Dissertation: Effect of HRVB on Stress

Pages: 16 (4499 words)  ·  Bibliography Sources: 27  ·  Level: Master's  ·  Topic: Health  ·  Buy This Paper

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[. . .] Systems Theory and Negative Feedback Loops

Lehrer (2013) has pointed out that Systems Theory has a wide applicability, having been used in numerous fields -- from psychology to biology to sociology. Systems Theory in fact grew out of a mathematical approach developed by Wiener in the 1940s through work on antiaircraft systems during WW2 (so the analogy of the body being like an aircraft is not so random). The theory focused on the way in which negative feedback loops, oscillations and control all work together within a system (Lehrer, 2013).

What is a system? A system has been defined as "a variety of elements that interact with one another to form a whole entity" (Lehrer, 2013). It is an entity that cannot be broken up into characteristics but most be taken as a whole.

Negative feedback has been defined as the "hallmark of homeostatic regulatory activity" (Lehrer, 2013). The diverse systems that employ negative feedback loops all show the characteristic of oscillation. When oscillatory patterns emerge, it is the result of multiple feedback loops operating in conjunction with one another within a system. Oscillatory negative feedback loops all figure predominantly in both biological and behavioral controls, from processes in cells to whole, functioning societies (Lehrer, 2013).

Oxygen Metabolism and Parasympathetic Responses

As Gallego, Nsegbe and Durand (2001) have shown, the arterial pressure regulation feedback system provided by the relationship of carbon dioxide, oxygen and blood acid levels in effect is articulated via the metabolic control that is the breathing process. The parasympathetic response is essential in metabolizing oxygen and keeping the body in rhythm. Panneton (2013) has shown that parasympathetic responses are important in the body's regulation as well, especially with regard to the way in which the cardiovascular system assists in addressing the problem of anoxia:

A controlled reflex of onset bradycardia, a parasympathetic response, is foremost and reduces cardiac output dramatically, which by itself would induce a precipitous drop in arterial blood pressure. Thus the sympathetic nervous system counteracts the ensuing pressure drop, and a massive peripheral vasoconstriction commences redistributing circulating blood by reducing blood flow in cutaneous, muscular, and splanchnic circulations, but a maintained or augmented flow to the central nervous system and heart (Panneton, 2013, p. 284).

The interplay of the central nervous system and the breathing process is evident therefore in this case as well as others.

Measure of the Parasympathetic Function

Gibbons, Cheshire and Fife (2014) have shown that one common autonomic testing term is the cardiovagal. Here the parasympathetic response is measured by way of the cardiac function, which is controlled by the vagus nerve -- which in turn impacts the variability of the heart rate. Zygmunt and Stanczyk (2010) have also pointed out that "changes in heart rate during orthostatic testing and Valsalva manoeuvre, as well as during deep breathing or diving reflex, reflect parasympathetic modulation" (p. 11). Essentially, the cardiovascular system works because the vagal brake acts as a modulator: it is a restraint, as described by Dr. Stephen Porges in his Polyvagal Theory. As Porges (2001) has noted, the vagus places a limitation on the heart rate, though when vagal tone is taken away, rapid escalation of stress can result.

Heart Rate Control

Heart rate is controlled by the autonomic nervous system's two branches -- the sympathetic nervous system and the parasympathetic nervous system. The former produces hormones (epinephrine and norepinephrine) to boost heart rate, while the latter produces acetylcholine to decelerate heart rate. Heart rate can be impacted by various external factors, such as stress, coffee (caffeine), anxiety, excitement, environment, etc. Efforts to lower the heart rate or to steady it when it is impacted by stressors such as these include meditation and slow breathing. This is where the idea of HRV comes from -- the practice of taking slow, deliberate, thoughtful and purposeful breaths that can help the body to regain control of itself in the face of stressors that are external to it or that are part of the body's overall processes as a result of disease or infection. An example of how the heart is controlled by the body can be seen when one is engaging in exercise: when the body is actively moving in exercise, the sympathetic system actives and causes the heart rate to accelerate rapidly. The more that a person exercises, the more the heart itself is worked and the more the heart's actual size can even increase, just as a muscle in the arms or legs will increase when these particular parts of the body are used again and again regularly over a given period of time.

