8.5.1. Time-Domain Parameters #
HR (Heart Rate):HR = 60 / mean(RR) [bpm]
SDNN (Standard Deviation of NN Intervals):SDNN = std(RR) [s]
Interpretation: overall heart rate variability.
RMSSD (Root Mean Square of Successive Differences):RMSSD = sqrt(mean(diff(RR).^2)) [s]
Interpretation: parasympathetic activity (vagal influence).
pNN50:pNN50 = 100 * sum(abs(diff(RR)) > 0.05) / (length(RR) - 1) [%]
Interpretation: percentage of interval differences greater than 50 ms. High values = high variability.
8.5.2. Poincaré Plot Parameters #
SD1 and SD2:
- A Poincaré plot is constructed:
plot(RR(1:end-1), RR(2:end)) - Correlation and rotation angle are calculated.
- Coordinates are rotated to align with the principal axes.
- SD1: standard deviation along the minor axis (short-term variability)
- SD2: standard deviation along the major axis (long-term variability)
Formulas:SD1 = std(perpendicular_axis)
SD2 = std(major_axis)
SD1/SD2 ratio = SD1 / SD2
Interpretation:
- SD1: parasympathetic activity
- SD2: sympathetic and parasympathetic activity
- SD1/SD2: balance of the autonomic nervous system
8.5.3. Frequency-Domain Parameters #
Method: FFT spectral analysis of the RR tachogram.
Procedure:
- RR intervals are interpolated onto an even grid (spline interpolation).
- FFT is computed:
Y = fft(zscore(RR_interpolated)) - Spectral power density is calculated.
- Power is integrated over the following ranges:
- VLF (Very Low Frequency): 0–0.04 Hz
- LF (Low Frequency): 0.04–0.15 Hz
- HF (High Frequency): 0.15–0.4 Hz
Parameters:LF_nu = LF / (LF + HF) * 100 [%]
HF_nu = HF / (LF + HF) * 100 [%]
LF/HF ratio = LF / HF
Interpretation:
- LF: sympathetic and parasympathetic activity (baroreflex)
- HF: parasympathetic activity (respiratory arrhythmia)
- LF/HF: balance of the sympathetic and parasympathetic systems
Physiological explanation of HRV frequency components:
VLF (Very Low Frequency, 0–0.04 Hz):
- Physiology: very slow oscillations associated with thermoregulation and hormonal cycles
- Control: mainly humoral factors; less directly related to the autonomic nervous system
- Clinical significance: less well studied; often excluded from analysis
LF (Low Frequency, 0.04–0.15 Hz):
Physiology: associated with the baroreflex, the blood pressure regulation mechanism
Control: mixed—both sympathetic and parasympathetic systems
Mechanism:
- When blood pressure changes, baroreceptors send signals.
- The autonomic system adjusts heart rate.
- This creates slow oscillations (around 0.1 Hz, period about 10 seconds).
Interpretation:
- High LF may indicate activation (sympathetic + parasympathetic)
- Low LF may indicate reduced regulation
HF (High Frequency, 0.15–0.4 Hz):
Physiology: associated with respiratory arrhythmia
Control: predominantly parasympathetic (vagus nerve)
Mechanism:
- During inhalation: parasympathetic activity decreases → pulse increases
- During exhalation: parasympathetic activity increases → pulse decreases
- This creates fast oscillations synchronized with breathing
Interpretation:
- High HF = good parasympathetic activity = relaxation, recovery
- Low HF = reduced parasympathetic activity = stress, fatigue
LF/HF ratio:
Physiology: balance between sympathetic and parasympathetic systems
Interpretation:
- High LF/HF (>2): predominance of sympathetic activity, stress, activation
- Low LF/HF (<1): predominance of parasympathetic activity, relaxation, rest
- Normal LF/HF (1–2): balanced state
Normalized units (nu):
- LF_nu and HF_nu express the percentage of total power excluding VLF.
- This allows comparisons between people by removing the influence of total power.
- LF_nu + HF_nu = 100%
Clinical significance:
- Reduced HF and increased LF/HF are markers of stress and fatigue.
- Athletes at rest often show high HF and low LF/HF.
- Under chronic stress, LF/HF is persistently elevated.
- Recovery after exercise is accompanied by an increase in HF.
8.5.4. Functional Group #
Interpretation: Functional state of the autonomic nervous system.
Physiological explanation of the functional group:
The functional group is a comprehensive evaluation of autonomic nervous system state based on normalization of HRV parameters relative to age- and sex-specific norms.
How it works:
- Each person has “normal” SD1, SD2, and HR values based on age and sex.
- Actual values are compared with these norms (z-scores).
- The combination of z-scores determines the functional group.
See the document *Classification of Functional Groups*.
8.5.5. Additional Parameters #
Relaxation:Relaxation = 1e6 * SD1 * SD2 / HR [ms²/beat]
Interpretation: an integral relaxation indicator.
rrHRV median:
Median distance to the central point on the map of relative RR intervals.
Extra point:
Additional score in the functional group (0–100).
