Abstract
Analysis of heart rate variability (HRV) can be applied to assess the autonomic nervous system (ANS) sympathetic and parasympathetic activity. Since living systems are non-linear, evaluation of ANS activity is difficult by means of linear methods. We propose to apply the Higuchi fractal dimension (HFD) method for assessment of ANS activity. HFD measures complexity of the HRV signal. We analyzed 45 RR time series of 84 min duration each from nine healthy and five diabetic subjects with clinically confirmed long-term diabetes mellitus type II and with diabetic foot ulcer lasting more than 6 weeks. Based on HRV time series complexity analysis we have shown that HFD: (1) discriminates healthy subjects from patients with diabetes mellitus type II; (2) assesses the impact of percutaneous auricular vagus nerve stimulation (pVNS) on ANS activity in normal and diabetic conditions. Thus, HFD may be used during pVNS treatment, to provide stimulation feedback for on-line regulation of therapy in a fast and robust way.
Highlights
Analysis of heart rate variability (HRV) represents a common tool for assessment of autonomic cardiac regulation and provides information about pathophysiological changes in various diseases (Task Force of the European Society of Cardiology the North American Society of Pacing Electrophysiology, 1996; Ashkenazy et al, 1999; Klonowski, 2007; Pierzchalski et al, 2011; Bian et al, 2012; Jiang et al, 2013; Shaffer and Ginsberg, 2017)
In contrary to the linear methods, Higuchi fractal dimension (HFD) can be directly applied to HRV series in time domain and it is suitable for short time series analysis, i.e., of 100–200 data points of a non-stationary signal
By means of our implementation of Higuchi’s method, overall mean ± standard deviation (SD) of aggregated HFD values were found higher for diabetics than for healthy subjects (Figure 1 and Table 2)
Summary
Analysis of HRV represents a common tool for assessment of autonomic cardiac regulation and provides information about pathophysiological changes in various diseases (Task Force of the European Society of Cardiology the North American Society of Pacing Electrophysiology, 1996; Ashkenazy et al, 1999; Klonowski, 2007; Pierzchalski et al, 2011; Bian et al, 2012; Jiang et al, 2013; Shaffer and Ginsberg, 2017). Estimation of sympathetic and parasympathetic activity of ANS is necessary for an accurate adjustment of auricular VNS, the task that is difficult to achieve and potentially vulnerable to erroneous interpretation with standard linear methods (Skinner et al, 1992; Yeragani et al, 1993; Wagner and Persson, 1998; Klonowski, 2007; Sharma, 2009; Sassi et al, 2015). The value of 1 corresponds to a regular time series (simple curve has Euclidean dimension equal 1) while for Gaussian-type noise HFD may attain different values: 1.5 for Brownian, 1.8 for pink, and 2.0 for white noise (Klonowski, 2007, 2011)
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