Atrial fibrillation is an arrhythmia in which the electrical activity of the atrium becomes disorganized and therefore unable to produce a coordinated atrial contraction. The absence of an atrial pulse reduces the emptying of the atria during diastole and results in an enlarged left atrium and increased left atrial pressure. The venous A wave represents atrial contraction and disappears due to the absence of an atrial beat. Decreased filling of the heart results in a decrease in stroke volume. Heart rate increases because the continuous electrical activity of the atria initiates a high rate of ventricular activity Systemic blood pressure typically falls because of inadequate filling of the ventricles and the resulting decrease in stroke volume.