A 72-year-old woman with chronic hypertension and diabetes mellitus presents with syncope. ECG shows a regular rhythm with a PR interval of 200 ms, but every third P wave is not followed by a QRS complex. The QRS duration is 120 ms. She is hypotensive (90/60 mmHg) and has a heart rate of 40 bpm. What is the most appropriate immediate management?

## Clinical Context: Mobitz II AV Block with Hemodynamic Compromise ### ECG Interpretation **Key Point:** Every third P wave not followed by a QRS complex = 3:1 AV block (a form of high-degree AV block). The constant PR interval (200 ms) before dropped beats and the widened QRS (120 ms) indicate the block is **below the AV node** (His/Purkinje system). **Clinical Pearl:** Mobitz II (or high-degree block in the infranodal region) is a **pathological block** with high risk of sudden progression to complete heart block. ### Differential Diagnosis: Mobitz I vs. Mobitz II | Feature | Mobitz I | Mobitz II / High-Degree | |---------|----------|------------------------| | Site | AV node | Below AV node (His/Purkinje) | | PR interval pattern | Progressive lengthening | Constant, then sudden drop | | QRS width | Narrow (< 120 ms) | Wide (≥ 120 ms) | | Prognosis | Benign, self-limiting | Dangerous, may progress to complete block | | Hemodynamic effect | Usually well-tolerated | Often causes hypotension/syncope | | Pacemaker indication | Rarely | Usually required | ### Why This Patient Needs Immediate Pacing 1. **Hemodynamic instability:** BP 90/60 mmHg, HR 40 bpm → inadequate cardiac output 2. **Syncope:** Indicates cerebral hypoperfusion from severe bradycardia 3. **High-degree block:** 3:1 conduction with infranodal site → imminent risk of complete block 4. **Wide QRS:** Confirms infranodal pathology (not reversible AV nodal ischemia) **High-Yield:** Mobitz II and high-degree AV blocks with hemodynamic compromise require **temporary pacemaker insertion** as an emergency measure. This is a **class I indication** for temporary pacing. ### Temporary vs. Permanent Pacemaker - **Temporary:** Immediate stabilization in acute symptomatic block - **Permanent:** Considered if the block does not resolve or if recurrent high-degree block occurs **Mnemonic:** **SYMPTOMATIC MOBITZ II = PACE** — if the patient is Symptomatic (syncope, hypotension) with Mobitz II or high-degree block, Pacing is Appropriate and Crucial for Emergency stabilization. [cite:Harrison 21e Ch 297]