We report a rare case with a history of tetralogy of Fallot with dextrocardia, who presented with palpitation and cardiac arrest upon admission. This unusual complex condition and the atypical symptoms obscured the primary cause of his cardiac arrest and posed challenges on his diagnosis and treatment along with concomitant cardiac arrythmias.

Abstract Background The paced morphology of right bundle branch block (RBBB) pattern is one of the parameters which can be used to confirm the left bundle branch capture during left bundle branch pacing (LBBP) implantation. However, changes of paced morphology after LBBP performed in patients with RBBB have not been well discussed. Objective To compare the paced morphology in patients with RBBB before and after LBBP and to discuss the possible mechanisms behind such changes. Methods Patients with RBBB who underwent LBBP implantation were selected. The QRS duration was measured before and after the implantation of LBBP for each patient and compared Student t test. Results We included 34 consecutive patients (median age 72.76 years, 73.53% male), of whom 3 had sick sinus syndrome (SSS) and 31 had high-grade atrioventricular conduction block (AVB). The average QRS duration was 144.87±5.22ms before LBBP implantation and 116.18±6.15ms after LBBP implantation, with a mean decrease in QRS duration of 28.70±3.30ms (95% CI: 27.55-29.85 ms, P < 0.01). Conclusion In patients with RBBB, the QRS duration after LBBP is narrower than that before LBBP. The specific mechanism is still unclear.