Failure of blood cardioplegia to protect myocardium at lower temperatures

To assess the effect of temperature, oxygenated blood and crystalloid cardioplegic solutions (CPs) at infusion temperatures of 20°C, 10°C and 4°C were compared. The amount of potassium in each CP was 25 mEq/l. There were six study groups with seven canine hearts in each group, all of which were subjected to 90 minutes of global ischemia at 20°C, 10°C or 4°C, followed by 45 minutes of normothermic reperfusion. During ischemia, either blood or crystalloid CP was given every 30 minutes. With each infusion, any change in myocardial oxygen tension was recorded using mass spectrometry, and oxygen consumption (MV̇O₂) was calculated. Left ventricular (LV) function was assessed before and after ischemia in all hearts by measuring isovolumic developed pressure using an intraventricular balloon. Injection of 20° C blood CP resulted in a mean increase in intramyocardial oxygen tension (Po₂) of 7 mm Hg. At 10°C or 4° C, blood CP infusions did not cause a significant increase in intramyocardial Po₂, while with crystalloid CP, intramyocardial Po₂ did not increase at any of the three infusion temperatures. The mean MV̇O₂ with each blood CP injection, expressed as ml O₂/100 g LV wet weight, was 16.5 ± 0.8 for 20°C blood, 4.1 ± 0.3 for 10°C blood and 3.5 ± 0.4 for 4°C blood (p < 0.001, 20°C blood vs 10°C and 4°C blood). MV̇O₂ with each crystalloid CP injection was 0.9 ± 0.1 for 20°C CP, 0.8 ± 0.1 for 10°C blood CP, and 0.7 ± 0.1 for 4°C CP. Recovery of developed pressure after 45 minutes of reperfusion, expressed as a percentage of preischemic control, was 76.0 ± 3.4% for 20°C blood CP, 65.6 ± 2.3% for 10°C blood CP, and 54.0 ± 2.7% for 4°C blood CP (p < 0.05, 20°C blood CP vs 10°C and 4°C blood CP). Recovery of developed pressure, also expressed as a percentage of preischemic control, was 56.6 ± 1.4% for 20°C crystalloid CP, 72.9 ± 3.0% for 10°C crystalloid CP, and 72.0 ± 2.3% for 4°C crystalloid CP (p < 0.05, 20°C crystalloid CP vs 10°C and 4°C crystalloid CP). These data show that blood CP is most effective when infused at 20°C. The use of 10°C blood CP enhanced myocardial cooling, but was of no additional benefit, presumably because there was little oxygen delivery. Blood CP at 4° C resulted in significantly poorer preservation of LV function. Crystalloid CP infused at either 4°C or 10°C was as effective as 20°C blood CP, despite the absence of oxygen use by hearts treated with crystalloid CP.