• The vagus nerve is the tenth and most extensive cranial nerve. It emerges directly from the brain and runs right down to the abdomen, innervating major organs and systems including the heart, lungs and gastrointestinal tract [1]. The vagus nerve is made up of around 80% to 90% afferent fibers [2] – the kind of nerve fibers that carry information from the body back to the brain.
  • The vagus nerve is important because of the way it interfaces with the autonomic nervous system (ANS). The ANS has two branches, the sympathetic and parasympathetic nervous systems (SNS and PNS). In a healthy body these exist in homeostatic balance. Homeostasis is the process of fluctuation that occurs within the body’s systems in order to maintain a stable environment.
  • The autonomic nervous system is in charge of a series of largely unconscious bodily functions including respiratory rate, heart rate, and digestion. It’s the primary mechanism in control of the fight or flight response [3] – the instinctive, fear-driven reaction to stress.
  • The SNS regulates fight or flight, and the PNS, which is primarily driven by the vagus nerve, controls the opposing rest and digest functions.
  • When we experience chronic stress, the sympathetic nervous system goes into overdrive, perpetuating feelings of anxiety and causing a wide range of physical and mental health problems. These can be exacerbated by negative feedback loops between the brain and the body. Because the fibers of the vagus nerve predominantly send information from the body to the brain, it is possible to relieve psychological stress by addressing its physiological causes. This potentially redefines stress from a clinical standpoint.
  • By activating the vagus nerve, we can trigger the body’s balancing capabilities. Because the vagus nerve is the main driver of the PNS, good ‘vagal tone’ results in a more balanced system and greater resilience against stress. It has been demonstrated that vagus nerve stimulation can be achieved using slow breathing practice, with an optimum rate of six breaths a minute.


The vagus nerve releases a neurotransmitter – a chemical signal that sends messages to other cells – called acetylcholine or ACh. Acetylcholine is found in motor neurons where it triggers smooth muscle to contract, causes blood vessels to dilate and slows the heart rate. It is also found in brain neurons where it is important for memory and cognition.

ACh was discovered in 1913 by an English chemist called Arthur Ewins and his colleague the pharmacologist and physiologist Sir Henry Hallet Dale [4]. It was the first neurotransmitter to be identified. In 1921, Otto Loewi pinpointed its functional importance in an experiment in which, by isolating the heart of a frog, he discovered that activation of the vagus nerve triggered it to secrete a substance he called Vagusstuff [5]. Vagusstuff, or acetylcholine as it was later confirmed to be, caused the heart rate to slow, simulating relaxation or activation of the parasympathetic nervous system. The implication of this is that it is possible to activate the PNS in order to produce physiological conditions cohesive with a state of relaxation, simply by stimulating the vagus nerve. This produces a bottom-up feedback loop in which the brain receives information that alleviates feelings of stress, potentially relieving the physical and psychological complications familiar in chronic stress. It’s the physical and mental equivalent of having a reboot button. Loewi and Dale were jointly awarded the Nobel Prize in Physiology or Medicine 1936 “for their discoveries relating to chemical transmission of nerve impulses” [6].


Heartbeats are not meant to be regular. They change constantly. The more coherent variability there is in their speed and intensity, the more resilient your health is likely to be both mentally and physically [7].

This fluctuation is called heart rate variability (HRV). It’s measured using an ECG, which shows the differences in heartbeats to the millisecond.

HRV is driven by the autonomic nervous system. When the sympathetic branch is activated, so is the fight or flight stress response. In this heightened state, the heart rate is less flexible and the variation between beats is smaller. This signifies a system that will struggle to adapt, implying poor health. HRV can reduce when health is compromised [8,9]. People who are older, less fit, or have chronic mental or physical conditions often have low HRV.

Equally, increasing evidence suggests a link between autonomic imbalance and higher incidence of disease and mortality. This makes HRV a good measure for predicting and assessing health problems. People with higher vagal tone are likely to be healthier both mentally and physically.

