A patient with complete heart block (CHB) has a ventricular rate of 38 bpm and an atrial rate of 80 bpm on ECG, with P waves bearing no consistent relationship to QRS complexes. The QRS is wide (0.14 s). From which pacemaker focus do the QRS complexes most likely originate, and why?

• A Bundle of His — His bundle escape rate is 40–60 bpm and characteristically produces a narrow QRS because the His bundle conducts to both bundle branches simultaneously
• B AV node — the AV nodal escape rhythm at 38 bpm generates the wide QRS because nodal tissue depolarises slowly
• C Purkinje fibres/ventricular myocardium (idioventricular escape rhythm) — the AV node has failed to conduct; the wide QRS reflects abnormal ventricular activation not using the His-Purkinje system's normal pathways ✓
• D Sinoatrial node — the SA node escape rhythm at 38 bpm represents sick sinus syndrome with high-grade block

Explanation

In complete heart block where the block is below the bundle of His (infra-Hisian), ventricular escape pacemaker cells (Purkinje fibres or ventricular myocardium) fire at intrinsic rate of 20–40 bpm. Activation spreads slowly through working ventricular myocardium rather than the specialised His-Purkinje system, producing a wide, bizarre QRS (>0.12 s). Bundle of His escape has a narrow QRS (40–60 bpm). AV nodal escape also produces a narrow QRS (40–60 bpm). A rate of 38 bpm with wide QRS specifies an infra-Hisian or idioventricular focus. CHB is an absolute indication for permanent pacing.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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