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Position the exact blockage

Reading File
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Searching PubMed

"inferior wall myocardial infarction" AND "atrioventricular block"

Likely culprit: acute occlusion of the right coronary artery (RCA), proximal to mid segment, with the block at the AV node. This is my reading of the photos, which are rotated and taken at an angle. Confirm it on the original tracing.
ECG findings (21:05:39 tracing)
  • The rate is 33-36/min, regular, with a wide-ish QRS of 123-126 ms. The machine reads third-degree AV block with a ventricular escape rhythm. It printed P = 0 and PR = 0, so it found no P waves. I could not confirm P waves reliably in the photo. The first tracing (21:06:32) is labelled "sinus bradycardia, complete RBBB", which conflicts with the second, so check the rhythm strip by eye.
  • ST elevation is present in II, III and aVF.
  • ST depression is present in aVL, which is reciprocal, and in V1-V3, which suggests posterior involvement.
  • The T waves in the inferior leads are peaked, and there is right axis deviation.
This pattern is an inferoposterior STEMI with complete heart block.
Why the RCA
  • Marked reciprocal ST depression in aVL points to the RCA rather than the circumflex. Check that ST elevation in III is greater than in II, and that lead I shows no ST depression.
  • The AV node is supplied by the AV nodal branch of the RCA. The SA node is also usually supplied by the RCA, which fits the sinus bradycardia on the first tracing. [Tintinalli's Emergency Medicine, p. 3681] says the conduction system is supplied by the AV branch of the RCA and the septal perforators of the LAD.
  • A culprit lesion proximal to the RV marginal branches would also cause RV infarction. Record right-sided leads (V4R) and posterior leads (V7-V9).
  • The circumflex is the alternative culprit if the patient has a left-dominant system. Angiography will settle it.
Level of the conduction block
  • In inferior MI the block is usually in the AV node, with a more stable, narrow escape rhythm [Harrison's 22E, AV Block in the setting of myocardial ischemia].
  • It is located proximal to the His bundle in about 90% of cases. Narrow junctional escape rates are typically above 40/min, and the block is often reversible.
  • This tracing has two features against a typical nodal block: a rate of 33-36 and a QRS of about 125 ms with an RBBB pattern. That could be a junctional escape with a pre-existing or new RBBB, or a lower, infranodal escape. These features make the escape less reliable, so don't assume a benign nodal block.
  • Harrison's also notes that CHB in anterior MI may be preceded by RBBB. This tracing is inferior, but the RBBB and wide escape warrant a careful look.
Next steps
  • Treat this as a STEMI and arrange primary PCI of the RCA.
  • Prepare for temporary pacing, given the rate of about 35 with a wide escape.
  • Atropine may help if the block is nodal, but use it cautiously.
  • Check potassium. Peaked T waves with bradycardia and a wide QRS can also be hyperkalemia, which can mimic or add to this picture.
  • Check for RV infarction before giving nitrates or diuretics.
A recent systematic review looked at aVR ST deviation for predicting the infarct-related artery in inferior STEMI [PMID 41581264]. It does not contradict the above.
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