Because it’s connected with the autonomic nervous system, particularly the parasympathetic branch, HRV is used to indicate how well the vagus nerve is working. This is called vagal tone. Interestingly, vagal tone in the heart is closely linked to both breath and emotions, affecting interpersonal qualities such as empathy, emotional regulation and attachment [10]. While good vagal tone indicates a functional autonomic nervous system, ANS dysfunction is frequently found in patients with anxiety, panic disorder, PTSD and other stress-related physical and mental illnesses.

HRV is incredibly valuable because it allows clinicians to quantify stress and susceptibility to stress related conditions based on physiological considerations. This makes it possible to define stress in terms of the direct effect it has on organs like the heart rather than as a purely psychological or emotional issue.


When vagal tone is good there are constant small variations in the heart rate dependent on where you are in the breathing cycle. When you inhale, the heart rate speeds up, when you exhale it slows down. This phenomenon is called respiratory sinus arrhythmia (RSA), and it’s used to define how well the autonomic nervous system is working.

Most people take between 9 and 24 breaths every minute [7]. This can change when we experience stress, during exercise or because of alterations to breathing patterns. RSA fluctuates for the same reasons – like most of the body’s functions it is flexible and constantly adapting to external and internal circumstances.

During the breathing cycle, RSA regulates gas exchange in the tiny air sacs in the lungs (the alveoli). When the lungs are richest in oxygen the heart rate is higher. Build up of carbon dioxide prompts exhalation. If you work to develop a low sensitivity to blood carbon dioxide, your exhalation will naturally become longer than your inhalation during normal breathing. This allows you to benefit from the slower heart rate and feelings of calm, activating your parasympathetic nervous system. This slowing of the heart rate during exhalation is driven by the vagus nerve, which secretes acetylcholine when you breathe out.

Scientists have found that this exchange of gases is most efficient when the heart rate and breath are synchronized. It has also been proven that this cardiac synchronicity is most effectively achieved at a breathing rate of 6 breaths a minute. The efficacy of this slow breath rate is underlined by the fact RSA maximizes during relaxation, slow, deep breathing and sleep, and lessens during periods of stress and anxiety [11].


In the 1990s, a research psychologist and clinician called Paul Lehrer devised a technique called heart rate variability biofeedback (HRVB). The main idea behind the procedure is to slow down the breath in order to get beat-by-beat heart rate data that allows the patient to maximize RSA, synchronizing their heart rate and breathing pattern.

There is significant evidence to show the benefits of HRVB for many common illnesses. It has been found to reduce levels of stress in otherwise healthy people [12], and can be used to improve athletic performance [7]. There are potential applications for conditions as wide-ranging as asthma, COPD, irritable bowel syndrome, fibromyalgia, hypertension, cardiac rehabilitation, long-term muscle pain, anxiety, depression, PTSD and insomnia. The improvements in gas exchange that come from HRVB training have the potential to improve symptoms of respiratory diseases [7], reduce stress induced hyperventilation, restore the balance of the autonomic nervous system and regulate the inflammatory response [13].


Given that respiratory sinus arrhythmia represents the connection between the breath and the heart rate, it naturally follows that there is a close link between RSA and the blood pressure.

The baroreflex is another homeostatic function. It provides the negative feedback loop in which heightened blood pressure reflexively causes the heart rate to slow so the blood pressure can decrease [14], and low blood pressure causes the heart rate to increase, raising levels back to normal. This constant balancing is necessary to keep the blood pressure at a sustainable level, not too high and not too low. The blood pressure doesn’t just react to stressful events. It changes constantly when we breathe due to the changes in thoracic pressure.

Researchers believe that the strengthening of the baroreflex is the most tangible effect of HRVB [7]. When the breath to heart rate ratio is optimized in terms of blood gas exchange, the baroreflex at its most efficient. In order to achieve this efficiency, breathing provides a natural way to stimulate the necessary increase in HRV via the vagus nerve.

The baroreflex is not just a control mechanism. Its functioning affects the ease with which blood vessels contract and dilate (vascular tone). And baroreflex sensitivity is also linked to the body’s sensitivity to blood carbon dioxide. The baroreceptors work most effectively when you have reduced sensitivity to CO2.

If you are more sensitive to CO2 you will find it harder to hold your breath after an exhalation. This is because the body responds to CO2 build-up with the desire to inhale. You can reduce your sensitivity to CO2 by increasing your BOLT score. A BOLT of more than 25 seconds indicates a functional baroreflex. Lower baroreflex sensitivity to CO2 produces a slower heart rate and better vagal tone.

