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 ?
- Irregularity is observed immediately at the onset of VT as the re- entrant circuit warms up and tries tosettle down
- AV dissociation can make the VT irregular but it is subtle .(This AV dissociation is absent if retrograde VA conduction is intact)
- Multiple reentry circuits with two morphological VTs dissociating themselves
- VT with multiple exit points and epicardial breakthroughs
- 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.
- Multi- focal VT (We have MAT in atria do we really have MVT ? (Why not , are we missing it ?)
Statistically , as well as realistically , the commonest cause for any highly irregular tachycardia turns out to be AF , whether QRS is wide or narrow !