THE EFFECT OF REGULAR AND IRREGULAR TACHYCARDIAS ON THE SIZE OF THE HEART

Atrial fibrillation in the intact unanesthetized dog: hemodynamic effects during rest, exercise, and beta-adrenergic blockade

The effects of atrial fibrillation were studied in 12 healthy unanesthetized dogs, 9 to 49 days after surgical implantation of transducers for measurement of aortic flow and left ventricular diameter. Atrial fibrillation and pacing at comparable ventricular rates were induced by electrical stimulation of the right atrial appendage, and their effects were compared with observations made during sinus rhythm in each dog. At rest, cardiac output and mean arterial pressure were not significantly different during sinus rhythm, atrial fibrillation, and atrial pacing. After beta-adrenergic blockade with propranolol, cardiac output during fibrillation was significantly less than that during pacing at comparable ventricular rates. Arterial pressure was not detectably altered. During moderately severe treadmill exercise by six dogs, cardiac output fell significantly upon induction of fibrillation. After pentobarbital anesthesia fibrillation caused decrements in cardiac output and arterial pressure that were accentuated after thoracotomy.These observations suggest the existence of compensatory mechanisms that maintain an essentially constant cardiac output when atrial fibrillation is induced in healthy unanesthetized dogs at rest. These mechanisms appear to fail during moderately severe exercise, beta-adrenergic blockade, and pentobarbital anesthesia.

Ventricular dynamics in atrial fibrillation

During atrial fibrillation there are beat-to-beat changes of ventricular rate, end-diastolic volume, end-diastolic pressure, and arterial pressure or resistance to ventricular ejection. The effect of these variables on left ventricular dynamics in man has been approached by studying single sequences of beats in patients with atrial fibrillation. An index of changes of left ventricular volume has been determined from the electrokymogram of left cardiac border. Left ventricular pressures have been measured at surgery or by the transbronchial technic. These studies provide a better understanding of the hemodynamics of the left ventricle during atrial fibrillation and demonstrate another approach to the study of left ventricular function in man.