The effect of rVNS breathing

To understand this, a basic knowledge of the human body's nervous system is necessary, which we'll start with right away.

Neurobiological principles [ 1 ]

The nervous system of vertebrates, including humans, is divided into 2 main subsystems:

Central nervous system (CNS) and peripheral nervous system (PNS).

To understand how rVNS respiration works, the CNS is not yet that important, which is why we leave out an explanation for it.

We are now focusing more on the PNS.

The PNS is again divided into 2 functional subsystems:

The somatic and the vegetative/autonomic part.

Somatic system

Controls everything we can consciously control, such as our muscles and other reflexes. Walking, speaking or smelling are all typical tasks.

Autonomous system

Controls everything that is not consciously happening, such as your heartbeat, digestion or breathing most of the time.

It is getting interesting now. Because the autonomic nervous system is responsible for the positive effects of rVNS breathing.

One final subdivision and then we're done.

The autonomic nervous system is again divided into 3 parts:

• Sympathetic nervous system/sympathetic nervous system: Responsible for anxiety and stress reactions
• Parasympathetic nervous system/parasympathetic nervous system: Responsible for relaxation and digestion. Generally conserves energy.
• Enteric nervous system: A separate nervous system for the stomach. Also sometimes called the 2nd brain.

Now the vagus nerve (VN) comes into play. This nerve is a central part of the parasympathetic nervous system. This is a cranial nerve complex with widespread fibers that connect to glands and organs such as the heart or lungs [ 2 ].

VN activity is suppressed during inhalation and increased during exhalation, especially with slow breathing cycles [ 3 ].

You can even do a test on your own body to see how strongly the sympathetic and parasympathetic nervous systems affect your heart. When you breathe in, you may notice that your heart beats a little faster and stronger than when you breathe out. If you feel this, then you have experienced live that the sympathetic nervous system increases your heart rate and, conversely, the paraympathetic nervous system slows your heart rate when you exhale. As mentioned above, when you breathe out, VN activity increases, which relaxes the body and you can notice this, for example, in a slower heartbeat.

Slow breathing techniques with long exhalations signal a state of relaxation through the VN, leading to more VN activity and further relaxation. This is a form of respiratory biofeedback.

Exactly how VN & rVNS work

In the direct route, slow breathing and prolonged exhalation are caused by vagal activity (VN activity). This follows from the previously mentioned role of the VN in respiratory affective and effective processing (slowing and expiration). Controlled breathing uses the vagus nerve as an effector and intentionally increases its activity, even if only for a short time.

The indirect pathway involves stimulation through biofeedback and follows from physiological feedback theory: by adopting physiological body patterns associated with relaxation and low-threat situations (e.g., slow breathing), vagal afferents project this state to the CNS , which initiates a “rest-and-digest” state — again via the VN. The indirect pathway is responsible for more long-term tonic changes in vagal tone (VN activity). In both pathways, breathing styles with low respiratory rate and low inhalation/exhalation ratio show an increase in vagal tone, albeit at slightly different time periods [ 4 ]. As a result, VN efferent pathways further increase VN activity and produce associated physiological consequences (e.g., decrease in heart rate, blood pressure, increase in HRV). A relaxation loop follows.

This indirect pathway can be viewed as a form of biofeedback and is responsible for long-term changes in VN activity. Breathing patterns play a key role here: a recent study using electroneurograms to map left VN breathing pattern signaling showed an almost perfect overlap between this mapping and actual breathing cycles [ 5 ].

The first physiological mechanism for CNS control of breathing patterns to stimulate VN (as biofeedback) is the baroreceptor reflex [ 6 ; 7 ]. This reflex is responsible for regulating blood pressure and is triggered by stretch-activated mechanoreceptors (baroreceptors) in blood vessels, leading to activation of the vagal branch of the cardiac node, thereby reducing heart rate and subsequently blood pressure. The threshold for triggering this reflex (cardiovagal baroreflex sensitivity) can be reduced by a respiratory rate of 0.1 Hz or about 6 breaths per minute. Interestingly, this is the exact same breathing rate that is reported in breathing studies to have the highest HRV increase/VN activity and therefore relaxation. A reduction in sensitivity leads to more frequent reflexes, a lower heart rate and increased vagal tone [ 8 ; 9 ; 10 ; 11 ]. This mechanism is a faster indirect pathway between respiratory rate and heart rate as mediated by VN than the biofeedback pathway through VN afferent subcortical projections that signal broad relaxation.

The slowest of the indirect pathways is biofeedback, in which a low respiratory rate and a small inhalation/expiration ratio signal a resting state to the CNS. This is consistent with the James-Lange hypothesis of physiological feedback from emotions and similar representations [ 12 ]. The theory, independently proposed by William James and Carl Lange, states that the identification and experience of an emotion follows peripheral physiological responses (e.g., arousal), rather than the other way around. The type of emotion experienced depends on the interpretation of the physiological state and the assessment of the context in which it is triggered. The physiological stress reaction therefore precedes the subjective emotional experience of fear or sadness. Following this line of reasoning, bottom-up changes to dysfunctional emotional states can be created by altering the physiological state of the body.

