Highlights
- Cervical vagus nerve stimulation has an immediate effect of reducing heart rate, but then increases immediately following a session.
- Auricular vagus nerve stimulation has no impact on resting heart rate during treatment, but results in a reduced heart rate immediately following a session.
- Auricular vagus nerve stimulation demonstrated a greater impact on sympathetic nervous system recovery (increased HRV) than cervical vagus nerve stimulation.
Introduction
The vagus nerve, also known as the tenth cranial nerve, has recently attracted substantial attention within the scientific community and beyond due to its unique role in modulating physiological and psychological wellbeing. As the longest autonomic nerve in the human body, it extends from the brainstem to the colon and innervates various organs along its course, playing a crucial role in controlling heart rate, digestion, mood, and immune response.
The vagus nerve is composed of several branches, with the auricular and cervical branches being of particular interest. The auricular branch, also known as the Arnold's nerve, is a small branch of the vagus nerve that provides sensory innervation to the external ear and external auditory canal. On the other hand, the cervical branch of the vagus nerve, also known as the superior, inferior, or recurrent laryngeal nerve, innervates structures within the neck and thorax, including the larynx, trachea, esophagus, and cardiac plexus.
Despite their common origin, the distinct anatomical and functional characteristics of the auricular and cervical branches may impart different physiological effects upon stimulation. Heart rate (HR) and heart rate variability (HRV) are two metrics commonly used to assess autonomic nervous system function and are known to be influenced by vagus nerve activity. Understanding the differential effects of stimulating the auricular versus cervical branch on these parameters could have significant implications for the clinical application of vagal nerve stimulation.
This case study of one individual aims to explore potential differences between the auricular and cervical branches of the vagus nerve with respect to HR and HRV. By investigating these variations, we hope to provide valuable insights into the optimal choice and clinical applications of vagal nerve stimulation.
Methods and Materials
This study employed a within-subjects design where a single subject was exposed to six different sessions across six consecutive days, with one session per day. The sessions were divided into three sessions of auricular vagus nerve stimulation and three sessions of cervical vagus nerve stimulation, administered in alternating order starting with the cervical stimulation.
Equipment
Stimulation was administered using the VeRelief Prime nerve stimulation device, manufactured by Hoolest Performance Technologies. This device utilized proprietary dry hydrogel electrodes, which eliminated the need for conductive gel or fluid. Stimulation was constant at a frequency of 25Hz and a current of 10mA.
Heart rate (HR) measurements were obtained using a Scosche Rhythm24 HR monitor. HRV was analyzed using the HRV Logger app, an app developed by Marco Altini, data advisor to Oura Ring.
Environment and Subject Condition
The experimental sessions were conducted in a minimally noisy office environment with the subject seated at a desk. The subject was instructed to keep their eyes open, focusing on a blank screen throughout the duration of each session.
Procedure
Each session comprised of a continuous 3 minute recording broken up into three distinct periods. The initial period involved a one-minute baseline HR recording with no stimulation. This was immediately followed by a one-minute HR recording during active stimulation, and concluded with a one-minute resting HR recording post-stimulation.
For auricular vagus nerve stimulation, the device was targeted at the junction containing the great auricular and the auricular branch of the vagus nerve, located near the tympanomastoid fissure just below the earlobe on the side of the neck.
In the case of cervical vagus nerve stimulation, the device was positioned so that the electrodes were parallel to the right collar bone, pressed into the neck just above the right collar bone and in close proximity to the palpable pulse. Incorrect placement was identified by any observed muscle contraction, at which point the device was repositioned accordingly.
Results
Effects on Heart Rate
For the cervical vagus nerve stimulation trials, the average resting heart rate during the 60-second pre-stimulation measurement was 81.3 bpm. The average resting heart rate during the 60-second stimulation period was 76.0 bpm, indicating a 6.4% reduction in heart rate during stimulation. Finally, the average resting heart rate immediately after stimulation was 81.1 bpm, effectively a 0% change in heart rate from the baseline measurement.
For the auricular vagus nerve stimulation trials, the average resting heart rate during the 60-second pre-stimulation period was 85.1 bpm. The average resting heart rate during the 60-second stimulation period was 86.8 bpm, indicating a 2% increase in heart rate during stimulation. Finally, the average resting heart rate immediately after stimulation was 82.8, indicating a 2.7% reduction in heart rate compared to the baseline resting level.
Effects on Heart Rate Variability (rMSSD)
For heart rate variability, rMSSD was used to indicate HRV and parasympathetic tone. For the cervical vagus nerve stimulation trials, the average resting HRV during the baseline 60-second measurement period was 38.3. The average HRV during the 60-second stimulation period was 39.7, indicating a 3.8% increase compared to baseline. The average HRV during the 60-second rest period immediately after stimulation was 35.4, indicating a 7.4% reduction in HRV compared to the baseline resting HRV level.
