Vagal tone

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Vagal tone is activity of the vagus nerve (the 10th cranial nerve) and a fundamental component of the parasympathetic branch of the autonomic nervous system. This branch of the nervous system is not under conscious control and is largely responsible for the regulation of several body compartments at rest. Vagal activity results in various effects, including: heart rate reduction, vasodilation/constriction of vessels, glandular activity in the heart, lungs, and digestive tract, liver, immune system regulation as well as control of gastrointestinal sensitivity, motility and inflammation. [1]

Contents

In this context, tone specifically refers to the continual nature of baseline parasympathetic action that the vagus nerve exerts. While baseline vagal input is constant, the degree of stimulation it exerts is regulated by a balance of inputs from sympathetic and parasympathetic divisions of the autonomic nervous system, with parasympathetic activity generally being dominant. Vagal tone is frequently used to assess heart function, and is also useful in assessing emotional regulation and other processes that alter, or are altered by, changes in parasympathetic activity. [2] [3]

Measurements of vagal tone can be performed by means of either invasive or noninvasive procedures. Invasive procedures are in the minority and include vagus nerve stimulation by specific manual, breathing or electrical techniques. Noninvasive techniques mainly rely on the investigation of heart rate and heart rate variability. [4] [5] [6]

Noninvasive vagal tone quantification

In most cases, vagal tone is not measured directly. Instead the processes affected by the vagus nerve – specifically heart rate and heart rate variability – are measured and used as a surrogate for vagal tone. Increased vagal tone (and thus vagal action) is generally associated with a lower heart rate and increased heart rate variability. However, during graded orthostatic tilt, vagal tone withdrawal is an indirect indicator of cardiovascular fitness. [7]

Vagal innervation of the heart

Heart rate is largely controlled by the heart's internal pacemaker activity. In a healthy heart, the main pacemaker is a collection of cells on the border of the atria and vena cava called the sinoatrial node. Heart cells exhibit automaticity, the ability to generate electrical activity independent of external stimulation. The electrical activity spontaneously generated by the sinoatrial node sets the pace for the rest of the heart. [1]

In absence of external stimuli, sinoatrial pacing generally, while awake, maintains the heart rate in the range of 60–100 beats per minute (bpm). [8] The two branches of the autonomic nervous system work together to increase or slow the heart rate. The vagus nerve acts on the sinoatrial node, slowing its conduction and modulating vagal tone, via the neurotransmitter acetylcholine and downstream changes to ionic currents and calcium of heart cells. [4] Because of its effect on heart rate, and cardio health, vagal tone can be measured and understood by examining its correlation to heart rate modulation and heart rate variability. [5]

Respiratory sinus arrhythmia

Respiratory sinus arrhythmia (RSA) is typically a benign, normal variation in heart rate that occurs during each breathing cycle: the heart rate increases when breathing in and decreases when breathing out. [1] RSA was first recognized by Carl Ludwig in 1847 [9] but is still imperfectly understood. [10] It has been observed in humans from the early stages of life through adulthood, [11] [1] and is found in several different species. [12] [13] [14]

During inhalation, the intra-thoracic pressure lowers due to the contraction and downward movement of the diaphragm and the expansion of the chest cavity. Atrial pressure is also lowered as a result, causing increased blood flow to the heart, which in turn decreases baroreceptors firing response which diminishes vagal tone. This causes an increase in heart rate. [1]

During exhalation, the diaphragm relaxes, moving upward, and decreases the size of the chest cavity, causing an increase in intrathoracic pressure. This increase in pressure inhibits venous return to the heart resulting in both reduced atrial expansion and increased activation of baroreceptors. This relieves the suppression of vagal tone and leads to a decreased heart rate. [1]

