Ventricular tachycardia is a regular wide qrs tachycardia.It can be monomorphic or polymorphic.
General diagnostic rules In VT (Gross though, with considerable overlap)
- Polymorphic VT is more often Ischemic , drug induced, electrolytic, and includes many inherited VTs .Most primary ischemic VT are polymorphic.
- Monomorphic VT occurs more common with structurally abnormal heart.(DCM, HOCM, ARVD etc .Please note late scar induced VT are often monomorphic , which is also being referred to as Ischemic VT in literature )
*Its important to realise any VT will transform to polymorphic just prior to degenerating into VF.
Management Issues.
The management of VT in acute setting is same irrespective of morphology of QRS complex.Either you DC shock or administer Amiodarone, Lignocaine , and other reserve drugs.
The issue comes only in stable VTs.In stable VT or if VT recurrence it’s advisable to bother about the ethology and choose a drug.Its believed , Amiodarone is contraindicated in true polymorphic VT that was precipitated by prolonged QT interval or Brugada syndrome.
In Ischemic VT , lignocaine may be preferable over Amiodarone as the later may prolong the QT interval and VT could recur if the index VT was triggered by ischemia induced prolonged QT and subsequently gets worsened by the drug Amiodarone.(please note, Lignocaine has neutral or even shorten the QT)
Let me conclude with a controversial observation, many of VT storms we are witnessing only in the era of Amiodarone.Most episodes of VT Storm are polymorphic and often precipitated after blouses of Amiodarone punctuated with DC shock. With an explicit pro arrhythmic potential of this drug, I strongly believe some of the episodes of VT Storm is iatrogenic and it tends to disappear as the drug effect of Amiodarone weans off.
Final message
In monomorphic VTs, drug choice and selection may not be that important , polymorphic it could make a big difference !
Always ask a query whether the VT you are tackling (in any setting )is likely to have precipitated by prolonged Action potential duration (Read as QT Interval ) ? Of course ,one can’t get a clear answer to this in bed side .But, if you have strong reasons to suspect , better to avoid drugs that prolong action potential duration ( Amiodarone comes top in this list, though it can terminate VT of any ethology with any morphology because it has all 4 group action of Waghaun williams !)
Comments welcome from EP experts , still to understand things in perspective.
Dr.S.Venkatesan MD 