CLINICAL PROBLEM:
A 43-year-old lady with no evidence of structural heart disease
underwent an electrophysiological study for the evaluation of recurrent
episodes of palpitations. A narrow complex regular tachycardia was
documented, which was not reverted with intravenous adenosine
administration. There was no basal pre-excitation. The AH and HV
intervals during sinus rhythm were 70ms and 48ms respectively. A regular
narrow complex long RP tachycardia with concentric atrial activation was
induced which was incessant in nature. Ventricular overdrive pacing from
RV septum during tachycardia at 20 ms earlier than tachycardia cycle
length showed pseudo-V-A-A-V response with VA linking on cessation of
pacing. A His refractory ventricular premature beat (VPB) was delivered
during the tachycardia (Figure 1). A spontaneous catheter induced VPB
was noted after 2 beats. What are the observations and mechanisms?