The connection across all these systems is slow breathing. The breath rate defined by HRVB is 6 breaths a minute. By reducing the rate of respiration, you can increase vagal tone, reduce stress and improve your physical and mental health.

Slow breathing practice benefits the heart and respiratory system in many ways. It stimulates the baroreceptors to become more responsive to changes in blood pressure. It makes the heart rate more flexible and enlivened. It awakens the reactivity of the vagus nerve and shifts the balance of the autonomic nervous system away from the stressed out fight or flight mode towards an open, relaxed, socially oriented outlook that allows you to thrive on a physical, mental and social level.

Another connection between the baroreceptors and the breath was found in a study that looked into the potential analgesic effect of deep breathing [15]. Researchers discovered that the perception of pain was lower when patients held their breath after a deep inhalation – where the diaphragm was lowered or extended. This pain perception was found to involve the baroreceptors in some way, suggesting that the pain response is more complex than its neural components.

There is a close relationship between baroreceptor function, the breath and the emotions. Chronic stress can cause depression and anxiety. It’s known that these psychological conditions can adversely affect the working of the diaphragm15. At the same time, changes in the psychological or emotional state can create a more intense feeling of pain. Researchers concluded that the diaphragm influences the perception of pain and the function of the baroreceptors, and vice versa. It is not yet clear why this is the case, but it is suggested that the way the blood flows in the body in response to changes in pressure created when the diaphragm moves may determine the response of the baroreceptor neurons.


The vagus nerve fibers innervate the heart and the lungs, as well as other systems and organs throughout the body. The breath is another function that is controlled by the autonomic nervous system (ANS), but equally it is interrelated with the two branches of the ANS in the way it connects with the heart. During inhalation [16], the sympathetic nervous system causes the heart rate to speed up slightly. During exhalation, the vagus nerve secretes acetylcholine, causing the heart to slow slightly via the parasympathetic nervous system. In simple terms, the out breath is directly controlled by the vagus nerve [17], while vagal activity is suppressed during the in breath [18,19]. This means that by increasing the length of the exhalation, it is possible to stimulate the vagus nerve, activating the rest and digest PNS and effectively accessing acetylcholine ‘on demand.’ This creates feelings of relaxation and rebalances the system away from chronic fight-or-flight stress.

Scientists have demonstrated that it is possible to stimulate the vagus nerve using slow breathing exercises. It is thought that this stimulation is prompted by the baroreflex mechanism. There is also a weight of evidence, both documented and empirical, that people who practice slow and controlled breathing exercises gain significant benefits due to a long-term improvement in vagal tone. Balance between the SNS and PNS requires the body and all its systems to consistently maintain the ability to relax. When the PNS is dominant, you will experience freedom from chronic stress and the alleviation of stress-related illnesses and symptoms.

The rate of six breaths a minute has been proven to be best for the optimum working of many different systems within the body:

  • It is the best rate for reducing dead space in the lungs – because breathing is slower, air has more time to get right into the tiny air sacs in the lungs.
  • It addresses the poor breathing pattern of chronic hyperventilation. By reducing the breath rate, the amount of air breathed also reduces, even though tidal volume per breath increases because the diaphragm is engaged.
  • It engages the diaphragm. Diaphragm breathing has the effect of slowing down the breath [10]. Equally it is not possible to achieve a respiratory rate of 6 breaths a minute without employing the diaphragm muscle. One study showed that people who practice diaphragm breathing exercises are more likely to achieve a respiration rate of between 3 and 7 breaths per minute than those who breathe less deeply [11].
  • It’s best for increasing oxygen uptake, because more air gets into the alveoli where gas exchange takes place. It is also best for optimizing gas exchange [11].
  • It’s best for ease of sustainability – it’s not slow enough to make it too difficult for most people to practice.
  • It provides optimal functioning of the SNS and PNS, increasing vagal tone and creating a ‘tonic shift’ in autonomic balance.
  • It increases the filling of the heart and the amount of blood pumped per minute.
  • It causes blood pressure oscillations to synchronize with the heartbeat.
  • It increases baroreflex sensitivity.
  • It optimizes release of acetylcholine.
  • It creates the best results in HRV and RSA.

