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. FROM a hemodynamic point of view atrial fibrillation is characterized by 2 abnormalities , the fibrillating, ineffectively contracting atria and the irregularly contracting ventricles. A number of studies have been reported describing the effects of atrial fibrillation on cardiac output,'-'3 venous pressure, 13-16 circu-coronary flow.7 21 In general, these studies have indicated a decreased ability of the heart to function effectively as a pump when atrial fibrillation is present. To what extent this is due to the lack of normal atrial contraction and to what extent due to the irregular ventricular rhythm is not clear from studies to date. It has been shown that when the ventricular rate is rapid the phenomenon of pulse deficit is more marked and that the mean cardiac output is diminished.6 9 Since digitalis was used to slow the ventricular rates in these studies, it is difficult to be certain that the observed effects are due to the changes in ventricular rate and not to the positive inotropic effect of digitalis. This is a study of one of the abnormalities of atrial fibrillation, ventricular function under the conditions of the irregular ventricular rhythm as seen in this arrhythmia. The approach has been to evaluate ventricular function in terms of single beats. Studies by others have demonstrated that the beat-to-beat variations of arterial pressure, duration of ventricu-lar systole, and rate of ventricular ejection, although related to the length of the preceding From the Clinic of General Medicine and Experi-R-R interval and duration of ventricular diastole, show considerable beat-to-beat variation for any given R-R interval or period of diastole within a single sequence of beats.14' 22-26 A number of mechanisms have been suggested to explain why variations in the duration of just the preceding R-R interval or period of ventricular …