Analysis of Heart Rate variability (HRV) in Frequency Domain and Recurrence Quantification Analysis during the Treatment by Transcutaneous Vagus Nerve Stimulation

Research Article

Ann Depress Anxiety. 2018; 5(1): 1093.

Analysis of Heart Rate variability (HRV) in Frequency Domain and Recurrence Quantification Analysis during the Treatment by Transcutaneous Vagus Nerve Stimulation

Conte S¹, Wang F², Altamura M1,3, Bellomo A³, Serafini G4, Orsucci F5, Kaleagasioglu F1,6, Mendolicchio L1,7, Casciaro F1,8, Norman R1 and Conte E1*

1School of Advanced International Studies on Applied Theoretical and non Linear Methodologies of Physics, Italy

2College of Science/Agricultural Mathematical Modeling and Data Processing Center, Hunan Agricultural University, China

3Department of Biomedical Science, University of Foggia, Italy

4Tor Vergata University, Italy

5Division of Psychology, University College, England

6Faculty of Medicine, Near East University, Turkish Republic of Northern Cyprus

7Clinic Villa Miralago, Italy

8Department of Biomedical Sciences and Human Oncology, University of Bari, Italy

Corresponding author: Conte E, School of Advanced International Studies on Applied Theoretical and non Linear Methodologies of Physics, Italy

Received: May 21, 2018; Accepted: June 18, 2018; Published: June 25, 2018


The aim of the present work has a methodological profile. It aims to analyze the HRV during thirty minutes transcutaneous Vagus Nerve Stimulation tVNS treatment. We used the linear methodology of the Fast Fourier Transform and an analysis of the variability of the R-R signal in frequency domain under such conditions of stimulation. As specialized method of analysis we used also the Recurrence Quantification Analysis (RQA). The importance to use a combination of such linear and non linear methodologies was to give results of physiological interest that is to say to ascertain, as well as possible, the influence of such tVNS treatment on HRV. We examined 50 young subjects in normal health conditions and 50 young subjects with Autonomic Nervous Dysfunction (ANS) as well as it is evidenced by HRV. The experiments were arranged in three phases, recording the data before, during, and after a 30-minute treatment. The analysis of the data was performed in the frequency domain evaluating the VLF, the LF, and the HF and the variability of the tachogram according to the 1996 Task Force Standards of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. In addition the Recurrence Quantification Analysis (RQA) was used. The indication arising from such methodological study is that the unbalanced initial ANS dysfunction of the examined subjects, as mirrored from the HRV analysis, compensates and balances with also a prevailing tendency to parasympathetic HRV modulation during the tVNS treatment.

Keywords: HRV; Transcutaneous Vagus Nerve Stimulation (tVNS); Ear clip and finger plethysmography; Fast Fourier Transform (FFT); Heart Rate Analysis of Variability; Heart Rate Analysis; Recurrence Quantification Analysis (RQA)


The idea that we can influence neuronal activity with electrical pulses is not new. Earlier this century, patients were treated, and still are, with electroconvulsive therapy as a treatment for severe depression [1].

Presently, electrical stimulation of the Vagus Nerve (VNS) is an FDA-approved therapy tool for both refractory depression and epilepsy [2-4]. It has been used, as well, for a number of psychological and psychiatric disorders. Furthermore, it recently has emerged as a promising therapeutic approach for cardiac diseases [5,6]. As supported by neuroimaging studies, we also maintain that it broadly affects different parts of the brain, including the thalamus, cerebellum, orbito-frontal cortex, limbic system, hypothalamus, and medulla [7- 10].

