Figure 1 showing pattern of group beating followed by initiation
of narrow complex tachycardia.
We put a decapolar catheter in coronary sinus, a quadripolar in HIS
location. Below is the electrograms. Initial beat is a sinus complex
with AH interval of 79 msec. Middle one is also sinus beat as P wave is
of same morphology and atrial activation pattern are same as that of
previous sinus beat, but it is a preexcited beat and HV interval is
negative here. The third beat is also a sinus beat but conducting to
ventricle with prolonged AH interval of 127 msec. So what is the
mechanism of prolonged AH interval in the third beat? Most probably the
preexcited beat is retrogradely activating the HIS because the AH
interval in preexcited beat is less than that of sinus beat. The
preexcited ventricular stimulus is penetrating AV node and resetting its
refractoriness (concealed conduction)1. By the time
next sinus beat is reaching the AV node The HIS is in relative
refractory period so it is conducting with prolonged AH interval.
Now what is the cause of pause after the third beat ? The third beat is
producing an echo beat which is resetting the sinus node (Fig 3). Here
the pause is incomplete ( A1-A3 less than twice A1-A1). The response of
sinus node to this extrastimulus is falling in zone II or zone of
reset2.
Orthodromic AVRT was induced and intermittent preexcitation was noticed.
Earliest atrial signal during tachycardia was seen in His proximal
signal suggesting parahisian pathway. Pathway successfully ablated from
noncoronary cusp successfully and post ablation ECG showed sinus rhythm
with no preexcitation.