Rebuttal from Marek Malik, Katerina Hnatkova, Heikki V. Huikuri, Federico Lombardi, Georg Schmidt and Markus Zabel.

The article highlights the association between heart rate variability (HRV) and the autonomic nervous system (ANS). Topics discussed include estimating ANS responses to provocations by assessment of ANS responsiveness; SDNN which is a measure of total heartbeat interval variance; and absolute measurements of low (LF) and high (HF) frequency HRV components from their proportions or normalized expressions.