Since I’m hopeless at the math part of HRV, I contacted the developers of the software I use and they thought that Correlation Dimension was the calculation most akin to the Higuchi Fractal Dimension. I’ve been looking at that in some of the patients I’ve studied but can’t see much, at least yet. The authors feel that using the Higuchi Fractal Dimension is a simpler way to get robust data that can be used as feedback for a closed loop feedback system for TAVNS. These authors use needle probes. I’m not convinced that it necessary, but time will tell.
Higuchi Fractal Dimension of Heart Rate Variability During Percutaneous Auricular Vagus Nerve Stimulation in Healthy and Diabetic Subjects.
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.