HEART RATE VARIABILITY TEST
Introduction
Heart Rate Variability (HRV) is defined as the beat to beat timing changes that occur between heart
beats. In other words the time between heart beats changes from one beat to the next. Changes in
beat intervals are measured in milliseconds or 1/1000th of a second. It is this timing change or rate
change (variability) that gives us the term Heart Rate Variability. For most of us, the idea that the time
between heart beats is variable, contradicts what we have commonly known. Most of us learned to take a
radial, carotid, or popliteal pulse, and that the pulse rate equaled the heart rate, and that rate was fairly
constant. This is not the case, and PULSE RATE ≠ HEART RATE.
In short, heart rate variability is just what the name implies. It is the continuous (variable) changes in
heart rate or heart beat frequency over a given period of time. To clarify, most of us assume that if
we take our pulse, and the pulse rate is 80 beats in a 60 second period, than the heart is beating 80
times per minute at a fairly constant rate. This is not true. Pulse rate is the number times that the you
feel blood surging through the artery that is being palpated. For the purpose of HRV, Heart Rate is
the number of times the ventricles depolarize, or the number of electrical discharges that occur in the
ventricles in a 60 second period.
History of Heart Rate Variability
Heart rate variability was invented in 1965 when Hon and Lee noted that fetal distress was preceded
by alterations in interbeat intervals before any appreciable change occurred in the heart rate itself.
Then Sayers and others focused attention on the existence of physiological rhythms imbedded in the
beat-to-beat heart rate signal. During the 1970s, Ewing et al. devised a number of simple bedside
tests of short-term heart beat differences to detect autonomic neuropathy in diabetic patients. The
association of higher risk of post-infarction mortality with reduced HRV was first shown by Wolf et al.
in 1972. In 1981, Akselrod et al. introduced power spectral analysis of heart rate fluctuations to
quantitatively evaluate beat-to-beat cardiovascular control. These frequency–domain analyses
contributed to the understanding of the autonomic nervous system's effect on RR interval
fluctuations in the heart rate record. The clinical importance of HRV became apparent in the late
1980s when it was confirmed that HRV was a strong and independent predictor of mortality following
an acute myocardial infarction.
Physiological Basis of Heart Rate Variability
The physiological basis for HRV rest in the fact that in addition to the normal heart cells responsible
for contraction, there are also very specialized cells that are responsible for starting each
contraction and for rapid and coordinated excitation of the heart muscle. These cells are collectively
called the conduction system, which is comprised of the following structures: Sinoatrial node,
atrioventricular node, atrioventricular bundle (Bundle of His), right and left bundle branches, and
small conduction fibers called Purkinje fibers. The sinoatrial node (SA node) is often called the
pacemaker of the heart, and depolarizes or fires to start the contraction of the heart muscle. It is
located in the right superior aspect of the right atrium, and is under direct control of the autonomic
nervous system. At rest it fires about 75-100 times in a 60second period. After the SA node fires the
electrical signal travels to both atria and results in their simultaneous contraction. The electrical
signal then passes through an area of nonconductive fibrous tissue via the AV node which slows
down the speed of the electrical signal.
This delay is what allows the ventricles to fill with blood before their collective contraction as
discussed earlier. The signal then reaches the bundle of HIS and is divided into left and right bundle
branches, which carry the electrical impulse to the bottom portions of the ventricles, causing the
lower portion of the ventricles to contract first. This is followed by the middle and upper portion of the
ventricles which therefore squeezes the blood out of the heart and off to the lungs and the systemic
circulation
Additionally, the SA node is under direct control of the Autonomic Nervous System which is the part
of the nervous system that non-voluntarily controls all organs and systems of the body. Remember
that the human nervous system is divided in two ways, structurally, and functionally. Structurally, the
nervous system is divided into two parts, the central nervous system (CNS) and the peripheral
nervous system (PNS). The central nervous system is comprised of the brain and spinal cord. The
peripheral nervous system is comprised of all of the other nerves that branch from the brain and
spinal cord, including the cranial nerves, and spinal nerves.
Functionally, the nervous system is divided into the autonomic nervous system and the voluntary
nervous system. The voluntary nervous system is involved with actions or movements that are under
our direct conscious control. The autonomic nervous system, or automatic nervous system, is the
portion of the nervous system that controls all of the bodily functions that are not under direct
conscious control i.e., heart rate, breathing, eye blinking, etc. It is this portion of the nervous system
that we will be focusing on throughout the rest of this course
The autonomic nervous system is divided into two parts or branches, the sympathetic branch and
the parasympathetic branch. We will concentrate on the sympathetic branch first. In general the
sympathetic branch is responsible for increasing or speeding up bodily functions, such as heart rate,
respiratory rate, pupil dilation, and increase blood flow to the extremities. At the SA node, the
sympathetic nervous system’s primary function is to increase SA node depolarization or firing, which
in turn speeds up the heart rate.
The parasympathetic branch is responsible for slowing down heart rate and respiratory rate,
decreasing blood pressure, and shunting blood from the extremities to the digestive organs. The
parasympathetic nerve supply to the heart is primarily derived form cranial nerve ten (X) also know
as the vagus nerve. The vagus nerve sends preganlionic fibers directly to the terminal ganglia
located on the heart muscle and then postganglionic fibers influence the SA node from there.
Parasympathetic stimulation of the SA node decreases its depolarization rate, or firing rate, and
thereby slows down the heart rate.
At rest both sympathetic and parasympathetic systems are active with parasympathetic dominance.
