Sampling frequency of the RR interval time series for spectral analysis of heart rate variability

Spectral analysis of heart rate variability (HRV) is an accepted method for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. Various non-parametric methods for HRV estimation have been developed and extensive literature on their respective properties is available. The RR interval time series can be seen as a series of non-uniformly spaced samples. To analyse the power spectra of this series using the discrete Fourier transform (DFT), we need to interpolate the series for obtaining uniformly spaced intervals. The selection of sampling period plays a critical role in obtaining the power spectra in terms of computational efficiency and accuracy. In this paper, we shall analyse the RR interval time series from selected subjects for different sampling frequencies to compare the error introduced in selected frequency-domain measures of HRV at a constant frequency resolution for a specific duration of electrocardiogram (ECG) data. It should be pointed out that, although many other error causes are possible in the frequency-domain measures, our attention will be confined only to the performance comparison due to the different sampling frequencies. While the choice of RR interval sampling frequency (f(s)) is arbitrary, the sampling rate of RR interval series must be selected with due consideration to mean and minimum RR interval; f(s = )4 Hz was proposed for a majority of cases. This is an appropriate sampling rate for the study of autonomic regulation, since it enables us to compute reliable spectral estimates between dc and 1 Hz, which represents the frequency band within which the autonomic nervous system has significant response. Furthermore, resampled RR intervals are evenly spaced in time and are synchronized with the samples of the other physiologic signals, enabling cross-spectral estimates with these signals.     

Sampling frequency of the RR interval time series for spectral analysis of heart rate variability

Spectral analysis of heart rate variability (HRV) is an accepted method for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. Various non-parametric methods for HRV estimation have been developed and extensive literature on their respective properties is available. The RR interval time series can be seen as a series of non-uniformly spaced samples. To analyse the power spectra of this series using the discrete Fourier transform (DFT), we need to interpolate the series for obtaining uniformly spaced intervals. The selection of sampling period plays a critical role in obtaining the power spectra in terms of computational efficiency and accuracy. In this paper, we shall analyse the RR interval time series from selected subjects for different sampling frequencies to compare the error introduced in selected frequency-domain measures of HRV at a constant frequency resolution for a specific duration of electrocardiogram (ECG) data. It should be pointed out that, although many other error causes are possible in the frequency-domain measures, our attention will be confined only to the performance comparison due to the different sampling frequencies. While the choice of RR interval sampling frequency (f_s) is arbitrary, the sampling rate of RR interval series must be selected with due consideration to mean and minimum RR interval; f_s?=?4 Hz was proposed for a majority of cases. This is an appropriate sampling rate for the study of autonomic regulation, since it enables us to compute reliable spectral estimates between dc and 1 Hz, which represents the frequency band within which the autonomic nervous system has significant response. Furthermore, resampled RR intervals are evenly spaced in time and are synchronized with the samples of the other physiologic signals, enabling cross-spectral estimates with these signals.     

Power spectral analysis of heart rate and blood pressure variability in anaesthetized dogs.

Short-term oscillation of heart rate and blood pressure are mainly regulated by the automatic nervous system. It has been proposed that non-neural factors, such as changes in intrathoracic pressure, can strongly modulate this rhythmicity. Our aim was to evaluate the effect of changing intrathoracic pressure and central autonomic nervous activity on heart rate and blood pressure variability. Evaluation was performed by using spectral analysis techniques with autoregressive modelling. The variability in heart rate and blood pressure remained in animals with open chest or paralysed respiratory muscles. After vagotomy, the variability in heart rate decreased, but not that of blood pressure. Total spinal anaesthesia elicited a decrease in the variability in blood pressure. The pharmacological blockade of alpha- and beta-receptors further decreased both variabilities. It was concluded that in anaesthetized dogs heart rate and blood pressure variability are mainly of central origin and non-neural factors have only minor effect on these central rhythms. High (> 0.15 Hz), medium (0.07-0.15 Hz) and, obviously low (0.00-0.07 Hz) frequency variations in heart rate are mostly mediated vagally. In blood pressure, medium and obviously low frequency variations are modulated by sympathetic nervous system, whereas high frequency variations are secondary to the heart rate variation.     

Circadian systemic haemodynamics in borderline and mild hypertension.

