Power Spectrum Of Heart Rate Variability Research Articles

Evaluation of the autonomic nervous system function in children with primary monosymptomatic nocturnal enuresis--power spectrum analysis of heart rate variability using 24-hour Holter electrocardiograms.

The purpose of this study was to evaluate the circadian rhythm of the autonomic nervous system functions of children with primary monosymptomatic nocturnal enuresis. The subjects were allocated to three groups--enuresis group (n = 72), normal control group (n = 26), post-treated and cured enuresis group (n = 13). In the three groups. we evaluated the autonomic nervous system function by the power spectrum analysis of heart rate variability (HRV) using 24-hour Holter electrocardiograms. Frequency components of the power spectra of HRV (% power) were divided into three factors; the very low frequency (VLF), the low frequency (LF) and the high frequency (HF) by computer analysis using Fast Fourier Transformation (FFT). In the three groups, the HF component reflecting parasympathetic activity was significantly higher, compared with the LF during sleep. The HF in the enuresis group was significantly higher, compared with the HF in the control group during sleep. The HF in the control group was significantly lower, compared with the LF during waking. However, the HF in the enuresis group was significantly higher, compared with the LF during waking. It showed more increased parasympathetic tone in the enuresis group, compared with that of the control group during sleep and waking. In the post-treated enuresis group, the HF during sleep and waking were significantly decreased, compared with those before treatment. These findings suggest the parasympathetic hyperfunction through 24 hours and abnormal circadian rhythm of autonomic nervous system functions in the enuresis group. The post-treated enuresis group showed normal autonomic nervous system functions.

Reversal of deteriorated fractal behavior of heart rate variability by beta-blocker therapy in patients with advanced congestive heart failure.

The slope of the power spectrum in heart rate variability (HRV) reflects the fractal or scaling behavior in HR dynamics and recently was confirmed as an independent predictor of postmyocardial infarction survival. Whether or not the new measurement in HRV foresees the functional evolution in patients with advanced congestive heart failure treated by beta blockers is unclear. Sequential 24-hour Holter ECG recordings were obtained at baseline, and 1 and 3 months after addition of atenolol therapy for advanced congestive heart failure in 10 patients. The slope and intercept of the regression line of power-law behavior, the short- and intermediate-term of detrended fluctuated analysis (DFA), the approximate entropy (ApEn), and the standard frequency spectra of the 24-hour HRV were compared sequentially as well as with those in 12 age-matched normal controls. The results showed that the slope (-1.70 +/- 0.45 vs -1.22 +/- 0.21; P < 0.05) and the intercept (5.11 +/- 0.46 vs 5.62 +/- 0.24; P < 0.05) of the regression line of power-law behavior and the short-term DFA (for 4 to 11 beats) (0.78 +/- 0.18 vs 1.13 +/- 0.21; P < 0.05) increased after 3 months of atenolol treatment. However, the change in intermediate-term DFA (>11 beats) and ApEn was not apparent (1.24 +/- 0.21 vs 1.22 +/- 0.15 and 1.34 +/- 0.14 vs 1.36 +/- 0.11; both P > 0.05). The evolution of the slope or intercept of the regression line of the HRV power spectrum did not correlate with the echocardiographic or clinical cardiac function, or with the frequency spectral components of the HRV (P > 0.05). Additional beta-blocker therapy upregulated the fractal behavior control of the HRV in patients with advanced congestive heart failure. The improvement was independent of subjective and objective global cardiac performance.

Microvascular Reactivity in Normotensive Subjects with a Familial Predisposition to Hypertension

Using the laser-Doppler method we measured blood flow on the nailfold skin to compare the reactivity of cutaneous microcirculation in three groups of normotensive subjects: 11 subjects with a familial predisposition to hypertension without a previous record of high blood pressure, 6 predisposed subjects with a previous record of high blood pressure, and 13 subjects with no predisposition to hypertension. The flow was measured after direct and indirect skin cooling and heating and during postocclusive reactive hyperemia (PRH) after a 10-min occlusion of digital arteries. The frequency of flow oscillations in the second part of the PRH was established. Heart rate spectral analysis was performed based on the monitoring of the peripheral pulse frequency by means of the finapres device. In comparison to the other two groups of subjects, the group with a predisposition and a previous record of high blood pressure displayed a larger surface area in the low frequency band (0.05 to 0.15 Hz) of the heart rate variability power spectrum (the Bonferroni test, P < 0.05). As compared to subjects without predisposition, both groups of predisposed subjects exhibited higher frequency of flow oscillations in the second part of the PRH (the Bonferroni test, P < 0.05). Our results indicate that there could be a change in cutaneous microvascular reactivity of local (most probably myogenic) origin even in normotensive subjects with a predisposition to hypertension, whereas in normotensives with a predisposition and a previous record of high blood pressure there could be also a different cutaneous microvascular reactivity of central (nonvascular) origin.

