MYOCARniAL IRRITABILITY— HEGNAUKR and COVINO 



345 



If the main trunk of the left coronary artery is occluded for 2 minutes all 

 uonnolhcrnn'r dogs develop ventricular fihrillation (hg. 1). Since the funclidn of 

 the heart and lungs is taken over hy the instrument, the rhythm oi the heart is of 

 little cimcern. Ventricular fibrillation has lost its nightmarish aspect and is just a 

 cosmetic defect of the electrocardiogram ! After 2 hours, ventricular fibrillation can 

 be stopped by electrical shock and the pump-oxygenator can be turned off. 



In hypothermic dogs with by-pass of heart and lungs the period of complete 

 coronary occlusion can be progressively prolonged (fig. 2). At 23° C. the empty, 

 Ijeating heart can be deprived of coronary flow for 25 minutes without ventricular 

 fibrillation. Longer periods of coronary occlusion at lower temperatures were not 

 tested in this series of experiments. After occlusion of the aorta the heart rate de- 

 creases in a linear fashion with time (fig. 3) and the electrocardiogram shows signs 

 of severe ischemic injury (fig. 4). This electrical injury is of a functional nature 

 only, because after release of the aortic occlusion and after perfusion with oxy- 

 genated blood, normal electrocardiogram and heart function can be established. 

 The degree and dynamics of myocardial ischemia and the rapidity of its recovery 

 can be demonstrated by measuring myocardial oxygen tension polarographically 

 with a platinum electrode (fig. 5). 



It should be pointed out that the changes recorded are not absolute ones because 

 no corrections are made for temperature and pH effects. In this experiment clamp- 

 ing of the ascending aorta was followed by a gradual decrease in myocardial 

 oxygen tension until about half of the available oxygen was used up after 6 minutes. 

 After release of the aorta it took the heart muscle about one minute only to return 

 to its previous baseline of oxygen tension. The slowing of the heart rate and the 

 decrease of amplitude of excursions during the ischemic period can also be 



BEFORE OCCLUSION 



3:36 

 2 MIN. AFTER OCCLUSION 



1@ ^P 



3:46 



4:25 



Fig. 1. — Temporary occlusion of main trunk of left coronary artery during partial by-pass of 



heart and lungs (36° C). 



