The objectives of the study were to determine how gas exchanges indices including alveolar-arterial oxygen tension difference(A-aDO2), PaO2/PAO2, PaO2/FIO2 and pulmonary shunt ratio (QS/QT) were changed after open heart surgery according to the change of inspired fraction of oxygen(FIO2) and to identify the reliable parameters for postoperative respiratory care in the intensive care unit.

Blood gases were measured from radial and pulmonary arterial blood for arterial and mixed venous blood in 15 patients, with a mean age of 45.1 yr, after open heart surgery, in mechanically ventilated states with 1.0, 0.6 and 0.4 of FIO2 with normocarbia maintained (PaCO2 30~40 mmHg). Above mentioned gas exchange indices were calculated and compared.

After decreasing the FIO2 from 1.0 to 0.6 and 0.4 in order, the major changes were significant decrease in pulmonary shunt ratio : 24±3%, 17±3% and 11±3% at FIO2 1.0, 0.6 and 0.4, respectively and A-aDO2 : 347±34 mmHg, 184±16 mmHg and 94±11 mmHg at FIO2 1.0, 0.6 and 0.4 respectively, while PaO2/PAO2 showed the only significant increase at FIO2 0.4 compared with those at FIO2 1.0 and PaO2/FIO2 was unchanged.

In cases of open heart surgical patient with cardiovascular stability, pulmonary shunt ratio and A-aDO2 appear to be more sensitive on gas exchange, but the oxygen tension indices such as PaO2/PAO2 as well as PaO2/FIO2 might be suggested as possible alternatives for the shunt measurement.