Effect of xenon on autonomic cardiovascular control—comparison with isoflurane and nitrous oxide

Study Objectives: To clarify the effect of xenon on the autonomic nervous system by comparing similar effects of isoflurane and nitrous oxide. Design: Prospective, randomized study. Setting: Operating room at a university hospital. Patients: 39 ASA physical status I and II patients scheduled for general anesthesia. Interventions: Patients were randomly allocated into one of three groups and received one of the following inhalational anesthetics: 56% of xenon (Group X), 0.94% of isoflurane (Group I), or 70% of nitrous oxide and 0.15% of isoflurane (Group N). Phenylephrine (pressor test) and nicardipine (depressor test) were given to assess baroreflex sensitivity. Measurements and Main Results: Continuous blood pressure (BP) and electrocardiogram (ECG) were recorded before and during anesthesia to analyze heart rate (HR) variability and baroreflex sensitivity. Power spectrum of HR variability was calculated by fast Fourier transformation and power spectrum densities at low frequency (LF: 0.04–0.15Hz) and high frequency (HF: 0.15–0.40 Hz) were compared. Baroreflex sensitivity was calculated from the slope of regression for BP changes versus associated changes in R-R intervals. For HR variability, Group X showed lower power spectrum densities (ms 2.Hz −1) in LF and HF than did Group I (LF: 0.09 ± 0.06 vs. 0.35 ± 0.53; p < 0.05; HF: 0.40 ± 0.34 vs. 0.98 ± 0.68, p < 0.01). Group X had the lowest baroreflex sensitivity (ms.mmHg −1) via pressor test of the three study groups (Group X: 2.00 ± 0.87, Group I: 3.53 ± 2.14, Group N: 3.78 ± 2.17, p < 0.05). Conclusions: Xenon depressed both sympathetic and parasympathetic transmission more than isoflurane at 0.8 MAC. Xenon was also suggested to be relatively vagotonic.

Effects of esophageal stimulation in patients with functional disorders of the gastrointestinal tract.

We studied the effects of esophageal electrical stimulation on cortical-evoked potentials (EPs) and power spectrum of heart rate variability (PS/HRV) in patients with diabetes and non-cardiac chest pain (NCCP). We also recorded cognitive-evoked potentials (P300 EPs) in response to an odd-ball stimulation in patients with NCCP. Diabetic patients did not yield reproducible cortical EPs. Their power spectra of heart rate variability (PS/HRV) showed an increased vagal modulation during stimulation. In patients with NCCP the P300 EPs were of greater amplitude (17 +/- 3 microV vs. 12 +/- 1 microV in controls, p < 0.04), while peak latencies were slightly elongated in patients (382 +/- 22 ms vs. 354 +/- 12 ms in controls). The PS/HRV in these patients also showed an increased vagal modulation of the sinus node activity. Our results suggest the following: (1) in patients with diabetes, afferent pathways and processing of sensory signals are likely to be impaired; (2) an increased perception of esophageal stimulation reflects an exaggerated brainstem response and altered cortical processing of visceral sensation in patients with NCCP.

Effects of esophageal stimulation in healthy subjects.

We studied the effects of esophageal electrical stimulation on heart rate variability power spectra (PS/HRV) and cortical evoked potentials (EPs) in healthy subjects. The intensity of stimulation was varied from 2.7 to 20 mA. We found that the amplitude of the cortical evoked potentials (amplitude of the N2/P2 peak) increased from 5.1 +/- 0.7 microV at 5 mA to 16.3 +/- 1.1 microV at 20 mA. The PS/HRV showed an increase in the vagal modulation of the sinus node. When the stimulation frequency was varied from 0.1 to 1 Hz at a constant intensity of 15 mA, the amplitude of cortical EPs (N2/P2 peak) decreased with increase in the frequency of stimulation (p < 0.05). The LF:HF ratio decreased significantly for all frequencies of stimulation (p < 0.005). An experimental paradigm to evoke the cognitive component in the cortical EPs yielded a peak around 354 ms following the stimulus.

Effect of constant and intermittent vagal stimulation on the heart rate and heart rate variability in rabbits.

