Inferior MI and Associated Heart Block
Inferior myocardial infarction is most commonly associated with first-degree AV block and Type I second-degree AV block (Mobitz I/Wenckebach), though complete (third-degree) heart block can also occur, all typically occurring at the level of the AV node with a favorable prognosis compared to anterior MI. 1, 2
Pathophysiology of AV Block in Inferior MI
The mechanism differs fundamentally from anterior MI:
- Increased vagal tone from ischemia of the inferior wall (supplied by the right coronary artery which also perfuses the AV node) is the primary mechanism 2
- The block occurs above the His bundle (at the AV node level) in the vast majority of inferior MI cases 2, 3
- Direct ischemia to the AV node from decreased blood supply contributes to conduction disturbances 2
- The Bezold-Jarisch reflex activation triggered by reperfusion can also lead to bradycardia 2
Specific Types of Heart Block in Inferior MI
Most Common Patterns
- Sinus bradycardia is extremely common in the first hour, especially in inferior infarction 1, 2
- First-degree AV block (PR interval >200 ms) occurs frequently but needs no treatment 1, 2
- Type I second-degree AV block (Mobitz I/Wenckebach) is usually associated with inferior infarction, seldom causes adverse hemodynamic effects, and is generally well-tolerated with narrow QRS escape rhythm >40 bpm 1, 2, 3
Less Common but Important
- Complete (third-degree) AV block can occur in 20% of inferior MI patients, though it is typically transient 4, 5, 6
- Advanced AV block in inferior MI is characterized by narrow QRS complexes, indicating the block is at the AV node level 2, 7
Critical Clinical Distinction: Inferior vs Anterior MI
This distinction is life-saving:
- Inferior MI with AV block: Located above the His bundle, produces transient bradycardia with narrow QRS escape rhythm, and has relatively low mortality 2, 3
- Anterior MI with AV block: Located below the AV node (His-Purkinje system), produces unstable wide QRS escape rhythm, and carries extremely high mortality up to 80% due to extensive myocardial necrosis 2, 3
Management Approach
Initial Treatment
- Atropine 0.3-0.5 mg IV (repeated up to total 1.5-2.0 mg) is first-line for symptomatic bradycardia or hypotension 1, 2
- Use atropine cautiously as increased heart rate may worsen ischemia 1
- If atropine fails and hypotension persists, temporary pacing should be instituted 1, 2
Pacing Indications
Class IIa indications for temporary pacing in inferior MI include: 1
- Symptomatic bradycardia unresponsive to atropine 1
- Mobitz type II second-degree AV block (though this is rare in inferior MI) 1
Permanent pacing is seldom necessary unless the block persists for more than 14-16 days 1, 3
Common Pitfalls
- Do not routinely place temporary pacemakers in all inferior MI patients with heart block—only when specific hemodynamic indications are present 5
- Type I second-degree AV block in inferior MI is generally benign and does not require pacing unless causing hypotension or heart failure 1
- Transient AV block without intraventricular conduction defects does not require permanent pacing 3
Prognostic Considerations
- Patients with AV block in inferior MI have higher in-hospital mortality (24-28%) compared to those without block (4-13%) 5, 6
- However, 3-year post-hospital mortality is similar between those who had AV block and those who did not, indicating the block itself does not worsen long-term prognosis in survivors 6
- Early-appearing advanced AV block (during hyperacute stage) carries worse prognosis with more syncope, heart failure, and cardiogenic shock compared to late-appearing block 4
- The presence of bundle branch block with AV block in inferior MI significantly worsens prognosis and increases mortality 6