Posts Tagged ‘polymorphic vt’

A  classification of  Polymorphic VT .

It can be classified according the etiology,  morphology  or hemodynamic stability . However , for some reason classifying  PVT with reference to the preceding QT interval and  the manner in which  tachycardia twists its axis along the qrs axis has been the most popular theme . ( Obviously ,the classification  process should  evolve further , and management strategies should be linked with it )

classification of polymorphic vt, torsades de pointes , long qt syndrome

Read a related article :Different Avatars of polymorphic VT

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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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It is one of the greatest innovation in medicine  . . . that is . . . electric current being  used as  a  drug to  treat disorders of heart . Of course ,  it is not a surprising finding  when we know heart is an  electro mechanical organ ,  and electricity can be used  to treat various disorders of heart by delivering it  in an optimal dosage and site.

Devices  that help administer  electric  current in cardiac disease.

  1. External  cardiovertor and defibrillator
  2. Implanted defibrillator
  3. Anti bradycardia  pacemaker
  4. Anti tachycardia pacing
  5. Cardiac  resynchronisation device

What  is  the  difference  between cardioversion and  pacing ?

Cardioversion  is reverting  a tachycardia with  a electric shock that is delivered diffusely throughout the heart This  electrical wavelets traverse the  focus of tachycardia  and the adjoining myocardium  which is called critical electrical mass (Usually reentrant) .This depolarises the cells responsible for tachycardia and extinguishes the abnormal electrical activity.

Defibrillation is same as cardiversion except that it is a high energy shock  and delivered without synchrony with qrs complex . In VF, we defibrillate in all others  we cardiovert .

What are the disadvantages of cardioversion ?

Eventhough it is a very successful modality for treating cardiac arrhythmias it also has some issues.

  • Cardioversion is not infallible. It rarely works in tachycardia due to enhanced automaticity (Multifocal atrial tacycardia , Automatic junctional tacycardia , Digoxin induced tacycardia it may even be dangerous !)
  • Many times multiple shocks are required and may result in myocardial damage, stunning , and elevated cardiac enzymes.
  • In susceptible patients, especially in elderly it may depress the natural pacemaker ie the sinus node and dangerous  bradycardia

over drive pacing paired pacing anti tachycardia

What is difference between cardioversion   applied externally on the chest wall and intracardiac cardiversion as in Implantable cardiovertor defibrillator(ICDS) ?

The underlying principle is same except that the energy required is a fraction of that applied in the chest wall . The average energy required is up to 20 joules . while it requires up to 300 joules

What is anti tachycardia pacing ?   Why this concept came into vogue ?

When it became clear , cardioversion may not work in all forms of tachycardia and risks of multiple shocks  on the myocardium  not be taken lightly , experts in those times (1970s)   thought  a pace maker lead in a optimal site can do the job of cardiovertor. .

Pacing rapidly  in the tachycardia zone  provide us an opportunity  to  enter  the  tachycardia circuit , interfering , interrupting  and blocking the reentrant circuit  (We call it entrainment)  . If it is an automatic tachycardia pacing in close vicinity of the tachycardia   focus result in a  electrical  line of  barrier  which acts as an  exit  block ( Like the lakshman  reka !  in Ramayana )

The term ATP is used as a  general term as anti tachycardia pacing .Over drive pacing  can be used synonymously.

What is the  main advantage of ATP ?

  • Less injury as it avoids recurrent shocks  .
  • Can be administered as many time as  required .
  • Some tachycardias specifically respond to ATP only (Read below)

How to perform overdrive pacing ?

  • Conventional  temporary  pacing lead can be used .
  • Special  leads  may be required for paired pacing .
  • The rate of pacing is generally between 50- 150 /mt
  • Technically we call it as  over drive pacing ,   in reality most  of the  time   pacing is domne at a lesser heart rate than the tachycardia itself .This should be recognised because, the term overdrive pacing connotes a meaning of pacing at more than the tachycardia rate.
  • The duration of pacing  may be 30 seconds to 2 mts as necessary .

Can we use the external transcutaneous pacemaker paddles for overdrive  pacing ?

Yes we can, it may be termed a  non invasive external overdrive pacing .This  mode is not popular among cardiologists  not because it is ineffective  , rather we have not fully realised it’s  potential .

