Parsimonious Correction of Heart Rate Variability for Its Dependency on Heart Rate

In their article, Monfredi et al 1 addressed the relationship between mean heart rate (HR) and its variability (HRV).In their carefully conducted experiments and with clear reasoning, they demonstrated a universal exponential decay-like relationship between HRV and HR and concluded from this that HRV cannot be used in any simple way to assess autonomic nerve activity to the heart.Like Stauss, 2 we tend to agree with this conclusion as it points out a highly relevant and important issue in the field of cardiovascular medicine.However, we note that the complex biophysical model presented is merely an example of a restriction-of-range phenomena, which has been acknowledged in psychophysiological research 3,4 .These insights were not picked-up and included in the guiding HRV Task Force paper 5 and likely, therefore, largely missed. Ceiling Effect and Restriction of RangeThirty years ago, Akselrod et al 6 elegantly explained that the nature of the interbeat interval (IBI) time series can explain the relationship between HRV and HR.In short, because IBI is simply an interval between 2 events (viz., the R-peaks in the ECG), R-peaks per definition occur sequentially in time.When IBI values decrease toward a (lower or upper) limit, a ceiling effect will cause a restriction of range for variation and reduce variability of the IBI.To compensate for ceiling effects, the variability for the mean IBI level, the variation coefficient of IBI (VC IBI, 100% multiplied by SD and divided by its mean) is suggested.6,7 To calculate spectral values of HR that include an HR level compensation for variability, Akselrod et al 6 suggested to normalize the power of HR by division by (mean HR*mean HR).She explains, in the footnote (p 869 6 ), that this normalization is equivalent to the SD of IBI divided by its mean.