Effect of hemodilution on the adequacy of cerebral perfusion under hypothermic cardiopulmonary bypass

Abstract

Objective: Open heart surgery without transfusion has been performed even in children. However, the critical limit of the hemoglobin level has not yet been ascertained. Here, we have assessed experimentally the effect of the hemoglobin level on brain metabolism under hypothermic cardiopulmonary bypass.Methods: Brain tissue pH was measured in 14 rabbits that were put on bypass with a different degree of hemodilution. Cardiopulmonary bypass was started at 37°C and cooled down to 25°C. After maintaining the bypass at 25°C for 60 minutes, the animal was rewarmed to 37°C for 30 minutes and then kept on-bypass for another 30 minutes. The perfusion flow was maintained as 100 ml/kg/min.Results: The lowest hemoglobin level in each rabbit was from 2.5 through 8.5 g/dl. During hypothermic bypass, brain tissue pH increase from 7.21 ±0.16 (mean ±SD, at the normothermic baseline) to 7.55±0.27 except 2 cases (6.91±0.16) whose hemoglobin level was lower than 3.0 g/dl. The brain tissue pH after 60 minutes on hypothermic bypass had a good correlation with the hemoglobin level (r=0.831). After rewarming for 60 minutes, the brain tissue pH was decreased to 7.18±0.31. In 4 rabbits with less than 4.0 g/dl of hemoglobin, the brain tissue pH (6.67±0.24) was lower than the baseline level. In the other 10 rabbits, the brain tissue pH (7.22±0.16) was almost the same as the baseline level. The correlation coefficient between the brain tissue pH and the hemoglobin level after rewarming for 60 minutes was 0.778.Conclusions: These results indicated that severe hemodilution in cardiopulmonary bypass promoted acidosis in brain even during hypothermia.