Vagus nerve - the heart and breathing

The Vagus Nerve (10th Cranial Nerve) is the longest and most important autonomic nerve bundle in the body.   It is for instance the main information path between the brain and the gut. It has a number of branching nerves that go to the heart, lungs, voice box, stomach, ears, and other organs.   It monitors the condition of these organs and it is also considered a vital part of the immune system. Its most important ‘internal-organ’ role is to keep up homeostasis in the body. In everyday life we know it mostly for its calming effect on stress and our ‘fight-or-flight’ responses. From this it is known as the rest-and-digestnerve.

Breathing, Heart Rate and the Vagus Nerve:

The vagus nerves are responsible for the lowering of the heart rate and in the lungs the vagus nerve causes bronchoconstriction and expansion to enable efficient oxygenization periodicity. When the autonomic nervous system is working normally, the vagus nerve is speeding up the heart rate during inhalation and decreasing heart rate during exhalation. This variation is critically important for determining oxygenization and in its ‘derived’ forms are currently used as the basis in all wearable sports- and health diagnostics involving heart rate. One traditional way to see this vagal tone is by measuring Heart Rate Variability (HRV) but for it to be accurate the measurement should be done for longer times and not by optical heart rate sensors which are inherently inaccurate.   

The Vagus ECG Watch Test:

The Vagus ECG Watch is the first wearable that can make a quick, precise and detailed diagnosis of this interaction between the heart rate and breathing.   Our algorithms can detect breathing from the ECG data (Cardiac Brody Effect, (21)) and the we can calculate the Respiratory Sinus Arrythmia (RSA) which is considered a better measurement of vagal tone than HRV.   Based on our own scientific research  we have in addition to this RSA (15), developed a method to detect and diagnose the periodicity of heart rate increases and decreases in relation to each inhale and exhale from hand-to-hand ECG (22).  We call these values the Respiratory Sinus Arrythmia Synchronization (RSAsync, pat.pend).  In the Vagus Watch analytics it is expressed as an index between 0-100 where 100 is indicating a very well functioning Respiratory-Cardiac oxygen vagal synchronization.  The Vagus Test is a 90 seconds test during which the users ECG is first measured during a 30 seconds ‘normal’ breathing state and then during a 60 seconds ‘controlled breathing’ phase.


Important parts of the vagus nerve:

The dorsal vagus nerve – which is the unmyelinated part of the vagus nerve sends which communicates with the viscera, especially to the intestines.  This part of the vagus nerve is also responsible for extreme fear-reactions such as ‘freeze’ reactions and is overactivated in states of chronic fear.
The ventral vagus nerve is evolutionarily the newer part which is myelinated. Its extensions are mainly situated in the upper part of the body.  It is considered responsible for many autonomic emotional- and human interaction functions.
The nucleus ambiguus – which gives rise to the branchial efferent motor fibers of the vagus nerve and preganglionic parasympathetic neurons that innervate the heart.
The solitary nucleus – which receives afferent taste information and primary afferents from visceral organs
The spinal trigeminal nucleus – which receives information about deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa and the mucosa of the larynx.

The – ‘Rest & Digest’  – Vagus nerve is the most important nerve you have.  It is more than 10km long and forms the main part of the parasympathetic nervous system.  It for instance controls the ‘resting-phases’ of the heart and lungs. The vagus nerves are responsible for the lowering of the heart rate and in the lungs the vagus nerve causes bronchoconstriction to enable efficient oxygenation. A person usually dies quite quickly if the vagus nerve remains in full shutdown.  

When the autonomic nervous system is working normally, the vagus nerve is speeding up the heart rate during inhalation and decreasing heart rate during exhalation. This variation is critically important for optimal oxygenation.  One traditional way to see this vagal tone is by measuring Heart Rate Variability (HRV) but for it to be accurate the measurement should be done for longer times and not by optical heart rate sensors which are inherently inaccurate.    Currently there are also many other indirect forms to quantify this oxygenation with wearable sports- and health devices.

Vagus Test: Measure breathing and pulse/respiration synchronization


The Vagus ECG Smartwatch is the first wearable that can make a quick and precise diagnosis of the second-to-second interaction between heart rate and breathing.   Our algorithms can detect the timing and strength of every breath from the ECG data by using the Cardiac Brody Effect (1). The breathing data is then compared to pulse changes and from peak-to-peak timing correlations we calculate the Respiratory Sinus Arrhythmia (RSA).  RSA is considered the best current scientific measurement of vagal tone = how well your ‘rest & digest’ nervous system is working. Based on our scientific research we have then, in addition to this RSA, developed a method to detect and diagnose the periodicity of heart rate increases and decreases in relation to each inhale and exhale (2).  We call these cardiac/respiratory correlation values the Respiratory Sinus Arrhythmia Synchronization (RSAsync, pat.pend).   

