Measurement of Maximal Inspiratory Pressure: A Comparison of Three Methods

Abstract

Maximal inspiratory pressure (MIP) is commonly measured by respiratory care practitioners to determine a patient's respiratory-muscle strength and/or ability to maintain unassisted ventilation. We compared the results of three different methods for measuring MIP. METHODS: In Part 1 of the study, we compared MIP values obtained by Methods I and II in 75 patients. Method I used an aneroid manometer connected to a Y-piece, and Method II used a modified Y-piece that included a one-way valve to permit expiration after airway occlusion. In Part 2 of the study, we compared MIP measurements (n = 50) made by Method II and Method III; Method III used the negative inspiratory pressure function of the Puritan-Bennett 7200a ventilator. RESULTS: Part 1-Using Method II resulted in a consistently higher MIP (39 ± 9 cm H₂O) [3.8 ± 0.9 kPa] (mean ± SD) than did using Method I (28 ± 11 cm H₂O) [2.7 ± 1.1 kPa]; the difference was statistically significant (p < 0.027). There were no differences between Methods I and II in number of efforts or time to reach MIP. Part 2-MIP measured by Method III (7200a) was 17 ± 9 cm H₂O [1.7 ± 0.9 kPa], and was consistently less than MIP measured by Method II (39 ± 11 cm H₂O) [3.8 ± 1.1 kPa]; the difference was statistically significant (p < 0.0067). The time to reach MIP after airway occlusion in this study ranged from 12 to 25 seconds (mean, 17 s). CONCLUSIONS: The addition of a one-way valve to the MIP-measurement system results in more accurate estimation of respiratory-muscle strength. The time to reach airway occlusion is at least 15 seconds. MIP measured by the 7200a ventilator grossly underestimates true MIP.