Different types of  overdrive pacing

What is coupled pacing ?

It is a type of overdrive pacing where   pateint’s own sponataneous  rhythm   is used trigger a  pacemaker stimulus    and  hence only alternate beats or pacing beats which is coupled with the pateint,s own rhythm it is called coupled pacing . This  is different from from paired pacing  in that only single pacemaker stimulus per cycle .

What is paired pacing ?

Two pacing stimulus are given .The first impulse is maintained constant and the second impulse is done with varying coupling  interval to scan the entire cardiac cycle .It is expected at some point of paired pacing the second impulse would  block the reentrant circuit.

What is random paired pacing ?

The atrium is   delivered a   pair of random stimulus ( . . Like a bite of snake !) is  delivered into the atria .This can revert many of the reentrant atrial and ventricular  re entrant tachycardia.

What is the  unique value of  sinus paired  pacing ?

In patients  with persistent sinus tachycardia,  especially  in patients with  high MVo2 situations or dysfunctional ventricle we have no option to control the heart rate without depressing myocardial contraictility . Most of the negative chronotropic drugs have negative inotropic action also.  In these situations pairing a pacemaker stimulus with a sinus impulse can produce a compenstatry pause  and result  in reduction in net heart rate as well  as increased  contractility due to post extrasystolic potentiation.

How does a  catheter whip inside the atria   terminate many of the procedure related  tachycardias in cath lab ?

It is a common maneuver  in cath lab ,  to  forcibly whip the   catheter for   terminating  many of the transient procedure related  SVTs and outflow tract VTs . The arrhythmias get terminated  either due to catheter hit induced mechano  electrical  cardioversion   (5 joules ?) or  the atrial subendocardial stretch due to the  whip lash .

What are the tachycardias that may  respond to overdrive pacing ?

It is logical to expect any of the reentrant tachycardia  might respond to ATP. The  exact success rate can  not be established  since this modality  is not applied  in vast majority of  patients . Only if a patient  is not responding to drugs or multiple DC shocks ATP is thought off . Of course ATP can not  considered  a first option   unless othe  patient is  on a temporary pacer.

What is the caution for using ATP ? Why  atrial overdrive pacing   is preferred over  ventricular  overdrive pacing ?

Pacing a ventricle rapidly carries a risk of inducing ventricular fibrillation . So whenever  possible ATP  should be administered  through  an  atrial lead. This may not be possible always as in the presence of AV block a VT  can not be captured  by atrial pacing  .

It is also  a fact  many times   when the    ventricular overdrive pacing  fails to revert a VT , an  atrial overdrive pacing has been successful . This is due to the  more uniform    depolarization  wave fronts , that reach the ventricle and reset the VT .

Currently ATP is useful in

  • Recurrent atrial tachycardia
  • Refractory ventricular tachycardia especially with enhanced automaticity (Early ischemic VT )
  • Digoxin induced tachycardias
  • Some cases of Tachy brady syndrome

In some of the modern pacemakers and  in all ICDs ATP is a an important programmable parameter .In fact using this mode liberally would conserve battery life .Many times a simple hemodynamically stable VTs are shocked by ICDs  instead an ATP will  do the job . It is a well recognised fact that   ATP is underutilsed in ICDs .This issue needs to be addressed.

Final message

Pacemakers are not only meant to treat bradycardias but also tachycardia . Even though it is a well known fact for over 3 decades , for some reason this simple and effective concept is not getting  the  attention of  the current generation cardiologists  which it definitely deserves!


  1. Overdrive Pacing for Ventricular Tachyarrhythmias: A Reassessment    P. R. KOWEY andT. R. ENGEL
    ANN INTERN MED November 1, 1983 99:651-656
  2. Pacing Techniques in the Treatment of Tachycardias  I. WIENER  ANN INTERN MED August 1, 1980 93:326-329
  3. Treatment of Recurrent Symptomatic Ventricular Tachycardia R. A. WINKLE, E. L. ALDERMAN, J. W. FITZGERALD, and D. C. HARRISON ANN INTERN MED July 1, 1976 85:1-7
  4. Treatment of Tachyarrhythmias by Pacing J. E. Batchelder andD. P. Zipes

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