Influence of current strength on excitability and conduction of human atrium and atrioventricular node

The influence of current strength on excitability and conduction of atrium and atrioventricular node was assessed in 25 patients using different current strengths (2, 3, 4, 5, 7, 10, 15 mA) and introducing extrastimuli (parasinusal zone) after the eighth paced complex of a basic drive (100 beats X min-1). Bipolar stimulation with the distal pole as cathode was performed so that effective and functional refractoriness of atrium and atrioventricular node, and the maximum value of atrial latency (interval between the extrastimulus and the beginning of atrial activity), intra-atrial conduction time, and AH interval could be determined at each current strength. In some patients atrioventricular nodal effective refractoriness could or could not be determined at each current strength, whereas in others the determination was possible only at the highest or the lowest current strengths. Moreover, the increase in current strength induced a progressive parallel reduction in both atrial effective and functional refractoriness; induced a progressive lengthening of intra-atrial conduction time (this was seen only in patients with a history of atrial arrhythmias); allowed the maximum possible lengthening of AH interval; and did not visibly influence atrioventricular nodal refractoriness and atrial latency. By altering atrial refractoriness and intra-atrial conduction time current strength affects the prematurity of the atrial impulse and the time at which it reaches the atrioventricular node. These findings should be taken into account when diagnostic and therapeutic electrophysiological procedures are performed.