Sham taVNS application was performed by stimulating a sterile acupuncture needle (0.25x25 mm) placed on the left lobulus auriculae of the left ear (which is not innervated by the vagus nerve) with a stimulator. Sham taVNS group received for 10 days, 30 minutes with a current intensity of 1 mA, pulse width of 200 µs, frequency of 25 Hz and biphasic sinusoidal waveform.

The ULF is also one of the frequency-domain HRV parameters. ULF represents the ultra low frequency. It indicates frequencies <0.003 Hz. It does not directly reflect sympathetic or parasympathetic activity.

In our study, the short-term effects of taVNS on ULF will be evaluated using data obtained from 24-hour Holter ECG recordings on the pre-stimulation day (day 0) and on day 11.

ULF (ms²): ultra low frequency (<0.003 Hz)

The VLF is also one of the frequency-domain HRV parameters. VLF represents the very low frequency. It indicates frequencies between <0.003 and 0.04 Hz. It has been associated with baroreceptor activity, thermoregulation, the renin-angiotensin system, and endothelial functions.

In our study, the short-term effects of taVNS on VLF will be evaluated using data obtained from 24-hour Holter ECG recordings on the pre-stimulation day (day 0) and on day 11.

VLF (ms²): very low frequency (frequencies between <0.003 and 0.04 Hz)

The LF is also one of the frequency-domain HRV parameters. LF represents the low frequency. It indicates frequencies between 0.04 and 0.15 Hz. It reflects a combination of sympathetic and parasympathetic activity and is particularly associated with sympathetic activity in seated or standing positions.

In our study, the short-term effects of taVNS on LF will be evaluated using data obtained from 24-hour Holter ECG recordings on the pre-stimulation day (day 0) and on day 11, while the acute effects will be assessed using data from 15-minute Holter recordings on day 1 and day 10.

LF (ms2): low frequency power (0.04 to 0.15)

The HF is also one of the frequency-domain HRV parameters. HF represents the high frequency. It indicates frequencies between 0.15 and 0.40 Hz. It reflects parasympathetic activity. An increase in HF indicates a high vagal tone.

In our study, the short-term effects of taVNS on HF will be evaluated using data obtained from 24-hour Holter ECG recordings on the pre-stimulation day (day 0) and on day 11, while the acute effects will be assessed using data from 15-minute Holter recordings on day 1 and day 10.

HF (ms²): high frequency power (0.15 to 0.40)

Time-domain HRV parameters are based on the analysis of the time intervals between normal heartbeats in 24-hour ECG recordings. In our study, the time-domain HRV parameters, including SDNN, SDNNi, SDANN, RMSSD, and pNN50, will be calculated from 24-hour Holter ECG recordings obtained before stimulation and after 10 days of stimulation.

SDNN (ms): It is the standard deviation of all NN (interval between two adjacent R waves) intervals in the 24-hour ECG recording.

Time-domain HRV parameters are based on the analysis of the time intervals between normal heartbeats in 24-hour ECG recordings. In our study, the time-domain HRV parameters, including SDNN, SDNNi, SDANN, RMSSD, and pNN50, will be calculated from 24-hour Holter ECG recordings obtained before stimulation and after 10 days of stimulation.

SDNN index (ms): It is the mean of the standard deviations of all NN intervals for all 5-minute segments within 24 hours.

Time-domain HRV parameters are based on the analysis of the time intervals between normal heartbeats in 24-hour ECG recordings. In our study, the time-domain HRV parameters, including SDNN, SDNNi, SDANN, RMSSD, and pNN50, will be calculated from 24-hour Holter ECG recordings obtained before stimulation and after 10 days of stimulation.

SDANN (ms): The standard deviation of the mean of the normal RR intervals of the 5-minute segments in the 24-hour ECG recording.

Time-domain HRV parameters are based on the analysis of the time intervals between normal heartbeats in 24-hour ECG recordings. In our study, the time-domain HRV parameters, including SDNN, SDNNi, SDANN, RMSSD, and pNN50, will be calculated from 24-hour Holter ECG recordings obtained before stimulation and after 10 days of stimulation.

RMSDD (ms): is the root of the mean of squares of consecutive NN interval differences over the 24-hour recording.

Time-domain HRV parameters are based on the analysis of the time intervals between normal heartbeats in 24-hour ECG recordings. In our study, the time-domain HRV parameters, including SDNN, SDNNi, SDANN, RMSSD, and pNN50, will be calculated from 24-hour Holter ECG recordings obtained before stimulation and after 10 days of stimulation.

pNN50 (%): The proportion of differences between consecutive NN intervals greater than 50 ms during the 24-hour recording

The Ewing's battery parameters (EBP) are the most popular among clinical autonomic test batteries. In our study, the tilt test, heart rate and blood pressure changes, respiratory heart rate changes, and the Valsalva maneuver selected from the EBP clinical autonomic tests.

This test is used to evaluate the hemodynamic response to changes in body position. When moving from a supine to an upright position, venous pooling in the lower extremities leads to a decrease in stroke volume, resulting in a rapid increase in heart rate within the first 30 seconds. The normal response is characterized by a slight increase in heart rate (by 5-20 beats per minute), and the 30/15 ratio in the test is used to assess the initial phase of adaptation to standing. After tilting to an 80-degree upright position using a tilt table, the ratio of the longest R-R interval around the 30th beat to the shortest R-R interval around the 15th beat is calculated. A ratio lower than 1.04 is considered abnormal.

It is another of the clinical autonomic tests in our study. This is another test used to evaluate the hemodynamic response to changes in body position. When moving from a supine to an upright position, blood moves to the lower extremities, reducing venous return and stroke volume. This test is performed to identify the presence of orthostatic hypotension (OH). After measuring the patient's baseline blood pressure in the supine position, the patient is brought to an 80-degree upright position using a tilt table and is kept in this position for 5 minutes. A decrease in systolic blood pressure by ≥20 mmHg or in diastolic blood pressure by ≥10 mmHg is considered a positive test result.

It is another of the clinical autonomic tests in our study. The Valsalva maneuver is a test used to evaluate baroreceptor function and reflects parasympathetic activity. In this test, the patient lies in a supine position with the head elevated to 30 degrees, takes a deep breath, and then performs a forced expiration against a closed airway for 15 seconds, followed by a 45-second rest period of normal breathing. The ratio of the longest R-R interval to the shortest R-R interval is recorded. Three maneuvers are performed, and the average of the three ratios is calculated. A value below 1.2 indicates parasympathetic dysfunction.

It is another of the clinical autonomic tests in our study. This test is based on the phenomenon of respiratory sinus arrhythmia, which becomes particularly evident when breathing at a rate of six breaths per minute. The test is used to assess the effect of the parasympathetic nervous system on heart rate. In this test, the patient lies in a supine position with the head elevated to 30 degrees and is instructed to breathe in and out six times over one minute, with a 5-second inspiration and a 5-second expiration cycle. The difference between the maximum and minimum heart rate within the one-minute cycle is measured. The cycle is repeated three times, and the average of the heart rate differences is calculated. A variability of less than 10 beats per minute indicates parasympathetic dysfunction.