Page 388 - Clinical Application of Mechanical Ventilation

354 Chapter 11

from auto-PEEP, which will be readily apparent when graphics are available. A patient
may have to be sedated and paralyzed temporarily to be mechanically controlled at low
frequencies (6 to 8/min) and low V (e.g., 4 to 7 mL/kg IBW) to prevent severe auto-
T
PEEP and lung damage (ARDSNet, 2000).
Loss of Elastic Recoil

Figure 11-33 shows the waveforms for a patient with emphysema (solid lines of
(Figure 11-33) The solid expiratory flow) compared to one with normal lung functions (dashed lines).
lines show that in conditions
of high compliance or loss of The expiratory flow wave is similar to the example in Figure 11-32 for the
elastic recoil (e.g., emphy- asthmatic/bronchitic patient, but with noted changes. First, the spiked peak
sema), the peak expiratory
flow is decreased (loss of the expiratory flow is not present because ventilating pressures for emphysema pa-
spiked peak). The PIP is
also decreased because less tients are typically low where PIP is 25 cm H O (solid-line pressure wave) for a
2
pressure is needed to ventilate substantially high V of 1.0 L being delivered. Also, lung tissue recoil pressure
lungs with high compliance. T
(peak P ALV ) and the consequent expiratory flow driving pressure is reduced
(approximately 15 cm H O in this example since initial P is approximately
2
TA

80
V (L/min)

1 2 3 4 5 6 7

P (cm H 2 O) 280

40

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1 2 3 4 5 6 7
Time (sec)
Figure 11-33 The effects of normal elastic recoil (dotted lines) and loss of elastic recoil (solid
lines) on the expiratory flow and pressure-time waveforms. When the elastic recoil is decreased
(i.e., low elastance or high compliance), a lower expiratory flow and a lower peak inspiratory pres-
sure are noted.

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