Airway resistance measured by the interrupter technique: expiration or inspiration, mean or median?

The measurement of airway resistance by the interrupter technique (Rint) ne eds standardization. Should measurements be made be during the expiratory o r inspiratory phase of tidal breathing? In reported studies, the measuremen t of Rint has been calculated as the median or mean of a small number of va lues, is there an important difference? Subjects were 2.5-5.0 yrs (median 4.0 yrs) who had previous respiratory sym ptoms. The Rint in expiration (RintE) and inspiration (RintI) pre and posts albutamol, the coefficient of variation (CV) of values contributing to meas urements, and bronchodilator responsiveness(BDR) in both phases were compar ed. Measurements using median and mean were compared. RintE was higher than RintI by 4% (p <0.01). The CV of values making up Rin tE and RintI, and BDR measured in expiration and inspiration were similar. The median difference between means and medians of values making up measure ments was 0.6% (range -6-11%). RintE has been shown to be consistently greater then RintI but the differen ce in this study is small. It is suggested that one or the other is chosen as the standard. In the present data the mean of a set of values contributi ng to a measurement was not significantly different from the median. Howeve r, the use of the median has been recommended since it is less affected by possible outlying values such as might be included by fully automated equip ment.