Circadian variations in blood pressure (BP), stroke volume (SV), heart rate (HR), cardiac output (CO) and total peripheral resistance (TPR) were determined by a pulse contour method from the intra-arterial pulse wave in 32 normotensive (NT), 32 borderline hypertensive (BHT) and 31 hypertensive (HT) middle-aged men. Daytime averages were used as the reference levels. The nocturnal decrease in BP and HR were similar in the three groups. In the night, SV did not change in the NT group, but was increased in the BHT and HT groups. The nocturnal increase in SV may reflect reduced venous capacity causing increased cardiac filling. As a consequence of the difference in SV, the nocturnal CO fall was diminished in the HT group as compared with the NT group. Moreover, TPR had a tendency to decrease in the HT group, which may be considered as a baroreflex response to buffer the expected rise in BP. Five years later, 25 NT, 24 BHT and 19 HT subjects were reassessed using casual BP measurements. In the NT and BHT groups, six and 17 subjects, respectively, had progressed to hypertension. In a logistic regression model for those who became HT, the nocturnal increase in SV was a significant predictor for future hypertension. In conclusion, the results suggest that circadian systemic haemodynamics may be altered before BP is markedly elevated, and that haemodynamic studies might be useful in predicting the development of sustained hypertension.     

Effects of sino-aortic denervation on spectral characteristics of blood pressure and pulse interval variability: a wide-band approach

Sino-aortic denervation (SAD) is employed in cats to evaluate the baroreflex influence on blood pressure (BP) and pulse interval (PI) spectral components from 0·00008 to 0·9 Hz as assessed by FFT wide-band spectra and their 1/f modelling; and the linear coupling between BP and PI and between systolic and diastolic BP as assessed by coherence analysis. Specific procedures have been developed to obtain an effective smoothing of spectra and coherence functions. SAD induced an increase in BP powers from 0·03 to 0·0006 Hz and a power reduction of most of the remaining BP components; a reduction of PI powers at all frequencies; marked deviations of BP spectra from the 1/f trend; a reduction of the coherence between BP and PI from 0·12 to 0·5 Hz and a coherence enhancement at lower frequencies. These findings indicate that the arterial baroreflex modulates both fast and slow spectral components of BP and PI; homogeneously enhances PI fluctuations at all frequencies; produces differentiated effects on BP fluctuations along the frequency axis; and at low frequencies exerts the buffering action on BP through strategies which reduce the BP-PI linear link.     

Baseline and reactivity measures of blood pressure and negative affect in borderline hypertension

Measures of blood pressure (BP) and negative affect were taken prior to, immediately following, and ten minutes after a cognitive stressor in groups of normotensive (group N; n = 15) and borderline hypertensive (group B; n = 15) young adults. Two testing sessions, one week apart, involved performance of a simple, congruent color-word ("easy") and a cognitive stress-inducing, incongruent color-word ("difficult") version of the Stroop test. In both sessions, group B showed significantly higher baseline systolic and diastolic BP, higher systolic responses to the difficult Stroop task, and higher recovery measures of systolic and diastolic BP than group N. In general, group B had significantly elevated baseline and reactivity scores on state measures of negative affect in both sessions. Correlations revealed strong positive associations between blood pressure and mood reactivity only for borderline subjects. Depressed mood was more strongly and consistently related to blood pressure reactivity than was hostility. Borderline hypertension appears to be associated with stable, perhaps clinically significant elevations in negative affect, and with dysphoric response to mild cognitive stress.     

The effect of ventilation on spectral analysis of heart rate and blood pressure variability during exercise

Heart rate variability (HRV) and systolic blood pressure variability (BPV) during incremental exercise at 50, 75, and 100% of previously determined ventilatory threshold (VT) were compared to that of resting controlled breathing (CB) in 12 healthy subjects. CB was matched with exercise-associated respiratory rate, tidal volume, and end-tidal CO(2) for all stages of exercise. Power in the low frequency (LF, 0.04-0.15 Hz) and high frequency (HF, >0.15-0.4 Hz) for HRV and BPV were calculated, using time-frequency domain analysis, from beat-to-beat ECG and non-invasive radial artery blood pressure, respectively. During CB absolute and normalized power in the LF and HF of HRV and BPV were not significantly changed from baseline to maximal breathing. Conversely, during exercise HRV, LF and HF power significantly decreased from baseline to 100% VT while BPV, LF and HF power significantly increased for the same period. These findings suggest that the increases in ventilation associated with incremental exercise do not significantly affect spectral analysis of cardiovascular autonomic modulation in healthy subjects.     

