A 58-year-old man from Delhi presents to the emergency department with acute-onset syncope during exertion. On examination, he is haemodynamically stable with a regular heart rate of 45 bpm. ECG shows a prolonged PR interval (240 ms) with progressive lengthening until a P wave is not followed by a QRS complex. After the dropped beat, the PR interval resets and the cycle repeats. Which anatomical structure of the cardiac conduction system is most likely affected?

Explanation

## Clinical Presentation & ECG Findings The patient exhibits **Mobitz I (Wenckebach) second-degree atrioventricular block**, characterized by: - Progressive PR interval prolongation - Eventual dropped QRS complex (non-conducted P wave) - PR interval resets after the dropped beat - Regular rhythm with cyclic pattern ## Anatomical Basis **Key Point:** Mobitz I block occurs at the level of the **atrioventricular (AV) node**, which is the slowest-conducting part of the conduction system. The AV node has the following properties: - **Location:** Between the right atrium and right ventricle, in the AV groove - **Conduction velocity:** 0.02–0.05 m/s (slowest in the heart) - **Refractory period:** Longest of all conduction tissues (100–120 ms) - **Blood supply:** AV nodal artery (branch of RCA in 90% of people) ## Pathophysiology of Mobitz I ```mermaid flowchart TD A[Atrial impulse enters AV node]:::action --> B[Conduction through AV node progressively slows]:::action B --> C[PR interval lengthens on successive beats]:::outcome C --> D{AV node refractory?}:::decision D -->|Yes| E[P wave not conducted]:::outcome D -->|No| F[Normal AV conduction resumes]:::action E --> F F --> G[PR interval resets to baseline]:::outcome ``` ## High-Yield Facts **High-Yield:** Mobitz I is typically benign and often seen in athletes or with increased vagal tone (sleep, rest). It rarely progresses to complete heart block. **Clinical Pearl:** The syncope in this patient during exertion is unusual for Mobitz I (which is typically asymptomatic). However, the ECG pattern is diagnostic of AV nodal block. **Key Point:** The AV node is the **only site where conduction can be progressively delayed** in a cyclic manner — this is why Mobitz I is pathognomonic for AV nodal disease. ## Why Other Sites Don't Fit | Structure | Conduction Pattern | Why Not Mobitz I | | --- | --- | --- | | SA node | Generates impulse | Dysfunction causes sinus bradycardia or arrest, not progressive PR prolongation | | Bundle of His | Rapid conduction (1–1.5 m/s) | Blocks suddenly (Mobitz II); does not show progressive delay | | Purkinje fibres | Fastest conduction (2–4 m/s) | Blocks abruptly; no progressive delay mechanism |