Traditionally, vagus nerve stimulation (VNS) has been performed by implanting a neurostimulating device connected to an electrode located along the cervical branch of the vagus nerve. In principle, this procedure may have negative aspects, such as coughing during stimulation, hoarseness, general operational and anesthesiological risks, and high costs. In order to minimize adverse effects, a new non-invasive neuro stimulating procedure, called transcutaneous Vagus Nerve Stimulation (tVNS) has been developed. We used the NuCalm device ( which applies two gel electrode patches to the neck below the ear lobe (right and left) for transcutaneous stimulation of the afferent auricular branch of the vagus nerve (ABVN). This device has received FDA approval, satisfying essential health and safety requirements. The employed tVNS targets the cutaneous receptive field of the ABVN at the outer ear [11]. Ear lobe vagal stimulation does not create adverse effects [12], but it does induce modulating effects on heart rate, blood pressure, or peripheral microcirculation during the stimulation procedure. The use of applied electrodes near both right and left ear lobes assures afferent and efferent induction since the right vagus nerve has efferent fibers to the heart. Of course, use of the device is not recommended in the case of pregnancy, and is contraindicated for persons with metal in the body, such as pacemaker and ventricular assist devices or brain and cochlear implants.

The aim of the present paper has a physiological-methodological profile, to evaluate the influence of the tVNS on the HRV during a single treatment. As known, HRV is the modulation of heart rate dynamics through the Autonomic Nervous System (ANS) and is profoundly modified in a great variety of experimental conditions and, in particular, in subjects having Psychological Disorders (P.D.). A way to examine ANS dysfunction in P.D. is the frequency domain analysis of Fast Fourier Transform (FFT), which enables us to identify three bands in the obtained power spectrum frequency domain. These bands are the Very Low Frequency (VLF), the Low Frequency (LF), and the High Frequency, respectively. The LF and HF bands relate mainly to ortho-sympathetic and parasympathetic ANS modulation. Therefore, the focus of the present study was to select a group of P.D. subjects and investigate their HRV before, during and after the tVNS treatment, with the only and important goal of ascertaining whether this tNVS treatment really induces a modification of HRV in such subjects during such single treatment. We used different methods of linear analysis, in particular the R-R signal and the signal obtained from the R-R with estimation of the variability of such signal [13,14] as well as clinicians need accurate determination in such frequency field when the tVNS treatment is acting. As general non linear method of analysis was added; it is the Recurrence Quantification Analysis (RQA). Such three employed methods of analysis were able to give a satisfactory evaluation of the acting ANS modulating dynamics accounting for linear and non linear physiological components.

Materials and Methods

The NuCalm device was furnished by Solace Life sciences, Inc., of Wilmington, Delaware, USA. Two groups of subjects were examined: 50 young subjects in normal health conditions and 50 young subjects affected by anxiety, or depression.

To test the subjects, each potential participant was given a nonstructured interview following the criteria of the Diagnostic and Statistical Manual of Mental Disorders (DSM). Participants selected for the study showed symptomatology of anxiety or depression. Those selected were divided into two groups and administered tests to evaluate the level of their anxiety and depression symptoms. The scales used to measure their scores were the Hamilton Anxiety Scale (HAM-A) and the Hamilton Depression Scale (HAM-D), respectively. Both tests were administered during patient recruitment and showed medium scores, with HAM-A at 42 and HAM-D at 18. Specifically, we examined subjects all having ages between 25 and 45. All the subjects gave their written informed consent. According to the 1996 Task Force Standards of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, we selected the subjects excluding a priori those affected from well established other pathologies as we fixed in detail in [13].

The obtained tachograms were recorded by finger plethysmography (PG) and each time it was compared to those obtained by ECG, accounting for Pulse Wave Transit Time (PWTT).

The NuCalm device was employed for the transcutaneous stimulation, applying gel electrode patches near the right and left ear lobes, and setting the current value at 0.1 MA. The NuCalm device output was constantly monitored by a Welleman PCLAB 2000 SE computer interface. The scheme of the experiment was as follows. Each subject was recorded for three minutes by GSR in tone and phase values, just before the start of recording tachogram data. This was performed to establish the general condition of each subject. Soon after the experiment started a PG recording of five minutes was performed without tNVS. Then the PG recording was extended for 30 minutes with the subject under NuCalm stimulation. After 30 minutes of treatment, a final recording of five minutes was done on the subject without treatment. This scheme enabled us to estimate HRV before, during and after the tVNS.

