AV dissociation is the most specific diagnostic clue in VT.But this is not a constant finding. In fact one would be lucky to spot a fusion beat which denotes AV dissociation . It occurs in less than 30% of patients with VT .
Technically , for AV dissociation to occur atria and ventricle should not be related in either direction .
If there is a retrograde VA association ante grade AV conduction is not possible and hence one can not get a fusion beat or so.
What happens to p waves during VT ? How does atria depolarise during VT ?
Atrial chambers can not sit idle during VT .It has to somehow get depolarized and contract but the timing may not be appropriate .
P waves during VT can either be antegrade or retrograde .
Theoretically both can be present but most times it is the retrograde p waves we see.
The occurrence and timing of p waves is related to the VA conduction .
If there is 1 :1 VA conduction during VT there can not be AV dissociation for the simple reason we have VA association.In fact there is constant vigil to depolarise the ventricle through the normal AV node and his purkinje in spite of the VT .SA node is aware of this fact , how difficult it is going to be confront the upcoming rapid ventricular impulse . Usually the ventricular impulse prevails over the atrial impulse and much part atria is controlled by the VT . In fact the VT reaches all the way to SA node and simply overdrive it . At these fast heart rates retrograde p waves are not visible. ( But surprisingly one may see a regular cannon wave in the neck with 1: 1 VA conduction.
Mean while , the SA is always under look out for a opportunity to sneak into the ventricles thorough AV node. This happens when the VT focus slightly slows down or shifts to a new site . this sis the time we are able to witness the AV dissociation . When the atrial impulse capture fully or partially the ventricle fusion beats occur confirming AV dissociation .
Final message
AV dissociation is present in less than 30% of VT because in 70% there is a VA association.(Retrograde VA conduction ) . When V is associated with A there can not be AV dissociation.
Dr.S.Venkatesan MD 
I did not realize that VA conduction was anywhere near that common. I did know that it was not common to see AV dissociation, but that has always been my favorite confirmation of VT – maybe because some people will see P waves and assume that it cannot be VT.