Introduction
Over the past decade, there has been great progress in research into methods to assess heart rate variability (HRV) and its clinical application. HRV is believed to be a parameter that represents autonomic modulation of the cardiovascular system. Recommendations for the use of HRV in clinical practice and its translation into potential therapeutic use have not yet been established. The heart rate variability is measured by analysing the variability in the duration of each heart cycle. HRV assessment is carried out on the basis of an ECG test, which, due to its accessibility and non-invasiveness, is a common and technically simple method. Evaluation of time and frequency analysis indicators allows interpretation of test results.
Due to the accessibility of HRV, it is important to establish the correct methodology for analysing and reporting results. The lack of consistency in the descriptions of the research methodology is a significant problem for the scientific community. Standardising HRV reporting will help to design future experiments and collect data. In his 2016 article, Quintana proposed guidelines to provide a minimum set of criteria on the basis of which HRV research in psychiatry should be designed and reported. The appropriate selection of participants in both the research and control groups, data on the type of data recording equipment, sampling frequency, recording time, analysis of collected data, calculation of HRV, and data archiving were taken into account [1]. In his article, Ernst Gernot pointed to the difficulties in translating HRV into clinical condition and subsequent therapy. Heart rate variability is very often used to describe sympathetic (SNS) and parasympathetic (PNS) function. However, this is based on the assumption that the SNS and PNS are in balance, and this consistency is often not proven. The balance between them is more complex [2]. Another important issue to consider is whether and which HRV parameters reflect the autonomic nervous system (ANS) status, the SNS, and PNS, respectively.
Physiology of the ANS – the balance between the SNS and PNS
This system, in terms of function and anatomy, is divided into parts: parasympathetic – the parasympathetic system, sympathetic – the sympathetic system, the intestinal and visceral-sensory system. All internal organs are innervated by the sympathetic and parasympathetic systems. These parts complement each other, leading to the harmonious operation of the selected organ. Unlike the somatic nervous system, the ANS causes involuntary reactions. It is known that the ANS innervates internal organs, regulates their work, and synchronises their operation with the endocrine system, behaviour, and emotions, which facilitate adaptation. The main tasks of the ANS include the following: maintaining homeostasis of the body, regulating physiologic processes, i.e. digestion, breathing, excretion, thermoregulation, blood circulation, regulating immune system function, reflex reactions, i.e. acceleration or slowing of heart rate. The parasympathetic and sympathetic parts differ in structure and function. The PNS centres are located in the brain stem and spinal cord. The biggest nerve is the vagus nerve. It innervates the organs of the chest and abdomen. The SNS centres are located in the lateral horn of the spinal cord of the thoracic and lumbar segment. The most important sympathetic nerve is the visceral nerve. The ANS is controlled by the chemical transmitters acetylcholine and norepinephrine [3-5].
Automatism of the heart
The ability of the heart to produce stimuli that cause heart systole and diastole is called heart automatism. The heart’s electrical system is composed of the sinoatrial node, the atrioventricular node, the Bundle of His, and Purkinje fibres. In the case of normal heart function, there is a sinus rhythm generated by the sinoatrial node, and the frequency of the produced stimulus is 70/min. The effects of the ANS are visible in the effectors located in the cardiovascular system, including the sinoatrial node, which affects the rhythm of the heart. Stimulation of the sympathetic part and mediators, i.e. adrenaline and noradrenaline, increase the HR value, while activation of the PNS, represented by the heart branches of the vagus nerve and the participation of the mediator of acetylcholine, cause the heart to slow down. The influence of the parasympathetic system prevails in the heart, and the influence of the sympathetic system prevails in blood vessels [3-5].
