Influence of Manual and Electrical Vagal Stimulation on Autonomic Activity and Other Dysautonomous Symptoms in a Healthy Population.

Non-invasive electrical and manual stimulation of the vagus nerve can have varying effects on the regulation of the autonomic nervous system, as assessed by heart rate variability in healthy individuals. This stimulation could have beneficial effects in reducing dysautonomic symptoms such as fatigue or improving sleep quality in healthy individuals.

The study will consist of two groups of 8 people each, healthy volunteers of equivalent age and sex:

• Transcutaneous auricular stimulation group: 8 people.
• Manual stimulation group: 8 people.

Study demographic and clinical variables:

• Age
• Sex
• Heart rate variability (HRV): Heart rate variability analysis is proposed as a non-invasive and reliable measure to indirectly assess autonomic nervous system dysfunction. HRV is the measurement of the fluctuations in the intervals between heart beats. The oscillation between cardiac cycles is controlled by the balance of the autonomic nervous system.

Two HRV parameters/domains are assessed: frequency and time. Regarding frequency parameters, the high-frequency (HF) measurement is an indicator of parasympathetic nervous system activity, the low-frequency (LF) measurement is an indicator of sympathetic nervous system activity and finally the ratio between the two parameters LH/HF indicates the balance between both systems.

Regarding time parameters, a significant decrease in the Standard deviation of the normal-to-normal interval (SDNN) and the Root mean square successive difference (RMSSD) and pNN50 indicators would indicate a decrease in parasympathetic nervous system activity.

A Polar® H10 chest strap connected to a mobile phone will be used to collect HRV data and the data will be analyzed using Elite software.

The Polar® H10 chest strap, previously moistened to ensure better conductivity, will be placed around the patient's chest so that the sensor is located below the inframammary groove and above the sternum. The patient will then be placed in a supine position where they will remain at rest for 5 minutes to stabilize their vital signs. Once these minutes have passed, the short-term measurement of HRV will begin, recording the values for 5 minutes, while the patient remains at rest and breathing freely.

• Quality of life (Eqol5)
• Fatigue: FACIT Fatigue Scale:
• Sleep quality questionnaire: Pittsburgh sleep quality index.
• Treatment Satisfaction Scale (CRES-4)

Inclusion criteria:

• Healthy volunteers between 40 and 70 years of age.
• No known neurological pathology.
• Willingness to participate voluntarily in the study.

Exclusion criteria:

• Cognitive impairment objectively assessed by a neurologist and considered relevant.
• Contraindications for the application of transcutaneous auricular stimulation (implanted cardiac pacemaker or deep brain stimulation treatment).
• Consumption of tea, caffeine, energy drinks, alcohol or tobacco in the two hours prior to the intervention.

On the first day, participants will receive:

• Participant information sheet and informed consent form: clearly explaining the intervention they will undergo and the inclusion and exclusion criteria, and requesting their signature.
• Data protection form (ARSOPOL).

Subsequently, demographic and clinical variables will be collected:

• Age
• Gender
• Heart rate variability
• Quality of life: Eqol5 questionnaire
• Fatigue: FACIT fatigue scale
• Sleep quality questionnaire: Pittsburgh Sleep Quality Index.

Once the initial measurements of the variables have been taken, participants will be randomly divided into two groups similar in age and sex: transcutaneous auricular stimulation group and manual stimulation group:

- Group to which electrical stimulation of the vagus nerve will be applied: Transcutaneous auricular stimulation: The intervention consists of applying electrical stimulation to the auricular branch of the vagus nerve with a 2-pole stimulation electrode. A commercial transcutaneous electrical stimulation device approved for use in humans (tVNS R (Hook), from tVNS® Technologies GmbH, Erlangen (Germany)) will be used. lectrode will be placed on the left ear in contact with the cymba conchae, an area innervated by the auricular branch of the left vagus nerve.

Stimulation protocol: Stimulation will be administered in cycles of 60 minutes/day for 10 days (5 days a week). The stimulation parameters are: Pulse width: 500 μs, Frequency: 25 Hz, Alternating duty cycle: 60 seconds ON followed by 10 seconds OFF, Total time: 60 minutes per day. Current intensity: 200% of the perception threshold, provided it is tolerated and does not cause pain.

Contingency plan in case the intensity of the stimulation is not tolerated and causes discomfort and/or pain: if the participant reports discomfort and/or pain, the intensity of the stimulation will first be reduced until the discomfort and/or pain disappears. If the discomfort and/or pain persists after this action, the stimulation will be withdrawn and the participant will be removed from the study.

A total of 10 interventions will be carried out, from Monday to Friday for 2 consecutive weeks.

- Group to which manual vagus nerve stimulation will be applied. The intervention consists of applying a protocol of manual therapy techniques to the vagus nerve and along its path, during 4 sessions of 30 minutes each, 2 interventions per week for 2 consecutive weeks.

The therapeutic protocol is structured around a total of nine specific manoeuvres. The intervention will begin with the application of three techniques targeting the cervical region, followed by two manoeuvres focusing on the anterior region of the neck. This will be followed by two techniques performed on the thorax and, finally, two mobilisations targeting the abdominal region.

The protocol will be based on previously published studies: Bayo-Tallón V and Giles PD.

