Inferior STEMI is as common as Anterior STEMI .Unlike the anterior STMI which auto localises to LAD , inferior STEMI has to be fixed either RCA or LCX.
Following ECG features help localize Inferior STEMI .
- ST elevation in lead 3 > lead 2 suggest RCA (Not always true )
- ST depression in lead V1,V2,V3 strongly suggest LCX. (More objectively the sum of ST depression in V1, 2 , 3 divided by sum ST elevation in 2,3, AVF , if less than 1 indicate LCX. Or simply ST depression V3 > Lead 3 indicate LCX.)
- ST depression in lead 1 indicate RCA
- ST elevation in lead V6 strongly suggest LCX
Finally , and most importantly RV infarction as documented by ST elevation in V4R almost always localises the lesion in proximal RCA.
Role of Echo
If ECG features are not clear , a rapid bed side echo has a very good localizing value. To fix RCA look specifically for wall motion defect between “6 to 8” O-clock position .It corresponds to infero basal septum that is invariably supplied by RCA. For LCX involvement concentrate on “3 to 6” o clock position.
Which has better outcome RCA or LCX STEMI ?
- Though RV infarction does not occur with LCX , incidence of MR is more with LCX and can be truly troublesome. This probably negates the potential advantage of “protected RV” in LCX STEMI.
- Since LV lateral free wall involvement is extremely rare with RCA STEMI , it has a lesser impact on LV function while LCX STEMI can give a double blow to LV (MR and LV dysfunction)
- On the down side ,coronary artery spasm and thrombus load are more with RCA .
Interventions in RCA is fairly straightforward ,while acute LCX PCI has some issues . Apart from technicalities of intubating the posteriorly curving LCX ,realistically it involves fishing in troubled waters , as we need to cross the left main , likely physical contacts with LAD ostium , which is the sole supply chain for the injured and ischemic LV myocardium . Meanwhile , If RCA is the culprit , its a well cordoned crime scene where one can spend time liberally and fix the lesion.
It is easier to localisethe culprit artery in inferior STEMI ,but its a tricky to predict outcome .Both can be troublesome .It depends on dominance of the RCA/LCX ,proximal nature of lesion, the number and caliber of OMs, and PLVs and RV branch .However, it remains a fact LCX STEMI has a overall turbulent course.