Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 181

root of the mean squared difference of successive RRs, the number of pairs of suc-
cessive RRs that differ by more than 50 ms (so called NN50), the proportion of NN50
divided by total number of NNs (pNN50). These markers and other tools helpful in
HRV quantification may be found in PhysioNet [Goldberger et al., 2000].
The series of RR intervals can also be represented by density distribution of RR
interval durations or density distribution of differences between adjacent RR inter-
vals. In construction of the density distribution a reasonable number of RR intervals
is needed. In practice, recordings of at least 20 minutes (but preferably 24 hours) are
recommended to ensure correct performance. The discrete scale of histogram should
be appropriate—not too fine or too coarse, permitting the construction of smoothed
histograms.
The deviations from normal distribution may be quantified by fitting to it some ge-
ometrical shapes or by means of higher order moments: skewness and kurtosis [Clif-
ford, 2006]. However the higher the moment, the more sensitive it is to outliers and
artifacts. Skewness—the indicator of asymmetry—is used to detect possible sud-
den acceleration followed by longer deceleration in the heart rate, the phenomenon
which may be connected with clinical problems. Another measure quantifying RR
distribution is the HRV triangular index measurement; it is the integral of the density
distribution (that is, the number of all RR intervals) divided by the maximum of the
density distribution [Malik and Camm, 1995].

4.2.2.2 Frequency-domain methods of HRV analysis

In HRV several rhythms can be distinguished, which are conventionally divided in
four frequency bands:
• Ultra low frequency (ULF) ): 0.0001 Hz ≤ ULF < 0.003 Hz

• Very low frequency (VLF): 0.003 Hz ≤ VLF < 0.04 Hz

• Low frequency (LF): 0.04 Hz ≤ LF < 0.15 Hz

0.4 Hz
Fluctuations in VLF and ULF bands are thought to be due to long-term regulatory
mechanisms such as thermoregulatory system, systems related to blood pressure, and
chemical regulatory factors [Cerutti et al., 1995]. VLF appear to depend primarily
on the parasympathetic system. HF is a measure of respiratory sinus arrythmias and
can be considered an index of vagal modulation [Malliani et al., 1991]. Some studies
suggest that LF, when expressed in normalized units, is a quantitative marker of sym-
pathetic modulations; other studies view LF as reflecting both sympathetic activity
and vagal activity. Consequently, the LF/HF ratio is considered by some investigators
to mirror sympathovagal balance or to reflect the sympathetic modulations [Malliani
et al., 1991]. It is important to note that HRV measures fluctuations in autonomic
inputs to the heart, rather than the mean level of autonomic inputs.
In a spectrum calculated from short-term recordings of two to five minutes three
main spectral components can be distinguished: VLF, LF and HF. The ULF compo-
nent may be found from long-term recording, usually 24 hours. In case of long-term

• High frequency (HF): 0.15 Hz ≤ HF <