The additive protective effects of hypothermia and chemical cardioplegia during ischemic cardiac arrest in the dog.

In clinical use cardioplegia is usually combined with local cardiac hypothermia, whose powerful protective effects make it difficult to assess the contributions made by the cardioplegic solution. The additive protective effects of hypothermia and of an experimental cardioplegic infusate were studied in 20 dogs which were subjected to 120 minutes of myocardial ischemia at 20 degrees C. In the hypothermic group 10 hearts were infused with a noncardioplegic solution at 20 degrees C at the onset and after 60 minutes of ischemia. In the cardioplegia plus hypothermia group of 10 hearts identical infusion conditions were followed, with the cardioplegia solution at 20 degrees C. Measurements of ventricular function before and after bypass revealed significantly better recoveries in the cardioplegic group than in the hypothermic group. Recoveries of cardiac output, left ventricular minute work, and dP/dt in the cardioplegia group were 92%, 62% and 91%, respectively, whereas in the hypothermia group the values were 38%, 17%, and 43%, respectively. Parallel biochemical assessments from biopsies revealed that postischemic myocardial adenosine triphosphate (ATP) content was unchanged from control in the cardioplegia group but fell significantly to 56% of control in the hypothermia group. Assessment of myocardial integrity by birefringence showed no change in the cardioplegia group but a deterioration in the hypothermia group. These results demonstrate that chemical cardioplegia when combined with hypothermia affords additional protection to the ischemic heart.