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CHAPTER 48: Ventilator Waveforms: Clinical Interpretation 421

ml Volume-pressure cm H O smaller tidal volume or less PEEP. In a small patient study, adjusting
2
500 the ventilator according to the stress index reduced overdistention and
measures of lung inflammation when compared with ARDSnet strategy-
guided ventilation. 25
There are problems with using VP curves or the stress index to guide
ventilator management in patients with ALI and ARDS. First, these
methods depend on a passive patient. Second, the presence of an LIP
(or stress index <1) may not correlate with recruitment and derecruit-
ment. Most importantly, these approaches have not been shown to
26
improve meaningful outcomes, despite the theoretical elegance.
VENTILATOR WAVEFORMS
AND HEMODYNAMIC INTERPRETATION

Respiratory muscle activity greatly affects intrathoracic pressure, which
40 alters measured hemodynamic values. By convention, hemodynamic
values such as Pra and Ppw are measured at end-expiration since the
FIGURE 48-24. Several volume pressure loops are superimposed while the inspiratory respiratory muscles are most likely to be passive at end-expiration. It can
flow rate is reduced from 60 L/min (largest loop) to 45 to 30 L/min and finally to 12 L/min. be quite difficult to determine the point of end-expiration from a hemo-
Notice that what appears to be a LIP moves leftward and becomes progressively less evident dynamic tracing, mostly because of respiratory activity (Fig. 48-26).
as flow is reduced, showing that this is not a LIP but rather and artifact of the changing flow This can lead to incorrect measurement of important pressures, perhaps
early in the breath. prompting incorrect treatments. Further, dynamic fluid-responsiveness
predictors such as pulse pressure-, stroke volume-, or inferior vena caval
diameter-variation depend on the pleural pressure changes expected in
It is technically simpler to judge the stress index, a measure of the lin- passively ventilated patients. When patients are breathing actively, these
earity of the pressure-time waveform during inspiration (Fig. 48-25). predictors are generally less accurate or even misleading. In the mod-
24
The stress index relies on two assumptions: that flow is constant during ern era of low tidal volume ventilation, reduced reliance on sedatives,
inspiration (and this is guaranteed by the ventilator) and that inspiratory and sparing use of therapeutic paralysis, effort is more the rule than
resistance does not change during tidal ventilation (and this is largely the exception.
true). In this case, airway pressure should rise quite linearly as long as End-expiration can often be detected in the hemodynamic waveforms
respiratory system compliance does not change. Deviations from linearity by paying attention to inspiratory to expiratory ratios, the nature of
imply that compliance is increasing (stress index <1), suggesting tidal the respiratory rise in pressure (which differs between the ventilator-
recruitment and a need for more PEEP, or that compliance is decreas- induced rise in the passive patient and the spontaneous expiratory rise
ing (stress index >1), suggesting lung overdistention and a need for a in the active patient), and the abruptness of the falls in pressure, as

Flow-time

Pressure-time

SI < 1 SI = 1 SI > 1

FIGURE 48-25. During mechanical ventilation of these passive patients with constant inspiratory flow, pressure should rise linearly (after the initial flow-related rise) as seen in the middle
panel, indicating a stress index (SI) of 1 (linear rise). In contrast, the first panel shows a pressure rise that is convex upward (SI <1). Notice the departure from linearity, especially early in the
breath at a time when tidal recruitment might be expected. In the third panel, pressure rises linearly for the initial portion of the breath, but then rises more than expected later, becoming
concave upward (SI >1). This departure from linearity shows that respiratory system compliance is falling late in the breath, possibly signaling overdistention.

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