Postoperative hypoxemia in obese patients

Abstract

Although obese patients represent a high risk group and reports document the occurrence of sudden unexplained deaths in the early postoperative period, no data exist to delineate the existence, extent or duration of hypoxemia in obese subjects. Twenty adults undergoing jejunoileal bypass for morbid exogenous obesity, with a mean weight of 144.0 +/- 24.7 kg and a mean age of 34.1 +/- 8.3 years, were studied. While breathing room air, arterial blood was collected from each patient, placed in ice and analyzed within ten minutes. A preoperative sample was taken; then samples were analyzed two hours postoperatively and on postoperative days one through five. Preoperatively in the obese patients there was a significant reduction in arterial oxygen tension, mean 75.1 +/- 11.1 mm Hg, as against 92.4 +/- 4.0 mm Hg in control patients matched for age. There was also a difference in the slope of the regression equation of PaO(2) vs. age; the regression coefficient being significantly greater in the obese group (0.9 vs. 0.46). Postoperatively there was a further fall in PaO(2). Decrease in mean PaO(2) in the obese group in mm Hg averaged -9.2 two hours postoperatively (p < 0.001), -15.0 on day one (p < 0.001), -15.5 on day two (p < 0.001), -11.7 on day three (p < 0.001) and -5.1 on day four (p < 0.01). PaCO(2) was not significantly different from the preoperative control. Base excess was elevated on days two, three and four; 3.1 mEq/ L(p. < .001), 3.6 mEq/L(p < .001) and 1.9 mEq/L(p <.05) respectively. Our findings indicate that in obese patients: 1) Preoperatively the fall in PaO(2) with increasing age is greater than in control subjects; 2) Starting with a low basal PaO(2) during postoperative days one through four, the fall in CaO(2) often reaches dangerous levels; and 3) Elevation of blood buffer on days two, three and four can further depress respiration at a time when the PaO(2) has been shown to be at its lowest levels. This hypoventilation due to metabolic alkalosis can further accentuate the already existent hypoxemia. Because of these findings, we suggest careful monitoring of arterial blood gases in obese patients and the administration of oxygen and appropriate electrolyte solutions postoperatively to reduce the increased morbidity and mortality.