Heart rate variability in healthy humans during night sleep

Polysomnography has been performed and heart rate variability has been studied during night sleep in healthy young subjects of both sexes. It has been shown that, on average, a shift in autonomic support towards parasympathicotonia occurs during sleep, with a maximum at stages III and IV of NREM sleep. At stage II of NREM sleep and during REM sleep, a short-term activation of the sympathetic nervous system comparable to wakefulness is observed.     

Heart rate variability in young hockey players at rest and during active orthostatic test

Heart rate variability of 27 young (10- to 11-year-old) athletes (ice hockey players) was studied. Special autonomic regulatory mechanisms that function both at a supine position and during active orthostatic test were found, which probably reflect the adaptation of the cardiovascular system to sports activity.     

Heart rate and arterial pressure variability and baroreflex sensitivity in ovariectomized spontaneously hypertensive rats

AimsThe present study evaluated the effects of ovariectomy on heart rate and arterial pressure variability and cardiac baroreflex sensitivity (BRS) in female spontaneously hypertensive (SHR) and Wistar–Kyoto rats (WKY).Main methodsSham-surgery animals were used as control. Sixteen weeks after ovariectomy or sham-surgery, animals were recorded. Time series of pulse interval (PI) and systolic AP (SAP) were analyzed by means of autoregressive spectral analysis, which quantifies the power of very low (VLF = 0.01–0.25 Hz), low (LF = 0.25–0.75 Hz) and high frequency (HF = 0.75–2.5 Hz) bands. BRS was assessed by means of linear regression between changes of PI and SAP induced by vasoactive drugs or calculation of α-index, a spontaneous BRS index.Key findingsThere was no difference in baseline PI or SAP between ovariectomized and sham SHR. Spectral analysis of heart rate variability suggested a shift of sympatho-vagal balance toward sympathetic predominance in ovariectomized SHR (LF/HF = 1.8 ± 0.2 versus 0.7 ± 0.2 in sham SHR, p < 0.05). Ovariectomy increased total variance and VLF power of SAP in SHR (29.1 ± 9.6 mmHg² and 18.6 ± 6.3 mmHg² versus 9.1 ± 2.1 mmHg² and 4.3 ± 1.4 mmHg², respectively, in sham SHR, p < 0.05). In addition, ovariectomy reduced reflex bradycardia in SHR (0.18 ± 0.03 ms/mmHg versus 0.34 ± 0.06 ms/mmHg in sham SHR, p < 0.05). Ovariectomy did not affect heart rate and SAP variability or BRS in WKY.SignificanceThese data showed that ovarian hormones deprivation induced marked changes on cardiovascular control, increasing SAP variability and cardiac sympatho-vagal balance and blunting BRS in female hypertensive animals, which reinforce the possible protective role of ovarian hormones on the cardiovascular system.     

Heart rate variability and short duration spaceflight: relationship to post-flight orthostatic intolerance

Background  

Upon return from space many astronauts experience symptoms of orthostatic intolerance. Research has implicated altered autonomic cardiovascular regulation due to spaceflight with further evidence to suggest that there might be pre-flight autonomic indicators of post-flight orthostatic intolerance. We used heart rate variability (HRV) to determine whether autonomic regulation of the heart in astronauts who did or did not experience post-flight orthostatic intolerance was different pre-flight and/or was differentially affected by short duration (8 – 16 days) spaceflight. HRV data from ten-minute stand tests collected from the 29 astronauts 10 days pre-flight, on landing day and three days post-flight were analysed using coarse graining spectral analysis. From the total power (P_TOT), the harmonic component was extracted and divided into high (P_HI: >0.15 Hz) and low (P_LO: = 0.15 Hz) frequency power regions. Given the distribution of autonomic nervous system activity with frequency at the sinus node, P_HI/P_TOT was used as an indicator of parasympathetic activity; P_LO/P_TOT as an indicator of sympathetic activity; and, P_LO/P_HI as an estimate of sympathovagal balance.     

Heart rate variability in exercising humans: effect of water immersion

Power spectrum analysis of heart-rate variability was made in seven men [mean age 22 (SEM 1) years] in head-out water immersion (W) and in air (A, control) at rest and during steady-state cycling to maximal intensity (maximum oxygen uptake, V˙O_2max). At rest W resulted in a trebled increase in the total power (P < 0.05), coupled with minimal changes in the power (as a percentage of the total) of the high frequency peak (HF, centred at 0.26 Hz; 18% vs 28%) and of the low frequency peak (LF, 0.1 Hz; 24% vs 32%). A third peak at about 0.03 Hz (very low frequency, VLF) represented the remaining power both in W and A. These changes as a whole indicated that immersion caused a vagal dominance in cardiac autonomic interaction, due to the central pooling of blood and/or the pressure of water on the trunk. Exercise caused a decrease in the total power in W and A. The LF% did not change up to about 50% V˙O_2max, thereafter decreasing towards nil in both conditions. The HF% decreased in similar ways in W and A to about half at 55%–60% V˙O_2max and then increased to reach 1.5 times the resting values at V˙O_2max. The central frequency of HF increased linearly with oxygen uptake, showing a tendency to be higher in W than in A at medium to high intensities. The VLF% remained unchanged. The lack of differences in the LF peak between W and A during exercise would suggest that blood distribution had no effect on the readjustments in control mechanisms of arterial pressure. On the other hand, the findings of similar HF powers and the very similar values for ventilation in W and A confirmed the direct effect of the respiratory activity in heart rate modulation during exercise. Accepted: 25 August 1997     

Does nitric oxide buffer arterial blood pressure variability in humans?

