Electrocardiogram of the month Supernormal excitability and conduction in the human heart – electrocardiographic myth or reality?

Abstract

Three cases of possible supernormal conduction and excitability are described. Case one: subthreshold stimulation with ventricular capture occurring only at the end of the T wave. Case two: advanced atrioventricular block with conduction present only when P waves fell at the downsloping part of the T wave of ventricular escape beats. Case three: 2 : 1 atrioventricular block at atrial pacing rate of 115 bpm with improvement of conduction during higher pacing rates (3 : 2 block at 130 bpm and above; with negligible PR prolongation). Possible alternative mechanisms: gap phenomenon, ventriculophasic modulation of vagal discharge, ventriculophasic pacing lead movement and concealed 2 : 1 atrioventricular block due to superimposed ventricular escape rhythm, are discussed and considered either impossible or unlikely.