Heart Rate Variability basics
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.
Short-term HRV analysis and assessment of the autonomic regulation
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.
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.
Physiological Basics of HRV
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
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
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
Schema showing the baroreflex functionality
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