Slow breathing is more efficient. Breathing from the diaphragm at 6 breaths a minute reduces the body’s response to imbalances in blood gases [20,21,22] – hypercapnia (too much CO2) and hypoxia (too little oxygen). On the other hand, when you breathe faster than normal, despite the fact you are taking in more air per minute, less of that air reaches the alveoli where gas exchange occurs.

In one study, scientists measured the levels of arterial oxygen during normal respiration and at rates of 15, 6 and 3 breaths a minute. The results showed that 6 breaths a minute was most effective for reducing dead space in the lungs. Another report concluded that diaphragmatic breathing, which equates to a slower breath rate, could potentially relieve the impact of stress in otherwise healthy people [11] .

Physiologically, this was found to be because diaphragmatic breathing causes an increase in HRV. Diaphragm breathing at 6 breaths a minute has also been found to optimize the balance between the two branches of the autonomic nervous system [11], making the system better able to adapt to both physical and mental stress.

Breathing exercises at 6 breaths a minute enhance the functions of the respiratory system. More air gets deeper into the lungs, breathing is more efficient, oxygenation of the cells and muscles is better and more blood flows back to the heart.

A study of athletes at high altitude showed the positive long-term effects of this breathing practice. Climbers summiting Everest and K2 without oxygen supplies displayed similar blood oxygen levels with slower rates of respiration after two weeks acclimatization to lower levels of atmospheric oxygen, demonstrating that breathing efficiency in terms of oxygenation is more important than how much air is breathed or how much oxygen is in that air [23]. Breathing at 6 breaths a minute optimizes the release and hydrolysis of acetylcholine [11], the neurotransmitter released by the vagus nerve.


If you’ve ever practiced any form of yoga, meditation or other form of contemplative routine, you will be familiar with the idea of slow breathing creating a feeling of physical and mental wellbeing, calmness and focus. The thing all of these activities have in common is that breath is controlled. Even by giving attention to the breath it starts to slow down, and slow breath exercises with breathing from the diaphragm rather than the upper chest are common. Breathing during these practices is also always through the nose, which is also an important detail to note, because breathing always should be through the nose.

Many traditions teach slow, deep breathing techniques with an emphasis on longer exhalations. When you understand that the vagus nerve is stimulated during the exhalation and passive on the inhalation, this explains why these practices are so effective in terms of mental health and cognition. However, one study looked in detail at respiration rates and ratios and came up with an even more specific answer. While countless trials have found 6 breaths a minute to be optimal for a whole catalogue of reasons [24], the parameters of these often compared disparate respiration rates like 13, 6 and 3 breaths a minute. This particular study examined rates that are very similar – 6 and 5.5 breaths a minute. Researchers also tested varying inhalation to exhalation ratios – 5:5 (equal) and 4:6 (with a longer exhalation than inhalation). The expectation prior to the study was that slow breathing with an emphasis on the exhalation would provide greater relaxation and a bigger increase in HRV. However, contrary to this theory, the results showed that a breathing rate of 5.5 breaths per minute with an equal ratio of inhalation to exhalation (5:5) increased HRV most significantly [24]. It also improved baroreflex function and enhanced activation of the vagus nerve, and therefore the parasympathetic nervous system, more than the other breathing rates or ratios. This has exciting implications for patients with conditions where low HRV is an issue.


Several papers on the topic of slow breathing explain that slow breathing practice can be uncomfortable. Many people commonly over breathe – breathing fast, from the upper chest and taking in too much air. When you slow down your breath significantly, the volume of air that reaches the air sacs in your lungs will increase, and the amount of air per breath may well increase, but you are likely to breathe a lower volume of air overall per minute. This might result in feelings of air hunger, especially if your sensitivity to blood carbon dioxide is high.

It’s a good idea if you want to begin a practice of 5.5 or 6 breaths a minute breathing exercises, to learn to slow down your breath a bit at a time over the course of several weeks. Breathing techniques to activate your vagus nerve will help you enjoy the calming effects of the parasympathetic nervous system and give you a sense of control over chronic anxiety.


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