Heart rate variability
Heart rate variability is defined as changes in the length of successive RR intervals, after identification and correction of additional ventricular and supraventricular beats and elimination of artifacts. Only RR intervals between the correct QRS intervals are interpreted. HRV is a natural mechanism of continuous adaptation of physiological processes to changing environmental conditions. Heart rate variability analysis is a non-invasive measure of heart modulation by the ANS. A high variability of HRV is interpreted as normal, while a reduced HRV indicates ANS imbalances and poor prognosis of patients. Interpretation of HRV is based on the evaluation of an electrocardiogram. The recommended sampling rate is between 250 and 500 Hz. While early studies on HRV mainly used 24-hour monitoring using the Holter method, the latest research uses short-term 2-5-minute records [6]. HRV is assessed on the basis of time (statistical and geometric) and spectral analysis. Timing parameters are useful for assessing daily heart rate variability or comparing the effects of internal and external factors on HRV. Time analysis describes the range of changes in heart rate over a specific period. Time analysis parameters are based on direct RR interval measurements (SDNN, SDANN) or on differences between intervals (rMSSD, NN50, pNN50):
- SDNN is the standard deviation of the NN intervals, one of the most useful parameters to describe the total variability of the heart rate. This feature depends on the length of the ECG;
- SDANN is the standard deviation of the average NN intervals;
- RMSSD is the root mean square of successive RR interval differences. This figure corresponds to the HF component for spectral analysis. On the basis of this, the differences in subsequent NN intervals are determined;
- NN50 is the number of adjacent NN intervals that differ from each other by more than 50 ms;
- pNN50 is the percentage of adjacent NN intervals that differ from previous ones by more than 50 ms relative to the number of all NN intervals [7-9].
Spectral analysis was introduced by Akselrod, and it allows us to visualise cyclical changes taking place in the circulatory system under the influence of various stimuli. It is performed using a fast Fourier transform or autoregressive method.
For the assessment of the total spectral power (TP) the following parameters are used:
- HF – high frequency (0.15-0.4 Hz) – describes HRV variability modulated by the parasympathetic system, dependent on sinus respiratory arrhythmia (RSA) and changes in arterial pressure (BPV);
- LF – low frequency (0.04-0.15 Hz) – describes variability modulated by the sympathetic and parasympathetic systems and indicates the preservation of cardiovascular reactivity, including normal baroreceptor activity. Higher LF values indicate a relative advantage of the sympathetic system. A decrease in LF value after atropine administration indicates a simultaneous influence of the parasympathetic system on the values of this parameter;
- VLF – very low frequency (< 0.04 Hz) – coexists with excessive stimulation of the renin-angiotensin system;
- LF/HF – the ratio of low-frequency power to high-frequency power – reflects the sympathetic-parasympathetic balance;
- HFnu – normalised high-frequency component HFnu = HF/TP – VLF × 100;
- LFnu – normalised low-frequency component LFnu = LF/TP – VLF × 100.
An alternative method of HRV analysis comprises nonlinear methods. Such methods include the following:
- Poincaré charts – a graphical representation of the correlation between consecutive NN intervals. They allow the assessment of short- and long-term cardiac variability. Thanks to this method, the asymmetry of the heart rhythm was discovered - slowing down and accelerating the heart rhythm in the generation of HRV;
- entropy index – approximate entropy and entropy of the sample. Entropy, as an indicator of disorder, allows us to estimate the complexity of a series of NN intervals [7-9].
Experimental control
The issue of controlling external factors affecting HRV registration in study participants is potentially the most problematic. The results of measurements can be influenced by such variables as time of day, age of participants, gender, weight, medications taken, and environmental stimulants, such as caffeine, alcohol, and tobacco products. There was also a correlation between eating, subsequent digestion and water consumption, and the effects on HR. In the paper, information on co-morbidities, in particular the cardiovascular and circulatory system diseases, as well as the type of medicines taken should also be included. The exclusion of life-threatening diseases as well as medicines other than painkillers such as ibuprofen and paracetamol is considered controlled [2]. Carl Ludwig also described a relationship between RSA and HRV [10, 11].
HRV – use in practice
Studies on HRV are not used in diagnostics, but they are important in forecasting. In 1963, 2 scientists, Hon and Lee, identified a link between HRV scores and fetal risk. We use this dependence to this day by performing cardiotocographic examinations in the third trimester of pregnancy. The cardiotocography (CTG) test consists of 2 parameters – fetal heart rate variability and uterine activity recording. The fetal brain, like that of an adult, affects heart function through the interaction of the SNS and PNS. By monitoring FHR, we can assess whether the child receives the right amount of oxygen, which allows for early detection of the threat to the life of the fetus [12].
HRV provides significant prognostic information. Lower heart rate variability is associated with higher mortality after heart attack, cardiovascular failure, and among patients with diabetes. In the general population, there is a correlation between decreased HRV and increased overall mortality and higher incidence of cardiovascular diseases such as coronary heart disease, myocardial infarction, or congestive heart failure. One of the studies assessing the prognostic importance of HRV is the ATRAMI study. In this study, higher cardiovascular mortality after a recent heart attack, where the SDNN value was less than 70 ms, was demonstrated [13-15].
