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The effects of chloroquine on the electrocardiogram and heart rate in anaesthetized dogs.

The effects of intravenous administration of chloroquine on the electrocardiogram (ECG) and heart rate were studied in anaesthetized dogs. Prolongation of the PR interval on the ECG and bradycardia occurred from a dose of 4 mg kg-1. Prolongation of the QRS interval occurred from 6 mg kg-1. Pacing the heart at a constant rate or administration of atropine had no effect on the cardiac changes observed during chloroquine administration. Administration of propranolol potentiated the effects of chloroquine on the ECG intervals and heart rate, whereas adrenaline opposed these effects. It is suggested that prolongation of the ECG intervals is not secondary to the reduction in heart rate, whilst beta-adrenergic blockade renders the heart more vulnerable to the actions of chloroquine.