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).


METHODS
Estimation of these two variables in normal subjects and postoperative cardiac patients.


METHODS
The pulmonary function laboratory for the normal subjects and the intensive care unit for the cardiac patients.


METHODS
Nine postoperative cardiac patients and six healthy subjects.


METHODS
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.


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.
Andreassen
S
S
Department of Medical Informatics and Image Analysis, Aalborg University, Denmark. sa@miba.auc.dk
Rees
S E
SE
Kjaergaard
S
S
Thorgaard
P
P
Winter
S M
SM
Morgan
C J
CJ
Alstrup
P
P
Toft
E
E
D000328:Adult; D001026:Coronary Artery Bypass / Q000009:adverse effects*; D003325:Coronary Care Units; D005260:Female; D006801:Humans; D000860:Hypoxia / Q000097:blood* / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D010092:Oximetry; D010100:Oxygen / Q000097:blood*; D010102:Oxygen Inhalation Therapy; D013990:Tidal Volume; D014692:Ventilation-Perfusion Ratio
D010100:Oxygen