Conclusion

This Section of the Statement has reviewed the tests of respiratory muscle function, direct and indirect, that are relevant to the particular environment of the ICU. The scientific basis and methodology of many of these tests are discussed in earlier Sections of the Statement. This article also focuses on the crucial issue of weaning from mechanical ventilation.

Abnormalities of the pattern of breathing are common in ICU patients, especially in those with respiratory muscle dysfunction. Tachypnea is a sensitive marker of deteriorating clinical status, but is not specific. Paradoxical motion of the rib cage and abdomen occurs with elevated respiratory load, but is not diagnostic of respiratory muscle fatigue. Measurements of maximum inspiratory pressure have poor reproducibility in critically ill patients and are of limited use for decision making in the ICU. Airway occlusion pressure (P0.1) is easy to measure in patients receiving assisted ventilation, and high values signal increased respiratory motor output. The most direct measures of patient effort are work of breathing and PTP, but these measurements require considerable attention to detail and their use is confined to research settings. A scooped contour of the airway pressure tracing during volume and flow preset assisted ventilation indicates patient effort. The relative

TABLE 2. LIKELIHOOD OF SUCCESSFUL WEANING RELATED TO

fR/VT RATIO*

TABLE 2. LIKELIHOOD OF SUCCESSFUL WEANING RELATED TO

fR/VT RATIO*

fr/Vt Ratio

Likelihood Ratio

< 80

7.53

80-100

0.77

> 100

0.04

Definition of abbreviations: fr = respiratory frequency; Vt = tidal volume. * Based on data of Yang and Tobin (171).

Definition of abbreviations: fr = respiratory frequency; Vt = tidal volume. * Based on data of Yang and Tobin (171).

swings in esophageal and gastric pressures provides insight into the recruitment of different respiratory muscle groups. Respiratory muscle contractility can be assessed by recording transdiaphragmatic pressure provides in response to phrenic nerve stimulation, a complex and invasive technique. intrinsic PEEPi, common in critically ill patients, can be measured using the EEO pressure in ventilated patients and by an esoph-ageal balloon catheter in patients breathing spontaneously; with both techniques, however, the value can be falsely elevated because of expiratory muscle activity.

The icu setting where tests of respiratory muscle function have greatest application is in the prediction of weaning outcome. Traditional predictors, such as maximum inspiratory pressure, vital capacity, and minute ventilation, are frequently falsely positive and negative. Although patients failing a weaning trial may have an elevated tension-time index and airway occlusion pressure, these tests have not gained popularity in everyday icu practice. The ratio of respiratory frequency to tidal volume is the most reliable simple predictor of weaning outcome.

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