How the threshold “r” influences approximate entropy analysis of heart-rate variability

Calculation of approximate entropy (ApEn) requires to select the correct threshold ldquorrdquo. Previous studies recommended r to be between 0.1 and 0.25 times the signal standard deviation, and now r=0.2 is used in almost all HRV studies. Recently it has been claimed that for fast signal dynamics, r=0.2 may lead to erroneous conclusions, while r maximizing ApEn, rMAX, correctly assesses entropy. We verified 1) if rMAX differs from r=0.2 also for HR; and 2) if all r values in the 0.1-0.25 range provide similar ApEn measures. For this aim, we recorded R-R intervals in 10 young subjects for 10psila, in supine and sitting positions, and calculated ApEn(r) for r between 0.02 and 1.20, identifying rMAX and ApEn(rMAX). rMAX felt into the recommended range, but it significantly differed from 0.2. At the extremes of the range, the effects of posture change on ApEn were even opposite: ApEn(0.25) decreased while ApEn(0.1) increased. Therefore the choice of r is critical even in HRV studies.