A new field is the use of HRV to predict systemic infections in intensive care medicine. The infection causes an inflammatory reaction in the body that is supposed to restore homeostasis. Immunogenic stimuli stimulate vagus nerve fibres directly through cytokines and indirectly through chemoreceptors. A negative correlation was found between TNF alpha and IL-6 and SDNN, SDANN variables. Reduced HRV parameters were also achieved with an increase in CRP [2, 6].
It has been proven that patients with cerebral palsy have an increased likelihood of dying from cardiovascular diseases and changes in the respiratory system. In a 2020 article by Gąsior et al. based on a review of 12 studies, it was shown that children with cerebral palsy are characterised by impaired ANS function. These patients are characterised by significantly higher HF, decreased HRV, and weaker adaptation to physical activity, and in the spirometry test they obtain weaker results compared to healthy children [14]. Cohen-Holzer et al. and Amichai et al. showed that exercise and training positively affect ANS regulation in paediatric patients with cerebral palsy.
In the last decade, there have been numerous dissertations concerning the evaluation of HRV in psychiatry. Analyses showed that people with mental disorders have reduced HRV, with the greatest decrease observed in psychotic disorders. HRV is also used as a method in research on anxiety disorders. It has been noted that anxiety disorders are associated with disturbed processes that inhibit the ANS. This inhibition is associated with a constant state of anxiety. Reduced HRV shows reduced resistance to stress, and increased HRV promotes behavioural adaptation. Considering this relationship, the effectiveness of treatment methods aimed at increasing HRV and thus alleviating the symptoms of the disease is currently being studied [16, 17].
Cardiovascular autonomic neuropathy (CAN) is one of the most serious forms of diabetic neuropathy. Clinical symptoms of CAN occur in patients with diabetes with a long duration of the disease. HRV analysis is used in CAN diagnostics, and reduction of HRV is considered one of the first parameters of the disease. Diabetes has been shown to cause disturbance of daily variability of HRV. SDNN and rMSSD parameters are reduced during hyperglycaemia, while LF HRV is used to predict hypoglycaemia [2, 6].
Pregnancy and the circulatory system
Pregnancy is a period during which the cardiovascular system undergoes adaptive changes in order to adapt to the changed requirements and needs of the pregnant woman and fetus. The circulatory system is subject to hormonal and mechanical influences associated with pregnancy. Changes occurring within this system usually precede the actual metabolic demand of the pregnant woman and the fetus. Hormones, mainly progesterone and oestrogen, acting via receptors, are decisive for the course of changes taking place in the body of the pregnant woman.
The adaptation of the cardiovascular system to the changing conditions of pregnancy is dynamic and gradual, adequate to the growing needs. Already at 6-8 weeks of pregnancy, there is a gradual increase in the volume of circulating blood, which reaches its highest values at about 32-34 weeks of pregnancy. This growth is part of the body’s adaptation to the increased volume of the uterine muscle, mammary glands, kidneys, and muscles, and also facilitates the maternal-fetal exchange of oxygen and nutrients.
The measure of systolic function is the minute capacity of the heart, which corresponds to the volume of blood that the heart pumps per minute. In the first trimester, it increases by about 30%. In the early weeks of pregnancy, this increase is possible by increasing the volume of pumped blood per stroke, and from the 20th week of pregnancy by increasing the heart rate. In the second trimester, as a result of adaptive processes, the minute capacity increases to 130%. This increase is rapid up to 24 weeks, then remains constant. In the third trimester, the volume of circulating blood changes slightly, while the heart rate continues to increase. The minute capacity is maintained at a similar level as at the end of the second trimester, and blood pressure values normalise.
During childbirth, there are further important haemodynamic changes. During birth, uterine contractions induce an increase in cardiac output, and pain activating the sympathetic system contributes to an increase in minute capacity and blood pressure by about 10%.
In the case of caesarean section, although it does not cause such significant haemodynamic changes as natural childbirth, it can lead to transient changes in minute volume, heart rate, and blood pressure through the influence of general anaesthesia.
The direct result of labour is an increase in minute capacity resulting from the outflow of blood from the uterus and from increased venous return resulting from reduced pressure on the inferior vena cava through the pregnant uterus. Usually, within 1-3 days after childbirth, the cardiovascular system normalises [18, 19].