Allostasis

Allostasis (stability by variability) in stress research has received some attention over recent years. It plays a role with hormonal mediators in stress response as McEwen (2000) has shown. However, one of the challenges that researchers are uncovering is that the allostatic load center around the brain can be lead to impairment over time as environmental challenges trigger a continuation of stress for persons in certain instances where stressors are predominant factors of life: "In anxiety disorders, depressive illness, hostile and aggressive states, substance abuse, and post-traumatic stress disorder (PTSD), allostatic load takes the form of chemical imbalances as well as perturbations in the diurnal rhythm, and, in some cases, atrophy of brain structures. In addition, growing evidence indicates that depressive illness and hostility are both associated with cardiovascular disease (CVD) and other systemic disorders" (McEwen, 2000, p. 108). The allostatic load can be described as the wear and tear that a body undergoes over time as situations of chronic stress appear. The physiological consequence of long-term exposure to repeated stressors can lead to allostatic overload.

In terms of resilience, which is characterized by the ability to respond to environmental stressors and return the body to a state of normalcy. However, in chronic cases, resilience is repeatedly tried and can be broken, just as in war, a wall may be destroyed through repeated use of a battering ram. In the case of the body undergoing constant stress, the wall of resilience that it puts up to fend off the attacks of stressors can wear out eventually. Beaton and Simon (2011) note that one big event that is stressful in one's life can be enough to harm resilience -- just as in the case of modern warfare a battering ram is replaced by the use of explosives: one wall-shattering bomb will do the trick in bringing the wall (or one's resilience) down in tatters. Specifically, Beaton and Simon (2001) state that "traumatic early life experiences can shape the physiological stress response over development and are predictive of atypical hypothalamic -- pituitary -- adrenal (HPA) axis activation and neuroendocrine dysregulation" (p. 69). The rise of a traumatic event, however, is not necessary in contributing to allostatic overload, though it certainly can. Allostatic overload can occur as a result of continued stress over time on the body.

Allostatic Overload

The concept of allostatic overload is important to this study because it essentially equates to a decrease in HRV. Beaton and Simon (2011) highlight how allostatic overload can negatively impact the body:

Allostatic load is the physiological cost of maintaining homeostasis when faced with severe or chronic stress. Allostasis can occur via physiological (e.g., cortisol release or blood pressure elevation) or behavioral (e.g., avoiding stress-inducing stimuli) mechanisms that are elicited in response to real or perceived challenges. While modulated by individual differences in genotype, experience, and environment, allostatic load can increase over time to the point of overload and exhaustion. Allostatic overload exhausts coping resources and overexposes the organism to the hormonal, immunological, and neural mediators released via chronic activation and dysregulation of the HPA axis. Allostatic overload can also manifest as an inability to habituate to stressors, a failure to inhibit the stress response when not needed or as the lack of an effective stress response when one is truly needed.

Allostatic overload is, in other words, what happens when the body is pushed past the point of resilience. It is important for medical practitioners to be able to know the extent to which the allostatic load is under pressure. Fortunately, HRV is a good measure of allostatic capacity, as resilience and allostasis go hand in hand. HRV is known to improve baroreflex gain (Lehrer 2007) and huge acute effects on HRV can increase chronic variability over time.

The Reason for the Review

The reason for the review is that many clinical outcome measures have been studied and can be linked to resilience. As HRVB is a measure of resilience and what resilience is, the reason this meta-analysis review was conducted is that there were very few studies on HRVB that were large enough to compare to clinically significant treatments. Because of that absence, a closer examination of the available literature on the subject was… [END OF PREVIEW]

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