Increased respiratory work as a determinant of weaning outcome

A number of investigators have examined the utility of respiratory work measurements in predicting the outcome of a trial of weaning from mechanical ventilation.

Fiastro.efa/ (1988) measured the work of spontaneous breathing in patients who were successfully weaned from mechanical ventilation and in patients who were ventilator dependent. Only the work being performed on the lungs was assessed. All the ventilator-dependent patients had work per minute values above 15.7 J/min and four (66 per cent) had work per liter values above 1.37 J/l (the remaining two patients each had work values of 1.08 J/l). Importantly, neither index alone (work per minute, work per liter) was as good at predicting weaning outcome as the combination of the two indices.

H.e.n.nin.a M..aL (1.9.7.7,), reported that patients remained ventilator dependent when the work rate was greater than 16.66 J/min, whereas spontaneous ventilation could usually be sustained if the work rate was 9.80 J/min or less. Again, these investigators measured work performed on the lungs only. P^0cto^.aQd..W0.0.!.§.0Q.,.í19Z3) found that a work rate of 13.3 J/min provided the best separation of ventilator-dependent and ventilator-independent postoperative patients (657 measurements in 168 patients). However, this threshold value was associated with false-positive and false-negative rates of 1! per cent.

In conclusion, work of breathing is higher in ventilator-dependent than ventilator-independent patients, but the predictive value of work measurements as an index of weaning has not been prospectively determined. Furthermore, accurate measurements of work of breathing are difficult to perform, and there are several limitations to their calculation and interpretation (T.o,b.i.n,..,a.Qd...yaQ.,d.e Graaff.. . 199.!). Whether work of breathing measurements provide information which cannot be obtained by simpler predictors of weaning outcome, such as the degree of rapid shallow breathing, is unclear. Therefore the ability of work of breathing measurements to enhance clinical decision-making needs to be examined in a prospective fashion.

Chapter References

Fiastro, J.F., Habib, M.P., Shon, B.Y., and Campbell, S.C. (1988). Comparison of standard weaning parameters and the mechanical work of breathing in mechanically ventilated patients. Chest, 94, 232-8.

Henning, R.J., Shubin, H., and Weil, M.H. (1977). The measurement of the work of breathing for the clinical assessment of ventilator dependence. Critical Care Medicine, 5, 264-8. Marini, J.J., Capps, J.S., and Culver, B.H. (1985). The inspiratory work of breathing during assisted mechanical ventilation. Chest, 87, 612-18.

Proctor, H.J. and Woolson, R. (1973). Prediction of respiratory muscle fatigue by measurements of the work of breathing. Surgery, Gynecology and Obstetrics, 136, 367-70.

Tobin, M.J. and Van de Graaff, W.B. (1994). Monitoring of lung mechanics and work of breathing. In Principles and practice of mechanical ventilation (ed. M.J. Tobin), pp. 967-1003. McGraw-Hill, New

York.

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