Comparison between intracellular and extracellular direct current recordings of sinus node activity for evaluation of sinoatrial conduction time.

For evaluation of sinoatrial conduction time in humans, study of extracellular direct current (DC) electrograms from the sinus node has been proposed. To validate this method, we compared transmembrane potentials from multiple sites (40 to 60, "mapping" of sinoatrial activation by microelectrode technique) and extracellular DC electrograms of the sinus node in 12 isolated rabbit atria. Sinoatrial conduction time, measured by microelectrodes and by extracellular electrograms, was essentially the same if the DC electrode was positioned over the pacemaker center (35 +/- 15 and 33 +/- 15 msec, respectively; deviation less or equal to 2 msec). While in all experiments phase 4 and phase 0 depolarization of dominant pacemaker fibers was reflected in the DC electrogram, it shape was influenced by pacemaker location and duration of sinoatrial impulse propagation. If sinoatrial conduction time was long (greater than 25 msec) the transition from the diastolic to the upstroke slope was smooth and the sinus node potential was clearly separated from atrial activity. If sinoatrial conduction time was short (less or equal to 25 msec) the onset of the upstroke slope was well defined and the upstroke slope directly merged into atrial activity. Extracellular recordings 0.2 mm away from the pacemaker center were fairly unchanged in shape; however, sinoatrial conduction time was significantly underestimated. Underestimation also occurred when the tip size of the extracellular electrode was increased from 0.2 to 0.5 and 1.0 mm. Thus sinus node activity is reflected in extracellular DC recordings; however, measurement of sinoatrial conduction time by this technique requires exact localization of the electrode over the pacemaker center, which cannot be controlled in humans.