The strength-duration curve and its importance in pacing efficiency: a study of 325 pacing leads in 229 patients.

Pacemaker battery life is dependent on programmable parameters, principally pulse amplitude and pulse duration. High factory default settings cause excessive current drain. The strength-duration curve relates pacing threshold to pulse duration. The most energy efficient pacing occurs at chronaxie, a value of pulse duration derived from the curve. Strength-duration curves were calculated for 325 acutely implanted pacing leads. Chronaxie and rheobase were compared for atrial and ventricular leads. Chronaxie was compared with actual programmed pulse duration. There were 101 atrial and 224 ventricular leads, all passive fixation. The curve fit was good, (mean error +/- SD) 0.024 +/- 0.06 V for atrial curves and 0.008 +/- 0.034 V for ventricular curves. Mean (+/- SD) atrial and ventricular chronaxies were 0.24 +/- 0.07 ms and 0.25 +/- 0.07 ms, respectively. A "Z" value of 1.4 indicated that chronaxies might have been from the same population. Mean (+/- SD) atrial and ventricular rheobases were 0.51 +/- 0.2 V and 0.35 +/- 0.13 V, respectively. A "Z" value of 7.1 (P < 0.001) suggested atrial and ventricular rheobases were from differing populations. All patients had factory default pulse durations of 0.45 ms or 0.5 ms, exceeding acute chronaxie by a factor of two, thus, demonstrating suboptimal pacing. We conclude that understanding the strength-duration curve is critical. Sensible programming of other pacing functions optimizes longevity. Battery drain is reduced by programming pulse duration to chronaxie with a doubling of voltage threshold at this point to achieve a safety margin. Further study of chronaxie drift with time is required.