Systolic time intervals in induced atrial fibrillation in the dog

The effect of induced atrial fibrillation on ventricular performance in the dog, as measured by changes in the systolic time intervals (STI), was investigated. Atrial fibrillation was induced by enhancement of vagal tone with morphine sulfate, followed by direct mechanical stimulation of the atrium. Dogs received 15 mg. per kilogram of morphine sulfate subcutaneously, followed by 3 mg. per kilogram of pentobarbital sodium. ECG, atrial electrogram, phonocardiogram, and direct arterial blood pressure were recorded during periods of sinus rhythm and during periods of induced atrial fibrillation. Data were analyzed by selecting 20 representative cardiac cycles from each condition in each of 15 dogs. Cycles were selected so that the broadest spectrum of rates was examined for each animal. Three hundred cycles were examined from each condition, or a total of 600 cycles. Heart rate (HR), left ventricular end-diastolic pressure, and aortic end-diastolic pressure were unchanged during atrial fibrillation. The left ventricular pre-ejection period (LVPEP), externally derived isovolumic contraction time (EICT), and total mechanical systole (S 1-S 2 interval) were all found to increase significantly in duration after the induction of atrial fibrillation. The left ventricular ejection time (LVET) and the electromechanical delay (Q-S 1 interval) were significantly decreased in duration following the induction of atrial fibrillation.