Hypertension in pregnancy
From 3% to 10% of pregnant women are diagnosed with hypertension, which is the most common health problem during pregnancy and is a real threat to the mother and fetus. Women in pregnancy complicated by hypertension belong to the group at increased risk of serious complications, such as cerebrovascular accidents, organ failure, placental detachment, as well as disseminated intravascular coagulation. The fetus is at risk of delayed intrauterine development, premature birth, and death. To identify hypertension in pregnancy, values exceeding 140 mmHg systolic and 90 mmHg diastolic should be obtained with a minimum of 2 measurements taken at least 4 hours apart. According to the recommendations of the European Society of Hypertension and the European Society of Cardiology, hypertension diagnosed during pregnancy should be divided into pre-pregnancy-induced hypertension, chronic hypertension, and hypertension that cannot be qualified on the basis of assessment during pregnancy.
Pre-pregnancy hypertension (PPH) is the one that we find before pregnancy or before 20 weeks of pregnancy, lasting more than 42 days after childbirth. This hypertension can be primary or secondary and rarely occurs with proteinuria.
Pregnancy-induced hypertension (PIH) is elevated blood pressure without proteinuria, occurring after 20 weeks of pregnancy and subsiding more than 42 days after delivery.
The reason for this is the incorrect adaptation to changes occurring during the developing pregnancy.
If pregnancy-induced arterial hypertension is accompanied by significant proteinuria, this is the preeclampsia state. The definition of preeclampsia lists the triad of symptoms: hypertension, oedema, and proteinuria. Factors contributing to the development of preeclampsia include the following: multiple pregnancy, kidney disease, family preeclampsia history, diabetes, high body mass index (BMI), antiphospholipid syndrome, history of preeclampsia, chronic hypertension, and history of first pregnancy. Growth impairment following placental failure and premature birth are potential risks for the developing fetus.
If hypertension existing before pregnancy is accompanied by hypertension induced by pregnancy and proteinuria, in relation to this condition, we use the term “chronic hypertension with imposed preeclampsia”.
Pre-pregnancy hypertension is hypertension with or without systemic symptoms, diagnosed after 20 weeks of pregnancy, often during the first measurement of blood pressure. In such cases, a re-evaluation is necessary 42 days after birth.
It should be remembered that women with hypertension during the first pregnancy show an increased risk of hypertension in the next pregnancy. The earlier it appears, the higher the risk of recurrence in the next pregnancy. Women with pregnancy-induced hypertension also have an increased risk of hypertension and stroke later. In addition, an increased risk of coronary heart disease has been shown in women with preeclampsia [10, 20].
Heart rate variability in pregnancy
During pregnancy, there is a reorganisation in the functioning of the ANS, which results in changes in the components of HRV. Such changes help to ensure the proper development of the fetus. Changes in HRV, in addition to physiological causes, may also reflect mental or physical health problems. Many studies have shown that HRV during pregnancy can indicate blood pressure abnormalities or preeclampsia. Pregnant women with gestational hypertension have a higher ratio of LF power to HF power in early pregnancy compared to women with uncomplicated pregnancies. In addition, by examining the impact of depression and anxiety on HRV parameters, it was shown that it may reflect the mental health of a pregnant woman. HRV parameters can illustrate the level of stress experienced by a pregnant woman.
During the study, Iman et al. performed continuous monitoring of HRV in daily home conditions, collecting PPG signals from 58 pregnant women throughout the duration of pregnancy and for 3 months after delivery. Continuous monitoring of HRV allowed the detection of trends of HRV changes.
Continuously measured HR followed physiological trends, increasing as pregnancy progressed and returning to normal in the postpartum period. The study results showed that all HRV parameters in the time domain measured in the study and frequency domain parameters decreased during the second trimester of pregnancy. In the postnatal period, some parameters, i.e. SDNN, RMSSD, LF, and HF, increased as expected as the body recovered from pregnancy and childbirth. It has been suggested that the autonomic nervous system regenerates about 4 months after birth [21]. Heiskanen et al. also found similar results for HRV parameters in the frequency domain. The results of the study show that HR increased significantly during the second trimester, while during the third trimester it decreased slightly. The HRV time domain parameters and their normalised values fell significantly in the second trimester, followed by significant increases in the third trimester [22].
