Intermittent rate-dependent retrograde conduction over a concealed atrioventricular accessory pathway: what is the mechanism?

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.