Neuromuscular response to resistive unloading: helium vs. bronchodilation.

Abstract

To evaluate the neuromuscular response to resistive unloading, we compared the ventilatory and occlusion pressure (P100) response of normal subjects breathing 20.9% O2 in helium (He-O2) with their response to unloading produced by inhaled atropine sulfate. During He-O2 breathing airway resistance (Raw) fell by 49% of the base-line value on air, and P100 decreased by 20.8%. Minute ventilation, tidal volume, respiratory frequency, end-tidal Pco2, inspiratory and expiratory duration, and mean inspiratory flow were not significantly different when air was replaced by He-O2. In contrast, although atropine reduced Raw by an equivalent amount, there was no change in P100. Atropine had no significant effect on other respiratory variables, although a trend toward higher minute ventilation was noted. Fowler dead space increased after atropine but was not affected by He-O2. We conclude that, unlike He-O2 unloading, atropine unloading does not cause a reduction in occlusion pressure. This may be due to the effect of atropine on anatomical dead space which stimulates ventilation sufficiently to offset the fall in neuromuscular output due to resistive unloading.