Background: Maximal inspiratory mouth pressures are suitable for non-invasive evaluation of respiratory muscle function. Different studies on PIMAX give predicted normal values and their relation to anthropometric data. Due to a large inter-subject variation of PIMAX, predicted values, however, maximal inspiratory mouth pressures are not suitable to define the individual expected normal PIMAX. What is the lower limit of the normal range?
Methods: PIMAX has been prospectively measured in a representative sample of 504 healthy volunteers (248 males and 256 females) between 18 and 82 years of age with normal lung function. Age, height, weight, body mass index (BMI) and smoking status were recorded and incorporated stepwise in a multiple regression analysis to determine prediction equations. Lower limits of the normal range were defined as the fifth percentile of the residuals derived from the regression model.
Results: Mean values of PIMAX were 9.95 kPa for men and 7.43 kPa for women. Significant correlations were found with height, weight, BMI, FEV1, PEF and FVC (P<0.01). The strongest correlation appeared with sex and age (P<0.001). Smoking status and smoked pack-years were not independent predictors of inspiratory pressures. Lower limits of normal were 59% for women and 60% for men of the predicted PIMAX.
Conclusions: In the interpretation of maximal inspiratory mouth pressures, normal values should represent the lower limit of the normal range derived from the regression model in order to avoid false pathological results. Prediction equations as well as lower limits of normal resulting from a study cohort of healthy 18-82-year-olds are given and are recommended to be used by pulmonary function laboratories in young and old patients.