Maximal inspiratory mouth pressures (PIMAX) in healthy subjects - what is the lower limit of normal?

Background: Maximal inspiratory mouth pressures are suitable for non-invasi ve evaluation of respiratory muscle function. Different studies on PIMAX gi ve predicted normal values and their relation to anthropometric data. Due t o a large inter-subject variation of PIMAX, predicted values, however, maxi mal inspiratory mouth pressures are not suitable to define the individual e xpected normal PIMAX. What is the lower limit of the normal range? Methods: PIMAX has been prospectively measured in a representative sample o f 504 healthy volunteers (248 males and 256 females) between 18 and 82 year s of age with normal lung function. Age, height, weight, body mass index (B MI) and smoking status were recorded and incorporated stepwise in a multipl e regression analysis to determine prediction equations. Lower limits of th e normal range were defined as the fifth percentile of the residuals derive d 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 an d FVC (P < 0.01). The strongest correlation appeared with sex and age (P < 0.001). Smoking status and smoked pack-years were not independent predictor s of inspiratory pressures. Lower limits of normal were 59% for women and 6 0% 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. Pre diction 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.