The influence of heart coherence on synchronization between human heart rate variability and geomagnetic activity

2.1. Participants

The experiment location had a local site coordinator who was responsible for participant recruitment, logistics coordination and participant training in the study procedures and use of the HRV recorders. Study coordinator in Lithuania recruited a total of 20 participants. The mean age (sd) of the 20 participants in the analysis was 23.3 (0.6) years consisting of 16 females and 4 males.

During the 14 day period between 26 February and 12 March 2015, 20 participants located in Lithuania participated in the study. All of the participants were healthy and either worked or attended classes during daytime hours. The Lithuanian group consisted of 20 medical students attending the Lithuanian University of Health Sciences.

2.2. Ethics statement

The research met all applicable standards for the ethics of experimentation in accordance with the Declaration of Helsinki. The permit to perform biomedical investigation was granted by the Kaunas Regional Ethics Committee for Biomedical Investigations, No. BE-2-51, 23.12.2015 (copies of documents are enclosed as supplemental data). Participants provided written informed consent prior to the experiment.

2.3. HRV data collection

HRV is a noninvasive measure that reflects ANS activity and dynamics. All participants underwent daily 24-hour ambulatory HRV recordings during a two-week period between 26 February and 12 March 2015 (Bodyguard2, Firstbeat Technologies Ltd., Jyväskylä, Finland). Prior to the start of the study, each participant received instructions on attaching, starting, and stopping the recorders as well as instructions to participants for logging day to day activity, sleep, waking, etc. from the local site coordinator.

The group coordinator was instructed in how to retrieve data from the HRV recorders and upload it to the data collection FTP site. Participants were instructed to stop the recorder each morning after waking up to start the day, and allowed up to 50 minutes to shower or bathe before reattaching the recorder and starting the new day’s recording. Ambu Blue Sensor VL microporous breathable disposable electrodes were used for all of the recordings. The electrodes were placed in a modified V5 position. To minimize skin irritation over the two weeks, participants were encouraged to locate the electrodes around three different positions near the V5 electrode sites. The HRV recorder calculates the RR Interval (R is a point corresponding to the peak of the QRS complex of the ECG wave; and RR is the interval between successive Rs) from the electrocardiogram sampled at 1000 Hz. The RR interval data were stored locally in the device memory, and downloaded to a computer workstation at the completion of the study.

All of the HRV recordings were downloaded from the FTP site to a computer workstation and analyzed using DADiSP 6.7. Inter-Beat-Intervals greater or less than 30 % of the mean of the previous four intervals were considered artifacts, and were removed from the analysis record. Following an automated editing procedure, all of the recordings were manually reviewed by an experienced technician, and, if needed, corrected. Daily recordings were processed in consecutive 5-min segments in accordance with the standards established by the HRV Task Force. Any 5-min segment with >10 % of the IBIs either missing or removed in editing were excluded from the analysis. The local time stamps in the HRV recordings were converted to Coordinated Universal Time (UTC) to enable synchronization between locations and magnetic field data sets.

2.4. Magnetic field data

The local magnetic field intensity was measured using magnetometer located in Lithuania which is a part of the Global Coherence Monitoring Network [21]. Two magnetic field detectors (Zonge Engineering ANT-4) at the site are positioned in the north-south and east-west axes to detect local time-varying magnetic field strengths (sensitivity 1 pT) over a wide frequency range (0.01-300 Hz) while maintaining a flat frequency response. The data acquisition infrastructure captures, then stamps, the global positioning system time, and transmits the data to the common server. Each magnetometer in the network is continuously sampled at a rate of 130 Hz.

2.5. Heart Lock-In procedure

On March 5th, the group participated in the Heart Lock-In technique for a 15-minute period. The Heart Lock-In technique focuses on building the capacity to sustain heartfelt positive emotions. The technique is generally practiced for five to fifteen minutes at a time, although longer sessions may be used as well. The steps of Heart Lock-In are:

Step 1: Focus your attention in the area of the heart. Imagine your breath is flowing in and out of your heart or chest area, breathing a little slower and deeper than usual.

Step 2: Activate and sustain a regenerative feeling such as appreciation, care or compassion.

