Breathing pattern during induced bronchoconstriction

Abstract

The breathing patterns of normal subjects monitored with respiratory inductive plethysmography were investigated after mild increases in respiratory resistance provoked by aerosolized methacholine during natural breathing and while breathing on a mouthpiece to a pneumotachograph. First, during natural breathing, comparisons of inspiratory ventilation (VI), tidal volume (VT), frequency (f), inspiratory time (TI), fractional inspiratory time (TI/TT), and mean inspiratory flow (VT/TI) were made before and after aerosolized buffered saline and methacholine in a dose that reduced specific airway conductance (sGaw) by 35% (PD35). There was a significant increase in VT/TI and VI after methacholine, whereas VT, f, TI, and TI/TT were not consistently modified by saline or methacholine. Pretreatment with bronchodilators prevented changes in respiratory resistance (Rrs) as well as in breathing pattern after PD35 methacholine. On another day, Rrs, end-expiratory lung volume level, and breathing pattern during natural breathing were monitored after administration of predetermined doses of methacholine that reduced sGaw by 25% (PD25), PD35, and 55% (PD55). Increases in VT/TI and end-expiratory lung volume level paralleled the increases in Rrs after each dose of methacholine but not with saline control. VI increased along with Rrs at the PD25 and PD35 doses but plateaued at the PD55 dose while Rrs continued to rise. There were no changes in breathing pattern in subjects who breathed on a mouthpiece to a pneumotachograph after PD55 methacholine. Thus alterations of the breathing pattern due to mild-to-moderate degrees of bronchoconstriction are characterized by progressive rises of mean inspiratory flow (an index of respiratory center drive) and end-expiratory lung volume level (a measure of pulmonary hyperinflation), but VI plateaus at the more severe degree of bronchoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)