Hypoxemia after coronary bypass surgery modeled by resistance to oxygen diffusion

OBJECTIVE: To evaluate a model describing postoperative hypoxemia after cardiac surgery by using two variables, i.e., shunt and resistance to oxygen diffusion (Rdff). DESIGN: Estimation of these two variables in normal subjects and postoperative cardiac patients. SETTING: The pulmonary function laboratory for the normal subjects and the intensive care unit for the cardiac patients. PATIENTS/SUBJECTS: Nine postoperative cardiac patients and six healthy subjects. INTERVENTIONS: Inspired oxygen fraction was varied in normal subjects and in cardiac patients 3-6 hrs after surgery. This variation occurred in four to seven steps to achieve arterial oxygen saturations in the range 0.90-1.00. MEASUREMENTS AND MAIN RESULTS: Measurements were taken of arterial oxygen saturation, cardiac output, ventilation, and end-tidal gases at each inspired oxygen fraction. These measurements gave the following estimates for the normal subjects: shunt = 3.9+/-5.4% (mean +/- SD) and Rdiff = -5+/-16 torr/(L/min) [-0.7+/-2.2 kPa/(L/min)]; for the cardiac patients: shunt = 7.7+/-1.8% and Rdiff = 212+/-230 torr/(L/min) [28.2+/-30.6 kPa/(L/min)]. The increase in Rdiff (P = .01) was sufficient to explain the observed hypoxemia in these patients. The value for shunt was not significantly increased in the patients (p = .09). The two-variable model (shunt and Rdff) gave a better prediction of arterial oxygen saturation than a model with shunt as the only variable (p = .02). CONCLUSIONS: In cardiac patients requiring supplementary oxygen, the respiratory abnormality could, in our model, be best described by an increased Rdiff, not by an increased shunt value.