Heart rate (HR) (first row), tidal volume (Vt) (second row), Vt and superimposed HR (third row). The HR modulation is clearly visible: HR increases with inspiration and decreases with expiration. VivoSenseHR vs Vt.jpg
Heart rate (HR) (first row), tidal volume (Vt) (second row), Vt and superimposed HR (third row). The HR modulation is clearly visible: HR increases with inspiration and decreases with expiration.
Heart rate (HR) (first row), ECG signal (ECG) (second row), and respiration (third row) for a newborn subject in a 15-seconds recording. HR expresses oscillations synchronous with respect to respiration. RSA neonatal ECG+RESP+HR.png
Heart rate (HR) (first row), ECG signal (ECG) (second row), and respiration (third row) for a newborn subject in a 15-seconds recording. HR expresses oscillations synchronous with respect to respiration.
RSA magnitude estimation based on a multivariate approach based on joint analysis of ECG and respiration. The green line shows the heart rate variations averaged over several breathing cycles. This clearly shows the trends that are typical of RSA. RSA Bartsch method.png
RSA magnitude estimation based on a multivariate approach based on joint analysis of ECG and respiration. The green line shows the heart rate variations averaged over several breathing cycles. This clearly shows the trends that are typical of RSA.

RSA as a vagal tone estimator

Respiratory sinus arrhythmia (RSA) is frequently used as a noninvasive method for investigating vagal tone, in physiological, behavioral, and several clinical studies. [16] [17] [18] This can be done using electrocardiography (ECG) recording, [19] although other methods are also being developed that take advantage of the interactions between ECG and respiration. [20] [15] Interpretation of RSA measurements must be done with care, however, as several factors including differences between individuals can change the relationship between RSA and vagal tone. [21]

Evolution and physiology

It has been suggested that RSA may have evolved to save energy for both cardiac and respiratory systems by reducing the heart rate [22] and by suppressing ineffective ventilation during the ebb of perfusion (delivery of blood from arteries to capillaries for oxygenation and nutrition). [23] [24]

RSA has been found to increase in subjects in resting state and to decrease in states of stress or tension. It is increased in supine position and decreased in prone position, and is on average higher and more pronounced during the day as compared to the night. [22] RSA has also been extensively used to quantify vagal tone withdrawal in graded orthostatic tilt. [7] [25]

Typically, expression of RSA decreases with age. [26] However, adults in excellent cardiovascular health, such as endurance runners, swimmers, and cyclists, are likely to have a more pronounced RSA. Professional athletes on average maintain very high vagal tone and consequently higher RSA levels. RSA is less prominent in individuals with diabetes and cardiovascular disease. [27]

Insights into psychology and disease

Vagal tone research has the potential to offer insight into social behavior, social interactions, and human psychology. Much of this work has been focused on newborns and children. [26] Baseline vagal tone can be used either as a potential predictor of behavior or as a signal of mental health (particularly emotion regulation, anxiety, and internalizing and externalizing disorders). [28] [29]

The polyvagal theory by Porges is an influential model of how the vagal pathways respond to novelty and to stressful external stimuli. [30] [31] [32] The theory proposes that there are two vagal systems, one that is shared with reptiles and amphibia and a second, more recent, system that is unique to mammals. The two pathways behave differently and can work against each other. This theory can account for several psychophysiological phenomena and psychosomatic illnesses. [30] [26] However, recent studies indicate that the vagal "system" described by Porges as being unique to mammals existed long before the evolution of mammals. [33] [34]

Other estimates of vagal tone

There are several methods of estimating vagal tone other than measuring RSA, including:

See also

Related Research Articles

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<span class="mw-page-title-main">Vagus nerve</span> Main nerve of the parasympathetic nervous system

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory fibers that create a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract.

<span class="mw-page-title-main">Autonomic nervous system</span> Division of the nervous system supplying internal organs, smooth muscle and glands

The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.

<span class="mw-page-title-main">Parasympathetic nervous system</span> Division of the autonomic nervous system

The parasympathetic nervous system is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

<span class="mw-page-title-main">Heart rate</span> Speed of the heartbeat, measured in beats per minute

Heart rate is the frequency of the heartbeat measured by the number of contractions of the heart per minute. The heart rate varies according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide. It is also modulated by numerous factors, including genetics, physical fitness, stress or psychological status, diet, drugs, hormonal status, environment, and disease/illness, as well as the interaction between these factors. It is usually equal or close to the pulse rate measured at any peripheral point.

