Factors affecting A-aDo2 after open-heart surgery.

Intrapulmonary shunt, cardiac output, and O2 consumption as factors contributing to alveolar-arterial O2 tension difference (A-aDo2) were tested in 11 patients after open-heart surgery. A-aDo2 was well correlated with intrapulmonary shunt, as expected, but no correlation was found between cardiac index (CI) and intrapulmonary shunt. When total shunt (venous admixture) was above 11%, there was an inverse correlation between A-aDo2 and CI, and it was observed that CI falling below 2.5 L/min/sq m contributed significantly to the enlargement of A-aDo2. Low Pao2 (less than 65 torr, breathing air) was characterized by larger arterial-mixed venous O2 content difference than high Pao2 (is greater than 65 torr, on room air), indicating that A-aDo2 was influenced by cardiac output relative to metabolic demand. Intrapulmonary shunt played a main role in increasing A-aDo2 after open-heart surgery, and systemic factors, including changes in cardiac output and O2 consumption, also contributed to development of hypoxia.