COMPARISON OF 4 METHODS OF ASSESSING AIR-FLOW RESISTANCE BEFORE AND AFTER INDUCED AIRWAY NARROWING IN NORMAL SUBJECTS

Four methods for assessing airflow resistance were compared in seven n ormal adults at baseline and after inducing airway narrowing with inha led methacholine. Airway resistance (Raw) was measured during panting at 1-2 Hz within a body plethysmograph; total lung resistance was meas ured by using an esophageal balloon during quiet breathing (RLq) and w ith doubling of frequency while maintaining the original tidal volume; total respiratory resistance (Rrs) was measured at 6 Hz during forced oscillation applied at the airway opening, and interruption resistanc e (Rint) was measured at mid-tidal expiratory flow. Three methods of o btaining Pint after airflow interruption were compared [smooth curve f it of mouth pressure (Pm) back extrapolated to valve closure; two-poin t linear fit of Pm back extrapolated to 15 ms after closure; and Pm at 100 ms after valve closure]. We found similar basal median values (cm H(2)O . 1(-1). s) of Raw (1.3), RLq (1.4), RL of double resting freque ncy (1.9), Rrs (1.7), and smooth curve fit of Pm back extrapolated to valve closure (1.5); basal values of two-point linear fit of Pm back e xtrapolated to 15 ms after closure (2.4) and Pm at 100 ms after valve closure (4.4) were considerably larger. After induced airway narrowing , all methods of measuring resistance showed significant increases; th ese were largest with RLq (median %change of 265) and smallest with th e three Rint methods (median %change of 62-72). Rint and Rrs methods h ad poorer sensitivity for detecting bronchoconstriction than lung resi stance or Raw. Of the Rint methods, end interruption pressure was the most sensitive. We conclude that the convenience of measuring Rrs and Rint is associated with a relative lack of sensitivity in detecting in duced airway narrowing.