The effect of electrode position on atrial sensing for physiologically responsive cardiac pacemakers

Fully automatic pacing systems rely on accurate identification of spontaneous atrial signals for physiologically responsive pacing. These signals must be discriminated from far-field ventricular activity, which might otherwise be sensed in the atrium. To amplify on the previously reported superiority of bipolar signals and high-impedance circuitry for atrial sensing, we studied the effects of various intraatrial electrode positions on the atrial and ventricular contribution to electrograms recorded in this chamber. Compared with other intraatrial endocardial sites, right atrial signals were greatest in amplitude and slew rate in the appendage (RAA), averaging 3.3 +/- 0.41 mV and 1.15 +/- 0.16 V/sec (mean +/- SEM), respectively. These values were substantially higher than in the low atrium (p less than 0.001 and 0.0005 for amplitude and slew rate, respectively) and the high lateral atrium (p less than 0.05 for slew rate). Appendage atrial electrograms also had significantly higher amplitude and slew rate than far-field R waves recorded here (p less than 0.0001 for both). Additionally, the greatest difference in spectral content between atrial and far-field ventricular signals was also observed in the RAA. Thus, parameters in the domain of both time and frequency identified the RAA as the superior location for atrial sensing. Except for phrenic nerve problems with pacing, the HRA also appears to be a suitable electrode location for sensing. These considerations are germane in light of a growing number of atrial active and passive fixation leads now being employed for physiologic pacing.