Does lower systemic temperature enhance cardioplegic myocardial protection?

This study was performed to determine if systemic temperature affects the rate of cooling and rewarming of the myocardium between infusions of cold potassium cardioplegic solution and if it alters the degree of myocardial protection. Twenty-one dogs underwent cardiopulmonary bypass and 3 hours of ischemic arrest produced by infusing 0 degrees to 4 degrees C potassium cardioplegic solution every 20 minutes. The animals were perfused at different systemic temperatures--Group A, 23 degrees C; Group B, 30 degrees, C; and Group C, 37 degrees C. Myocardial temperature decreased to 11 degrees C in Groups A and B as compared with 13 degrees C in Group C immediately after infusion of the potassium cardioplegic solution (p < 0.0007). Myocardial temperature increased to 19 degrees, 21 degrees, and 26 degrees C in Groups A, B, and C (p < 0.000005) 20 minutes after infusion. Myocardial adenosine triphosphate (ATP) concentration significantly decreased, from 6.69 to 3.56 mumoles/gm (p < 0.0003) in Group B and from 4.99 to 2.80 (p < 0.005) in Group C at the end of 3 hours of cardioplegic arrest. Myocardial glycogen levels also significantly decreased, from 1.156 to 446 mg% (p < 0.003) in Group B and from 811 to 439 mg% (p < 0.04) in Group C. Myocardial ATP and glycogen did not decrease significantly in Group A during the period of arrest. Myocardial lactate values increased more in Groups B and C during the arrest period than in Group A. 12.6 versus 6.5 mumoles/gm (p < 0.03). Moderate systemic hypothermia, therefore, appears to enhance the myocardial protection of potassium cardioplegia.