The chronaxie time and its practical importance.

Describing mathematically, the intensity duration curves of electrostimulation by hyperbolas, Lapicque introduced two terms which characterized the functional relationship: "Rheobase" was the lowest intensity with indefinite pulse duration which just stimulated muscles or nerves. "Chronaxie" was that pulse duration at which the threshold intensity was twice that of the rheobase. Up to now, both terms have never played an important role in cardiac pacing. However, it can be shown that the chronaxie, especially, is an important parameter--influenced by several factors such as electrode size, material, and stimulation mode--which may help match the generator to its electrode. Practical consequences may be derived from the concept of chronaxie: 1) A pulse duration longer than the chronaxie is not desirable because current consumption is increased without decreasing the threshold significantly. 2) Pacing with constant current needs twice that pulse duration of constant voltage stimulation. 3) Smaller electrodes are more favorable because the pulse duration they need may be reduced without losing safety. 4) Estimation of the safety margin with decreasing output of the generator or with programmable pulse duration is possible if the chronaxie of a specific electrode is known.