| Abstract
| - SUMMARY. Resting electrocardiograms were recorded in 18 male adult rats injected subcutaneously with two doses of isoproterenol (200 mg·kg−1 body weight) 10 days before the animals were submitted to the ajmaline test (1 mg·kg−1 body weight iv). After the ajmaline test all rats were killed and the hearts examined histologically. Electrocardiographic changes were detected at rest in 72% of the isoproterenol injected rats: pathological Q waves, lengthening of the QRS complex, and QRS abnormality were found in 50%, 44%, and 44% of these animals respectively. Ajmaline induced similar changes in both control and isoproterenol treated rats (P wave enlargement (p<0.01 and p<0.001 respectively), increased PR interval (p<0.003 and p<0.001 respectively), and increased QaT interval (p<0.001 in both groups)). However, ajmaline caused an increase in heart rate only in isoproterenol treated rats (p<0.05). A pronounced increase in PR interval, not observed in control rats, was detected in one of five isoproterenol injected rats with a normal resting ECG but showing microscopical cardiac lesions. Apical aneurysm of the left ventricle was found in 16% of isoproterenol injected rats. A mononuclear inflammatory reaction was observed in 13 (72%) of the isoproterenol injected rats and was multifocal in at least three regions of the myocardium in six (46%) and disseminated throughout the myocardium in seven (53%) of these animals. When the electrocardiographic and pathological findings were compared, the ECG changes were found to have a 91% sensitivity, 83% specificity, and 91% positive predictive value. Thus it is concluded that the resting ECG is a reliable method of detecting myocardial lesions in isoproterenol injected rats.
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