Oxygenation of cardioplegic solutions: Potential for the calcium paradox

Abstract

Oxygenation of crystalloid cardioplegic solutions is beneficial, yet bicarbonate-containing solutions equilibrated with 100% oxygen become highly alkaline as carbon dioxide is released. In the isolated perfused rat heart fitted with an intraventricular balloon, we recently observed a sustained contraction related to infusion of cardioplegic solution. In the same model, to record these contractions, we studied myocardial preservation by multidose bicarbonate-containing cardioplegic solutions in which first the calcium content and then the pH was varied. An acalcemic cardioplegic solution (Group 1) and the same solution with calcium provided by adding calcium chloride (Group 2) or blood (Group 3) were equilibrated with 100% oxygen. Ionized calcium concentrations were 0, 0.10 +/- 0.06, and 0.11 +/- 0.07 mmol/L and pH values were 8.74 +/- 0.07, 8.54 +/- 0.08, and 8.40 +/- 0.07, all highly alkaline. Hearts were arrested for 2 hours at 8 degrees +/- 2.5 degrees C and reperfused for 1 hour at 37 degrees C. At end-arrest, myocardial adenosine triphosphate was depleted in all three groups, significantly in Groups 2 and 3. In Group 1 the calcium paradox developed upon reperfusion, with contracture (left ventricular end-diastolic pressure = 60 +/- 7 mm Hg), creatine kinase release up to 620 +/- 134 U/L, a profound further decrease in adenosine triphosphate to 1.9 +/- 1.7 nmol/mg dry weight, and either greatly impaired or no functional recovery (17% +/- 10% of prearrest developed pressure). Three hearts in this group released creatine kinase during arrest and did not resume beating during reperfusion. In Groups 2 and 3, the calcium paradox did not occur; functional recovery was 61% +/- 4% and 71% +/- 9% at 5 minutes of reperfusion. In two additional groups (4 and 5), the pH of the acalcemic cardioplegic solution was decreased by equilibration with 2% and 5% carbon dioxide in oxygen to 7.53 +/- 0.03 and 7.11 +/- 0.02. Contractions during arrest were smaller than in Groups 1, 2, and 3; adenosine triphosphate was maintained during arrest; functional recovery was 101% +/- 3% and 96% +/- 4% at 5 minutes of reperfusion. We conclude that acalcemic solutions with carbon dioxide are superior to highly alkaline calcium-containing solutions. If oxygenation of cardioplegic solutions, of proved value, causes severe alkalinity, then calcium paradox may result even with hypothermia. This hazard is prevented by adding calcium or blood to the solution or carbon dioxide to the oxygen used for equilibration.