Measurement of maximal inspiratory pressure in ventilated children.

Maximal inspiratory pressure (PIMAX), the maximum negative pressure generated during temporary occlusion of the airway, is commonly used to measure inspiratory muscle strength in mechanically ventilated infants and children. There are, however, no guidelines as to how the PIMAX measurement should be made. We compared the maximum inspiratory pressure generated during airway occlusion (PIMAX(OCC)) to that when a unidirectional valve (PIMAX(UNI)), which allowed expiration, but not inspiration was used. Twenty-two mechanically ventilated children (mean (SD) age 4.8 (4.5) years) were studied. Three sets of end expiratory occlusions were performed for each method in random order. The expired volume during PIMAX(UNI) was assessed and related to the functional residual capacity (FRC) measured using a helium dilution technique.The mean (SD) PIMAX(UNI) (45.5 (15.2) cmH(2)O) was significantly greater than mean (SD) PIMAX(OCC) (30.9 (9.0) cmH(2)O) (P < 0.0001). The mean (SD) expired volume during PIMAX(UNI), was 98 ml (62.3), a mean reduction in FRC of 33.1% (SD 13.9). There were no significant differences between techniques in the baseline respiratory drive, the number of efforts required and the time to reach PIMAX. Regardless of technique, PIMAX was reached in 10 inspiratory efforts or 15 sec of airway occlusion.A unidirectional valve allowing expiration, but not inspiration yields greater PIMAX values in children. Occlusions should be maintained for 12 sec or eight breaths (99% CI of mean).