Sinus Bradycardia is the Most Common Rhythm in Acute Inferior Wall MI
Sinus bradycardia is the rhythm most commonly associated with acute inferior wall myocardial infarction, occurring frequently in the first hour after symptom onset. 1
Why Sinus Bradycardia Predominates in Inferior MI
The European Society of Cardiology guidelines explicitly state that "sinus bradycardia is common in the first hour, especially in inferior infarction." 1 This occurs due to:
- Increased vagal tone from ischemia of the inferior wall, which is supplied by the right coronary artery that also perfuses the AV node 1
- Bezold-Jarisch reflex activation triggered by reperfusion 1
- Direct ischemia to the sinus node or AV node from decreased blood supply 1
The ACC/AHA guidelines confirm that "SND and atrioventricular block in the setting of an inferior wall MI may be attributable to a transient increase in vagal tone or decreased blood supply to the atrioventricular node or less commonly the sinus node." 1
Understanding the Other Rhythm Disturbances
While the question asks about the most common rhythm, it's critical to understand the full spectrum:
Type I Second-Degree AV Block (Wenckebach)
- Usually associated with inferior infarction and located above the His bundle 1
- Seldom causes adverse hemodynamic effects 1
- Generally well-tolerated with narrow QRS escape rhythm >40 bpm 2
- Transient, typically resolving within 7 days 1
First-Degree AV Block
- Can occur but needs no treatment 1
- Less specific to inferior MI location
Type II Second-Degree AV Block (Mobitz II)
- Has not been reported in inferior MI according to research evidence 3
- Almost always infranodal and associated with anterior MI, not inferior 3, 4
- Indicates extensive myocardial damage when present 1
Torsades de Pointes
- Not a characteristic rhythm of acute inferior MI
- Typically associated with QT prolongation from other causes
Critical Clinical Distinction
The key anatomic principle: AV block associated with inferior wall infarction is located above the His bundle in the vast majority of cases, whereas AV block with anterior MI is located below the AV node. 1 This explains why:
- Inferior MI produces transient bradycardia with narrow QRS and low mortality 1
- Anterior MI produces unstable wide QRS escape rhythm with extremely high mortality (up to 80%) 1
Management Implications
For sinus bradycardia in inferior MI:
- Atropine 0.3-0.5 mg IV (up to 1.5-2.0 mg total) is reasonable if symptomatic or hemodynamically significant 1
- Later in the course, it's usually a favorable sign requiring no treatment 1
- Temporary pacing may be needed if atropine fails and hypotension persists 1
Common pitfall: Do not confuse the most common rhythm (sinus bradycardia) with rhythms requiring pacing. Type I second-degree block, while also associated with inferior MI, is less frequent than simple sinus bradycardia but may require intervention if hemodynamically significant. 1