Authors

• Sara Milanović Litre — University Hospital of Split, Split, Croatia — ORCID: 0000-0003-3097-5047
• Anđelo Vukić — University Hospital of Split, Split, Croatia

Keywords

pacing, QRS morphologies, left bundle branch block, right bundle branch block

DOI

Full Text

**Introduction:** Left bundle branch area pacing (LBBAP) is increasingly used to treat bradycardia and deliver cardiac resynchronization therapy (CRT) (1). During LBBAP, QRS morphology in lead V1 is characterized by a terminal R wave, known as the right bundle branch block (RBBB) pattern; however, different QRS complexes have been described (2). This study aimed to analyze QRS morphology in lead V1 during LBBAP in patients with bradycardia and CRT after implantation. **Patients and Methods:** This study retrospectively reviewed 55 electrocardiograms (ECGs) recorded in the ward, immediately after the LBBAP procedure in consecutive patients with bradycardia (n = 46, 83.6%) and CRT indications (n = 9, 16.4%) from January to September 2025. The morphology of the QRS complex in lead V1 was analyzed to evaluate its features, particularly to distinguish RBBB-like patterns from other morphologies. **Results:** RBBB-like pattern was observed in the majority of ECGs (n = 35, 63.6%). The most common morphology within the RBBB-like pattern was “Qr“ (n = 15, 42.9%), followed by „QR“ (n = 10, 28.6%), „rsR“ (n = 4, 11.4%), „qr“ (n = 3, 8.6%) and „qR“ (n = 3, 8.6%), respectively. Other patterns appeared in two morphologies, mainly „QS“ (n = 16, 80.0%) and „Qrs“ (n = 4, 20.0%). The RBBB-like pattern was found to be significantly more prevalent among patients with bradycardia (n = 33, 71.3%) in comparison to those with indications for CRT (n = 2, 22.2%, p = 0.005). Conversely, other morphologies were notably more common in the CRT group (n = 7, 77.8%) than in the ECGs of patients with bradycardia indications (n = 13, 28.3%, p = 0.005). Further assessment revealed that other QRS morphologies in bradycardia ECGs were caused by deep septal pacing rather than LBBAP (n = 7, 53.8%) and anodal capture in LBBAP (n = 6, 46.2%). The fusion of LBBAP with intrinsic rhythm was the most common cause of other morphologies in CRT ECGs (n = 4, 57.1%), while there was one case each (14.3%) of anodal capture in LBBAP, a faster intrinsic rhythm, and non-selective LBB pacing. **Conclusion:** LBBAP pacing in bradycardia indications is associated with an RBBB-like pattern in ECG lead V1, particularly the ‚Qr“ type. In CRT, LBBAP is associated with different QRS morphologies. The absence of an RBBB-like pattern may not indicate loss of LBBAP capture.

Literature

1. Glikson M, Burri H, Abdin A, Cano O, Curila K, De Pooter J, et al. European Society of Cardiology (ESC) clinical consensus statement on indications for conduction system pacing, with special contribution of the European Heart Rhythm Association of the ESC and endorsed by the Asia Pacific Heart Rhythm Society, the Canadian Heart Rhythm Society, the Heart Rhythm Society, and the Latin American Heart Rhythm Society. Europace. 2025 March 28;27(4):euaf050. https://doi.org/10.1093/europace/euaf050
2. Sato T, Togashi I, Ikewaki H, Mohri T, Katsume Y, Tashiro M, et al. Diverse QRS morphology reflecting variations in lead placement for left bundle branch area pacing. Europace. 2023 August 2;25(9):euad241. https://doi.org/10.1093/europace/euad241