Posts Tagged ‘how to recognise multiple focus in a VT’

Ventricular tachycardia can be classified in a variety of ways. Monomorphic VT  and polymorphic VT  is one such classification based on VT morphology.Polymorphic VT  generally conveys a meaning of origin from multiple focus .But in reality  bulk of the polymorphic VT originate from a single focus .

How does a single focus have a multiple QRS  morphologies ?

This is possible because ,  even though VT arises from a single focus , the route it takes to exit  from the myocardium is different and hence they inscribe different QRS morphology for each  beat. It is also possible ,  as the conduction time varies with each  exit route  the VT  becomes  irregular. This  phenomenon   is called  polymorphic VT.

It is assumed the VT focus  is often located in the sub endocardium and breakthrough occur in the epicardial side as  we record  the activity  in the surface ECG. Electrophysiology of VT is not that simple ,  focus  of VT can be anywhere ,  the focus can be single or multiple and exit pathways  can also be multiple it and  it may even exit into endocardial cavity . Please note , even  a single  electrically abnormal cell  shall  act as a  focus . To confuse us further ,  some of  VTs  may not exit at all , extinguishes before reaching the surface.

It is  a  difficult  job to fix  a given  polymorphic VT  to arise   from a single focus or from multiple focus  .  Multifocal VT  can be  diagnosed  with  confidence only  after a through electro physiological  study.Clinically few clues are there. Electrolytic disorders and ischemia  are usually  multifocal,  while scars VT gives single focus .Other famous  example  of polymorphic VT are Torsedes de pointes .

Polymorphic VTs are  usually hemodynamically unstable but it is not a rule. Surprisingly ,some of the polymorphic  VTs are well tolerated . This is especially common in  multi focal polymorphic  VTs  which are hemodynamically and electrically  better off  . Ironically ,   presence of  multiple focus may be  a blessing  as they    compete with each other ,  in the process  pulling  down the other focus from triggering a VF . It  is possible  one focus acts like a natural anti tachycardia pacing for a  VT from another focus.

Ventricular fibrillation can be termed as an extreme form of irregular polymorphic VT as the wavefront breaks into innumerable fragments  each exiting the myocardium at will in a random fashion bringing ventricles to a standstill.

Final message

The term multifocal polymorphic  VT  is generally been abandoned at the bedside  as distinguishing  it from single focal  polymorphic VT  is a difficult task*.Still  the concept of  multifocal VT  is alive and  kicking in the EP labs ,  giving sleepless nights to  our Electro-cardiologists!

*Please note , multi focal VPDs can be recognised  with ease by different coupling intervals, but  it is difficult to identify during a  run of VT .

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