A person can measure these above mentioned vagal tone values by doing the ‘Vagus Test‘ application on our ECG smartwatch.  The test lasts only 90 seconds and it is very easy to do since it is done by sitting still while measuring the ECG heart rate and doing simple controlled breathing as instructed on the watch screen.   

The Vagus ECG smartwatch with its RSA and RSAsync values represent the most complete vagal tone home analytics system available on the market today and these biomarkers can be used for tracking a wide variety of health conditions affected by the vagus nerve.  

(1) ‘The Brody Effect to detect hypovolemia in clinical practice’,  Giraud et al, 2012

(2) ‘Vagal Tone Diagnostics with hand-to-hand ECG’, Study presented by Gustaf Kranck in April 2019 at the BFE conference in Wales.


Medical device and diagnostics:  When sold and delivered directly to consumers, the Vagus ECG smartwatch is for health tracking only.  It is not a medical device and it has no medical diagnostics with its Vagus Health smartphone apps. For medical diagnostics, the users are advised to contact their own doctors who can make the medical diagnostics when the user is for instance sending them the ECG recordings as PDF or raw data.  This is easy to do from the Vagus Health smartphone app.  



Nerve Explorer is a premium subscription for the Vagus Watch Tests where the user gets personalized analytics and commentary from the Vagus team.   We know that each user has personal health-, immune- and nerve-related issues and interests.  With the Nerve Explorer concept, we want to work together with our ‘explorers’ to learn and help.   During the 1st month of free Nerve Explorer subscription, our team will analyse each user’s test-data and be in contact with the users after 2 weeks of use (and the user having done at least 10 Vagus ECG Tests).   We send selected test graphics and our summary by email and communicate if we find irregularities in the test data which we think needs more ‘exploration’.  
The Nerve Explorer is 4.99 USD/month or 49.99 USD/year.   


Coronavirus COVID-19 crowd sourced open data study

Vagus.co together with research scientists are launching a crowd sourcing open data science study for home monitoring and early detection of respiratory infectious disease such as Coronavirus (Covid-19).  There is no previous study of systematic ECG recordings in humans for both pre- and post Coronavirus contraction.   We believe that this type of large data collection can only be obtained with crowd sourced methods.   According to the study hypothesis it is necessary to do a specific ‘Vagus ECG test’ protocol with controlled breathing to detect the subtle cardiac/respiratory changes which can occur early in the disease progress.  

The aim for this study is to obtain ECG data from people who have not yet been infected by COVID-19 but during the course of the data-collection maybe are infected by it or by some other viral or bacterial disease.    The study hypothesis is that this data can provide vital quick access to information for scientists worldwide so that they better can follow and understand the disease progress and find out new early detection methods for cardiac-respiratory infections such as COVID-19 (caused by SARS-CoV-2). 

The study is done with participants own Vagus hand-to-hand ECG smartwatches and its ‘Vagus ECG Test’ recorded data. To participate you need to have a Vagus ECG smartwatch or buy one from the shop.   If you have a watch – email us that you register to participate.   For those buying a watch, please write COVID-19 as reference in the order.   

The participating persons commit to doing three 90 seconds ‘Vagus ECG Tests’ per day.  One in the morning after getting up from bed, one around noon before meal and one in the evening before going to bed.   One test only lasts 90 seconds and the user needs to sit with hands resting on a table (or own legs) and follow the instructions displayed on the watch.   After each test there is an option to include a note of health status and for instance body temperature measurements if the person is self-monitoring it. We recommend such monitoring.  After a possible disease onset we recommend more frequent testing and regular downloads of the ECG recordings to a medical professional or relevant health care provider.  These downloads and sendouts are free for all study-participants.  People in the same houshold can use the same watch as long as they use a separate smartphone, Vagus App and user account.  Monitoring and analysis is free for persons in the same houshold.   The watch is waterproof and it can be disinfected between use.

The study data collection period is one year.   All study data will be anonymized and made available for free to researchers and individuals.   The participants and data providers are not paid for doing this data collection and study.   Study researchers receive no financial compensation from Vagus Health Ltd. We hope for a large and diverse participants group since the success of the study depends on finding at least several persons who contract a disease while collecting data.

Study team members will monitor that the full and unselected data is provided to the open data platform access and publish a monthly research report.  The study data will be uploaded bi-weekly on the open access data portal at www.vagus.co.  Vagus Health Ltd and data collected by this study is GDPR compliant.

