Diaphragmatic energy expenditure in chronic respiratory failure: The effect of assisted ventilation with body respirators

Abstract

Patients with chronic respiratory failure secondary to disorders which increase the work of breathing often benefit from assisted ventilation with body respirators. To test the hypothesis that body respirator therapy works by relieving the respiratory muscles of an excessive load, we recorded diaphragmatic and/or accessory inspiratory muscle electrical activity before and during assisted ventilation in 11 patients. All patients had chronic hypercapnia; seven had advanced obstructive pulmonary disease and four had severe thoracic cage restriction. When these patients used the tank respirator, their diaphragmatic activity fell to 9 ± SD 3 per cent of the level measured during spontaneous breathing. The reduction was highly significant in each patient (P < 0.001). Similar reductions of diaphragmatic activity occurred in the cuirass respirator. The body respirators also reduced accessory muscle activity (P < 0.02). Coincident with loss of diaphragmatic activity, the patients obtained relief from dyspnea. These results were independent of changes in arterial blood carbon dioxide tension (PaC0 2). Seven normal subjects were studied to further investigate the mechanism by which body respirators reduced inspiratory muscle activity. In the tank, these subjects displayed only minor reductions of diaphragmatic activity despite large decrements in end-tidal carbon dioxide pressure (P ETCO 2). However, when their upper airway resistance was increased, diaphragmatic activity promptly ceased in the tank. We conclude that capture of the ventilatory cycle by the body respirator is facilitated by loading the inspiratory muscles. We suggest that the beneficial effects of body respirator therapy for chronic respiratory failure are mediated in part through reduction of excessive inspiratory muscle contractile effort.