For the auricular vagus nerve stimulation trials, the average resting HRV during the baseline 60-second measurement period was 45.4. The average HRV during the 60-second stimulation period was 40.2, indicating an 11.4% reduction in HRV compared to the baseline resting HRV level. The average HRV during the 60-second post stimulation rest period was 53.7, indicating an 18.3% increase in HRV compared to the baseline resting HRV level.
Discussion
One of the many questions that continues to be asked within the vagus nerve stimulation community is what is the difference between stimulating the auricular branch of the vagus nerve and the cervical branch of the vagus nerve. Do both nerve branches have the same physiological pathways and therefore the same potential clinical applications? Or is one a better target for stimulation to treat various conditions compared to the other? The purpose of this case study was to shine some light on the true answer to that question.
The data from this study demonstrates that there is a clear difference between auricular vagal stimulation and cervical vagal stimulation with respect to the effects on heart rate and heart rate variability. In particular, it looks like cervical vagus nerve stimulation has the most profound immediate impact on reducing heart rate compared to auricular vagus nerve stimulation, while auricular vagus nerve stimulation has the most profound effect on increasing heart rate variability.
In the three heart rate graphs for the three cervical vagus nerve stimulation trials, there is a clear visual reduction in heart rate right around the 60s period when the stimulation was first applied. This suggests that cervical vagus nerve stimulation can be a powerful tactic to immediately reduce an elevated heart rate. However, it may also be dangerous for people with low resting heart rates as it may cause bradycardia, or those with high blood pressure, as the sudden change in blood pressure may cause a person to faint.
There is also a clear increase in heart rate the moment the cervical vagus nerve stimulation is removed, suggesting that stimulating the cervical vagus nerve can have a sympathetic rebound effect where heart rate will actually spike immediately following treatment. This is also validated by looking at the heart rate variability data, which shows a 7.4% reduction in rMSSD (parasympathetic output) between the baseline and post stimulation measurement periods. Sympathetic rebound post vagal stimulation has also been validated by other researchers Badran et al., 2018. The reduction in rMSSD suggests the nervous system shifted more towards a sympathetic state immediately after stimulation, suggesting that cervical vagus nerve stimulation may not be the best protocol for someone looking for immediate relief from a fight or flight state, such as those experiencing severe stress or even panic.
In the three heart rate graphs for the three auricular vagal stimulation trials, there was no clear reduction in heart rate immediately during the stimulation phase, but there does appear to be a visual gradual decrease in heart rate during the post stimulation period compared to baseline. There was an average reduction in heart rate by 2.7% between the baseline and post stimulation periods in the auricular vagal stimulation group, suggesting that auricular vagal stimulation has a slowing effect on the body immediately after treatment, but not during the stimulation itself. This is also validated by the HRV data that demonstrated an 18.3% increase in parasympathetic output between the baseline and post-stimulation measurement periods after a single 60-second stimulation session. Another sham-controlled study conducted by Hoolest (Study Three) with 24 individuals demonstrated that 10 minutes of auricular vagal stimulation showed a 31% increase in HRV (rMSSD) between the post stimulation and baseline measurement periods, whereas the sham group showed no change in HRV.
This suggests that auricular vagus nerve stimulation could be an effective tool to help people quickly recover from a fight or flight response, such as those experiencing severe stress or even panic.
Conclusion
Based on this extremely limited data set, it does appear that there is a clear difference between how stimulating the cervical vagus nerve vs. the auricular vagus nerve impacts the body. Cervical vagus nerve stimulation demonstrates an immediate reduction in heart rate, but also has a sympathetic rebound effect via an increase in heart rate and reduction in HRV immediately following treatment. Auricular vagus nerve stimulation showed effectively no impact on heart rate during treatment, but did demonstrate a gradual reduction in heart rate and a much larger increase in HRV than cervical vagal stimulation immediately following a stimulation session.
These results suggest that auricular vagal stimulation is a better option to help people gain rapid and also sustained relief from acute fight or flight responses. There are many tools available for people to find relief from stress and anxiety, such as mindfulness, breathing techniques, and even skin vibration tools. Auricular vagus nerve stimulation is a promising new tool with its own unique benefits that should also be considered by people looking for rapid relief.
Future studies aiming to validate the results of this case study should obviously include more patients, but should also include other nerve targets like the tragus (auricular) and closer to the carotid artery (cervical). There should also be a sham-controlled study to see what the impact of applying pressure (no stimulation) to each stimulation site has on heart rate and HRV.