Ventricular-triggered pacemakers: clinical experience.

It is now generally agreed that the presence of symptoms is an indication for long-term pacing of patients with complete heart block, and that this treatment lowers the mortality considerably (Chardack et al., 1966; Sowton, 1967a). An appreciable proportion of paced patients has intermittent atrioventricular conduction so that if a fixed rate pacemaker is implanted there is competition between natural and artificial pacemakers. This frequently produces palpitation, sometimes causes large swings in blood pressure, and very occasionally results in dangerous ventricular arrhythmias due to stimulation within the vulnerable period (Tavel and Fisch, 1964). Competition can be avoided in several ways, one of which is the use of demand pacemakers which do not stimulate the heart unless a fixed period without spontaneous activity occurs (Parsonnet et al., 1966; Sowton, 1967b). An alternative approach, due to Neville (Neville et al., 1966), is for the pacemaker to be triggered by spontaneous ventricular potentials and then to deliver an impulse to the ventricle immediately. This impulse falls ineffectively within the absolute refractory period and so cannot cause a competing rhythm, nor stimulate during the vulnerable period. So long as sinus rhythm persists the pacemaker follows the ventricular rate exactly; if a spontaneous ectopic beat occurs this, too, will trigger the pacemaker so that the stimulus falls harmlessly during the refractory period. If complete heart block returns, so that no spontaneous QRS complex arises during a preset time interval, the pacemaker begins to stimulate the ventricle at a fixed rate. The time interval is directly related to the fixed pacing rate, and pacemakers of this type provide a limit below which the ventricular rate cannot fall. If conducted or ectopic beats return the pacemaker will again be immediately triggered at the spontaneous rate. The use of atrial-triggered pacemakers, which