Heart rate variability

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As a heart rate variability ( English heart rate variability , HRV) refers to the natural variation of the time between two successive heart beats. It is also an indicator of the ability of an organism (human, mammal) to adapt the heart rate to physical and mental requirements. Measures for the heart rate variability can be derived statistically in the time domain as scatter measures ( mean , standard deviation , variance ) or spectrally in the frequency domain (low frequency, high frequency); a third possibility is offered by non-linear methods (e.g. Poincaré mapping , fluctuation analysis ).

A healthy organism constantly adapts the heart rate to current requirements via autonomous physiological regulation pathways. Physical exertion or psychological stress therefore usually results in an increase in the heart rate , which normally decreases again when the stress is relieved and relaxed. A greater ability to adapt to stress is shown in a greater variability of the heart rate. Under chronic stress, however, both are more or less restricted and consequently reduced because of the constant high tension that is typical for them.

Inconspicuous resting ECG: In healthy people, the heartbeat frequency varies.

Beginnings and current status of HRV research

As early as the 3rd century AD, the Chinese doctor Wang Shu-he (also Wang Shu-ho or Wang Hsi ) recognized that a variable heartbeat was a sign of health. He documented this in his writings Mai Ching ( The Knowledge of Pulse Diagnosis ). Since there were no measuring instruments such as stethoscopes or EKGs , the doctor had to be very sensitive to the detection of the interaction of the patient's body signals in order to be able to diagnose an illness. The fact that Wang Shu-he did not use the variability of the pulse rate for prognosis can be read in "From woodpeckers, raindrops and heartbeats ... (see below)".

There is currently a wide range of research on heart rate variability, which is mainly focused on three areas:

  • Clinical area : risk stratification and health prognosis with parameters of HRV
  • Rehabilitative medicine : classic and non-linear HRV methods for objectifying prognosis and performance
  • Stress medicine and psychophysiology : HRV biofeedback

New methods have been developed in the field of sports and training sciences for performance diagnostics and exercise control.

definition

The distance between two heartbeats is usually defined as the time between the start of two contractions of the heart chambers. This beginning of the ventricular contraction appears in the electrocardiogram (EKG) as a so-called R-wave . The distance between two R-waves is therefore called the RR interval (we also speak of NN-intervals: a) to avoid confusion with the blood pressure value RR (according to Riva-Rocci) and b) to identify R-waves that originate from a regular heart excitation, i.e. from the sinus node - in contrast to z. B. to supraventricular and ventricular extrasystoles ). The RR interval can be converted into the heart rate as a reciprocal value (60 BPM ~ 1000 ms: 60 beats per minute ~ 1000 milliseconds RR interval). As a rule, the RR intervals are not of the same length, but are subject to fluctuations. The quantification of these fluctuations is known as heart rate or heart rate variability (HRV).

Heart rate variability physiology

In a healthy individual, a heartbeat is triggered by an impulse from the sinus node as the central clock of the heart's autonomic excitation system. This in turn is under the influence of the superordinate vegetative nervous system , with an activating influence being exerted via the sympathetic nervous system , which u. a. increases the heart rate. Physical and mental stress go hand in hand with an increase in the activity of the sympathetic nervous system, while at the same time bodily functions that are regulated by the vagus , such as digestion , are reduced . To simplify matters, one can say that the sympathetic nervous system activates the subsystems (circulation, muscles, sugar) for attack and flight, while the vagus in turn helps to build up the necessary resources when the person is in a relaxed state of rest. External influences (stimuli), psychological processes (thoughts) or mechanical processes (breathing) interlock in a complex manner, but can also have different effects on the heartbeat depending on your own weight.

Measurement method

Spectral analysis
histogram

The EKG is still the central diagnostic method in cardiology . A time series of the RR intervals can be determined from it. The fluctuation of this time series can be quantified with the help of various methods with regard to its strength, time scale or internal pattern. Compared to the normal electrocardiogram, in which the curve shape is of diagnostic importance, the time resolution of the RR intervals is in the foreground when measuring the heart rate variability.

The standard deviation of the RR intervals is a simple statistical quantity for determining the variation . Today, a distinction is made between three areas (domains) that are used to analyze heart rate variability:

  • Time range (e.g. standard deviation of the RR intervals)
  • Frequency range (e.g. spectrum of heart rate variability)
  • non-linear area (e.g. Poincaré maps ).

With regard to its time scale, the fluctuations in the heart rate can be characterized in more detail using spectral analysis methods . More recently, complex empirical parameters such as B. used the fractal dimension .

Spectral analysis is a very precise method for determining the frequency components that make up the variability of the heart rate. For example, it provides information about the coupling of breathing and heartbeat (i.e. their coherence ) in a relaxed state. If breathing and heartbeat are well coupled, the spectral analysis shows a clear peak (peak value). In HRV research, the relevant measurement spectrum is divided into three frequency bands, VLF ( very low frequency ), LF ( low frequency , sometimes also referred to as MF ( middle frequency )) and HF ( high frequency ), in some cases plus a fourth frequency band : ULF ( ultra low frequency ). These frequencies represent

(see.)

