Implications of constant-energy pacing

Some cardiac pacemakers maintain constant energy per stimulus by automatically increasing the stimulus duration as battery depletion causes the stimulus amplitude to decline. This feature does not guarantee an invariant capture safety margin, however, because the stimulus energy at threshold increases for stimulus durations exceeding the geometric mean of the voltage and current chronaxie times. A method is described for identifying the impulse duration at which the initial capture safety margin begins to be lost with constant-energy pacing.