Obesity, respiratory function and breathlessness.

Oxygen consumption dedicated to respiratory work (V̇O2RESP) during quiet breathing is small in normal patients. In the morbidly obese, at high minute ventilations, V̇O2RESP is greater than in normal patients, but V̇O2RESP during quiet breathing in these patients is not known. We postulated that such patients have increased V̇O2RESP at rest which may predispose them to respiratory failure when additional respiratory workloads are imposed. We measured baseline V̇O2 in morbidly obese patients immediately prior to gastric bypass surgery and again after intubation, mechanical ventilation, and paralysis, and compared their change in V̇O2 to nonobese patients scheduled for elective abdominal surgery. Baseline V̇O2 was higher in the obese patients compared with control patients (354.6 versus 221.4 ml/min; p= 0.0001) and the change in V̇O2 from spontaneous breathing to mechanical ventilation was significant in the obese patients (354.6 versus 297.2 ml/min; p=0.0002) but not the control patients (221.4 versus 219.8 ml/min; p=0.86). We conclude that morbidly obese patients dedicate a disproportionately high percentage of total V̇O2 to conduct respiratory work, even during quiet breathing. This relative inefficiency suggests a decreased ventilatory reserve and a predisposition to respiratory failure in the setting of even mild pulmonary or systemic insults. (Am J Respir Crit Care Med 1999;160:883–6)