STEMI is the major cardiac emergency .The acute mortality is about 20% (Both prehospital and in CCU ) STEMI occurs whenever a coronary artery is occluded suddenly in toto .We traditionally believe that STEMI occurs only in the major epicardial vessels. (LAD/LCX/RCA .)
The total length of coronary tree is much longer than the length of the three vessels put together.The diagonals , the ramus, the OMs ,the septals run for varied distances. The caliber of these vessels can be quite large.It is estimated the diameter of first diagonal , the first OM or the ramus can be as big as LAD proper in 30% of CAD population . Law of statistics tells us sudden occlusion can occur any where in the coronary tree in ACS prone patients.
What is the real incidence of side branch STEMI ?
The dogmatic answer is ” We do not know”
Will we ever know ?
How will a Diagonal / OM /Ramus or PDA STEMI behave ?
It is surprising this question is not addressed by us for so long . Some may even question the existence of such an entity(Side branch STEMI ). This is most likely , reflect our ignorance on the issue . We know bifurcation lesions at the side branch origin is very common . Further , thrombus can migrate from a main stem to a side branch immediately after formation .
Clinical presentation of side branch STEMI
- Acute presentation is identical to that of a major main branch STEMI . The pain can be severe , the primary arrhythmic threat is real . Ischemic VF , once initiated does not modify it’s character according to the quantum of insult .
- ECG is the major variable. You , don’t expect gross , ST elevation in many leads as one would see in LAD MI /RCA MI.
- The age old teaching that an ECG can be entirely normal in acute MI , could actually imply the side branch STEMIs . When a small D2 or D3 gets occluded the ECG may not pick up the ST shifts .
- The commonest site of atherosclerosis apart from proximal LAD is the bifurcation of PDA in RCA. STEMI due to PDA occlusion is the most difficult thing to recognise. Many of them have very subtle ECG and clinical findings.
- There has been reports of acute complete heart blocks with isolated AV nodal infarcts. Here sudden cardiac deaths are reported
It is very much possible , many of the side branch MIs may be wrongly diagnosed as unstable angina by us , for the simple reason the myocardial necrosis is not large enough to produce ST elevation .They may actually respond to thrombolysis , as there is total occlusion in the coronary artery. Since, they do not manifest ST elevation there is a lost opportunity here . This , probably is the population in TIMI 3B trial that showed some ( statistically insignificant ) benefit for thrombolysis in NSTEMI.
Is primary PCI justified in side branch STEMI ?
May not be . The chances of side branch STEMI to result in LV dysfunction and progressive adverse remodeling is considerably less . The hazards of primary PCI for exceeds the risks of MI due to a septal or diagonal branch lesion .
Final message
- STEMI due to branch coronary artery occlusion is a less recognised entity among ACS.
- Cardiologists , need to look into this issue with little more seriousness as it could represent a new intermediate risk category among the much flaunted classification of acute coronary syndrome. Triaging and risk stratification of ACS needs a revamp.
- It is possible many of the UA patients , may in fact represent total occlusion of side branches.
- There is a definite case for showing less aggression in these patient subsets , provided we are sure about the location of lesion.
Counter point
* Identifying a side branch STEMI with confidence may be very difficult at bed side in an emergency . Implication of wrongly calling a STEMI as benign can be dangerous . So it will be argued , one need not do this exercise of traiging STEMI into main branch or side branch .
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Dr.S.Venkatesan MD 