[Low pacing thresholds of ventricular pacemaker leads: not a marker for large R-wave amplitudes and high impedance...

An optimal position of a ventricular pacing lead in the bottom or apex of the right ventricle is defined by low pacing thresholds and high R wave amplitudes. At similar pacing thresholds high pacing impedance additionally reduces the pacing current delivered by the pacemaker. Finding an optimal position for the pacing lead would be facilitated if locations with low pacing thresholds are usually associated with great R wave amplitudes and high pacing impedance. This study assessed the relationship between ventricular pacing thresholds, R wave amplitude, and pacing impedance for present-day pacing leads at implantation. For this purpose the acute lead performance of the unipolar lined lead "CapSure SP 4023" (Medtronic) was studied in 194 patients. PSA 5311 (Medtronic) was used to measure pacing thresholds at 0.5 ms in steps of 0.1 V and impedance at 5.0 V/0.5 ms and to register filtered R wave amplitude. Pacing thresholds were 0.4 +/- 0.2 V (range 0.1-1.2 V) and impedance 576 +/- 161 ohms (range 262-1200 ohms). An intrinsic rhythm was observed in 150 patients with an R wave amplitude of 13 +/- 5 mV (range 3-28 mV). There was no statistically significant correlation between pacing threshold and R wave amplitude (r = -0.09). In respect to impedance, neither the correlation with pacing threshold (r = 0.07) nor that with R wave amplitude (r = 0.18) was statistically significant. In conclusion, most patients had pacing thresholds < 0.5 V at 0.5 ms pulse duration and R wave amplitudes > 10 mV; fewer than 5% of the patients had pacing thresholds > 0.6 V or R wave amplitudes < 5 mV. Lead locations with low pacing thresholds are usually not associated with great R wave amplitudes or high impedance.