Threshold measurements: ten rules for good measuring practice.

Abstract

The following rules for professionally measuring thresholds are derived and discussed: RULE 1: Thresholds should be expressed as voltage averaged over pulse duration to get reproducible and comparable results! RULE 2: Pacing threshold measurements with exponentially decaying pulses should not be extended beyond 1.4 ms as that portion of the pulse below rheobase does not contribute to the stimulation effect! RULE 3: Threshold measurements are best carried out with fixed pulse duration and variable voltage! RULE 4: If threshold measurements are carried out in discrete steps, the steps should be chosen such that the relative step size is as equal as possible! RULE 5: Accuracy of threshold measurements is highly increased if the arithmetically averaged value of the last effective and the first ineffective pulse is defined as threshold! RULE 6: To determine strength-duration-curves, a linear regression of the quantity versus pulse duration should be calculated which yields simply the numerical values of the chronaxie and rheobase! RULE 7: To reach representative strength-duration-curves, measurements with at least four pairs of values must be carried out! RULE 8: Measuring defibrillation thresholds, the relative voltage step size should be chosen equally to have equal accuracy for all steps. RULE 9: If the result of a defibrillation threshold investigation does not reach significance, a too large voltage step size could be an explanation! RULE 10: Comparing intraindividually the threshold of two different defibrillation systems or parameter settings, the threshold ratios should be formed and averaged! Obeying these rules guaranties professional threshold measurements expressed as "rheobase" and "chronaxie" even with devices with discrete steps in parameter programming.