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Heart Rate Variability basics

Short-term HRV analysis and assessment of the autonomic regulation

It is believed that Heart Rate Variability (HRV) will become as common as pulse, blood pressure or temperature in patient charts in the near future. In the last ten years more than 2000 published articles have been written about HRV. HRV has been used as a screening tool in many disease processes. Various medical disciplines are looking at HRV. In diabetes and heart disease it has been proven to be predictive of the likelihood of future events. In 1996, a special task force was formed between the US and European Physiological associations to outline current finds on HRV and set specific standards on using HRV in medical science and future practice. Since then a steady stream of new information and value continues to come out of HRV research.

It all started in 1966 when a variation in the beat-to-beat intervals between heartbeats was noticed. Initially all recording devices were averaging heart rate data stream trying to get rid of any rapid HR fluctuations. Then there were very specific patterns in such fluctuations were noticed that had links to certain conditions way before any clinical symptoms appeared.

Physiological Basics of HRV

The origin of heartbeat is located in a sino-atrial (SA) node of the heart, where a group of specialized cells continuously generates an electrical impulse spreading all over the heart muscle through specialized pathways and creating process of heart muscle contraction well synchronized between both atriums and ventricles. The SA node generates such impulses about 100-120 times per minute at rest. However in healthy individual resting heart rate (HR) would never be that high. This is due to continuous control of the autonomic nervous system (ANS) over the output of SA node activity, which net regulatory effect gives real HR. In healthy subject at rest it is ranging between 50 and 70 beats per minute.

Schematic explanation of RA, LA, RV, LV parameters and their visualization on Heart Rate
Schematic explanation of  RA, LA, RV, LV parameters and their visualization on Heart Rate

Autonomic nervous system. The autonomic nervous system is a part of the nervous system that non-voluntarily controls all organs and systems of the body. As the other part of nervous system ANS has its central (nuclei located in brain stem) and peripheral components (afferent and efferent fibers and peripheral ganglia) accessing all internal organs. There are two branches of the autonomic nervous system - sympathetic and parasympathetic (vagal) nervous systems that always work as antagonists in their effect on target organs.

Sympathetic nervous system. For most organs including heart the sympathetic nervous system stimulates organ's functioning. An increase in sympathetic stimulation causes increase in HR, stroke volume, systemic vasoconstriction, etc. The heart response time to sympathetic stimulation is relatively slow. It takes about 5 seconds to increase HR after actual onset of sympathetic stimulation and almost 30 seconds to reach its peak steady level.

Schema explaining how parasympathetic and
sympathetic nervous systems inhibit functioning organs
Schema explaining how parasympathetic and sympathetic nervous systems inhibit functioning organs

Parasympathetic nervous system. In contrast, the parasympathetic nervous system inhibits functioning of those organs. An increase in parasympathetic stimulation causes decrease in HR, stroke volume, systemic vasodilatation, etc. The heart response time to parasympathetic stimulation is almost instantaneous. Depending on actual phase of heart cycle it takes just 1 or 2 heartbeats before heart slows down to its minimum proportional to the level of stimulation.

At rest both sympathetic and parasympathetic systems are active with parasympathetic dominance. The actual balance between them is constantly changing in attempt to achieve optimum considering all internal and external stimuli.

There are various factors affecting autonomic regulation of the heart, including but not limited to respiration, thermoregulation, humoral regulation (rennin-angiotensin system), blood pressure, cardiac output, etc. One of the most important factors is blood pressure. There are special baroreceptive cells in the hear and large blood vessels that sense blood pressure level and send afferent stimulation to central structures of the ANS that control HR and blood vessel tonus primarily through sympathetic and somewhat parasympathetic systems forming continuous feedback dedicated to maintain systemic blood pressure. This mechanism is also called baroreflex, which increases HR when blood pressure decreases and vice versa. This mechanism is also targeted to maintain optimal cardiac output.

Schema showing the baroreflex functionality
Schema showing the baroreflex functionality

Heart Rate Variability in research

The HRV analysis is a powerful, very accurate, reliable, reproducible, yet simple to do.

It is found that lowered HRV is associated with aging, decreased autonomic activity, hormonal tonus,specific types of autonomic neuropathies (e.g. diabetic neuropathy) and increased risk of sudden cardiac death after acute heart attack.

Other research indicated that depression, panic disorders and anxiety have negative impact on autonomic function, typically causing depletion of the parasympathetic tonus. On the other hand an increased sympathetic tonus is associated with lowered threshold of ventricular fibrillation. These two factors could explain why such autonomic imbalance caused by significant mental and emotional stress increases risk of heart attack followed by sudden cardiac death.

Aside from that, there are multiple studies indicating that HRV is quite useful as a way to quantitatively measure physiological changes caused by various interventions both pharmacological and non-pharmacological during treatment of many pathological conditions having significant manifestation of lowered HRV.

However it is important to realize that clinical implication of HRV analysis has been clearly recognized in only two medical conditions:

1. Predictor of risk of arrhythmic events or sudden cardiac death after acute heart attack

2. Clinical marker of diabetic neuropathy evolution

Nevertheless, as the number of clinical studies involving HRV in various clinical aspects and conditions grows, HRV remains one of the most promising methods of investigating general health in the future.

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