Erythrocyte sodium content and transport in borderline and mild hypertension

Summary Sodium content and transport of red blood cells were examined in 98 male blood donors. Regarding their blood pressure they were classified into the following groups: (a) 57 normotensives, (RR<140/90 mm Hg); (b) 24 borderline hypertensives (140/90RR<160/95 mm Hg); and (c) 17 hypertensives (RR>160/100 mm Hg). Compared with the normotensives the borderline hypertensives have significantly reduced red cell sodium content. The ouabain-resistant net Na⁺ uptake and the relative Na⁺ uptake, as a measure of the Na⁺/K⁺ pump, were significantly increased. With rising blood pressures the measured values turn to normal, so that no difference exists between the normotensive and hypertensive groups. It is supposed that in the initial or even prehypertensive state a considerable enhancement of the pump activity occurs, simultaneously accompanied by less marked increases in sodium influx, leading to a reduced intracellular sodium content. In the course of hypertension, possibly caused by the formation of a pump inhibitor, the sodium content of red cells turns to normal or supernormal values.Abbreviations BMI body mass index - BHT borderline hypertensive - Ca _ion ²⁺ ionized plasma calcium - HT hypertensive - k relative OR net Na⁺ uptake - [Na⁺]_i, [K⁺]_i intracellular sodium and potassium content in RBCs - NT normotensive - OR ouabain-resistant - RBCs red blood cells - Na OR net Na⁺ uptake     

高血压的血压变异性与靶器官损害的研究进展

有效的降低高血压患者心血管事件的发生率是我们一直追求的目标.近期的一些研究表明血压的升高和血压变异性的增高往往伴随着心、脑、肾、血管等靶器官损害的发生及发展,从而提升心血管事件的发生率及病死率.     

Extreme changes in dietary sodium effect daily variability and level of blood pressure in borderline hypertensive patients

This study examined the effect of large changes in dietary sodium on the average ambulatory blood pressure and its variability in 19 patients with uncomplicated borderline hypertension. Each patient participated in a 16-week protocol that consisted of four 4-week periods of different sodium intake (medium (120–160 mEq/day) during periods 1 and 3 and low (< 40 mEq/day) or high (> 225 mEq/day) during either period 2 or 4. The 24-hour urine sodium during the low and high periods averaged 18 and 327 mEq/day, respectively. Ambulatory blood pressure monitoring was done at the end of the fourth week of the low and high diet periods. During monitoring, pressures were recorded every 15 minutes while awake; in addition, patients kept diaries noting activities, posture, and situation at each measurement. The results show that there was a decline of 16/7 mmHg in the average ambulatory awake systolic and diastolic pressures from the high sodium to low sodium diets. Corresponding casual pressures decreased an average of 15 and 8 mmHg, respectively. In examining the factors associated with ambulatory pressure variability, systolic pressure showed greater variation by activity on a low sodium diet than on the high. The findings suggest that sodium restriction has a variable, but in some cases marked, effect on lowering the ambulatory blood pressure in borderline mildly hypertensive patients and that sodium balance may be important to consider when examining ambulatory blood pressure variability. © 1994 Wiley-Liss, Inc.     

Short and long term non-linear analysis of RR variability series

The complexity of RR variability is approached in the short and in the long term by means of black-box data analysis. Short term series of a few hundred beats are explored by means of informational entropy and predictability indexes. A correction to biases toward false determinism is performed assuming maximum uncertainty, whenever data do not furnish sufficient recurrences. Non-randomness and non-linearity are tested by means of surrogate data provided by random shuffling and phase randomization respectively. In the long term of the 24-h or of several hours, similar tests based on mutual information are applied and validated by means of surrogate series. In addition the state space reconstruction is carried out by means of state space non-linear filtering addressing directly the reconstructed trajectories. In this condition, parameters characterizing the hypothetical attractor, mainly the maximum Lyapunov exponent, can be reliably identified.     

Statistical analysis of RR series variability in spinal cord injured persons

Disabled persons with spinal cord injury are prone to cardiovascular dysfunction and an increased risk of cardiovascular disease. Rehabilitation of the disabled person is a critical task as it involves multiple therapies. Physical exercise is an important component of rehabilitation, and depends on cardiovascular health. Reduced RR variability is a marker of poor cardiac health. Time domain RR variability analysis of 38 normal healthy subjects and 20 spinal cord injured subjects has been carried out and compared. In this study, RR intervals were recorded in three different modes or positions: supine, sitting and five-second rhythm respiration. At a time of 150 s RR interval data were acquired in each mode and analysed. Statistical parameters (mean, HR, STD, NN50 and pNN50) were calculated. It was observed that most of the indices were significantly and substantially altered in spinal cord injured persons.     

Statistical analysis of RR series variability in spinal cord injured persons

Disabled persons with spinal cord injury are prone to cardiovascular dysfunction and an increased risk of cardiovascular disease. Rehabilitation of the disabled person is a critical task as it involves multiple therapies. Physical exercise is an important component of rehabilitation, and depends on cardiovascular health. Reduced RR variability is a marker of poor cardiac health. Time domain RR variability analysis of 38 normal healthy subjects and 20 spinal cord injured subjects has been carried out and compared. In this study, RR intervals were recorded in three different modes or positions: supine, sitting and five-second rhythm respiration. At a time of 150 s RR interval data were acquired in each mode and analysed. Statistical parameters (mean, HR, STD, NN50 and pNN50) were calculated. It was observed that most of the indices were significantly and substantially altered in spinal cord injured persons.     

