Heart rate turbulence
Encyclopedia
Heart rate turbulence is the return to equilibrium of heart rate
after a premature ventricular contraction
(PVC). It consists of a brief speed-up in heart rate, followed by a slow decrease back to the baseline rate. An important feature of HRT is that PVCs occur naturally in most adults, so that measuring the characteristics of a given person's HRT offers a noninvasive way to evaluate his or her cardiac function without applying artificial external stimuli.
The measured values of HRT parameters have been shown to be a statistically significant
predictor of the probability of dying from cardiac disease after a patient suffers a heart attack. There are indications that HRT may help predict a person's risk from dying suddenly, and that HRT can predict death not only in patients who have suffered a heart attack, but those who have other cardiac diseases as well.
HRT has nothing to do with turbulence
in fluid dynamics
.
See also Heart rate variability
.
. While studying PVC characteristics, Schmidt and his colleagues had noticed that heart rate seemed to speed up after a PVC
. To get a clearer picture, they listed the time from one heartbeat's R-wave to the next R-wave (called RR intervals) and synchronized these lists to the time of the PVC beat and averaged the values in the list. A plot of this averaged RR interval list (called a PVC tachogram) not only confirmed their observation that heart rate sped up for a few beats after a PVC, but highlighted another less obvious feature, that heart rate then slows down beyond what it was before the PVC, before returning to the original heart rate.
If Schmidt et al. had stopped there, it would have been an interesting observation to be found in the footnotes of an electrocardiography textbook. Instead, they reasoned that just as loss of variability in heart rate indicated patients more likely to be at high risk of dying after a heart attack, this phenomenon might also be an indicator of a healthy control of heart rate in such patients. They proceeded to test this hypothesis using 24 hour electrocardiogram (Holter monitor
) recordings from one hundred survivors of heart attacks with frequent PVCs. Greater turbulence seemed correlated with better prognosis. They then used this data to determine the optimal discriminating threshold between normal and abnormal HRT values, and came up with the values TS=2.5, TO=0%. Now came the test. Would HRT and these threshold values also work in the real world? These thresholds were applied to Holter records from a total of 1191 patients who had experienced a heart attack. There were 162 deaths (13.6%) during the follow-up period of about 2 years. It turned out that patients with abnormal HRT were approximately 3 times more likely to die than those with normal HRT, beating out some other commonly used predictors. Thus the field of HRT was born.
phenomenon. That is, a PVC interrupts the normal cardiac cycle
, so the ventricle
s of the heart
haven't had time to fill up to their normal level, before contracting and pumping their contents out. This results in a pulse (blood pressure
) weaker than expected and triggers normal homeostatic mechanisms that try to compensate by constricting arteries and increasing heart rate (the turbulence onset part of HRT). This is accomplished by the brain reflexively withdrawing the parasympathetic
nerve signals and increasing the sympathetic
nerve signals it sends to the heart. The compensatory constriction of the arteries and increased heart rate frequently cause blood pressure to oveshoot normal values (overcompensates), and activate the baroreflex in reverse. This time, the brain reinstates parasympathetic nerve signals and decreases sympathetic nerve signals, which cause the heart rate to slow (the turbulence slope part of HRT).
The exact quantitative contribution of the parasympathetic and sympathetic nervous flow to the heart to HRT is unknown. The simplistic view assumes that HRT is solely dependent upon parasympathetic activity because atropine
, a parasympathetic activity blocker abolishes HRT while a beta-blocker (sympathetic blocker) has no effect on HRT. The contribution of the compensatory pause, the pause between the PVC and the next normal beat, to HRT is also unknown. Some investigators believe that the compensatory pause produces a single beat blood pressure increase beyond normal only in compromised hearts, whereas other investigators have shown that the single beat surge occurs in normal hearts as well. To date, no physiological parameter has been linked in a quantitative manner to turbulence slope, whereas turbulence onset was shown by researchers in Calgary, Canada to be linearly dependent upon duration of subnormal blood pressure in a well designed experiment.
Finally, the reason the size of HRT after a PVC predicts cardiac death is suggested by its mechanism. Parasympathetic nervous activity to the heart is believed to be protective and sympathetic nervous activity, deleterious, to the heart. Especially after a heart attack, sympathetic nervous activity tends to be increased. A healthy HRT indicates the presence of a healthy amount of parasympathetic activity, countering sympathetic activity. To take a wider view, however, it may be that a healthy HRT is also an indication of a healthy brain, and is the reason small HRT also predicts a likelihood of death from non- cardiac causes as well as from cardiac causes.
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
after a premature ventricular contraction
Premature ventricular contraction
A premature ventricular contraction , also known as a premature ventricular complex, ventricular premature contraction , ventricular premature beat , or extrasystole, is a relatively common event where the heartbeat is initiated by the heart ventricles rather than by the sinoatrial node, the...
(PVC). It consists of a brief speed-up in heart rate, followed by a slow decrease back to the baseline rate. An important feature of HRT is that PVCs occur naturally in most adults, so that measuring the characteristics of a given person's HRT offers a noninvasive way to evaluate his or her cardiac function without applying artificial external stimuli.
The measured values of HRT parameters have been shown to be a statistically significant
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
predictor of the probability of dying from cardiac disease after a patient suffers a heart attack. There are indications that HRT may help predict a person's risk from dying suddenly, and that HRT can predict death not only in patients who have suffered a heart attack, but those who have other cardiac diseases as well.
