What’s the Connection Between STEMI and AV Block?
Basic things that you need to know about one of the most important complications post-STEMI!
Let’s start with a few questions for you! We’ve discussed this before: the principle of generation is very powerful when it comes to learning! So, please use it by answering these questions BEFORE you read the article.
Let’s dive in…
In my previous post, “Bradycardia | The 4 findings you MUST look for”, I presented an ECG showing 2nd-degree AV block (Mobitz I) and asked the following question:
Do you see any other major abnormalities? If so, could they be related to the AV block present in this case?
Here is the ECG again:1
Let’s analyse this ECG following the systematic approach that we’ve discussed before:
In summary, this ECG shows sinus rhythm with 1st-degree AV block and features consistent with an inferior STEMI.
Inferior STEMI and AV block
Is there a connection between the inferior STEMI and the AV block? Yes!
Firstly, let's define what STEMI is. It happens when a coronary artery is completely blocked. In this case, we need to determine which coronary artery is likely to be obstructed. In other words, which coronary artery provides blood to the inferior wall of the left ventricle?
The RCA perfuses the inferior wall of the left ventricle
As we can see in the following image showing the anatomy of the coronary arteries,2 the posterior descending artery (PDA) is the main branch that supplies blood to the inferior wall of the left ventricle.
This branch most commonly comes from the distal right coronary artery (RCA) (in 85% of the population).
The coronary circulation in the majority of people, where the PDA arises from the RCA, is termed right dominant.3
Based on the ECG shown above, which indicates an inferior STEMI (i.e., ongoing ischemia in the inferior wall of the left ventricle), we can predict that the right coronary artery (RCA) is most likely obstructed.
The RCA also perfuses the AV node
The AV nodal branch supplies blood to the AV node and also comes from the RCA (in 90% of the population).
So, here is the answer: the two findings (inferior STEMI and 1st-degree AV block) are connected because the same artery (RCA) supplies both the inferior part of the LV and the AV node.
Sinus bradycardia is also common following an inferior STEMI - here’s why
We have already talked about AV block following an inferior STEMI.
Sinus bradycardia is also common, especially after an inferior STEMI. This is because the sinoatrial (SA) node is supplied by the RCA in 60% of the population and by the left circumflex (LCx) artery in 40%. (4)
Conduction abnormalities following a STEMI: are they exclusive to inferior STEMIs?
An inferior STEMI is most likely secondary to occlusion of the RCA, as mentioned above. In view of the coronary artery anatomy, it’s more common to have an AV block following an RCA occlusion.
HOWEVER, AV block can also occur following an anterior STEMI, which is most likely due to occlusion of the left anterior descending (LAD) artery. In fact, this kind of AV block is usually infra-Hisian (below the His bundle) and has a higher mortality rate.4
The fact that it's infra-Hisian means that it doesn't allow the junctional area (which corresponds to the area of the AV node - His bundle) to produce and conduct a junctional escape rhythm to the ventricles (which is a rhythm that is more well-tolerated than the usually slower ventricular escape rhythm).
Summary
Conduction abnormalities are a significant complication of cardiac ischemia. They most frequently occur after an occlusion of the right coronary artery (RCA) but are associated with higher mortality following an occlusion of the left anterior descending (LAD) artery.
Jer5150, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Coronary.pdf: Patrick J. Lynch, medical illustratorderivative work: Fred the Oysteradaption and further labeling: Mikael Häggström, M.D. Author info - Reusing images- Conflicts of interest: NoneMikael Häggström, M.D., CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Lilly, L. S. (2021). Pathophysiology of Heart Disease (7th ed.). Wolters Kluwer.
Borja Ibanez and others, 2017 ESC Guidelines for the management of acute myocardial infarction in patients presenting with ST-segment elevation: The Task Force for the management of acute myocardial infarction in patients presenting with ST-segment elevation of the European Society of Cardiology (ESC), European Heart Journal, Volume 39, Issue 2, 07 January 2018, Pages 119–177, https://doi.org/10.1093/eurheartj/ehx393