Maximum Inspiratory Pressure

Measurement of maximum inspiratory pressure (Pi,max) has long been used in the clinical setting to assess inspiratory mus cle strength (see Section 2 of this Statement). The maneuver consists of a maximum inspiratory effort against a closed airway and requires a considerable degree of patient cooperation and coordination (60). Although ventilator-dependent patients may display poor cooperation in the execution of voluntary maneuvers, even in these patients values of Pi,max have been used to predict successful weaning (61). Marini and coworkers (62) suggested an approach for the standardization of the measurement of Pi,max in such patients. They used a unidirectional valve to permit exhalation while inhalation was blocked, thereby allowing patients to perform the maximal inspiratory effort at a lung volume approaching residual volume, where Pi,max is expected to be maximal; the highest Pi,max values were generally reached after 15-20 efforts or after 15-20 seconds of airway occlusion (62). However, subsequent work has shown that even employing such a standardized approach, the reproducibility of Pi,max values in ventilator-dependent patients is poor. "True" Pi,max in ICU patients is often significantly underestimated, being both patient- and investigator-dependent; even highly reproducible Pi,max measurements at any one sitting do not reliably reflect maximal efforts (63).

Although a high value of Pi,max, together with other measurements, may indicate that mechanical ventilation can be discontinued, a low Pi,max value may reflect a submaximal effort due to poor patient coordination and cooperation. In addition, the problem of insufficient reproducibility may limit the clinical usefulness of this test, at least when used in isolation.

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