Dogs which were areflexic and had induced heart block were studied and the consequences of atrial fibrillation at a constant, regular ventricular rate were examined. Alterations incurred by this arrhythmia could thus be related to the absence of effective atrial function per se. The observed hemodynamic consequences of atrial fibrillation were (1) an increase of mean left atrial pressure and an increase of mean left atrial pressure relative to left ventricular end-diastolic pressure; and (2) a decrease of aortic pressure and aortic flow. Evidence is presented which supports the view that the absence of effective atrial activity during atrial fibrillation results in early mitral regurgitation. Criteria for atrial fibrillation were also established by using two bipolar recording electrodes attached to the left atrium. These were (1) lack of uniform polarity, (2) lack of a phase relationship of the depolarization complex recorded at these two points, and (3) irregular rate of depolarization at each recording site.
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