Bradycardia and AV Block in Inferior STEMI
Up to 20% of patients with inferior STEMI will develop either second- or third degree heart block. Bradyarrhythmias and AV block in the context of inferior STEMI are usually transient (lasting hours to days), respond well to atropine and do not require permanent pacing.
There are three presumed mechanisms for this:
The conduction block may develop either as a step-wise progression from 1st degree heart block via Wenckebach to complete heart block (in 50% of cases) or as abrupt onset of second or third-degree heart block (in the remaining 50%).
Patients may also manifest signs of sinus node dysfunction, such as sinus bradycardia, sinus pauses, sinoatrial exit block and sinus arrest. Similarly to AV node dysfunction, this may result from increased vagal tone or ischaemia of the SA node (the SA nodal artery is supplied by the RCA in 60% of people).
The Bezold-Jarisch reflex is an inhibitory reflex usually denoted as a cardioinhibitory reflex defined as bradycardia, vasodilation, and hypotension resulting from stimulation of cardiac receptors. In acute inferoposterior myocardial infarction:
- Cardioprotective effect: It is thought that the cardiodepressor Bezold-Jarisch reflex (BJR) acts as a protective reflex to vasodilate the coronary arteries in the setting of inferoposterior infarction associated with coronary vasospasm.
- The BJR is thought to be responsible for up to 60% of bradyarrhythmia occurring within the first 60 minutes of infarction and for the occurrence of AV nodal block in the context of acute posterior or inferior myocardial infarction.
- Reperfusion: The BJR in acute inferior myocardial infarction represents a reliable prognosticator of timely reperfusion and sustained coronary patency. It occurs in 94% of cases of successful early thrmobolysis in inferior MI; and 89% of patent right coronary artery.
- Stimulation of vagal afferents in response to sympathetic overactivity may be the underlying pathogenetic mechanism promoting a BJR response. [Clin Cardiol. 2003]
