Fig. 5. Basic patterns in dual-chamber pacing. The first beat is fully paced and is an example of atrioventricular (AV) sequential pacing. In the second beat, a spontaneous R wave is sensed in the ventricular (V) channel before the atrioventricular interval times out, initiating a new VA interval (the interval between the ventricular stimulus or sensed spontaneous R wave and the subsequent atrial stimulus). Thus, it inhibits the ventricular stimulus that would have occurred (short vertical line in the ventricular-channel timing diagram). In the third beat, a P wave is sensed in the atrial (A) channel before the VA interval times out, initiating a new atrioventricular interval. It also inhibits the stimulus that would have occurred (short vertical line in the atrial-channel timing diagram). This is an example of atrial synchronous ventricular pacing, which is equivalent to the VAT mode (fig. 1). In the last beat, spontaneous P and R waves are sensed before the respective VA and atrioventricular intervals time out. ECG = electrocardiogram; PVARP = postventricular atrial refractory period; VRP = ventricular refractory period. Reprinted with permission from Bernstein AD, Parsonnet V: Fundamentals of antibradycardia-pacemaker timing. ACC Educational Highlights 1995; 11:5–9; copyright American College of Cardiology.