Estimation of inspiratory muscle strength in mechanically ventilated patients: The measurement of maximal inspiratory pressure

We examined the time course of prossure development after temporary occlusion of the airway to develop guidelines for estimating inspiratory muscle strength in mechanically ventilated patients. Twenty critically ill patients were tested by two methods, one involving total airway occlusion at end-exhalation (Method I) and the other involving use of a unidirectional expiratory valve to selectively permit exhalation while inspiration was blocked (Method II). With either technique, maximally negative pressures (MIP) were not achieved until approximately ten efforts or 20 seconds had elapsed postocclusion. Maximal pressures achieved by Method II almost invariably exceeded those achieved by Method I (mean Δ = 34%, P < .001), presumably because lower lung volumes improved the performance characteristics of the inspiratory muscles and enhanced ventilatory drive. No systematic difference emerged for either method between those patients able to cooperate with the maneuver and those unable to do so. We conclude that the outcome of the MIP procedure is influenced predictably by the method and duration of occlusion during critical illness. Attention to these technical considerations should help to improve the reliability of the MIP as an index of inspiratory muscle strength in this setting.