Characteristic variation in evoked potential amplitude with changes in pacing stimulus strength

The evoked potential, the intracardiac signal generated by a pacing stimulus, shows promise as a sensor for rate-responsive pacing and automatic threshold determinations. Thus, it is important to understand factors that may alter the morphology of evoked potentials and affect accurate signal analysis. Using a computer-based pacing system emulator, stimuli at 2.5, 5.0 and 6.9 V were delivered to 12 patients through permanent bipolar pacing leads. At 2.5 V, the evoked potential amplitude measured -12.63 ± 7.79 mV. When the pacing amplitude was increased to 5.0 and 6.9 V, the signal diminished in size or reversed in polarity, or both, averaging -0.83 ± 7.82 mV and 0.64 ± 7.0 mV, respectively (p < 0.01 vs 2.5 V). Pacing at 2.5 V was performed in an additional 8 patients with temporary quadripolar electrode catheters. With the distal pole of the catheter as the cathode and the proximal 3 poles as a common anode, the evoked potential averaged -9.01 ± 5.44 mV. With the proximal 2 poles of the catheter disconnected to make the anode equal in size and current density to the cathode, the evoked potential diminished to -0.94 ± 11.27 mV (p < 0.05). There is thus a decrease in the evoked potential at high stimulus amplitudes compared to that obtained at the cathodic threshold. This finding can be reproduced by manipulation of the size and current density of the anode, suggesting that anodal stimulation at the ring of permanent pacing leads may be responsible.