Heart rate variability and its determinants in patients with severe or mild essential hypertension.

How cardiac autonomic nervous control is related to the severity of essential hypertension in patients receiving long-term antihypertensive therapy is not well known. The aim of this study was to examine heart rate variability (HRV), a non-invasive measure of cardiac autonomic function, in patients with long-term and medically treated mild and severe essential hypertension and healthy control subjects, and to assess the clinical determinants of HRV in these patients. Thirty-four patients with severe essential hypertension (SEHT) and 29 with mild essential hypertension (MEHT) as well as healthy age- and sex-matched control subjects were studied. HRV was assessed from 10 min ECG-recordings during paced (0.2 Hz) breathing at rest and expressed as time and frequency domain measures. In the SEHT group time (SDNN, RMSSD) and frequency domain measures (total power, low-frequency (LF) power and high-frequency (HF) power of HRV in absolute units, and LF and HF power of HRV in normalized units) of HRV were significantly lower when compared with those of the control group. The MEHT and control groups did not differ from each other with respect to time or frequency domain measures of HRV. Comparison between the hypertensive groups showed that SDNN, total power, LF power and HF power were lower in the SEHT group compared with the MEHT group (P<0.05 for all). Among hypertensive patients RR-interval, age, gender, systolic finger blood pressure and diastolic office blood pressure as well as 24-h blood pressure were significant determinants of HRV. In conclusion, we found that the severity of chronic essential hypertension seems to be related to the severity of impairment of cardiac autonomic control.     

Determinants of circadian blood pressure rhythm and blood pressure variability in obstructive sleep apnoea

SUMMARY  The prevalence of hypertension in patients with obstructive sleep apnoea (OSA) is high and blood pressure profile is characterized by nocturnal blood pressure (BP) elevation and increased nocturnal BP variability. Ambulatory 24-hour-biood pressure monitoring (ABPM) is a valid, non-invasive method to describe circadian BP variation. Circadian BP profile and nocturnal BP variability were related to OSA severity (apnoea-hypopnoea index, mean low O₂), age and body mass index (BMI) in 73 patients with OSA. Prevalence of hypertension was 75%, and in 59% BMI was greater than 30 kg m^-2. A nocturnal decline of at least 10% from daytime mean BP values (night/day BP ratio <0.9; dipper) was found in only 25% of hypertensive patients and 39% of normotensive patients. Comparison between dippers and non-dippers showed significant differences in apnoea severity (apnoea-hypopnoea index 32 + 19 vs. 50 + 23/h, P <0.01; mean low O₂ 84.5 + 4 vs. 80.2 + 5.8%, P< 0.01) but not for age and BMI. In multiple regression analyses with age, body mass index, apnoea-hypopnoea index and mean low O₂ as independent and BP ratios and BP variability as dependent variables, sleep apnoea severity was the only independent predictor for circadian BP rhythm and nocturnal BP variability. The results presented here suggest that independent of age and obesity the severity of sleep apnoea is an important determinant of circadian BP variation and nocturnal BP variability.     

有氧运动对轻度原发性高血压患者血压和甲襞微循环的影响

目的研究有氧运动对轻度原发性高血压患者血压和甲襞微循环的影响。方法随机选取轻度原发性高血压患者21例为高血压组,同期健康体检者20例为正常对照组。高血压组进行规律的有氧运动训练,检测有氧运动前后血压、甲襞微循环的变化。结果高血压组甲襞微循环形态积分、流态积分、襻周状态积分和总积分值均高于正常对照组（P〈0．05）。有氧运动3个月后,高血压患者血压下降,甲襞微循环形态积分、流态积分、襻周状态积分和总积分值均降低（P〈0．05）。结论有氧运动能有效降低轻度原发性高血压患者的血压,改善其微循环状态。     

Clonidine normalizes low plasmaβ-endorphin concentration and blood pressure in young men with mild essential hypertension

Summary In young men with mild essential hypertension and age-matched normotensive volunteers, plasma concentrations of the endogenous opioid-endorphin were determined hourly from 9:00 p.m. to 2:00 a.m. The hypertensive patients' mean plasma-endorphin concentration was significantly lower in comparison with normotensive controls. After 14 days of treatment with clonidine, systolic and diastolic blood pressure was significantly reduced in both groups. Plasma-endorphin concentration increased in the hypertensive patients, but remained unchanged in the normotensive volunteers. The present findings point to a possible involvement of reduced-endorphinergic activity in blood pressure regulation of young men with essential hypertension.Abbreviation NTS nucleus tractus solitarii Dedicated to Prof. Dr. F. Krück on the occasion of his 65th birthday