Mark E Jospehson is the man who single-handedly carried the burden of teaching generations of electro-physiologists from Harvard Thorndike electro physiology services , Boston USA. Today , whatever we know about the mechanisms of VT , it is because of such great men who spent thousands of hours in the first generation EP labs in early 1970s and 80s , meticulously analysing the data emanating from over 600 scar mediated VT with complex circuitry .
He along with Miller published this seminal paper in circulation 1988 , which gave us the algorithm that localises Post MI VTs.
Following table summarises their finding.
The general principles of localisation of VT
- Localising VT following myocardial infarction is difficult but distinctly possible with about 60 % accuracy.
- Whenever we locate a focus we generally refer to epicardial site of exit not the focus of origin.
- Ischemic VTs with complex scars are difficult to locate .
- The rule that RBBB VT arise from LV and LBBB VT from RV is too simplistic in scar mediated VT.
- The fact that IVS is common to both RV and LV confounds the issue .Further, in a given clinical VT the origin , course and exit points of VT can considerably vary .For example septal VT can exit on either side and result in either RBBB or LBBB morphology (Epicardial break thorough )
- Multiple exit points are also possible.
- VT induced in EP lab may not be reproducing the same clinical VT. So we have to be careful in what we ablate and claim success !
- VT with structurally normal heart has more predictable behavior , for example RVOT VT almost always have LBBB morphology.
Other important rules of thumb are
- LBBB VT has more localising value .
- Superior axis is the most common axis.
- Bulk of the ischemic VT are located within the septum either in the apical or basal region .(75%)
- Infero posterior MI has more complex scars , hence VT morphology is heterogeneous.
The purpose of localising VT is important only with reference to ablation.(Of course for academic reasons as well ) With advent of electro anatomic imaging (Carto ) it is becoming easier to locate and track them . Still only a minority of VTs are amenable for RF ablation .
Please note , the most common modalities we use in the management of VT , Amiodarone and ICDs simply do not bother about focus of origin for it’s action ! That makes our job easy !