Hypoxemia after coronary bypass surgery modeled by resistance to oxygen diffusion

Objective: To evaluate a model describing postoperative hypoxemia after car diac surgery by using two variables, i.e., shunt and resistance to oxygen d iffusion (R-diff). Design: Estimation of these two variables in normal subjects and postoperat ive 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 subj ects. Interventions: Inspired oxygen fraction was varied in normal subjects and i n cardiac patients 3-6 hrs after surgery. This variation occurred in four t o seven steps to achieve arterial oxygen saturations in the range 0.90-1.00 . Measurements and Main Results: Measurements were taken of arterial oxygen s aturation, cardiac output, ventilation, and endtidal gases at each inspired oxygen fraction, These measurements gave the following estimates for the n ormal subjects: shunt = 3.9 +/- 5.4% (mean +/- sD) and R-diff -5 +/- 16 tor r/(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 h ypoxemia in these patients, The value for shunt was not significantly incre ased in the patients (p = .09), The two-variable model (shunt and R-diff) g ave a better prediction of arterial oxygen saturation than a model with shu nt as the only variable (p = .02). Conclusions: In cardiac patients requiring supplementary oxygen, the respir atory abnormality could, in our model, be best described by an increased R- diff, not by an increased shunt value.