A 72-year-old man with chronic hypertension and diabetes mellitus presents to the emergency department with a 3-hour history of severe chest pain radiating to the back. On examination, he is diaphoretic, blood pressure is 145/92 mmHg, heart rate is 58 bpm and irregular. Cardiac auscultation reveals an early diastolic murmur at the right upper sternal border. An ECG shows ST elevation in leads II, III, and aVF. Notably, the ECG also demonstrates some P waves that are not followed by QRS complexes, and some QRS complexes that appear without preceding P waves. The PR intervals of conducted beats vary. What is the most likely diagnosis?

Explanation

## Diagnosis: Third-Degree AV Block (Complete Heart Block) ### Clinical Presentation This patient has an acute **inferior MI with aortic dissection** (evidenced by the early diastolic murmur and chest pain radiating to the back). The combination of: - Some P waves **not** followed by QRS complexes - Some QRS complexes **without** preceding P waves - Varying PR intervals in conducted beats - Irregular rhythm at a slow rate (58 bpm) Indicates **complete dissociation between atrial and ventricular activity**. ### ECG Hallmarks of Third-Degree AV Block **Key Point:** Third-degree AV block is characterized by: 1. **Complete AV dissociation** — no relationship between P waves and QRS complexes 2. **Independent atrial and ventricular rates** — atria driven by sinus node, ventricles by escape pacemaker 3. **No conducted beats** — every P wave is blocked; every QRS is an escape beat 4. **Escape rhythm rate** typically 30–50 bpm (junctional) or 20–40 bpm (ventricular) **High-Yield:** In acute inferior MI, third-degree block is caused by **ischemia of the AV node** (RCA territory). It may be **transient** but requires urgent intervention. ### Pathophysiology **Clinical Pearl:** The inferior MI has caused: - Complete ischemic necrosis of the AV node - Loss of all AV conduction - Escape pacemaker (likely junctional given the relatively preserved rate) takes over - Atria continue to depolarize at sinus rate (~80–100 bpm) but cannot conduct - Ventricles beat independently at escape rate (~50–60 bpm) ### Management **Urgent:** Third-degree AV block in acute MI is a **Class IIa indication for temporary pacing**: - Transcutaneous pacing immediately - Transvenous pacing if hypotensive or symptomatic - Atropine 0.5–1 mg IV (may help if junctional escape; less effective for ventricular escape) - Prepare for possible permanent pacemaker if block persists >3–5 days ### Prognosis - In inferior MI: often **transient** (resolves within 3–7 days) - In anterior MI: often **permanent** (reflects extensive necrosis); worse prognosis **Mnemonic:** **"No Conduction, No Relation"** = Third-degree block: **N**o P-QRS relationship, **C**omplete dissociation