Electrocardiographic signs in experimental myocardial ischemia and infarction

Epicardial ECG signs have been studied in 26 anesthetized and thoracotomized dogs in an attempt to follow the progress of tissue damage during regional myocardial ischemia. Epicardial ECG's were recorded before and during 15 min, 1 and 5 h of severe left anterior descending coronary artery narrowing. Epicardial ST segment elevation followed a complicated natural history. An analysis of variance showed the significant effects of respiration, heart rate and changes in time during myocardial ischemia. Regional epicardial R waves showed a transient increase in amplitude following coronary narrowing. There was no loss of electrically active myocardium following 15 min of ischemia. Irreversible loss of R waves were noted at between 30 and 45 min and progressed to full development within 5 h following coronary artery narrowing. The loss of electrically active myocardium (R loss plus Q waves) at 5 h was closely related to the myocardial depletion of creatine kinase activity (mu/mg DNA-1) at 24 h in each dog. The early manifestation of myocardial ischemia (ST segment elevation at 17 min) was closely related in the later evidence of cell death (R loss plus Q waves) in each dog. These relationships were less precise when the results were combined and this showed the variability between dogs in heart size and infarct size. The study suggested that the individual complete natural history of these ECG signs must be studied before they can be used to assess the extent and progress of myocardial ischemia and cell death.