Dr.S.Venkatesan MD

We know , any wide QRS tachycardia  would argue us to make a default diagnosis of VT.But,  one has to be extremely cautious to apply this rule if  wide QRS  tachycardia shows  significant irregularity in RR interval .

All classical VTs are fairly regular tachycardia (Note the  key words , fair and regular) . Small cycle length variations are observed in VT,  but they are usually not discernible in surface ECG.

There are no practical rules .A well  appreciable  irregularity in RR interval will seriously  question the diagnosis of classical VT. To make an another statement, most of the  irregular wide QRS tachycardias infact turns out to be  atrial fibrillation with some form distal widening mechanism .(Preexisting blocks, or rate dependent  or antidromic conduction through accessory  pathways)

However , irregularity  is still possible during VT .(May be less than 10% of times)

When can VT can be irregular ?

1. Irregularity is observed  immediately at the onset of VT as the re- entrant circuit warms up and tries tosettle down
2. AV dissociation  can make the VT irregular but it is subtle .(This AV dissociation is absent if retrograde VA conduction is intact)
3. Multiple reentry circuits with two morphological VTs dissociating themselves
4. VT with multiple exit points and epicardial breakthroughs
5. A drugged VT.Amiodarone modified VT can be irregular as it can variably lengthens  the re-entrant  circuits and inducing VA block and precipitating AV dissociation.
6. Multi- focal VT  (We have MAT in atria do we really have MVT ? (Why not , are we missing it ?)

Final message

Statistically , as well as realistically  , the commonest cause for  any highly irregular tachycardia turns out to be AF , whether  QRS is wide or narrow !