The Impact of Morbid Obesity on Oxygen Cost of Breathing (V˙o 2RESP) at Rest

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Abstract
Oxygen consumption dedicated to respiratory work (V˙ o 2RESP) during quiet breathing is small in normal patients. In the morbidly obese, at high minute ventilations, V˙ o 2RESP is greater than in normal patients, but V˙ o 2RESP during quiet breathing in these patients is not known. We postulated that such patients have increased V˙ o 2RESP at rest which may predispose them to respiratory failure when additional respiratory workloads are imposed. We measured baseline V˙ o 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˙ o 2 to nonobese patients scheduled for elective abdominal surgery. Baseline V˙ o 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˙ o 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˙ o 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.