Discussion
In this case report, we have described a phenomenon of split AP potentials in a patient with a slowly-conducting atrioventricular pathway. The tachycardia can be clearly diagnosed as antidromic AVRT based on the intracardiac electrograms and atrial entrainment.
Usually, APs with stable excellent anterograde conduction are necessary for the initiation and maintenance of antidromic AVRTs. Free wall bypass tracts, including those with decremental conduction, are most commonly seen in antidromic AVRT [4,5]. Based on the findings above, our patient had a short atrioventricular pathway with baseline slow conduction in a rare (para-His) region. The evidence of decremental property was yet insufficient given the AP block during atrial pacing at long cycle lengths.
Recording of pathway potentials is useful in guiding AP ablation, which is usually manifested as a single sharp spike 10 to 30 ms before the onset of the delta wave during anterograde conduction[1]. However, two discrete potentials between A and V were recorded near the earliest ventricular activation site in our case, which were considered as AP potentials by careful analysis of electrograms during different levels of preexcitation. This pattern of double pathway potentials is similar to the split His potentials in the setting of intra-His conduction delay, which reflected the conduction disturbance within the slow atrioventricular pathway. Although mapping during tachycardia was not attempted, we believe that the conduction could be significantly improved by isoproterenol, as evidenced by the rapidly altered QRS configuration to a near maximal preexcitation pattern.
Catecholamine-dependent preexcitation and antidromic tachycardia was not uncommon[6,7]. In our patient, both of the anterograde AP and retrograde AV nodal conduction was simultaneously improved by isoproterenol, which served as the prerequisite of the tachycardia. To our knowledge, this is the first report of split AP potentials reflecting the slow conduction over the pathway in a case of antidromic tachycardia.
In addition, the fixed short H-V interval despite decremental conduction in AV node showed the characteristic of an extra fasciculoventricular AP[8], which would not participate in AV reentry and did not require ablation.
Informed consent was obtained from the patient.