1. Short-term heart rate variability in healthy young adults
Tuomas Koskinen, Mika Kähönen, Antti Jula, Tomi Laitinen, Liisa Keltikangas-Järvinen, Jorma Viikari, Ilkka Välimäki, Olli T. Raitakari 
Autonomic Neuroscience: Basic and Clinical 
Volume 145, Issue 1, Pages (January 2009)
DOI: /j.autneu
Copyright © 2008 Elsevier B.V. Terms and Conditions

2. Fig. 1
Spectral density curve of heart rate variability. Spectral density curve (PSD) is defined as an amount of power per unit (density) of frequency (spectral) as a function of the frequency (ms2/Hz). To generate the PSD, subject's electrocardiogram (ECG) is recorded and respective R–R-intervals, i.e. duration between two consecutive R-peaks of the ECG, are computed. Then estimation of PSD is modeled from all R–R-intervals by fast Fourier transformation, that is a mathematical process that transforms a waveform of different R–R-intervals into the components of its frequency spectrum. Number of analyzed R–R-intervals of current sample was 196. The PSD is distributed to three components to describe oscillations with different frequencies; very low frequency (VLF,≤0.04 Hz, i.e. >25 s), low frequency (LF, 0.04–0.15 H, i.e. 6.7–25 s) and high frequency (HF, 0.15–0.4 Hz, i.e. 2.5–6.7 s) component. Thus, PSD analysis provides the basic information how power distributes as a function of frequency. The vagal activity is the major contributor of the HF component (Pomeranz et al., 1985). Thus, oscillations in R–R-interval caused by breathing are seen at 0.25 Hz in PSD. The PSD integrals between above-mentioned frequencies gives us the amount of VLF, LF and HF components (ms2) of spectral HRV.
Autonomic Neuroscience: Basic and Clinical  , 81-88DOI: ( /j.autneu )
Copyright © 2008 Elsevier B.V. Terms and Conditions

3. Fig. 2
Lower 2.5% reference limits of HF and TP distributed by age, sex and heart rate. 60, 65 and 70 bpm represents mean heart rate during ECG recording. In women numbers of subjects stratified by heart rate (60, 65 and 70) were 131, 180 and 205, respectively. In men numbers of subjects stratified by heart rate (60, 65 and 70) were 166, 179 and 115, respectively. HF=high-frequency component and TP=total-frequency component.
Autonomic Neuroscience: Basic and Clinical  , 81-88DOI: ( /j.autneu )
Copyright © 2008 Elsevier B.V. Terms and Conditions

4. Fig. 3
Lower 2.5% reference limits of LF distributed by age, sex and heart rate. 60, 65 and 70 bpm represents mean heart rate during ECG recording. In women numbers of subjects stratified by heart rate (60, 65 and 70) were 131, 180 and 205, respectively. In men numbers of subjects stratified by heart rate (60, 65 and 70) were 166, 179 and 115, respectively. LF=low frequency component of spectral HRV.
Autonomic Neuroscience: Basic and Clinical  , 81-88DOI: ( /j.autneu )
Copyright © 2008 Elsevier B.V. Terms and Conditions

5. Fig. 4
Percentiles of Mean E/I in deep breathing test presented by age. MeanE/I=the mean ratio for longest and shortest R–R-interval during deep breathing cycle.
Autonomic Neuroscience: Basic and Clinical  , 81-88DOI: ( /j.autneu )
Copyright © 2008 Elsevier B.V. Terms and Conditions

6. Fig. 5
Percentiles of MeanDBD in deep breathing test presented by age. MeanDBD=the mean difference in instantaneous heart rate.
Autonomic Neuroscience: Basic and Clinical  , 81-88DOI: ( /j.autneu )
Copyright © 2008 Elsevier B.V. Terms and Conditions