Selin YÖNDEM, Özcan ÖZEKE, Ahmet KORKMAZ, Meryem KARA, Elif Hande Özcan ÇETİN, Duygu Koçyiğit BURUNKAYA, Fırat ÖZCAN, Serkan ÇAY, Dursun ARAS, Serkan TOPALOĞLU
Archives of the Turkish Society of Cardiology - 2026;54(4):359-360
A 58-year-old man with nonischemic cardiomyopathy and a dual-chamber implantable cardioverter-defibrillator (ICD) presented to the emergency department after two presyncopal episodes associated with ICD shocks triggered by premature ventricular contractions (PVC). During an electrophysiological study, the tracing shown in Figure 1 was obtained. What is the mechanism underlying the different QRS morphologies observed in this dual-chamber pacemaker tracing? Pseudomalformations such as fusion (F), pseudofusion (PF), and pseudo-pseudofusion (PPF) are considered normal pacemaker behaviors. Although these findings usually indicate normal device function, they may result in unnecessary energy consumption; therefore, it is important to determine whether pacing truly contributes to cardiac activation. Careful analysis of the electrocardigraphy reveals wide and similar QRS complexes in all beats except the second, seventh, and eighth beats, indicating that these are pure paced QRS complexes (P in Figure 2). Ventricular fusion represents the electrical summation of an intrinsic cardiac beat (sinus or premature beat) and depolarization from a pacing stimulus, resulting in a morphology that lies between a fully paced beat and a purely intrinsic beat (eighth beat in Figure 2). In contrast, a pseudofusion beat occurs when the intrinsic rhythm competes with the pacemaker timing cycle, producing a QRS shape similar to the intrinsic QRS activity with a superimposed pacing spike from the same chamber. This reflects inappropriate timing and occurs when pacing output coincides with intrinsic activation, resulting in a spike superimposed on an intrinsic QRS event (second beat in Figure 2). The QRS morphology is consistent with intrinsic depolarization. Pseudofusion does not confirm or exclude capture and is not dangerous to the patient; however, it may lead to unnecessary battery depletion. In contrast to PF, cross-chamber pacing in PPF does not alter the morphology of the intrinsic P wave or R wave. It is characterized by an intrinsic or fully paced QRS complex overlapping with a cross-chamber pacing spike originating from the atrium rather than the ventricle (seventh beat in Figure 2). Reprogramming the lower rate limit to avoid PPF and/or adjusting the atrioventricular (AV) delay to prevent pacing during vulnerable periods following PPF may successfully prevent further ventricular tachycardia or ventricular fibrillation (VT/VF) episodes. Alternatively, elimination of PVCs through medical therapy or ablation may also be considered.
