Atrioventricular interval optimisation is important in patients with dual-chamber pacing, especially with heart failure. In patients with CHD, especially in those with Fontan circulation, the systemic atrial contraction is supposed to be more important than in patients without structural heart disease.

We retrospectively evaluated two patients after Fontan procedure with dual-chamber pacemaker with a unique setting of optimal sensed atrioventricular interval.

The optimal sensed atrioventricular interval determined by echocardiogram was extremely short sensed atrioventricular interval at 25 and 30 ms in both cases; however, the actual P wave and ventricular pacing interval showed 180 and 140 ms, respectively. In both cases, the atrial epicardial leads were implanted on the opposite site of the origin of their own atrial rhythm. The time differences between sensed atrioventricular interval and actual P wave and ventricular pacing interval occurred because of the site of the epicardial atrial pacing leads and the intra-atrial conduction delay.

We need to consider the origin of the atrial rhythm, the site of the epicardial atrial lead, and the atrial conduction delay by using electrocardiogram and X-ray when we set the optimal sensed atrioventricular interval in complicated CHD.