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 (R-diff). 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 endtidal gases at each inspired oxygen fraction, These measurements gave the following estimates for the normal subjects: shunt = 3.9 +/- 5.4% (mean +/- sD) and R-diff -5 +/- 16 torr/(L/min) [-0.7 +/- 2,2 kPa/(L/min)]; for the cardiac patients: shunt = 7.7 +/- 1.8% and R-diff = 212 +/- 230 torr/(Vmin) [28.2 +/- 30.6 kPa/(L/ min)]. The increase in R-diff (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 R-diff) 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 R-diff, not by an increased shunt value.