Europace Advance Access originally published online on December 3, 2008
Europace 2009 11(2):260-262; doi:10.1093/europace/eun327
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SHORT COMMUNICATIONS
Intermittent rate-dependent retrograde conduction over a concealed atrioventricular accessory pathway: what is the mechanism?
Department of Cardiology, IRCCS Fondazione Policlinico S. Matteo, P.le Golgi 19, 27100, Pavia, Italy
* Corresponding author. Tel: +39 0382 501 276, Fax: +39 0382 503 161, Email: r.rordorf{at}smatteo.pv.it
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Abstract |
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Intermittent retrograde conduction over an accessory pathway (AP) is quite an uncommon phenomenon. We describe the case of a 60-year-old woman with recurrent episodes of palpitations. Atrioventricular re-entry tachycardia was diagnosed at electrophysiological study. Interestingly, retrograde conduction over the AP was detectable only when ventricular pacing was performed at fast rate. Supernormal retrograde conduction over the AP was considered the most likely explanation of this unusual finding.
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Case report |
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We report the case of a 60-year-old woman with a long-lasting history of narrow QRS tachycardia episodes. The patient was referred to our institution for radiofrequency (RF) catheter ablation. Electrophysiological study was performed in a non-sedated fasting state: a decapolar catheter was inserted in the coronary sinus (CS) and two quadripolar catheters were positioned in the right ventricle and His-bundle region. A supraventricular tachycardia with a cycle length (CL) of 300 ms was induced by programmed atrial stimulation (Figure 1). During tachycardia, first, atrial activation was recorded at distal CS, and premature ventricular beats delivered during the antegrade refractoriness of the His-bundle could reset the tachycardia. Both these observations militate in favour of an orthodromic atrioventricular re-entry tachycardia (AVRT) with retrograde conduction over a left-sided concealed AP. Atrial incremental pacing and programmed atrial stimulation failed to show antegrade conduction over the AP.
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Decremental pacing was performed from the right ventricular apex starting from a CL of 600 ms. Surprisingly, at pacing CL between 600 and 340 ms, complete ventriculo-atrial dissociation was observed; retrograde conduction over the left-sided AP appeared at CL between 330 and 285 ms. The same phenomenon was observed when pacing from the right ventricular outflow tract (Figure 2) and from the left ventricle, close to the presumed insertion of the AP, after transeptal catheterization. Similarly, during single ventricular extrastimulation delivered in sinus rhythm retrograde conduction over the AP was observed at coupling intervals between 340 and 250 ms; no retrograde conduction was present at longer and shorter coupling intervals from the three different pacing sites (Figure 3). Neither interventricular conduction delay nor decremental conduction over the AP was observed during programmed stimulation. Isoproterenol infusion did not affect the observed phenomenon. The AP was successfully ablated with 1 RF pulse delivered at the lateral aspect of the mitral annulus. After ablation no retrograde conduction was observed both at low and high pacing rate.
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Discussion |
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The reported case is of great interest because it emphasizes the importance of performing a comprehensive EP study to make the correct diagnosis in some cases of supraventricular tachycardias. As a matter of fact, the presence of a narrow QRS tachycardia with first atrial activation at distal CS, together with the demonstration of no basal retrograde conduction during ventricular pacing, could have lead to the false diagnosis of left atrial tachycardia. Entrainment from the ventricle played against this hypothesis, suggesting a reentrant mechanism. Incremental and programmed ventricular stimulation revealed the presence of intermittent retrograde conduction over a concealed AP leading to the correct diagnosis of orthodromic AVRT.
Intermittent retrograde conduction over a concealed AP is a rare occurrence with few previously reported cases.1
Catecholamine-induced changes in the AP refractory period, pacing site-dependent conduction block, supernormal retrograde conduction, and bradycardia-dependent block have been called into question as potential explanation of this unusual finding.
In our case, a role of the autonomic nervous system was very unlikely as isoproterenol infusion did not affect the observed phenomenon. Also pacing site-dependent block, as described by Rong et al.,2 could not justify our findings given that pacing from three different sites reproduced similar results.
Antegrade supernormal conduction over an AP is not a rare finding and several cases have been previously reported:3–4 supernormal conduction requires the presence of a very long refractory period of the AP and is usually characterized by a conduct–block–conduct pattern. In some cases of an abnormally prolonged antegrade refractory period, conduction at low CLs cannot be detected resulting in a block–conduct only pattern. At variance with antegrade supernormal conduction, retrograde supernormal conduction over an AP is very rare. To our opinion, the findings of the present case can be best explained by the occurrence of retrograde supernormal conduction over the AP: a very long retrograde refractory period of the AP was the most likely explanation that prevented us to show retrograde conduction at slow pacing rates resulting in a block–conduct only pattern. As it was not possible to demonstrate retrograde conduction at very low pacing rates, below the baseline sinus CL of about 700 ms, bradycardia-dependent block cannot be ruled out as a potential alternative electrophysiological explanation. Nevertheless, the wide range of the non-conducting zone (from 600 to 330 ms) makes this possibility less likely.
Supernormal conduction is a rare phenomenon that can justify intermittent rate-dependent retrograde conduction over an AP and only few cases have been reported ever since.1,5 The present case is of great clinical relevance because it emphasizes the importance of performing ventricular pacing both at low and high rates in order to make the correct diagnosis in some cases of atrio-ventricular re-entry tachycardias.
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[1] Suzuki F, Harada T, Nawata H, Ohtomo K, Satoh T, Hirao K, et al. Retrograde supernormal conduction, gap phenomenon in concealed accessory atrioventricular pathways. Pacing Clin Electrophysiol (1992) 15:1065–79.[Medline]
[2] Rong B, Tritto M, Di Biase L, Salerno JA. Pacing site and bradycardia dependent retrograde conduction block over an atrioventricular accessory pathway. Europace (2006) 8:438–42.
[3] Chang MS, Miles WM, Prystowsky E. Supernormal conduction in accessory atrioventricular connections. Am J Cardiol (1987) 59:852–6.[CrossRef][Web of Science][Medline]
[4] Przybylski J, Chiale PA, Sanchez RA, Pastori JD, Francos HG, Elizari MV, et al. Supernormal conduction in the accessory pathway of patients with overt or concealed ventricular pre-excitation. J Am Coll Cardiol (1987) 9:1269–78.[Abstract]
[5] Suguta M, Nogami A, Naito S, Oshima S, Taniguchi K, Aonuma K, et al. Retrograde supernormal conduction in concealed accessory atrioventricular pathway following catheter ablation. J Cardiovasc Electrophysiol (1997) 8:1291–5.[CrossRef][Web of Science][Medline]
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