L], which increases T to
T
I
2 sec. As a result of doubling V , peak P ALV is doubled (P ALV 5 1.0 L/0/0125 L/cm
T
H O 5 80 cm H O). Note that the initial flow-resistive pressure is sustained in the
2
2
second pressure waveform since the initial flow has not changed.
CMV during Descending Ramp Flow Ventilation
Figure 11-18 is an example of CMV during descending ramp flow ventilation.
The end-flow during The patient initiates the first two breaths and the ventilator triggers the next two
descending ramp flow ven-
tilation is zero. The alveolar breaths. During descending ramp flow ventilation, the peak alveolar pressure
pressure (P ALV ) or plateau (peak P , or plateau pressure) and C can be calculated without the need
pressure at this point equals ALV LT
the end-inspiratory pressure. for an end-inspiratory pause. This is because at end-inspiration, the flow is zero
End-inspiratory pause is not
needed to obtain the P ALV . (i.e., similar to end-inspiratory pause) and the peak alveolar pressure remains in
a plateau state.
Figure 11-18 shows descending ramp flow ventilation with a peak flow of 60 L/min
or 1 L/sec. For an inspiratory time of 0.5 sec, the tidal volume is 500 mL (1 L/sec 3
0.5 sec). With an end-flow pressure of 35 cm H O, the calculated C is 14.3 mL/cm
LT
2
H O (500 mL/35 cm H O).
2
2
120 1 sec ,3 sec 1 sec 3 sec 1 sec 3 sec 1 sec
E
I
I
E
I
I
E
V (L/min)
2 4 6 8 10 12 14

260
P (cm H 2 O)

40
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2 4 6 8 10 12 14
Time (sec)
Figure 11-18 Assist (first and second waveforms) and mandatory (third and fourth wave-
forms) mechanical ventilation during descending ramp flow ventilation.

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