Animal studies suggest that nitric oxide (NO) plays an important role in buffering short-term arterial pressure variability, but data from humans addressing this hypothesis are scarce. We evaluated the effects of NO synthase (NOS) inhibition on arterial blood pressure (BP) variability in eight healthy subjects in the supine position and during 60 degrees head-up tilt (HUT). Systemic NOS was blocked by intravenous infusion of N(G)-monomethyl-L-arginine (L-NMMA). Electrocardiogram and beat-by-beat BP in the finger (Finapres) were recorded continuously for 6 min, and brachial cuff BP was recorded before and after L-NMMA in each body position. BP and R-R variability and their transfer functions were quantified by power spectral analysis in the low-frequency (LF; 0.05-0.15 Hz) and high-frequency (HF; 0.15-0.35 Hz) ranges. L-NMMA infusion increased supine BP (systolic, 109 +/- 4 vs. 122 +/- 3 mmHg, P = 0.03; diastolic, 68 +/- 2 vs. 78 +/- 3 mmHg, P = 0.002), but it did not affect supine R-R interval or BP variability. Before L-NMMA, HUT decreased HF R-R variability (P = 0.03), decreased transfer function gain (LF, 12 +/- 2 vs. 5 +/- 1 ms/mmHg, P = 0.007; HF, 18 +/- 3 vs. 3 +/- 1 ms/mmHg, P = 0.002), and increased LF BP variability (P < 0.0001). After L-NMMA, HUT resulted in similar changes in BP and R-R variability compared with tilt without L-NMMA. Increased supine BP after L-NMMA with no effect on BP variability during HUT suggests that tonic release of NO is important for systemic vascular tone and thus steady-state arterial pressure, but NO does not buffer dynamic BP oscillations in humans.     

Analysis of heart rate variability and arterial blood pressure in different functional tests in men and women

Experiments with simultaneous recording of the heart rate, peripheral blood pressure, and respiration showed differences between the cardiovascular system parameters of men and women at rest and during various functional tests (cold, mental and physical exercise, spirometric mask testing). The results of several series of experiments using the same sample showed that women were characterized by a relationship between the state of the cardiovascular system and the psychoemotional status and well-being, as well as by a greater involvement of the central mechanisms in the regulation of the cardiovascular system when functional tests are performed. A high level of tonic activity and high reactivity of the sympathetic link of regulation of the cardiovascular system is typical of men.     

Heart rate variability in subjects with different respiratory rates

The variability of the cardiac rhythm was studied in males with different initial respiratory rates. At rest and during voluntarily controlled breathing, subjects with medium respiratory rates were found to have a less variable heart rate than their counterparts with low or high respiratory rates.     

Heart rate variability during head down tilt and lower body negative pressure in the Russian Tschibis

The cardiovascular function in space seems to be normal. However, abnormalities of cardiovascular responses have been found during lower body negative pressure suction in space. The etiology of the cardiovascular deconditioning in space is still unknown. A previous study showed, that short periods of head down tilt (HDT-6 degrees) induce changes in the spectral pattern of heart rate variabilty (HRV) and an increase in the sympathethic activation caused by orthostatic stress. The aim of this study was to test following hypotheses: 1. The dynamic of heart rate variability is different in the head down tilt and supine positions. 2. The application of lower body negative pressure (LBNP) during head down tilt induces similar heart rate variability patterns like the standing position. 3. After short term head down tilt the cardiovascular response to lower body negative pressure stressor is altered.     

Heart rate variability during long truck driving work

We recorded ambulatory electrocardiograms of 6 long distance truck drivers during their work period in order to observe the affect of autonomic nervous function and symptoms while doing their work. We also recorded their work patterns every minute. The RR50 value and the LFP/HFP ratio were calculated every two minutes based on R-R interval data. RR50 was significantly higher during taking naps than during other periods of work shifts, while, the LFP/HFP ratio showed significantly lower during taking naps than during other periods of work shifts. RR50 in the morning was significantly higher than that in the afternoon. On the contrary, the LFP/HFP ratio showed opposite tendency. Only on the times of driving, RR50 was significantly higher in the morning than that in the afternoon. On the other hand, the LFP/HFP ratio showed an opposite tendency. These results show that the parasympathetic nervous activities were more dominant than sympathetic nervous activities in the morning during the subjects were doing long distance truck driving including midnight work. Driving while in high parasympathetic nervous activity levels may add to cardiovascular stress and lead to drowsiness. And this may result in disrupted attention. It is necessary to decrease work time and improve working conditions of truck drivers working long-hour shifts.     

