Frequency analyses of heart rate variability often includes reports of high frequency (HF) or low frequency (LF) power, % power, and normalized power (nu). Given that absolute total power in the ECG can vary, I see the value in reporting "% power" and "(nu)", but what are the differences in "%" versus "(nu)" and does it matter??
- Vaughn McCall
Hello Dr. McCall,
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 within-and 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. Article Interpretation of Normalized Spectral Heart Rate Variability...
Both LF Low Frequency and HF High frequency band of the HRV spectrum power are often reported in units of milliseconds-squared
Normalized spectral band powers are derived measurements that include an autonomic activity, computed in HRV data analysis packages
For example Normalized power in LF band (Fnu), a derived index that is computed by dividing LF (Low Frequency band of the HRV spectrum, often between 0.04 – 0.15 Hz) by some suitable denominator representing the total relevant power.
Normalized power in HF band (HFnu), a derived index that is computed by dividing HF (Power in the High Frequency band of the HRV spectrum, often between 0.15–0.40 Hz) by some suitable denominator representing the total relevant power.
The normalized spectral indices are usually defined as
LFnu = LF / (LF + HF) or in case of HFnu = HF / (LF + HF)
Hope this helps,
F
Hi Filip,
Thank you for your answer. Very helpful. Our HRV software provides both a %-power and a normalized power
For example,
our HF output variables would be as follows:
HF Power
HF Power%
HF (nu)
Do HF Power% and HF (nu) essenentially provide the same information?
-Vaughn
Hi vaughn,
Basically, the power in normalized units (LF and HF) are calculated disregarding the VLF band, i.e., considering only the powers in LF and HF for normalization. In contrast, the percentage of LF and HF accounts for the total spectral power (includes VLF) for normalization.
The Task Force guideline for HRV, published in 1996, demonstrates the utility of assessing normalized units. In situations where the total power change, it is useful to see how the balance between LF and HF change. The percentage is a different metric, but I do not see it much on papers.
Best,
Luiz
In Heart Rate Variability (HRV) analysis, "%" and "(nu)" are two different ways of expressing the same HRV metric, which is the normalized units of the high-frequency (HF) component of HRV.
The HF component of HRV reflects parasympathetic (vagal) activity, which is one of the two branches of the autonomic nervous system involved in regulating heart rate. The HF component is usually measured in milliseconds squared (ms²), which is a raw metric that reflects the absolute power of the HF component in the frequency domain.
To facilitate comparison across different individuals and conditions, the HF component is often normalized by dividing it by the total power (TP) of HRV. This normalized metric is expressed as a percentage (%), which reflects the proportion of the total HRV power that is contributed by the HF component.
Alternatively, the HF component can be normalized by dividing it by the sum of the low-frequency (LF) and HF components, which is also known as the LF+HF power. This normalized metric is expressed in normalized units (nu), which reflects the proportion of the total variability that is contributed by the HF component.
Both "%" and "(nu)" are commonly used in HRV analysis and represent the same HRV metric, but they differ in the way the normalized value is expressed.
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