In other words, relaxing the body relaxes the mind and breathing exercises reduce stress [ 13 ; 14 ; 15 ].

Health effects of VN activity & rVNS

Most experimental studies show higher HRV (higher HRV indices correlate with higher VN activity) after breathing instructions, consistent with the involvement of rVNS. In particular, there is ample evidence that slow and deep breathing increases HRV indices of vagal tone [ 16 ; 17 ; 18 ; 19 ; 20 ; 21 ; 22 ] and lowers stress markers such as heart rate, blood pressure and salivary cortisol [ 13 ; 14 ; 15 ].

Van Diestet et al. [ 23 ] specifically examined the effects of different inspiratory/expiratory ratios at either slow or normal respiratory rates on various peak-valley (HRV) measures: higher HRV (both measures) was reported in the slow breathing condition, but only for prolonged expiration ( inhalation/exhalation ratio: 0.24) and not for prolonged inspiration (inhalation/exhalation ratio: 2.33). Although normal conditions were not included, this study demonstrates most clearly the stimulatory effects of specific breathing styles on VN. Another example of prolonged exhalation, albeit with a completely different goal and context: a study of Native American flute playing showed a significant increase in HRV during playing, contrary to what would be expected during physical exertion [24] . Needless to say, playing any pipe instrument requires an extremely extended exhalation.

As we review breathing techniques practiced in ContActs (contemplative activities such as yoga or meditation), studies looking at autonomic function through ContActs and using these types of techniques should report increased vagal tone. In fact, HRV increases with almost all forms of contact, consistent with the rVNS hypothesis. Various forms of meditation (e.g. Body Scan, FA, OM Acem, Zen) and mind-body exercises such as yoga all show an increase in vagal tone HRV in healthy participants [ 25 ; 26 ; 27 ; 28 ; 29 ; 30 ; 31 ; 32 ].

If the above pathologies are positively influenced in this way, there should be a clear negative relationship between vagal tone and the risk factors and symptoms of these diseases. In fact, increased HRV shows a negative correlation with cardiovascular disease in children and adults [ 33 ; 34 ] and even directly predicts hypertension [ 35 ]. It has an inverse relationship with inflammation [ 36 ; 37 ], inflammation in depression [ 38 ], depressive symptoms in children and adults [ 39 ; 40 ], perseverative cognition [ 41 ], symptomology of bipolar disorders [ 42 ], general anxiety and disorders [ 43 ; 33 ; 44 ] and has recently even shown a negative correlation with schizophrenia [ 45 ]. Although schizophrenia is not considered a stress-related disorder, the role of HRV in schizophrenia is intriguing when considering the interplay of dysfunctional emotional regulation and executive functions in its symptomology.

Although rVNS breathing causes a phasic change in PNS activity during and immediately after exercise, it also results in a tonic shift in autonomic balance in the long term. As PNS activity increases, SNS activity decreases. This shift is called vagal dominance. With vagal dominance, chronic stress and stress-related conditions are attenuated. The relaxation or rest and digestion behavior increases. Heart rate, blood pressure and inflammatory response decrease while HRV increases, which in turn affects (chronic) stress. This works directly via the tonal activity of the PNS, but also indirectly via the inhibition of the SNS by the VN. In particular, reducing the (chronic) stress response has positive effects on cardiovascular health and stress-related psychopathology. In addition, vagal dominance also leads to better immune function and attenuation of inflammatory conditions. [ 46 ]

Several studies on various ContActs also report a decrease in cardiometabolic risk factors and an increase in cardiopulmonary health and fitness. According to a meta-analysis, this is most consistently reflected in a reduction in heart rate, blood pressure and blood lipid profile across all practices [ 47 ; 48 ; 49 ; 50 ]. A meta-analysis also suggests increased erobic capacity [ 47 ].

ContAct reviews also report immunological improvements; Most studies find functional anti-inflammatory effects, with meta-analyses suggesting that the most commonly reported decreases in pro-inflammatory markers occur in C-reactive protein and pro-inflammatory cytokines such as tumor necrosis factor-a [ 51 ; 52 ; 53 ; 54 ]

Like other physical exercises, mind-body exercises improve overall physical function, particularly bone density, balance, strength and flexibility [ 49 ]. Mindfulness-based stress reduction, yoga and TCC appear to alleviate (chronic) pain conditions, as shown by pain scales in conditions such as migraine, fibromyalgia and osteoarthritis [ 55 ]. Since mindfulness-based stress reduction also includes yoga-like exercises, these results may be unique to mind-body exercises and are best interpreted as coming from the physical exercise portion of these programs, as exercise-induced analgesia is well established [ 56 ] and, according to a review of several Cochrane reviews [ 57 ], is even comparable to medications for chronic pain conditions.

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