The site of LAD occlusion (proximal versus distal) predicts both infarct size and prognosis.
- Proximal LAD / LMCA occlusion has a significantly worse prognosis due to larger infarct territory size and more severe haemodynamic disturbance
- The site of occlusion can be inferred from the pattern of ST changes in leads corresponding to the two most proximal branches of the LAD: the first septal branch (S1) and the first diagonal branch (D1).
Territories
- S1 supplies the basal part of the interventricular septum, including the bundle branches (corresponding to leads aVR and V1)
- D1 supplies the high lateral region of the heart (leads I and aVL)
Occlusion proximal to S1
Signs of basal septal involvement:
- ST elevation in aVR
- ST elevation in V1 > 2.5 mm
- Complete RBBB
- ST depression in V5
Occlusion proximal to D1
Signs of high lateral involvement:
- ST elevation / Q-wave formation in aVL and I
- ST depression ≥ 1 mm in II, III or aVF (reciprocal to STE in aVL)
In the context of anterior STEMI, ST elevation in aVR of any magnitude is 43% sensitive and 95% specific for LAD occlusion proximal to S1. Right bundle branch block in anterior MI is an independent marker of poor prognosis; this is due to the extensive myocardial damage involved rather than the conduction disorder itself.