How to participate if you don’t have a watch:   Order a watch, write in the order COVID-19 and then enter here with your email and the password we send you: https://dev-test.vagus.app/login.

Data Collection and -reporting team members: Gustaf Kranck, CEO and Founder, Vagus Health Ltd https://www.linkedin.com/in/gustafkranck/, Mattias Nordström, CTO and co-founder, Vagus Health Ltd https://www.linkedin.com/in/mattias-nordstr%C3%B6m-3987443/Johan Sellström, CEO, Carechain.io, Sweden https://www.linkedin.com/in/johansellstrom/,   Dr Fintan Nagle, Data Scientist, UCL, London UK https://www.ucl.ac.uk/~ucbpfsn/about.html, Prof. Johan Lundin, Professor at Karolinska Institutet and Research Director at FIMM https://www.researchgate.net/profile/Johan_Lundin.

UPDATE 26th April 2020:  The first data publication is estimated to be available end of June 2020, subject to that we by then have a sufficient amount of participants (>100) who have done on average 2 Vagus ECG Test per day for at least one month.

UPDATE 12th June 2020:  None of our study participants has to date been infected with SARS-Cov-2 and therefore we have no relevant data yet to publish.  We will be updating the situation as the amount of study participants increase and a sufficient amount of participants  have contracted COVID-19 in such manner that we know when the initial infection and/or symptomes occured.       

The study science background and hypothesis paper – press here to access.

Our Covid-19 study Press Release – press here for full text.

Open data licensing terms: https://creativecommons.org/licenses/by-nc/4.0/

Medical disclaimer: The Vagus ECG smartwatch and the Vagus ECG Test analytics are not yet approved medical devices or diagnostics tools.  They are in beta-testing phase.   There are no published medical trials to yet confirm the study’s hypotheses.  One aim for this study is to obtain data to confirm the hypothesis and publish a study.   The analytics or risk assessments presented by the Vagus App or in the user cloud interface should not be used as medical diagnostics.  They should not be used to decide alternations or additions to medical treatments without consulting a medical doctor or certified professional.   



Subscription classes:

1) Basic (free with all  Vagus ECG watches)

2) Nerve Explorer (free until August 2020. After that 4,99 USD/month (49.90 per year))

3) Coaching (from september 2020)

4) Trials and Studies


During the Coronavirus outbreak we will not charge any extra fees for the ECG recordings sendouts by email to doctors / healthcare providers.

This service is free in all subscription classes.




Recommended Reading:

Professor Stephen Porges books about Polyvagal Theory.  Dr. Porges is a Distinguished University Scientist at Indiana University where he directs the Trauma Research Center in the Kinsey Institute. In 1994 he proposed the Polyvagal Theory, a theory that links the evolution of the mammalian autonomic nervous system to social behavior and emphasizes the importance of physiological state in the expression of behavioral problems and psychiatric disorders.   There are several books published by Dr. Porges about the polyvagal theory and how ventral vagus nerve stimulation can help to alleviate symptoms in a number of diseases.

Professor Edward Bullmores, ‘The Inflamed Mind’ :   Cambridge University Professor Ed Bullmore is writing about how depression which will be the single biggest cause of disability worldwide in the next 20 years and how treatment for depression has not changed much in the last three decades.  In his book, ‘The Inflamed Mind: A radical new approach to depression’, he reveals the breakthrough links found between depression and inflammation of the body and brain, representing a whole new way of looking at how mind, brain and body all work together.  This book is a good description on how the vagus nerve, immune system and homeostasis are linked to depression and anxiety.

Professor Antonio Damasios books about how the brainstem formulate ‘will’

Oliver SacksAwakenings’, 1990 edition.   In the 1990 edition Oliver Sacks made a lot of additions to the original Awakenings book and many fine new conclusions.  Awakenings is generally considered on of the best ‘patient observer’ book written in neurology. 





Selected Science Papers:

There are thousands of science papers published annually which discuss the vagus nerve and vagus stimulation so it is difficult to give a comprehensive list.  Here are those used for our own vagal tone study presented in April 2019:


1)   Assessment of cardiac preload and left ventricular function under increasing levels of positive end-expiratory pressure.  Leucke, Thomas; Roth, Harry (1993).  Intensive Care Medicine. 30 (1): 119–126.

2) Mechanism of decreased left ventricular stroke volume during inspiration in man.   Pathophysiology and natural history.  Authors: M. Karam, M.D., R. A. Wise et al.

