Influence of Current Strength and Length of Basic Drive on Atrial Refractoriness in Man

Abstract

An analysis of the influence of current strength and length of basic drive on atrial refractoriness has not been performed sistematically in the human atrium. We studied 29 patients (24 males and 5 females), ranging in age from 23 to 78 yrs. Atrial effective (ERP) and functional (FRP) refractory periods were measured during atrial pacing (100/min) using: a) variable current strengths (2, 3, 4, 5, 7, 10, 15 mA) and introducing extrastimuli after the eight paced complex of the basic drive; b) a constant current strength (5 mA) and introducing extrastimuli after 8 beats, 1 minute and 3 minutes of the basic drive. A bipolar stimulation, with the distal pole as cathode was performed. In all patients the increase of both current strength and basic drive length produced a reduction of ERP and FRP. At current strengths higher than 7 mA ERP and FRP became nearly fixed. We conclude that: 1. as stimulation occurs earlier in the cardiac cycle, more current and/or a longer previous basic drive are required to initiate a response. This is noteworthy considering that the ability to initiate or terminate reentrant arrhythmias by programmed stimulation is dependent on the refractoriness of the limbs of the reentrant circuit; 2. the relation between stimulus strength and atrial refractoriness is non linear. This could imply that if refractoriness is determined at a single current strength, it would be more appropriate to do so at a current strength (> 7 mA) at which minimal changes in refractoriness are observed; 3. 8 beats of atrial pacing are not sufficient to achieve a steady state of atrial refractoriness.