Step 3: Radiate that renewing feeling to yourself and others [23].

Use of this technique is typically accompanied by feelings of peacefulness, harmony, and a sense of inner warmth, and is often an effective means to diffuse accumulated stress and negative feelings [24].

2.6. Computation of the power of the local magnetic field

The spectral power of the local magnetic field was computed using the magnetic field intensity values and applying the algorithm denoted in [14] as Algorithm A. The main steps of the algorithm included the computation of the spectrogram of the magnetic field intensity data signal (for one-second time intervals), clipping the spectrogram to a certain level in order to minimize the noise (manmade noise, lightning) in the magnetic field data and summing the values over the frequency range [0, 1] Hz. Empirically obtained optimal noise clipping level was 1.25 (pT)²/Hz.

2.7. Determination of geometrical synchronization between participants’ HRV and local magnetic field power time series

For the determination of synchronization between participants’ HRV and the signal of the power of local magnetic field, computed for 1s time intervals (see Section 2.6), we employed techniques developed, validated and described in detail in [14]. This approach is based on the fact, that it is possible to map a data signal into an integer which describes the geometrical dynamics of the data signal. The integer corresponds to the optimal time lag value that maximizes the area of the attractor constructed from the data series.

We slightly modified the algorithm described in [14] as Algorithm C to be relevant to this study to the following procedure:

Let $RR=\left({RR}_1,\dots ,{RR}_n\right)$ and $M=(M_1,\dots ,M_n)$ be synchronously sampled participant’s HRV and local magnetic field (MF) power signals of size$n$.

1) Divide series $RR$ and $M$ into $T$ 5-minute-long observation windows (according to HRV analysis standards [25]) and compute optimal time lag values for each observation window for HRV time series $\left({\tau }_{*}^{\left(RR\right)}=\left({\tau }_{*1}^{\left(RR\right)},\dots ,{\tau }_{*T}^{\left(RR\right)}\right)\right)$ and magnetic field power data signal $\left({\tau }_{*}^{\left(M\right)}=\left({\tau }_{*1}^{\left(M\right)},\dots ,{\tau }_{*T}^{\left(M\right)}\right)\right)$. This step reduces the analyzed HRV and MF time series to integer vectors, characterizing geometrical features of those series.

2) Smooth obtained optimal time lag vectors in order to identify averaged changes in time lags for HRV $\left({\stackrel{-}{\tau }}_{*}^{\left(RR\right)}\right)$ and magnetic field data $\left({\stackrel{-}{\tau }}_{*}^{\left(M\right)}\right)$.

3) Calculate the Pearson correlation coefficient between the obtained mean time lag vectors $C^{(RR,M)}=\rho \left({\stackrel{-}{\tau }}_{*}^{\left(RR\right)},{\stackrel{-}{\tau }}_{*}^{\left(M\right)}\right)$.

Consider a set of HRV time series ${RR}^{\left(k\right)}=\left({RR}_1^{\left(k\right)},\dots ,{RR}_n^{\left(k\right)}\right)$, $k=\stackrel{-}{1,K}$, corresponding to $K$ participants. In order to calculate the mean synchronization between the HRV of all $K$ participants and magnetic field, we compute HRV/Magnetic field correlation coefficients $C^{\left({RR}^{\left(k\right)},M\right)}$ for each participant $k=\stackrel{-}{1,K}$ and calculate the mean correlation coefficient $C^{\left({RR}^{\left(1..K\right)},M\right)}=\frac{1}{K}\sum _{k=1}^K{C}^{\left({RR}^{\left(k\right)},M\right)}$.

2.8. Identification of the effect of being in a coherent state using the Heart Lock-In technique on synchronization with the magnetic field

Procedures for the estimation of synchronization between participants’ HRV and local magnetic field power time series, described in Section 2.7, were applied to the data described above.

Since the Heart Lock-In technique took place once for a 15-minute period (on March 5th, 2015), we calculated participants’ HRV/MF synchronization for each day of the study separately in order to see if the synchronization between participants’ HRV and magnetic field activity on the day of the Heart Lock-In was different compared to days before and after the using the technique.