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).

33. 56 year old man recovering from pulmonary œdema requiring prolonged mechanical ventilation. His jugular pulse made waves.