HRT has nothing to do with turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
in fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
.
See also Heart rate variability
Heart rate variability
Heart rate variability is a physiological phenomenon where the time interval between heart beats varies. It is measured by the variation in the beat-to-beat interval....
.
History
The concept of HRT was introduced to the medical community by Georg Schmidt and colleagues from the Munich University of Technology in 1999 in the British medical journal the LancetThe Lancet
The Lancet is a weekly peer-reviewed general medical journal. It is one of the world's best known, oldest, and most respected general medical journals...
. While studying PVC characteristics, Schmidt and his colleagues had noticed that heart rate seemed to speed up after a PVC
Premature ventricular contraction
A premature ventricular contraction , also known as a premature ventricular complex, ventricular premature contraction , ventricular premature beat , or extrasystole, is a relatively common event where the heartbeat is initiated by the heart ventricles rather than by the sinoatrial node, the...
. To get a clearer picture, they listed the time from one heartbeat's R-wave to the next R-wave (called RR intervals) and synchronized these lists to the time of the PVC beat and averaged the values in the list. A plot of this averaged RR interval list (called a PVC tachogram) not only confirmed their observation that heart rate sped up for a few beats after a PVC, but highlighted another less obvious feature, that heart rate then slows down beyond what it was before the PVC, before returning to the original heart rate.
If Schmidt et al. had stopped there, it would have been an interesting observation to be found in the footnotes of an electrocardiography textbook. Instead, they reasoned that just as loss of variability in heart rate indicated patients more likely to be at high risk of dying after a heart attack, this phenomenon might also be an indicator of a healthy control of heart rate in such patients. They proceeded to test this hypothesis using 24 hour electrocardiogram (Holter monitor
Holter monitor
In medicine, a Holter monitor is a portable device for continuously monitoring various electrical activity of the cardiovascular system for at least 24 hours...
) recordings from one hundred survivors of heart attacks with frequent PVCs. Greater turbulence seemed correlated with better prognosis. They then used this data to determine the optimal discriminating threshold between normal and abnormal HRT values, and came up with the values TS=2.5, TO=0%. Now came the test. Would HRT and these threshold values also work in the real world? These thresholds were applied to Holter records from a total of 1191 patients who had experienced a heart attack. There were 162 deaths (13.6%) during the follow-up period of about 2 years. It turned out that patients with abnormal HRT were approximately 3 times more likely to die than those with normal HRT, beating out some other commonly used predictors. Thus the field of HRT was born.
Mechanism of HRT
Everyone agrees that HRT is essentially a baroreflexBaroreflex
The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms for maintaining blood pressure. It provides a negative feedback loop in which an elevated blood pressure reflexively causes heart rate to decrease therefore causing blood pressure to decrease; likewise, decreased...
phenomenon. That is, a PVC interrupts the normal cardiac cycle
Cardiac cycle
The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages...
, so the ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
s of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
haven't had time to fill up to their normal level, before contracting and pumping their contents out. This results in a pulse (blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
) weaker than expected and triggers normal homeostatic mechanisms that try to compensate by constricting arteries and increasing heart rate (the turbulence onset part of HRT). This is accomplished by the brain reflexively withdrawing the parasympathetic
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...
nerve signals and increasing the sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
nerve signals it sends to the heart. The compensatory constriction of the arteries and increased heart rate frequently cause blood pressure to oveshoot normal values (overcompensates), and activate the baroreflex in reverse. This time, the brain reinstates parasympathetic nerve signals and decreases sympathetic nerve signals, which cause the heart rate to slow (the turbulence slope part of HRT).
The exact quantitative contribution of the parasympathetic and sympathetic nervous flow to the heart to HRT is unknown. The simplistic view assumes that HRT is solely dependent upon parasympathetic activity because atropine
Atropine
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade , Jimson weed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects...
, a parasympathetic activity blocker abolishes HRT while a beta-blocker (sympathetic blocker) has no effect on HRT. The contribution of the compensatory pause, the pause between the PVC and the next normal beat, to HRT is also unknown. Some investigators believe that the compensatory pause produces a single beat blood pressure increase beyond normal only in compromised hearts, whereas other investigators have shown that the single beat surge occurs in normal hearts as well. To date, no physiological parameter has been linked in a quantitative manner to turbulence slope, whereas turbulence onset was shown by researchers in Calgary, Canada to be linearly dependent upon duration of subnormal blood pressure in a well designed experiment.
Finally, the reason the size of HRT after a PVC predicts cardiac death is suggested by its mechanism. Parasympathetic nervous activity to the heart is believed to be protective and sympathetic nervous activity, deleterious, to the heart. Especially after a heart attack, sympathetic nervous activity tends to be increased. A healthy HRT indicates the presence of a healthy amount of parasympathetic activity, countering sympathetic activity. To take a wider view, however, it may be that a healthy HRT is also an indication of a healthy brain, and is the reason small HRT also predicts a likelihood of death from non- cardiac causes as well as from cardiac causes.
Further reading
- The HRT website offers further information.
- Georg Schmidt, "Heart-rate turbulence after ventricular premature beats as a predictor of mortality after acute myocardial infarction. Lancet 1999; Vol. 353; No. 9162; 1390-96"
- Mari Watanabe, "Heart Rate Turbulence: a Review", Indian Pacing Electrophysiol. J. 2003;3(1):10