Sinus Bradycardia in Acute Inferior Wall Myocardial Infarction
Introduction to Sinus Bradycardia
- The European Society of Cardiology guidelines state that sinus bradycardia is common in the first hour, especially in inferior infarction, due to increased vagal tone from ischemia of the inferior wall 1, 2
- The American College of Cardiology/American Heart Association guidelines confirm that sinus bradycardia 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 3
Pathophysiology of Sinus Bradycardia
- Increased vagal tone from ischemia of the inferior wall, which is supplied by the right coronary artery that also perfuses the AV node, contributes to sinus bradycardia 3, 4
- Bezold-Jarisch reflex activation triggered by reperfusion can also lead to sinus bradycardia 5
- Direct ischemia to the sinus node or AV node from decreased blood supply can cause sinus bradycardia 3
Other Rhythm Disturbances
- Type I Second-Degree AV Block is usually associated with inferior infarction and located above the His bundle, seldom causing adverse hemodynamic effects 1, 2
- Type I Second-Degree AV Block is generally well-tolerated with narrow QRS escape rhythm >40 bpm 7, 8
- First-Degree AV Block can occur but needs no treatment, and is less specific to inferior MI location 1, 4
Critical Clinical Distinctions
- 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 7, 8, 9
- Inferior MI produces transient bradycardia with narrow QRS and low mortality, while anterior MI produces unstable wide QRS escape rhythm with extremely high mortality (up to 80%) 7, 8