Are spectral estimates of heart rate variability dependent on heart rate? a formula to correct when needed

Heart rate variability (HRV) investigated by spectral analysis is widely accepted as a tool for the investigation of the autonomic influence on heart activity. The possibility that HRV indexes should be corrected per the heart rate, before using them is currently overlooked. RR interval derived from EKG recording in 15 healthy subjects, during quiet supine mute resting has been analysed over consecutive 256 beat long windows by autoregressive spectral analysis. The formula ln(power spectral density)/ln(RR interval) was then applied when a dependence of power values from RR interval was found. Spectral total power, VLF, LF, and HF power were significantly related to RR interval. After the application of the correction formula no significant correlation was found. In conclusion, some spectral indexes (associated with parasympathetic drive to the sinus node and with sympatho/vagal balance) are dependent of heart rate and need a correction before analysis and interpretation. A simple effective formula. to make spectral indexes independent by heart rate is proposed which is effective.