Page 602 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

422 PART 4: Pulmonary Disorders

30

20

10

0
.5
FIGURE 48-26. Wedge pressure (Ppw) tracing in this ventilated patient who was making strong respiratory efforts. Since the Ppw varies from 20 to −5 cm H O, reading the tracing properly
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(at end-expiration) is vitally important. If this patient were passive, one would expect the end-expiratory pressure to be roughly −5 cm H O, but then the I:E ratio would be 2:1 (inverse ratio
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ventilation), the fall in pressure during (presumed) expiration is too abrupt, and the (presumed) inspiratory pressure should continue to rise (rather than plateauing) throughout the breath.
Instead, this ventilated patient is quite active and pulls the Ppw down during inspiration. The end-expiratory pressure is 15 cm H O.
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discussed in Figure 48-26. Additional confidence can be gained at the the hemodynamic waveform. End-expiration is readily identified on
bedside by examining the patient and ventilator while simultaneously the ventilator pressure waveform (Fig. 48-27). One then moves 200 ms
displaying the hemodynamic tracing in question. Perhaps the simplest earlier on the timescale (since patient effort begins before machine
and most accurate approach, however, is to connect the ventilator cir- inspiration, especially in the patient with autoPEEP, as discussed above)
cuit to a pressure transducer and display this on the same timescale as and measures the hemodynamic pressure there.

40
10 mmHg/cm
35
1 sec 30
25
20
CVP
10
5
0
−5
−10
10 mmHg/cm 35
30
25
20
PRS
10
5
0
200 msec
−5

FIGURE 48-27. The least equivocal method for identifying end-expiration on a vascular waveform is to display simultaneously the vascular pressure (here the CVP; top waveform) and the
Pao (PRS; bottom waveform). A point 200 ms before Pao rises (signaling inspiration) generally precedes the patient’s inspiratory effort and is a good marker for end-expiration. Because the CVP
varies during respiration in this patient, selecting end-expiration reproducibly is essential to getting reliable pressures. Here the CVP is 10 cm H O.
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