Heart rate variability
The structure of the heart allows changes in cardiac output depending on the current needs of the body and especially the CNS (Central nervous system) in order to maintain homeostasis. These changes in heart rate correspond to changes in parasympathetic and sympathetic tone and are a response to stress or total load. Even in the absence of external influences, heartbeats do not have the character of chronographic accuracy, and for this phenomenon the name Heart Rate Variability (HRV) has been adopted (Malik & Camm, 1995; Stejskal & Salinger, 1996; Salinger et al., 1998; Task Force, 1996).
The size of the HRV shows the extent to which the heart and cardiovascular system are able to respond to changes in the body's internal environment.
A number of methods and techniques can be used to measure heart rate variability, which have different informative and interpretative values. Because the variability of the length of R-R intervals reflects changes in the activity of the autonomic nervous system from a qualitative and quantitative point of view, its detection is used to monitor the tone of the parasympathetic and sympathetic system or its changes. Among the available HRV analysis methods, simple methods (Valsalva maneuver), time-domain method and frequency-domain method are used. With the development of new mathematical procedures, computer technology and understanding of this issue, other methods of HRV evaluation are being developed (Javorka, 2008).
HRV evaluation methods
We can use two methods of mathematical procedures in HRV analysis. First, time domain methods (further divided into statistical and geometric methods) and frequency domain methods, which includes parametric and nonparametric methods.
Time domain method
It is a method that is based on monitoring and comparing the length of R-R intervals of ECG recording in a certain period of time. We observe the difference in the length of R-R intervals in the entire length of the monitored time period. The observed variables are either modifications of the standard deviation of the length of cardiac intervals during the total ECG signal recording, or variables based on differences between successive RR intervals, which are rMSSD (square root of the mean of the differences between successive RR intervals), pNN50 (ratios between differences subsequent intervals at a difference greater than 50 milliseconds) and a pNN of 6.25% (pNN at a difference greater than 6.25% of the mean cardiac period). These variables reflect short-term changes in heart rate, do not depend on long-term trends, and thus indicate changes in the parasympathetic system (Javorka, 2008; Task Force, 1996).
Frequency domain method
Methods of frequency analysis of heart rate variability represent a procedure quantifying the activity of ANS, ie sympathetic and parasympathetic. Two methods are used to calculate the position and strength of the spectra, which usually give almost identical results. These are the Fast Fourier Transform - FFT (Fast Fourier Transformation) and the autoregressive method (Javorka, 2008; Task Force, 1996).
The mySASY system uses frequency analysis using the Fourier transform.
This method is most often referred to as Spectral Analysis of Heart Rate Variability (SA HRV).
