The MW/FEV, Ratio in Normal and Asthmatic Subjects

6 Oct

The MW/FEV, Ratio in Normal and Asthmatic SubjectsAsthmatic subjects and normal individuals differ in their response to a variety of bronchial challenges as well as to a deep breath. We hypothesized that a short hyperventilation test, quantitated as the ratio of a 12-second maximal voluntary ventilation/forced expiratory volume in one second (MW/ FEY^), also would differ between normal and asthmatic subjects. This speculation was based on previously unpublished observations in our laboratories and also on the following rationale: (1) In contrast to normal subjects, a deep inspiration (DI) commonly causes bronchoconstriction in asthmatics; this effect appears to correlate with bronchial sensitivity to methacho-line.

In one study, expiratory flow rates progressively decreased following sequential DIs. In another study, expiratory flow decreased after performance of a 15-second MW maneuver. (2) The bronchomotor effects of DI are dependent on the inspiratory flow rate. In asthmatic subjects with spontaneous bronchospasm, a rapid DI caused a greater increase in respiratory resistance (Rrs) than did a slow deep inspiration. Because differences between the effects of fast and slow DI on bronchomotor tone were attenuated by topically anesthetizing the airways with lidocaine, it was suggested that the bronchoconstrictor effects of DI were partially mediated via irritant receptors in the airways. Other studies also have suggested that mechanical or chemical stimulation of airway receptors causes vagally mediated reflex bronchoconstriction. Based on the above observations, we postulated that a series of rapid deep breaths (performed during the MW maneuver) might have greater effects on bronchomotor tone (in asthmatic subjects) than would the DI which precedes the FEVj maneuver. If so, MW would decrease relative to FEVp This sequence of events would not be anticipated in normal individuals since airways tone generally decreases after single or multiple deep inspirations in normal subjects. In this study, we prospectively compared the MW/ FEV, ratio in normal and asthmatic subjects as well as specific airway conductance (SGaw) before and after a single deep inspiration (SGawDI) and an MW maneuver (SGawMW). We also evaluated methacholine responsiveness and found that the MW/FEVj ratio correlated with the results of methacholine broncho-provocation testing.