Unusual electrocardiographic manifestations and clinical outcome of alternating bifascicular block after ventricular septal defect transcatheter closure: case report

Background Transcatheter occlusion of perimembranous ventricular septal defect (VSD) may exert mechanical compression and local irritation on the His-Purkinje conduction system, resulting in various bundle-branch blocks. In young children with bifascicular block, prolonged PR interval (i.e., first-degree atrioventricular block) suggests potential progression to trifascicular block after operation. This condition may predispose patients to paroxysmal complete atrioventricular block, which can lead to prolonged ventricular asystole and syncope. Such progressive electrocardiographic changes are rarely seen in young children, prone to misdiagnosis, and relevant clinical evidence remains insufficient. Case presentation A 3-year-old boy (body weight 14 kg, height 101 cm) presented with acute-onset persistent tachycardia (heart rate 240 beats/min; blood pressure 94/62 mmHg; body temperature 37.5 ℃). Laboratory tests were within normal limits. The 12-lead electrocardiogram demonstrated wide-QRS tachycardia. Adenosine failed to terminate the arrhythmia, whereas intravenous amiodarone restored sinus rhythm with QRS morphology identical to that at presentation. During hospitalization, sudden syncope occurred. Holter monitoring revealed alternating complete right bundle-branch block combined with left anterior fascicular block (RBBB+LAFB) and complete left bundle-branch block (LBBB), both complicated by first-degree atrioventricular block with distinct PR intervals. Paroxysmal complete atrioventricular block with ventricular asystole lasting up to 8.9 s was recorded, and a permanent cardiac pacemaker was subsequently implanted. No syncope or presyncope recurred during the 2-year follow-up. He had undergone transcatheter occlusion for perimembranous VSD two years previously. Conclusions In children with perimembranous VSD and pre-existing bifascicular block, post-operative progression to trifascicular block raises a plausible hypothesis that the perimembranous occluder might induce injury to the His-Purkinje system secondary to local traction or mechanical compression. Direct evidence of local ischaemia or fibrosis is lacking in this case. Patients may develop junctional tachycardia superimposed on bifascicular block, as well as transient complete atrioventricular block and ventricular asystole secondary to trifascicular block. This case suggests that individualized multidisciplinary risk assessment should be fully performed and guardians informed pre-operatively when conditions permit. Regardless of the surgical approach, long-term intensive Holter monitoring and regular follow-up in paediatric cardiology outpatient clinics are required post-operatively.