R-on-P Phenomena

Report:

Sinus rhythm

Runs of ventricular tachycardia

R-on-P “phenomenon”

Concealed retrograde conduction (top strip, end of VT)

Possible ventricular fusion beat (onset of VT in bottom strip)

Comment:

This patient did well following intubation; no specific anti-arrhythmic drugs were required.

The diagnosis of VT is supported by (i) R or RR’ QRS morphology, with the left rabbit ear taller than the right; (ii) concealed retrograde conduction for the first sinus beat after VT in the top strip; (iii) AV dissociation (R-on-P) at the onset of VT in all three strips; (iv) presence of a fusion beat.

The VT runs resemble, superficially, ventricular flutter. The rate, however, is too slow, and the QRS can still be separated from the ST/T segment. Another consideration with respect to ventricular flutter is that, while this lead may look like it, other leads may well show “ordinary” ventricular tachycardia.

The timing of a VEB - its coupling interval - has been the subject of considerable scrutiny; it now seems (pace Lown) that R-on-T (early) ectopics are no more likely to initiate VT or VF than R-on-P (late, late-diastolic) ones depicted here33.

The first beat of VT may be a fusion beat, but this is unlikely. The PR interval is too short to reasonably expect a contribution from above (usually is should be no shorter than 0.06” for that).