The actual balance between them is constantly changing in an attempt to achieve optimum
homeostasis despite ever changing external and internal conditions. In other words the two branches of
the ANS are constantly working against each other like an old married couple arguing.
Who Can Benefit From HRV Testing
Patient's with the following conditions may benefit from Autonomic Nervous
System testing.
Abnormal reflex
Migraines and other headaches
AIDS
Depression or Bipolar Disease
Anxiety
Post-traumatic Stress Syndrome
Attention Deficit Disorder
Fibromyalgia
Hypertension
Post-MI
Angina
Atherosclerosis
Mitral Valve Prolapse Syndrome
Cardiomyopathy
Cardiac Dysrhythmias
Congestive Heart Failure
Acquired Hypothyroidism
Thyroid Disorders
Premature Menopausal Symptoms
Menopausal Syndromes
Sleep Apnea
Asthma
COPD
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Sleep Disorders
Manifestations of thiamine deficiency
Paralysis agitans
Cerebellardegeneration
Spinocerebellar Amyotrophic diseases
Syringomyelia
Diseases of the spinal cord
Idiopathic peripheral autonomic neuropathy
Multiple sclerosis
Disorders of Vagus Nerve.
Mononeuritis
Peripheral & Poly-Neuropathies
Diabetes Mellitus
Chronic Pain
Orthostatic & Chronic Hypotension
Neurogenic bladder
Urogenital Dysfunctions
Anhidrosis
Sjögren’sdisease
Syncope and collapse
Chronic fatigue syndrome
Tachycardia (postural)
Methods of Measuring Heart Rate Variability
Time Domain Measurement
This method analyzes either the heart rate at any point in time, or the intervals between successive
normal QRS complexes. In a continuous electrocardiographic (ECG) recording, each QRS complex is
detected, and the so-called normal-to-normal (NN) intervals (that is all intervals between adjacent
QRS complexes resulting from normal sinus node depolarizations), or the instantaneous heart rate is
determined. The so called normal complexes are considered to be normal sinus rhythm beats of the
heart. This excludes any premature beats, late beats, or ectopic beats that are collectively know as
cardiac arrhythmia. Simple time domain variables that can be calculated include the mean or average
NN interval, the mean heart rate, the difference between the longest and shortest NN interval, the
difference between night and day heart rate, etc (only in a 24 hour recording). Other time domain
measurements that can be used are variations in instantaneous heart rate secondary to respiration,
90° passive tilt, Valsalva maneuver, spinal manipulation, acupuncture therapy, and injection or oral
administration of various medications.
The simplest variable to calculate is the standard deviation of the NN interval (SDNN), which is the
square root of variance. Variance is mathematically equal to total power of spectral analysis
(discussed below), therefore SDNN reflects all the cyclic components responsible for variability in the
period of recording. To put it another way SDNN represents both the sympathetic and
parasympathetic influences on heart rate. SDNN can be calculated over a 24 hour period or a short
term five minute period. As discussed further in this course, short-term 5-min recordings and 24 hour
long-term recordings both seem to be appropriate options for the accurate analysis of the autonomic
nervous system.
Other commonly used statistical variables calculated from segments of the total monitoring period
include SDANN, which is the standard deviation of the average NN interval calculated over short
periods, usually 5 min, which is an estimate of the changes in heart rate due to cycles longer than 5
min, and the SDNN index, the mean of the 5-minute standard deviation of the NN interval calculated
over 24 hours, which measures the variability due to cycles shorter than 5 min. However, the most
commonly used measures derived from interval differences include rMSSD, the square root of the
mean squared differences of successive NN intervals, NN50, the number of interval differences of
successive NN intervals greater than 50 ms, and pNN50 the proportion derived by dividing NN50 by
the total number of NN intervals. All of which you will see in most HRV analysis programs.
Frequency Domain Measurement
Another method of measuring heart rate variability is spectral analysis or power spectral density
(PSD). Power spectral density shows how the total power, or variance, is distributed as a function of
the various frequencies that affect the heart. In other words, PSD is the mathematical calculation of
the overall strength of the influence of both the parasympathetic and sympathetic nervous systems
on the heart.
Frequency domaina analysis is traditionally performed by means of Fast Fourier Transformation
(FFT). This method is simple to calculate and yields all of the following spectral frequencies in a
graphical display. In other words, power spectral analysis yields four frequencies that occur during
HRV assessment. These frequencies are defined as the ultra low frequency (ULF) 0.0001-0.003Hz,
very low frequency (VLF) 0.003-0.04Hz, low frequency (LF) 0.04-0.15Hz, and high frequency (HF)
0.15-0.4Hz. The HF power spectrum reflects parasympathetic (vagal) tone and fluctuations caused by
spontaneous respiration known as respiratory sinus arrhythmia. The LF power spectrum is evaluated
in the range from 0.04 to 0.15 Hz. This band reflects sympathetic nervous system activity . The VLF
power spectrum represents the thermoregulatory activity of the ANS on the body.
Another frequency domain measurment is the total power (TP).The TP is a net effect of all possible
physiological mechanisms contributing in heart rate variability that can be detected in 5-min
recordings. In other words the total power represents the overall variability of the nervous system,
and highly correlates to the SDNN measurement.
The LF/HF Ratio is used to indicate balance between sympathetic and parasympathetic tone. A
decrease in this score might indicate either increase in parasympathetic output, or a decrease in the
sympathetic output. In other words the LF/HF ratio is the ratio between the LF and HF power. A
higher number indicates increased sympathetic activity or reduced parasympathetic activity. This
ratio can be used to help quantify the overall balance between the sympathetic and parasympathetic
systems.
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