3) Effect of respiration on cardiac filling at rest and during exercise in Fontan patients: A clinical and computational modeling study.  IJC Heart & Vasculature, Volume 9, 7 December 2015, Pages 100-108    Authors: Alexander Van De Bruaenea

4) Respiratory change in ECG-wave amplitude is a reliable parameter to estimate intravascular volume status: Journal of Clin Monit Comput.  Giraud R.  Siegenthaler N. Morel DN,  

5) Two Distinct Responses of Left Ventricular End-Diastolic Pressure to Leg-Raise Exercise in Euvolemic Patients with Exertional Dyspnea, Korean Circulatory Journal, May 2016. Authors: SeongIl Choi, MD1 , Jeung-Hun Shin, MD1 , Whan-Cheol Park, MD1 , Soon-Gil Kim, MD1 , Jinho Shin, MD2 , Young-Hyo Lim, MD2 , and Yonggu Lee, MD3,    

6) Cytokine-specific Neurograms in the Sensory Vagus Nerve.   Bioelectron Med. 2016, Benjamin E Steinberg, Harold A Silverman, Sergio Robbiati, Manoj K Gunasekaran1, Téa Tsaava1, Emily .  Battinelli1, Andrew Stiegler4, Chad E Bouton5, Sangeeta S Chavan1,†, Kevin J Tracey1,

7) Mechanisms and Therapeutic Relevance of Neuro-immune Communication, Immunity, June, 2017 Sangeeta S. Chavan,Valentin A. Pavlov and Kevin J. Tracey

8) The vagus nerve and the inflammatory reflex—linking immunity  and metabolism. Nat Rev Endocrinol. 2012 December.  Valentin A. Pavlov and Kevin J. Tracey

9) Theoretical Analysis of Intracavitary Blood  Mass Influence on the Heart-Lead Relationship, 1956, By Daniel A. Brody, M.D.

10)  The Brody effect and change of the volume of the heart.  J. Electrocardiology 18, 1985, J. N. Amoore

11) The Brody Effect Revisited, Journal of Electrocardiology, October 1991, A. Van Oosterom, R. Plonsey.

12) Respiratory change in ECG-wave amplitude is a reliable parameter to estimate intravascular volume status, J. Clin Monit Comput (2013) 27:107–111, Raphael Giraud, Nils Siegenthaler, Denis R. Morel • Jacques-A Romand, Laurent Brochard • Karim Bendjelid

13) Respiratory sinus arrhythmia in humans:  how breathing pattern modulates heart rate, 1981, Judith Ann Hirsch, Beverly Bishop. 

14) Toward Understanding respiratory sinus arrythmia: Relations to cardiac vagal tone, evolution and biobehavioral functions. Biological Psychology 74, 2007.  Paul Grossman, Edwin W. Taylor.

15) The Polyvagal Perspective, Bil Psychol Feb 2007 and several other papers and books by Stephen Porges on Respiratory Sinus Arrythmia (RSA) and the Polyvagal Theory.

16) Accuracy of assessment of cardiac vagal tone by heart rate variability in normal subjects.  Am J Cardiol 1991, Hayano J, Sakakibara Y. Yamada A, et al.

17) Respiratory modulation of cardiac vagal tone in Lyme disease.  World J Cardiol, June 2014.  Basant K. Puri, Musadiq Shah, Jean A. Monro, Michele C. Kingston, Peter OO Julu. 

18)  Anatomic connections of the diaphragm influence of respiration on the body system.. Bordoni, B., & Zanier. (2013). Journal of Multidisciplinary Healthcare, 281.

19) Techniques of EMG signal analysis: detection, processing, classification and applications,  Raez, Hussain & Mohd-Yasin, 2006

20)  Physiology of the Autonomic Nervous System, Laurie Kelly McCorry, 2007

21) The Brody Effect to detect hypovolemia in clinical practice, R. Giraud et al, Critical Care, 2012

22) ‘Vagal Tone Diagnostics with hand-to-hand ECG, Gustaf Kranck et al, presented April 2019 at the Biofeedback Federation annual meeting in Wales.



Atrial Fibrillation (AFib) automated detection: There is much reason to believe that the majority of the automated Atrial Fibrillation (AFib) detections in current ECG smartwatches are false positives for less than 65 year old users.   There is not sufficient scientific research which would show that the benefits from such automated AFib detections in young people outweigh the adverse health effects from the excessive false positives. We have therefore decided not to provide or apply for medical AFib approval for the Vagus Health ECG smartwatch at this time. We strongly recommend consulting a cardiologist if the user suspects they have a repeating heart condition with AFib.

Want to know more?

Sign up for our blogposts and updates