A 55-year-old woman with hypertension and diabetes presents with palpitations and lightheadedness. ECG shows a PR interval of 280 ms with each P wave followed by a QRS complex, but the PR interval progressively lengthens until a P wave is not followed by a QRS. Which investigation is most specific for confirming the diagnosis and assessing the degree of AV nodal dysfunction?

Explanation

## Investigation of Choice for Mobitz II AV Block ### Clinical Presentation Analysis The ECG findings describe **Mobitz II second-degree AV block** (or possibly **Wenckebach/Mobitz I**): - Progressive PR prolongation - Dropped QRS after a P wave - Regular atrial rhythm ### Why Electrophysiological Study (EPS) with AV Nodal Assessment? **Key Point:** EPS is the **most specific investigation** for: 1. **Confirming the exact site of block** (AV node vs. infra-nodal) 2. **Quantifying AV nodal function** (Wenckebach cycle length, AV nodal refractory period) 3. **Assessing conduction reserve** and risk of progression to CHB 4. **Determining need for pacemaker** vs. observation ### EPS Parameters in AV Block | Parameter | Normal | Abnormal (Block Risk) | |-----------|--------|----------------------| | AH interval | 60–125 ms | >125 ms = AV nodal disease | | HV interval | 35–55 ms | >55 ms = infra-nodal disease | | Wenckebach cycle | >100 ms | <100 ms = rapid block progression | | AV nodal refractory period | <300 ms | Prolonged = conduction impairment | **High-Yield:** The distinction between **Mobitz I (AV nodal)** and **Mobitz II (infra-nodal)** is critical: - **Mobitz I** → AV nodal block → often benign → observation - **Mobitz II** → infra-nodal block → high risk of CHB → pacemaker indicated EPS with His bundle recording definitively localizes the block and guides this decision. ### Clinical Pearl In symptomatic patients (palpitations, lightheadedness), EPS risk stratification is essential because Mobitz II carries a **high risk of sudden progression to complete heart block**, which could be life-threatening. ### Mnemonic: EPS in AV Block **CONDUCTION** — Confirm site, Obtain HV interval, Nodal vs. infra-nodal, Determine prognosis, Utility for pacemaker, Conduction reserve, Therapeutic guidance, Identify risk.