A group of unpleasant symptoms associated with unsynchronized atrioventricular timing in patients who have single-chamber (ventricular) pacemakers. The symptoms may include syncope or presyncope, orthostatic dizziness, cough, dyspnea, palpitations, and others. The symptoms are produced by the contraction of the atria against closed atrioventricular valves and by the loss of cardiac output that the atria would normally contribute to ventricular filling during diastole. DDD pacing reduces the incidence of this condition by allowing restoration of atrioventricular synchrony.