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8.5. HRV Parameters

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Table of Contents

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).