For analysis of the data, we used the following methods

a) The fast Fourier Transform (FFT) spectrum analysis enabled us to estimate the VLF, LF and HF bands and the total power as frequency domain expression of the ANS modulation. This is the standard method, applied by re-sampling the original NN interval series and then applying the FFT. The VLF band was encoded between 0.003 and 0.04 Hz, the LF band between 0.04 and 0.15 Hz, and the HF band between 0.15 and 0.4 Hz. It is known that the VLF spectral component (0.003-0.05 Hz) is possibly related to long-term regulatory mechanisms (e.g. the renin-angiotensin system, the thermoregulatory peripheral blood flow adjustment). The LF spectral component is linked mainly to sympathetic modulation and baroreceptor activity, but also includes parasympathetic contributions. Finally, the HF spectral band reflects parasympathetic (vagal) activity.

b) We also used another method, the CZF method [13,15] to investigate variability of the R-R signal. We know that the standard FFT method in HRV contains limitations. The R-R time series is non stationary, non periodic and nonlinear. In principle, FFT should be applied only to linear, periodic and stationary signals. Consequently, basic information is missing, lost or approximated when using only FFT in HRV analysis. The situation improves by introducing some methodological innovation [13] based on appropriate physiological foundations. The combined use of two or more methods provides advancement in the reliability of the obtained results. A concept having valid support in physiology, in cardiology and in ANS and HRV studies, is the variability of heart rhythm. Variability may be formally expressed by RR(n)–RR(n-k), where RR(n) is the R-R time interval value at the beat (n), and RR (n-k) is the R-R value at the beat (n-k). Considered values for (k) are integers (k = 1, 2, 3, 4, 5…) if we consider k = 1, we are estimating the difference (Variability) (RR(n) - RR(n–1)) between two adjacent beats and for each beat (n). The concept of variability is not new. In 1996, the Task Force [16] recommended using such a variable. To this regard, we also currently use the Poincare Plot in HRV analysis. However, it gives only a bidimensional nonlinear representation of variability by plotting the previous R-R time value against the subsequent. It is unable to estimate VLF, LF and HF frequency domains. By using instead the CZF method, we estimate the previous conceptually-established idea of instantaneous variability of the RR(n) time series in the following manner:

The newly-obtained time series contains the difference (RR(n)- RR(n-1)). It is the variability of R-R intervals, and may be estimated in absolute value (CZF-A.V.) and without absolute value (CZFW. A.V.). In the first case of (CZF-A.V.), we have the measure of variability as the size of the variable itself. In the second case of (CZFW. A.V.), we estimate the dynamic heart rhythm profile, since we inspect when (RR(n)-RR (n-1))>0 and when (RR(n)-RR(n-1))<0. In this manner we have information on the dynamics between adjacent cardiac beats resulting, first, in a deceleration and, second, in an acceleration of the cardiac rhythm. Finally, performing the Discrete Fourier Transform (DFT) analysis on the time series (VaR(n)), we may evaluate variability in HRV in the three usual VLF, LF, and HF bands.

This method is of major importance in our study, since our aim is to analyze the variability induced in HR as a consequence of tVNS stimulation and, by this method, we have the possibility to compare it before, during, and after the treatment.

c) The final step of our investigation was realized by using the well-known method of Recurrence Quantification.

Analysis (R.Q.A.) [17]. This is a robust methodology that was originally was introduced by Zbilut and Webber was and subsequently elaborated by different authors [18-20], as we explain in detail in the next section. This method of non linear data analysis for investigation of dynamic systems has received, for many years, a constant and excellent international accreditation. It has been used by us in a number of papers [21].


All the results related to the frequency domain analysis are given in Tables 1 & 2 for statistical significance. The values are given in msec². Normal indicates states for normal subjects; P.D. indicates states for the subjects having psychological disorders. PD.DT relates the results obtained during the tVNS treatment and P.D.-A.T., those obtained soon after the treatment.