A 72-year-old woman with a history of hypertension and diabetes mellitus presents with recurrent syncope. 12-lead ECG shows PR interval of 280 ms with normal QRS duration (100 ms). There is a 2:1 AV conduction pattern on the rhythm strip. Which investigation is most appropriate to confirm the diagnosis and assess the need for pacemaker implantation?

## Investigation of Choice for High-Degree AV Block (2:1 Block) ### Clinical Presentation This patient has: - **PR interval 280 ms** (prolonged, >200 ms) - **2:1 AV conduction** (every 2nd atrial impulse conducts) - **Normal QRS** (100 ms, narrow) - **Recurrent syncope** (hemodynamic consequence) - **Chronic setting** (hypertension, diabetes — degenerative conduction disease) ### Diagnostic Challenge: Nodal vs. Infranodal Block? **Key Point:** In 2:1 AV block, the **site of block cannot be determined from surface ECG alone**: - Narrow QRS suggests nodal block, BUT - Infranodal block can also present with narrow QRS if the block occurs at the His level - This distinction is **critical** for prognosis and pacing indication ### Why His Bundle Electrogram with Atrial Pacing is Correct **High-Yield:** HBE with atrial pacing is the **gold standard** to: 1. **Localize the block** (measure A-H and H-V intervals) 2. **Determine prognosis** (nodal → often benign; infranodal → high risk of progression to CHB) 3. **Guide pacing decision** (infranodal → pacemaker indicated; nodal → observe) ### HBE Findings Interpretation | Finding | Nodal Block | Infranodal Block | |---------|------------|------------------| | **A-H interval** | Prolonged (>100 ms) | Normal | | **H-V interval** | Normal (<55 ms) | Prolonged (>100 ms) or absent | | **Block site** | AV node | His-Purkinje system | | **Prognosis** | Benign, transient | Ominous, progressive | | **Pacing need** | Rarely needed | Indicated | ### Atrial Pacing Protocol - Pace atrium at progressively faster rates (100, 120, 140 bpm) - Observe **Wenckebach phenomenon** (progressive PR prolongation then dropped beat) - **Nodal block:** Wenckebach occurs at AV node level (A-H prolongation) - **Infranodal block:** Wenckebach occurs at His-Purkinje level (H-V prolongation) — **ominous sign** ### Clinical Pearl **Syncope in 2:1 block = infranodal block until proven otherwise.** The presence of syncope indicates: - Inadequate escape rhythm - Risk of progression to CHB - **Pacemaker is indicated** (confirmed by HBE showing infranodal block) ### Diagnostic Algorithm ```mermaid flowchart TD A[2:1 AV Block + Syncope]:::outcome --> B[Surface ECG: Narrow QRS]:::outcome B --> C{Site of Block?}:::decision C -->|Cannot determine from ECG| D[HBE with Atrial Pacing]:::action D --> E{H-V Interval?}:::decision E -->|Normal| F[Nodal Block]:::outcome E -->|Prolonged/Absent| G[Infranodal Block]:::outcome F --> H[Observe, Atropine trial]:::action G --> I[Permanent Pacemaker]:::urgent ``` [cite:Harrison 21e Ch 276] ---