A 72-year-old man with chronic hypertension and diabetes presents with syncope. On examination, heart rate is 35 bpm and irregular, blood pressure is 85/55 mmHg. ECG shows P waves at a rate of 80 bpm and QRS complexes at 35 bpm, with no fixed relationship between them. The QRS duration is 0.12 seconds. What is the most appropriate initial management?

## Diagnosis: Third-Degree AV Block with Symptomatic Bradycardia ### Clinical Presentation The patient has complete dissociation between atrial and ventricular activity: - Atrial rate: 80 bpm (P waves) - Ventricular rate: 35 bpm (QRS complexes) - No fixed PR interval relationship - Syncope (indicating inadequate cerebral perfusion) - Hypotension (85/55 mmHg) - Wide QRS (0.12 s, indicating infranodal escape rhythm) ### Diagnosis: Third-Degree AV Block (Complete Heart Block) **Key Point:** Third-degree AV block is characterized by: - Complete dissociation of P waves and QRS complexes - No conducted beats - Independent atrial and ventricular rhythms - Escape rhythm originates below the block ### Pathophysiology The block is located below the AV node (infranodal), evidenced by the wide QRS (>0.12 s). This suggests disease in the His bundle or bundle branches, likely from degenerative conduction system disease in an elderly patient with comorbidities. ### Management Algorithm ```mermaid flowchart TD A[Third-Degree AV Block]:::outcome --> B{Symptomatic?}:::decision B -->|Yes| C{Hemodynamically stable?}:::decision B -->|No| D[Observation + ECG monitoring]:::action C -->|No - Hypotension/Syncope| E[Temporary pacing]:::action C -->|Yes| F[Prepare for permanent pacing]:::action E --> G[Transvenous pacemaker]:::action F --> G G --> H[Permanent pacemaker insertion]:::action ``` ### Why Temporary Transvenous Pacemaker? **High-Yield:** Symptomatic third-degree AV block with hemodynamic compromise requires **immediate pacing**. Temporary transvenous pacing is the bridge to permanent pacemaker implantation. **Clinical Pearl:** In third-degree AV block with infranodal block (wide QRS escape), atropine and isoproterenol are ineffective because the problem is below the AV node — these drugs act on the AV node. ### Why Not Other Options? | Option | Why Inappropriate | |--------|-------------------| | Atropine | Acts on AV node; block is infranodal (His/bundle branches). Ineffective. | | Isoproterenol | β-agonist increases AV nodal conduction; block is infranodal. Ineffective and may worsen ischemia. | | Digoxin | Negative chronotrope; will worsen bradycardia. Contraindicated in third-degree block. | **Warning:** Atropine and isoproterenol are only effective in **AV nodal** blocks (first-degree, Mobitz I, or nodal third-degree). This patient's wide QRS escape indicates an infranodal block — these drugs will not help. ### Permanent Management After stabilization with temporary pacing, the patient requires permanent pacemaker insertion (dual-chamber or single-chamber depending on clinical context).