Let us clear the confusion about ventricular tachycardia :Is it a regular or irreguar tachycardia?
Posted Nov 18 2009 10:01pm
Ventricular tachycardia is the most common wide qrs tachycardia.It is generally taught VT is a regular wide qrs tachycardia.It indeed appears regular most of the time but in reality it is not.
There are few situations it shows irregularity . Of course , this irregularity may be due to the associated phenomenon , even as the VT focus fire regularly. But for a physician what is manifested in the given ECG matters.
Frequent capture beats and fusion beats.
We know AV dissociation is sine qua non of VT. Not withstanding this fact , the sinus impulses always try to enter into the ventricle and looking for the door at AV node to open.Even a fraction of a second is enough for the sinus impulse to sneak through it , the only question is the timing .It should be noted that an intact VA conduction precludes antegrade AV conduction and fusion and capture beats are rare. So whenever the VA conduction lags behind or sluggish , more capture beats occur.This obviously make a VT irregular. This sort of irregular VT can occur in up to 20% .So it is indeed wrong to assume that the presence of irregularity one should make a diagnosis of VT.
VT induction phase cycle length fluctuations.
The other reason for VT may be irregular during the early minutes after the onset of VT. Here the electrical circuit fluctuates till it attains a steady state.
The VT often takes off with a turbulent course (Bumpy myocardial boulders due to electrical reactionaries and mechanical scars ) Then the circuit optimises .This happens more so in post MI scar induced VT where the tract can be long and circuitous with considerable delay in conduction. But, once the VT reaches the cruising speed it becomes fairly regular. In ischemia mediated micro reentry or automatic focal VT cycle length variations are less common and they maintain the classic property of the VT namely the regularity.
Drugged VT: The Amiodarone effect.
The effect of class 1 a or 3 drugs on VT is a complex one.Either they revert to SR or it may reduce the VT rate, widen the qrs complex, and make it little irregular.This is especailly common after the cumulative amiodarone effect.
The VT that has polymorphism obviously will be irregular.Torsades is the typical example.
Multifocal ventricular tachycardia(MVT)
Every one is aware of multi focal atrial tachycardia.It is surprising the MVT as an entity is rarely described in literature.Are the ventricles protected against such arrhythmia ? When multifocal VPDs occur very often why it is not transforming into MVT ? It is possible , once a VT is initiated it suppresses the other focus like a overdrive pacer and extinguishes all surrounding electrical activity. But as in parasytolic tachycardia MVT can occur occasionally when each focus is protected against the other by an entry block.
A VT may switch over from one focus to other.During the time of transition or competition between the two focus if you happen to record a ECG it can be really irregular and chaotic !
VT is a regular wide qrs tachycardia in majority.But this rule is applied only in monomorphic unifocal VTs. Even in monomorphic VTs there are occasions it may show irregularity due the associated phenomenon.A grossly irregular wide qrs tachycardia always indicate a antidromic AF with accessory pathway.
* Title talks: With due respect to the experts ,there is no reason for getting confused .For the practical working formula VT can be considered as regular tachycardia.