Software compensation for lung volume in assessment of inspiratory muscle strength and endurance.

BACKGROUND To increase the sensitivity of measurements of maximal inspiratory pressure (MIP) as a test of inspiratory muscle strength and endurance, software was developed to correct for variation in lung volume. METHODS Using a body plethysmograph to determine absolute lung volume during each manoeuvre, values for MIP were expressed as a percentage of the pressure "expected" from the unfatigued MIP/lung volume relation in each subject. RESULTS The method reduced the variance in peak and average pressure during a series of 18 MIP manoeuvres of 10 seconds duration separated by rest intervals of 10 seconds. CONCLUSIONS The correlation between average pressure and contraction number was improved significantly by the MIP/lung volume correction. This simple correction has many applications in measurements of the MIP.