One of the factors in the success of cardiac surgery is how well the myocardium is protected while the ascending aorta is occluded and the natural coronary blood flow is suspended. Five methods for myocardiac protection which are employed clinically were comparatively examined with a cardiac perfusion apparatus for isolated hearts of large animals in terms of left ventricular function and myocardiac metabolism. The apparatus was designed to exclude factors other than myocardium, including neural and secretory activities as well as to stabilize preload and afterload. Hearts were isolated from 56 mongrel dogs, classified into Groups 1,2,3, and controls, and treated as follows for 30,60, or 90 minutes.
The hearts of Group 1 were subjected to surfac e cooling in a physiologic saline at 10°C. Group 2 was subgrouped into 2-a and 2-b: the hearts of the first subgroup were perfused intermittently at 30-minute intervals with a lactated Ringer's solution (50 ml) at pH 8, as well as subjected to surface cooling; those of the second subgroup were intermittently perfused with a lactated Ringer's solution containing potassium (25 mEq/1). Group 3 was also subdivided into 3-a and 3-b: the first was a continuously perfused with GIK solution following cardioplegia induced by Young's solution; in the second group, diluted and cooled blood (Ht 20-25%,10°C) was used for the continuous perfusion. The perfusion rate was 10-15 ml/min for both subgroups. Myocardial ATP was measured 30,60, and 90 minutes after the beginning of the treatments by biopsy of the right ventricular free wall. The ATP content, which decreased with time in all the dogs, was significantly higher in Group 3-b than in the other 4 groups even 90 minutes after the treatments. Following the myocardial biopsy, left ventricular function was measured with the cardiac perfusion apparatus for isolated hearts using autologous blood. Left atrial pressure (LAPmin) and the angle of the left ventricular function curve (ASW) at the time of the earliest-observed ejection 30,60, or 90 minutes after the treatments were obtained on the left ventricular function curve of each group and comparatively studied. The left ventricular function of the hearts in Group 1 was similar to that in other groups after the 30-minute treatment, but a significantly higher LAPmin and a significantly lower ASW than Kl.5in other groups were observed after the 60-minute treatment. In Group 2-a, the left ventricular function declined with time during the 60-minute treatment. In Groups 2-b,3-a, and 3-b, by contrast, no significant difference was observed even after the 90-minute treatment.
The above results suggest that the surface cooling method is the least effective in protecting the myocardium, followed by the intermittent perfusion method with lactated Ringer's solution. The three other methods, namely, potassium cardioplegia, continuous coronary perfusion with GIK solution after Young's solution-induced cardioplegia, and continuous coronary perfusion with diluted blood, showed similar effects. According to the measurement of myocardial ATP content, however, the third seems to be slightly better.