Decay of inspiratory muscle activity in chronic airway obstruction

Relative decay rate of inspiratory muscle electrical activity (RDRI) in patients with chronic airway obstruction increased with decreasing expiratory time (TE), being faster than in normal subjects for a given TE. Time course of decay was similar in shape to that of normal subjects, whereas persistence time of activity during expiration was about half. Hence, braking action of inspiratory muscles in patients was smaller than in normal subjects. No tonic activity of inspiratory muscles was found in patients, even when frequency was increased and hyperinflation enhanced. Hence tonic activity of inspiratory muscles found by others in asymptomatic asthmatic or normal subjects after histamine inhalation seems elicited by histamine. In normal subjects breathing under resistive load, RDRI became similar to that of patients for a given TE: tonic activity of extradiaphragmatic inspiratory muscles occurred only if frequency was voluntarily increased at least three times, an unphysiological condition with resistive load. Under discontinuous inspiratory elastic load, RDRI of patients decreased or did not change, whereas previously that of normal subjects was found to increase.