Effect of alpha-stat versus pH-stat strategy on oxyhemoglobin dissociation and whole-body oxygen consumption during hypothermic cardiopulmonary bypass.

To determine whether alpha-stat or pH-stat strategy should be used, 20 patients undergoing coronary artery bypass grafting during moderate hypothermic hemodilutional cardiopulmonary bypass were studied. The carbon dioxide management during bypass was randomly done according to alpha-stat strategy in 10 patients (i.e., temperature-uncorrected PaCO2 was kept near 40 mm Hg and uncorrected pHa was kept at about 7.4) and according to pH-stat strategy in the other 10 patients (i.e., temperature-corrected PaCO2 was kept near 40 mm Hg and uncorrected pHa was kept at about 7.4). In both groups, when the central venous temperature was stable at 26.5 +/- 2.5 degrees C, the perfusion flow was altered sequentially from 2.4 to 1.8 and 1.2 L.min-1.m-2. The mixed venous oxyhemoglobin saturation at the different perfusion flows was monitored by the Oxy-Stat meter and was correlated with the corresponding mixed venous oxygen tension to construct an oxyhemoglobin dissociation curve. Also, the whole-body oxygen consumption at the different perfusion flows was computed. The whole-body oxygen consumption and the oxyhemoglobin dissociation were not significantly different between the alpha-stat and the pH-stat groups. In both groups, the dissociation curve is shifted to the left, but the oxygen consumption per unit time does not significantly change despite decreasing the perfusion flow from 2.4 to 1.2 L.min-1.m-2. The results suggest that oxygen delivery is not impaired during moderate hypothermic cardiopulmonary bypass independent of whether alpha-stat or pH-stat strategy is used.

Prediction of post-cardiopulmonary bypass cardiac output by venous oximetry.

The present study evaluates two equations for predicting the post-cardiopulmonary bypass cardiac output (CO) in 10 patients undergoing coronary artery bypass grafting. One equation is based on the relationship of CO with mixed venous oxygen saturation (SVO 2), while the second equation is based on the relationship with oxygen extraction (1 - SVO 2). Each patient served as his own control. During bypass, when the patients were normothermic and perfused with a pump flow of 2.4 L/min/m 2, the SVO 2 was monitored by an in-line Bentley oxystat Meter. Just before termination of bypass, the pump flow was decreased to 0.4 L/min/m 2 and the left atrial pressure was increased to 10-15 mmHg; the resulting SVO 2 was recorded. The post-bypass CO was predicted in every patient by the two equations. Immediately after weaning from bypass, the cardiac output was measured by thermodilution. The thermodilutional CO measurement was correlated with the CO predicted by the two equations. Correlation analysis suggests that CO prediction is more accurate and approaches the 1:1 ratio when the calculation of predicted CO is based on the relationship between cardiac output and oxygen extraction.