Cardiovascular · Year 1 · from Cardiovascular
Case 2: Complete Heart Block (Third-Degree AV Block)
Patient Presentation
Demographics: 72-year-old female
Chief Complaint: Syncope and fatigue
History of Present Illness: A 72-year-old female with history of hypertension presents after a witnessed syncopal episode while sitting in a chair. She lost consciousness for approximately 30 seconds without seizure activity and regained consciousness spontaneously. Over the past 2 weeks, she has noted progressive fatigue and dizziness. She denies chest pain or shortness of breath.
Physical Examination:
- Vital Signs: BP 140/85 mmHg, HR 35 bpm (regular), RR 14/min, SpO2 98% on room air
- General: Alert, no acute distress
- Cardiovascular: Bradycardic, regular rhythm, variable intensity S1 (cannon A waves in JVP), no murmurs
- Neurological: No focal deficits
Workup
- ECG: Complete AV dissociation with atrial rate 80 bpm and ventricular rate 35 bpm; narrow QRS escape rhythm; P waves bear no relationship to QRS complexes
- Labs: Troponin negative, electrolytes normal, TSH normal
- Echocardiogram: Normal LV function, no structural abnormalities
- Telemetry: Persistent complete heart block
Diagnosis
Complete (third-degree) atrioventricular block with junctional escape rhythm
Electrophysiology Correlation: Third-degree AV block occurs when no atrial impulses conduct to the ventricles, resulting in complete AV dissociation. The escape rhythm depends on where the block occurs:
- Block at AV node → junctional escape (narrow QRS, 40-60 bpm)
- Block below AV node (infranodal) → ventricular escape (wide QRS, 20-40 bpm)
This patient's narrow QRS escape suggests the block is at the AV node level. The variable S1 intensity occurs because of changing timing between atrial and ventricular contraction. Cannon A waves occur when the atrium contracts against a closed tricuspid valve.
Treatment
- Temporary transcutaneous pacing standby
- Atropine (limited efficacy for complete heart block)
- Permanent pacemaker implantation (definitive treatment)
- Dual-chamber pacemaker (DDD) indicated for AV block with sinus node function
- Hold any AV nodal blocking medications
Clinical Image
Image Description: ECG rhythm strip showing complete heart block with AV dissociation. P waves (atrial activity) occur regularly but independently of QRS complexes (ventricular activity). The ventricular rate is slow with a junctional escape rhythm.
Source: Wikimedia Commons - Third-degree AV block License: CC BY-SA 4.0 URL: https://commons.wikimedia.org/wiki/File:Third_degree_heart_block.png