In recent years, numerous analyses have been conducted on the correlation between HRV parameters and the development of preeclampsia in pregnant women. Shaza et al. found different HRV patterns and trends of changes in autonomic modulation in patients with preeclampsia compared to healthy pregnant women. Yang et al. observed higher LF/HF and LF and lower HF in pregnant women compared to non-pregnant women. In addition, a group of women with preeclampsia showed lower HF but higher LF/HF, compared with women with normal pregnancy and non-pregnant women. They showed that during normal pregnancy there is an advantage of sympathetic tone over parasympathetic tone, and this correction is further increased if pregnant women develop preeclampsia [23].
The results of Weber et al. show that late-onset preeclampsia, but not early-onset preeclampsia, is associated with increased heart rate variability and sensitivity to baroreceptor reflex (BRS). Hypertension, which is a cardinal symptom of preeclampsia, is usually accompanied by reduced BRS. The observed increased variability in heart rate and BRS in late preeclampsia may indicate that, unlike most cases of chronic hypertension and early-onset preeclampsia, the appropriate mechanism of regulation is largely intact in late-onset preeclampsia. This may indicate greater regulatory and compensatory capacity despite high blood pressure levels in the late preeclampsia [24].
Sharifiheris et al. reviewed HRV adaptation studies to assess ANS function in healthy pregnant patients. According to their findings based on data published in 8 papers, almost all time domain (TD) and most frequency domain (FD) bands were decreased during pregnancy, with the exception of LF (nu) and the LF/HF ratio. Increased LF/HF and LF during pregnancy indicate a dominance of the sympathetic nervous system over the parasympathetic nervous system. This result is consistent with findings in the existing literature. The parasympathetic nervous system has been shown to become less active over time in pregnancy [25].
In a recent study, Solorzano et al. found a link between antenatal HRV and postnatal depression, indicating that lower rMSSD values in the antenatal period predicted a higher risk of postnatal depression [26].
A study by Eriksson et al. investigated whether HRV indices measured before and after the onset of a stressor at 38 weeks’ gestation could predict the presence of depression and anxiety symptoms at 6 weeks postpartum [27].
In contrast to a recent study by Solorzano et al., it was shown that HRV indices alone did not predict postpartum depression or anxiety and did not improve the predictive power of psychological scale models in women with depression and pregnancy anxiety.
The study does not provide evidence for the possibility of using HRV indices to predict postpartum depression and anxiety in women with known depression and pregnancy anxiety. They suggest that testing the ability of HRV to predict postpartum affective disorders are warranted among women without pregnancy symptoms of depression or anxiety [27].
The Sharifi-Heris study included 12 studies involving 6656 participants: 5 for gestational diabetes mellitus (GDM), 4 for fetal growth, 2 for pulmonary function, and 1 for nervous system disorders.
The published results suggest that ANS is associated with some common complications of pregnancy, including hypertensive and fetal growth disorders. However, existing studies do not support an association between ANS and gestational diabetes. There was no difference between GDM and non-GDM pregnant individuals in terms of HRV measurement in 80% of the study [28].
The ANS is a regulatory system that responds to various stresses. Pregnancy is one of the stimuli that requires physiological changes to adapt to the corresponding requirements for fetal development. Bester et al. created an overview of autonomous changes by incorporating non-linear functions, HRF, and PRSA into a standard time and frequency domain analysis. In healthy pregnancy, the increasing stress of a progressive gestation is well tolerated [29].
Conclusions
In recent years, there has been an increase in the number of publications concerning HRV as a parameter reflecting the ANS activity. Analysis of heart rate variability despite certain limitations is a useful, non-invasive, and easy to use diagnostic tool. Scientific studies have confirmed that HRV is a prognostic factor for selected clinical conditions. The HRV parameter has the undoubted value that its changes may precede the occurrence of clinical symptoms. Therefore, the use of HRV analysis as a tool in screening and stratification of the risk of death from cardiovascular disease remains a worthy issue. Pregnancy is a period of increased interest in a woman’s health from herself, her family, and caring doctors. Therefore, during pregnancy, cardiovascular abnormalities that may have existed before, have been aggravated during pregnancy, or have appeared for the first time are often diagnosed. Detailed HRV analysis, based on a series of RR intervals, allows, among other things, the assessment of the risk of cardiac events and the prediction of the clinical progression of the patient. Natural pregnancy-related adjustment of the body causes a significant haemodynamic load, which in women with concomitant cardiovascular defects can lead to serious complications. Therefore, further research into the applicability of HRV measurements is needed.
Disclosures
Ethical considerations: none.
This research received no external funding.
The authors declare no conflict of interest.
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