REDUCED VOLUNTARY DRIVE TO BREATHE IN ASTHMATIC SUBJECTS

Maximal voluntary drive to the diaphragm and a non-respiratory muscle group (elbow flexors) was compared in 10 control subjects arid 11 asth matics who were studied when well. The degree of voluntary activation during repeated attempted maximal quasi-static efforts was determined using the twitch interpolation technique in the absence of contractile fatigue under both control conditions and following bronchial challen ge with histamine. Diaphragm activation was assessed using bilateral p hrenic stimulation at the normal resting end-expiratory lung volume af ter exhalation from TLC. Asthmatic subjects showed lower and more vari able voluntary activation than control subjects for both diaphragm (82 .0% +/- 18.4 [SD], vs 87.8% +/- 12.0, P<0.01) and elbow flexors (91.3% +/- 7.6 vs 95.8% +/- 4.1, P<0.01). Histamine challenge decreased FEV, in asthmatic subjects to 50% of the initial value, but had no signifi cant effect on voluntary activation in either subject group. The decre ased voluntary drive to the diaphragm observed in some asthmatic subje cts may predispose to rapid development of ventilatory failure during severe airway narrowing.