Heart Rate Variability in Sleep Studies

NORMALIZED SPECTRAL BAND POWERS/VARIANCES ARE DERIVED MEASURES REFLECTING AUTONOMIC ACTIVITY, COMMONLY COMPUTED IN MANY HEART rate variability (HRV) data analysis software packages available to sleep researchers, particularly for the low-frequency (LF: often 0.04-0.15 Hz) and high-frequency (HF: often 0.15-0.40 Hz) spectral power/variance bands. The normalized (or normalized unit) spectral indices are usually defined as LFnu = LF / (LF + HF) and HFnu = HF / (LF + HF). Note that the normalized measures are derived or computed indices that are not directly estimated from the raw R-R interval data themselves but are computed as a second step after the initial statistical estimation of the power in the LF and HF bands of the HRV spectrum. Less commonly, the normalizing denominator includes the power/variance in the very low frequency (VLF: frequencies < 0.04 Hz.) band, eg, LFnu = LF/(VLF + LF + HF). Alternatively, the denominator may include power in regions of the spectrum above the HF band (VHF: often frequencies > 0.40 Hz.), eg, LFnu = LF/ (LF + HF + VHF), or power contributions from both the VLF and VHF bands, eg, LFnu = LF/(VLF + LF + HF + VHF). All 4 versions are “unitless” ratios, due to the cancellation of the units in the equation of definition (ms2 / ms2). This report will concentrate primarily on the characteristics of the indices defined using the first of these 4 forms, although similar arguments about the simplex structure of normalized measures are relevant to all 4 definitions. Another decision in the practical definition of normalized HRV spectral indices is whether to compute proportional (fractional) values or percentage values (proportion×100%). The discussion in this report will be with respect to normalized values expressed as proportions, but it should be obvious that exactly the same arguments bear on normalized spectral band power values expressed as percentages. The most common notational acronyms for the normalized spectral HRV measures used in the sleep research-relevant literature are based on the conventions of either adding a “nu” as a suffix (eg, LFnu, HFnu) or an “n” or a “%”as a prefix (eg, nLF, nHF, %LF, %HF). Some authors simply use the standard power band symbols (LF, HF) but make it clear in the narrative that the quantities have been normalized. Part of the appeal of the normalized spectral HRV measures is that the normalization process expresses the quantities on a more easily understood proportion (0-1) or percentage (0%-100%) scale basis. Normalization removes most of the very large withinand across-subject variability in the total raw HRV spectral power, which theoretically and empirically tends to follow a long-tailed right-skewed exponential statistical distribution. The range of the normalized values is strictly bounded, so within-and across-subject statistical averages of normalized spectral indices tend to have distributions that converge more quickly to normal distributions, than do the raw measured spectral band powers, as study sample sizes increase. Normalized HRV spectral indices often have much smaller coefficients of variation when evaluated in repeatability/reproducibility studies. Normalization also tends to produce values that are more exchangeable across different laboratories, research studies, and spectral analysis algorithms. Many of the differences in the computed band power that are related to the practical issues of spectral analysis block length, pseudo-sample interpolation density, zero padding, windowing, and algorithm family (eg, discrete/fast Fourier transform versus autoregressive model-based spectrum) are significantly mitigated by normalization. Searching several of the major online biomedical research citation databases (MEDLINE, CINAHL, BIOSIS) for the Interpretation of Normalized Spectral Heart Rate Variability Indices In Sleep Research: A Critical Review