The patient will be placed in a supine position throughout the intervention:

1. Compression of the fourth ventricle:

   The physiotherapist will sit at the head of the stretcher. With the forearms resting on the stretcher, the thenar eminences of both hands are placed laterally on the external occipital protuberance, medially on the angles of the occipital scale and the tips of the thumbs are at the level of the first cervical vertebrae, adopting the shape of a "V". At the same time, the skull is compressed in a medial direction accompanying the extension. This compression is maintained until a decrease in respiratory rate is noted. The maneuver ends by gradually releasing the pressure.

2. Suboccipital muscles:

   The physiotherapist is seated at the head of the stretcher and places the 2nd, 3rd and 4th fingers of both hands in the space between the occipital and the spinous process of the second cervical vertebra. The fingers will be maintained with a flexion of the metacarpophalangeal joints while a constant/maintained pressure is applied in a ventral direction on the suboccipital muscles for 3 minutes.

3. Left jugular foramen:

   The physiotherapist is seated at the head of the stretcher and places his right hand on the occipital scale adapting the 2nd, 3rd and 4th fingers. The left hand is placed on the temporal bone: thumb and 2nd finger above and below the zygomatic apophysis respectively. The 3rd finger will be placed in the external auditory canal and finally the 4th and 5th fingers behind the ear at the level of the mastoid process. Keeping the hand stable on the occipital, the temporal is pulled ventrally using a supination movement of the grip located on the temporal.The technique will be performed for approximately 3 minutes, seeking decompression of the jugular foramen.

4. Mobilization of the structures of the anterior region of the neck/Larynx:

   The physiotherapist is standing on one side of the stretcher. The maneuver will be performed in two depth planes:

   • Superficial plane: the physiotherapist places both hands at the level of the thyroid cartilage with the thumbs, 2nd and 3rd fingers on either side of the larynx. A slow and gentle movement will be made to move the larynx laterally to the right and left alternately, until the relaxation of the superficial muscles of the anterior region of the neck is noted.
   • Deep plane: the same placement of the physiotherapist is maintained, placing the hands covering all the structures that are in front of the cervical spine. A slow and gentle movement will be made to move the structures laterally to the right and left alternately, until the relaxation of the deep muscles of the anterior region of the neck is noted.

5. Suprahyoid muscles:

   The physiotherapist is seated at the head of the stretcher and places the 3rd fingers of both hands on the floor of the mouth, at the level of the mental protuberance. The maneuver consists of applying pressure in a cranial direction and adding a lateral displacement, following the lower line of the jaw until reaching the angle of the jaw. They will be performed between 3-7 repetitions, until the relaxation of the suprahyoid muscles is noticed.

6. Expansion of the mediastinum area:

   The physiotherapist is standing on one side of the patient and places both hands overlapping on the sternum. The sternum is pressed dorsally and the areas that allow deeper compression are assessed. The maneuver is combined with the patient's breathing. During inspiration, the pressure is maintained and during expiration, the pressure is increased in the areas that allow greater depth. After 3-4 respiratory cycles, once the last, fast and deep inspiration has begun, the physiotherapist will suddenly release the pressure exerted on the sternum. There will be a sudden acceleration of the patient's inspiration and an expansion of the mediastinum area.

7. Diaphragm:

   The physiotherapist is standing on one side of the patient. Two maneuvers will be performed as a treatment of the diaphragm:

   • Cost closure maneuver: the last 4 fingers of both hands will be placed below the costal margin on one side. During exhalations, the thorax will be closed and the fingers will be introduced more and more under the ribs. During inspirations, the costal path and the stretch achieved during the exhalation phase are maintained. The maneuver is performed for 3 respiratory cycles. The same sequence will be repeated on the costal margin on the other side.
   • Rib opening maneuver: the last 4 fingers of both hands are placed below the rib cage on one side. On the first exhalation, the fingers are gradually introduced under the ribs. During the exhalations, the aim is to progressively increase the opening of the chest, which will facilitate the descent of the dome of the diaphragm. During the following inspirations, the degree of rib opening is maintained. The maneuver is performed for 3 respiratory cycles. The same sequence is repeated on the rib cage on the other side.

8. General visceral mobilization:

   The patient is placed with the knees bent and the feet on the stretcher. The physiotherapist is positioned on one side with the caudal hand on the patient's knees and the heel and palm of the cranial hand on the abdominal region, lateral to the small intestine. The patient's lower limbs are oscillated. When the lower limbs fall towards the physiotherapist, the abdominal grip displaces the intestine in the opposite direction and vice versa. Between 4-6 mobilizations will be performed.

9. Neurodynamic technique left vagus nerve:

The physiotherapist sits at the head of the stretcher and places the palm of his right hand supporting the occipital bone, placing the 2nd and 3rd fingers on the left occipital condyle. With this grip, the following sequence of movements will be performed in the cervical area with approximate amplitudes: 12º of flexion, 12º of inclination to the right and 52º of rotation to the left. The heel of the left hand is placed in the upper part of the abdomen at the level of the lesser curvature of the stomach. With this grip, gentle pressure will be performed in a dorsal direction. The maneuver will follow the patient's respiratory rhythm: during exhalation, the explained parameters will be set and during inspiration, both hands will reduce the tension to the initial resting position. 10 repetitions will be performed.

Once the two interventions have been completed, the variables collected at the beginning of the intervention will be measured again: heart rate variability, quality of life, fatigue, and sleep quality. A questionnaire will also be added to assess the degree of satisfaction with the intervention received.