Background: We have previously reported the superior protective properties of the University of Wisconsin (UW) solution compared with the St. Thomas solution (ST) in the rat heart subjected to the deep hypothermia (4 degrees C), thus demonstrating its possible use in cardiac transplantation. We thought it was important to evaluate the potential of the UW solution as a cardioplegic solution under the moderately hypothermic (20 degrees C) conditions of routine intraoperative myocardial protection.
Methods and results: Isolated rat hearts were subjected to 60 minutes of ischemia at 4 degrees C or 30 (or 60) minutes of ischemia at 20 degrees C with UW, ST, and ST plus 100 mM K (ST + 100) solutions. Coronary flow, mechanical function, endothelial function, and ultrastructure were observed. Mean time (seconds) to infuse 10 ml of cardioplegic solution under constant pressure, a measure of coronary vascular resistance at 4 degrees C and 20 degrees C, respectively, for each solution were ST, 69.2 +/- 6.9 and 64.7 +/- 3.8; UW, 142.2 +/- 8.8 and 187.2 +/- 10.0 (p < 0.01); and ST + 100, 78.2 +/- 8.0 and 176 +/- 8.1 (p < 0.001). Mean recovery values of cardiac output (expressed as percentage of its preischemic value) after 60 minutes of ischemia at 4 degrees C were ST, 95.5 +/- 2.1%; UW, 93.0 +/- 2.4%; and ST + 100, 96.5 +/- 1.5%. After 30 minutes of ischemia at 20 degrees C, values were ST, 88.0 +/- 1.3%; UW, 72.2 +/- 3.6% (p < 0.005 versus ST); and ST + 100, 53.3 +/- 1.8% (p < 0.001 versus ST).
Conclusions: The efficacies of UW and severely hyperkalemic cardioplegic solutions are affected by the degree of hypothermia under which they are used. Under moderate hypothermia (20 degrees C), severe hyperkalemia induces a marked increase in coronary vascular resistance that is associated with impaired myocardial protection. These studies discourage the use of UW for routine intraoperative cardioplegic arrest where the degree of hypothermia cannot be readily controlled. The ST solution does not share this constraint.