Agreement between two methods for assessment of maximal inspiratory pressure in patients weaning from mechanical ventilation

BACKGROUND: Respiratory muscle strength in patients with an artificial airway is commonly assessed as the maximal inspiratory pressure (MIP) and is measured using analogue or digital manometers. Recently, new electronic loading devices have been proposed to measure respiratory muscle strength. This study evaluates the agreement between the MIPs measured by a digital manometer and those according to an electronic loading device in patients being weaned from mechanical ventilation. METHODS: In this prospective study, the standard MIP was obtained using a protocol adapted from Marini, in which repetitive inspiratory efforts were performed against an occluded airway with a one-way valve and were recorded with a digital manometer for 40 seconds (MIP(DM)). The MIP measured using the electronic loading device (MIP(ELD)) was obtained from repetitively tapered flow resistive inspirations sustained for at least 2 seconds during a 40-second test. The agreement between the results was verified by a Bland-Altman analysis. RESULTS: A total of 39 subjects (17 men, 55.4±17.7 years) was enrolled. Although a strong correlation between MIP(DM) and MIP(ELD) (R=0.73, P<0.001) was observed, the Bland-Altman analysis showed a high bias of –47.4 (standard deviation, 22.3 cm H(2)O; 95% confidence interval, –54.7 to –40.2 cm H(2)O). CONCLUSIONS: The protocol of repetitively tapering flow resistive inspirations to measure the MIP with the electronic loading device is not in agreement with the standard protocol using one-way valve inspiratory occlusion when applied in poorly cooperative patients being weaned from mechanical ventilation.