<span class="mw-page-title-main">Palpitations</span> Perceived cardiac abnormality in which ones heartbeat can be felt

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The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure causes the heart rate to decrease. Decreased blood pressure decreases baroreflex activation and causes heart rate to increase and to restore blood pressure levels. Their function is to sense pressure changes by responding to change in the tension of the arterial wall. The baroreflex can begin to act in less than the duration of a cardiac cycle and thus baroreflex adjustments are key factors in dealing with postural hypotension, the tendency for blood pressure to decrease on standing due to gravity.

<span class="mw-page-title-main">Heart rate variability</span> Variation in the time intervals between heartbeats

Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.

The Hering–Breuer inflation reflex, named for Josef Breuer and Ewald Hering, is a reflex triggered to prevent the over-inflation of the lung. Pulmonary stretch receptors present on the wall of bronchi and bronchioles of the airways respond to excessive stretching of the lung during large inspirations.

<span class="mw-page-title-main">Wandering atrial pacemaker</span> Medical condition

Wandering atrial pacemaker (WAP) is an atrial rhythm where the pacemaking activity of the heart originates from different locations within the atria. This is different from normal pacemaking activity, where the sinoatrial node is responsible for each heartbeat and keeps a steady rate and rhythm. Causes of wandering atrial pacemaker are unclear, but there may be factors leading to its development. It is often seen in the young, the old, and in athletes, and rarely causes symptoms or requires treatment. Diagnosis of wandering atrial pacemaker is made by an ECG.

<span class="mw-page-title-main">Polyvagal theory</span> Proposed constructs pertaining to the vagus nerve

Polyvagal theory (PVT) is a collection of proposed evolutionary, neuroscientific, and psychological constructs pertaining to the role of the vagus nerve in emotion regulation, social connection and fear response. The theory was introduced in 1994 by Stephen Porges. There is consensus among experts that the assumptions of the polyvagal theory are untenable. PVT is popular among some clinical practitioners and patients, but it is not endorsed by current social neuroscience.

The Bezold–Jarisch reflex involves a variety of cardiovascular and neurological processes which cause hypopnea, hypotension and bradycardia in response to noxious stimuli detected in the cardiac ventricles. The reflex is named after Albert von Bezold and Adolf Jarisch Junior. The significance of the discovery is that it was the first recognition of a chemical (non-mechanical) reflex.

<span class="mw-page-title-main">Stephen Porges</span> Scientist and professor (born 1945)

Stephen W. Porges is an American psychologist. He is the Professor of Psychiatry at the University of North Carolina at Chapel Hill. Porges is also currently Director of the Kinsey Institute Traumatic Stress Research Consortium at Indiana University Bloomington, which studies trauma.

A vagal maneuver is an action used to stimulate the parasympathetic nervous system by activating the vagus nerve. The vagus nerve is the longest nerve of the autonomic nervous system and helps regulate many critical aspects of human physiology, including heart rate, blood pressure, sweating, and digestion through the release of acetylcholine. Common maneuvers that activate the vagus nerve include the Valsalva maneuver and carotid sinus massage, which can serve diagnostic or therapeutic functions.

Vagotonia is the state of the autonomic nervous system in which there is increased parasympathetic input through the vagus nerve, or the equilibrium between the sympathetic and parasympathetic is biased towards the latter. The opposite phenomenon has been referred to as sympatheticotonia.

Neurocardiology is the study of the neurophysiological, neurological and neuroanatomical aspects of cardiology, including especially the neurological origins of cardiac disorders. The effects of stress on the heart are studied in terms of the heart's interactions with both the peripheral nervous system and the central nervous system.

<span class="mw-page-title-main">Sinus arrhythmia</span> Irregular heart rhythm originating in the sinus node

Sinus arrhythmia is a commonly encountered variation of normal sinus rhythm. Sinus arrhythmia characteristically presents with an irregular rate in which the variation in the R-R interval is more than 0.12 seconds. Additionally, P waves are typically mono-form and in a pattern consistent with atrial activation originating from the sinus node.

Ganglionated plexi (GP) comprise the intrinsic cardiac autonomic nervous system composed of autonomic ganglia of the heart atrium and ventricles. Cholinergic neurons throughout the GPs project to all areas of the heart, The GP are embedded in the epicardial fat pads, consisting of only a few neurons or as many as 400 neurons. Vagus nerve stimulation has been shown to inhibit the activity of the GP, possibly through Nav1.8.

References

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