Heart rate variability responses to hypoxic and hypercapnic exposures in different mouse strains.

Heart rate variability (HRV) is a well-characterized, noninvasive means of assessing cardiac autonomic nervous system activity. This study examines the basic cardiac responses to hypoxic and hypercapnic challenges in seven strains of commonly used inbred mice (A/J, BALB/cJ, C3H/HeJ, C57BL/6J, CBA/J, DBA/2J, and FVB/J). Adult male mice, 8-12 wk of age, were chronically instrumented to a femoral artery catheter for the continuous measurement of systemic arterial blood pressure and heart rate. Mice were exposed to multiple 4-min periods of hypoxia (10% O2), hypercapnia (5% CO2), and combined hypoxia/hypercapnia (10% O2 + 5% CO2). HRV was derived from pulse intervals of the blood pressure tracings. Hypoxia induced increases in high-frequency HRV power and decreased low-frequency (LF) HRV power in most strains. Hypercapnia led to decreased high-frequency HRV power and increased LF HRV power in most strains. Strain differences were most notable in regard to the concomitant exposures of hypoxia and hypercapnia, with FVB/J mice mirroring their own response to hypercapnia alone, whereas CBA/J mice mirrored their own responses to hypoxia. As blood pressure is most likely the driving factor for heart rate changes via the baroreflex pathway, it is interesting that LF, considered to reflect cardiac sympathetic activity, was negatively correlated with heart rate, suggesting that LF changes are driven by baroreflex oscillation and not necessarily by absolute sympathetic or parasympathetic activity to the heart. These findings suggest that genetic background can influence the centrally mediated cardiovascular responses to basic hypoxic and hypercapnic challenges.     

Heart rate variability in weightlifters

On the basis of the literature and original data, heart rate variability (HRV) in weightlifters has been studied. The results showed that the distribution mode (a parameter of mathematical analysis that is equal to the most frequent length of RR intervals) indicates the intensity of physical exercise. Specific changes in the autonomic balance in athletes as dependent on their degree of training and sports qualification are important characteristics of adaptations to physical loads. For example, the degree of training of weightlifters is reflected by the level of the respiratory component as an index of the activity of the parasympathetic nervous system. Adaptation to physical exercise leads to an increase in the power of the spectrum of neurohumoral modulation and to changes in the ratio between the levels of the total spectral power of HRV.     

Heart rate variability and sports

This article presents a review of the literature and some author’s data on the problem of using heart rate variability in sports and sports medicine practice.     

Heart rate during exercise with leg vascular occlusion in spinal cord-injured humans.

Feed-forward and feedback mechanisms are both important for control of the heart rate response to muscular exercise, but their origin and relative importance remain inadequately understood. To evaluate whether humoral mechanisms are of importance, the heart rate response to electrically induced cycling was studied in participants with spinal cord injury (SCI) and compared with that elicited during volitional cycling in able-bodied persons (C). During voluntary exercise at an oxygen uptake of approximately 1 l/min, heart rate increased from 66 +/- 4 to 86 +/- 4 (SE) beats/min in seven C, and during electrically induced exercise at a similar oxygen uptake in SCI it increased from 73 +/- 3 to 110 +/- 8 beats/min. In contrast, blood pressure increased only in C (from 88 +/- 3 to 99 +/- 4 mmHg), confirming that, during exercise, blood pressure control is dominated by peripheral neural feedback mechanisms. With vascular occlusion of the legs, the exercise-induced increase in heart rate was reduced or even eliminated in the electrically stimulated SCI. For C, heart rate tended to be lower than during exercise with free circulation to the legs. Release of the cuff elevated heart rate only in SCI. These data suggest that humoral feedback is of importance for the heart rate response to exercise and especially so when influence from the central nervous system and peripheral neural feedback from the working muscles are impaired or eliminated during electrically induced exercise in individuals with SCI.     

Heart rate variability analysis

The expansion of heart rate variability analysis has been facilitated by the remarkable development of computer sciences and digital signal processing during the last thirty years. The beat-to-beat fluctuation of the heart rate originates from the momentary summing of sympathetic and parasympathetic influences on the sinus node. According to the extensive associations of the autonomic nervous system, several factors affect heart rate and its variability such as posture, respiration frequency, age, gender, physical or mental load, pain, numerous disease conditions, and different drugs. Heart rate variability can be quantitatively measured by time domain and frequency domain methods that are detailed in the paper. Non-linear methods have not spread in the clinical practice yet. Various cardiovascular and other pathologies as well as different forms of mental and physical load are associated with altered heart rate variability offering the possibility of predicting disease outcome and assessing stress.