The respiratory sinus arrhythmia - and thus the HF - are commonly viewed as a marker of parasympathetic activity. It was and is partially postulated that the LF is the sympathetic counterpart to the HF and the quotient LF / HF is therefore a suitable marker for the autonomous balance between the sympathetic and parasympathetic nervous systems. In the meantime, this has been adequately refuted, but LF / HF is still listed in current publications as a characteristic value of sympathovagal balance.

Another form of representation of the heart rate variability is the histogram . In a history diagram of a biofeedback measurement, it is counted how many of the heartbeats fall into a certain class. With a higher HRV, the heartbeats are evenly distributed over as many classes as possible. Under heavy stress, the vegetative balance shifts and the HRV is limited to a few classes.

Numerous studies use Poincaré or Lorenz plots to analyze HRV by means of two- or multi-dimensional point cloud representations. Various names are known for the display of successive RR intervals: Poincaré, Lorenz, recurrence and scatter plots as well as return maps. In this representation, each following data point relates to the previous one, with the RR time series being mapped onto itself in the simplest case of a two-dimensional representation.

To avoid misjudgments of the importance of various parameters of HRV, the Task Force of the European Society of Cardiology and The North American Society of Pacing and Electrophysiology have established guidelines for performing and interpreting HRV analyzes.

150Watt FreqSpectrum.gif

meaning

Since the heart rate variability has its origin in the function of the autonomic nervous system , diseases can in principle be identified which have an impact on the heartbeat. A distinction must be made between diseases that directly damage the autonomic nervous system and diseases that have an indirect effect on the autonomic nervous system, for example through permanently increased metabolic stress.

An example of the first group of diseases is diabetic neuropathy , and one of the second group is coronary heart disease. Mental illnesses can also have recognizable consequences on cardiac activity via an increase in the catecholamine level and activation of the sympathetic nervous system; the heart rate variability can therefore also be used for diagnostic purposes in the field of neuropsychiatry.

Other diseases with changes in heart rate variability are:

  • Asphyxia in newborns
  • sudden cardiac death after a heart attack (as a predictive value)
  • asthma
  • Trauma patients , as a predictive value for mortality, regardless of the mechanism, location or severity of the injury.
  • Sepsis , the HRV begins to drop even before the clinical diagnosis of sepsis.

HRV in stress medicine and psychophysiology

Various biofeedback techniques and devices have been developed over the past few decades to measure heart rate variability. Special emphasis was placed on the measurement of the coupling of heart and respiration in order to be able to determine the degree of coherence or synchronization of heart rhythm and respiratory rate.

The synchronization and chaotic course of breathing rhythm and heart rate can be represented visually or acoustically with this biofeedback method. The pulse is measured with the help of a chest strap or an ear clip, with the data being evaluated in a special way.

It was found that with such complex reactions as love or gratitude , which are connected with the emotional reaction of joy , there is a measurable synchronization of the rhythms of heart and breathing ( respiratory sinus arrhythmia ). This balance between breathing and heartbeat disappears, however, in reactions such as agitation ("stress"), anger or fear , which are associated with increased release of stress hormones.

Starting from the USA, research has been carried out in recent years to determine to what extent coherence of heart and breathing can be trained and which therapeutic successes can be achieved with different settings. In doing so, biofeedback techniques are used and, in various variations, attempts are made to specifically influence the emotional experience of the trainees. Special musical compositions are used, breathing techniques, mindfulness exercises, trance inductions or directed imaginations with concentration on the heart and breathing in connection with the activation of particularly positive, e.g. loving reactions.

Some studies, as well as reviews and meta-analyzes , indicate that HRV is reduced in people with mental illness compared to those who are mentally healthy. The HRV biofeedback is as a coaching method or complementary medical method for some time in the behavior therapy -oriented psychotherapy used. According to studies in the USA, depression , heart disease, asthma , anxiety disorders and insomnia can be positively influenced by this. Improving the coherence of breathing and heart can also help relieve tension, cope with stress and anxiety , and help you react more calmly in everyday life.

HRV biofeedback has been used in corporate health promotion for some time.

According to a study published in 2017, HRV biofeedback can also be used to improve the performance of athletes. The authors also mention the need for further studies.

HRV training for heart failure

The National Institutes of Health awarded Luskin of Stanford University a grant for research into training in HRV training in patients with severe heart failure. The patients suffered from shortness of breath, fatigue, edema and, in many cases, from anxiety and depression. After six weeks of treatment, the group that had learned to use HRV training had decreased stress levels by 22 percent and depression by 34 percent, while their physical condition for walking without shortness of breath had improved by 14 percent. However, no change in HRV was observed.

In the control group, in which conventional agents were used, all the indicators mentioned had worsened compared to the initial values.

HRV-controlled training in top-class sport

In top sport, the training load is increasingly controlled and controlled with HRV in order to avoid overloading. So have z. B. the four New Zealand rowing world champions 2015 periodized their training in the intensive phases before the world championship with HRV from day to day . They used the rMSSD (root mean square of successive differences) as a reference value. In this way, the effect of individual outliers can be mathematically minimized. Since endurance athletes usually already have a very low resting heart rate, the influence of the parasympathetic system is also taken into account.

literature

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Web links

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