The impact of morbid obesity on oxygen cost of breathing (Vo2RESP) at rest

Oxygen consumption dedicated to respiratory work ((V)over dotO(2RESP)) during quiet breathing is small in normal patients. In the morbidly obese, at high minute ventilations, (V)over dotO(2RESP) is greater than in normal patients, but (V)over dotO(2RESP) during quiet breathing in these patients is not known. We postulated that such patients have increased (V)over dotO(2RESP) at rest which may predispose them to respiratory failure when additional respiratory workloads are imposed. We measured baseline (V)over dotO(2) in morbidly obese patients immediately prior to gastric bypass surgery and again after intubation, mechanical ventilation, and paralysis, and compared their change in (V)over dotO(2) to nonobese patients scheduled for elective abdominal surgery. Baseline (V)over dotO(2) was higher in the obese patients compared with control patients (354.6 versus 221.4 ml/min; p = 0.0001) and the change in (V)over dotO(2), 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 (V)over dotO(2) 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.