Electrophysiological studies in four patients with atrial flutter with 1:1 atrioventricular conduction

Electrophysiological studies of atrioventricular conduction during rapid atrial overdrive pacing and during programmed premature atrial stimulation are reported in four patients with an unusually rapid 1:1 ventricular response to atrial flutter (ventricular rates 240 to 310 per minute). Second-degree AV block development during atrial overdrive pacing at rates well below those during spontaneous atrial flutter. Although none of the four patients showed evidence of pre-excitation on the standard 12-lead electrocardiogram, evidence suggesting a partial AV nodal bypass was demonstrated at electrophysiological study in one case. It is postulated that the profile of the atrial wavefront presented to the normal AV node by atrial flutter differs from that during high right atrial pacing and may account for the lower ventricular rates achieved during high right atrial overdrive pacing than during spontaneous atrial flutter in the remaining three cases.