To examine whether the high-frequency (HF) component of heart rate variability (HRV) reflects fluctuation or tonic level of vagal outflow, we investigated the effects of vagal efferent nerve stimulation (VS) on the heart rate and HRV in anesthetized open-chest rabbit under artificial respiration at a rate of 52 breaths/min (0.86 Hz). A power spectral analysis was performed at baseline and during VS (stimuli at 2 ms, 1-10 V and 5-25 Hz). VS was applied using two different patterns. The first was constant VS; continuous stimulation at graded frequency or voltage to simulate changes in the level of vagal "tone." The second pattern was intermittent VS; stimulation at 0.5 Hz of on-off cycle to simulate fluctuations in vagal efferent activity. The power spectrum at baseline showed a single narrow component at 0.86 Hz, identical to respiration rate. Both the constant and intermittent VS prolonged RR interval. The amplitude of the component at 0.86 Hz remained unaffected by either the constant or intermittent VS, whereas the latter evoked a distinct narrow component at 0.5 Hz, reflecting the on-off cycle of intermittent VS. Our results suggest that the HF component of the power spectrum of HRV measures the magnitude of fluctuations of vagal input associated with respiratory modulation.

Characterization of autonomic dysfunction in patients with irritable bowel syndrome by means of heart rate variability studies.

Our aim was to characterize autonomic dysfunction in patients with irritable bowel syndrome (IBS) using heart rate variability (HRV) studies. EKG signals were obtained from 35 patients (mean age, 39.1 +/- 9.5 yr, M:F ratio = 2.9:1) and 18 healthy controls (mean age, 38.2 +/- 6.5 yr, M:F ratio = 2:1) in supine, standing, and deep-breathing modes. Fast Fourier transformation and autoregressive techniques were used to analyze the HRV power spectra in very low (VLF, 0.0078-0.04 Hz), low (LF, 0.04-0.14 Hz), and high (HF, 0.14-0.4 Hz) frequency bands. In the supine position, the VLF power spectral density (PSD) in IBS was significantly higher than normal (3 vs 1.3 beats per minute [bpm]2/Hz, p < 0.01). On changing from the supine to standing position, the normals (NC) had raised median PSDs in the VLF (1.3 vs 12.8 bpm2/Hz, p < 0.01) and LF (1.6 vs 6.1 bpm2/Hz, p < 0.01) bands, as a sign of increased sympathetic tone, whereas the median HF PSDs (parasympathetic tone) remained unchanged (1.8 bpm2/Hz each, p = 0.8). Similarly, the IBS patients had increased VLF (3.04 vs 14.93 bpm2/Hz, p < 0.01) and LF (2.8 vs 8.7 bpm2/Hz, p < 0.01) PSDs on standing up, but the HF PSD was also raised (from 2.4 to 5.7 bpm2/Hz, p = 0.04). On changing from standing to the deep-breathing mode, the normals had a significant increase in the HF (from 1.8 to 10.3 bpm2/Hz, p < 0.001) and a significant reduction of the VLF (from 12.8 to 2.2 bpm2/Hz, p < 0.01) PSDs. The reduction of the LF PSD was not significant (from 6.1 to 5.6 bpm2/Hz, p = 0.6). In IBS, HF PSD remained constant (5.7 bpm2/Hz each, p = 0.6), whereas the LF PSD increased from 8.7 to 24.2 bpm2/Hz (p < 0.0001). The VLF PSD was reduced (from 14.9 to 4.1 bpm2/Hz, p < 0.0001). In IBS, the median sympathovagal outflow ratio was significantly lower in the standing position (1.4 vs 2.8, p < 0.02) and higher in the deep-breathing mode (7.33 vs 0.42, p < 0.0001) than normal. IBS patients have reduced sympathetic influence on the heart period in response to orthostatic stress and diminished parasympathetic modulation during deep breathing.

Characterization of autonomic dysfunction in patients with irritable bowel syndrome by means of heart rate variability studies

OBJECTIVE: Our aim was to characterize autonomic dysfunction in patients with irritable bowel syndrome (IBS) using heart rate variability (HRV) studies. METHODS: EKG signals were obtained from 35 patients (mean age, 39.1 ± 9.5 yr, M:F ratio = 2.9:1) and 18 healthy controls (mean age, 38.2 ± 6.5 yr, M:F ratio = 2:1) in supine, standing, and deep-breathing modes. Fast Fourier transformation and autoregressive techniques were used to analyze the HRV power spectra in very low (VLF, 0.0078–0.04 Hz), low (LF, 0.04–0.14 Hz), and high (HF, 0.14–0.4 Hz) frequency bands. RESULTS: In the supine position, the VLF power spectral density (PSD) in IBS was significantly higher than normal (3 vs 1.3 beats per minute [bpm] 2/Hz, p < 0.01). On changing from the supine to standing position, the normals (NC) had raised median PSDs in the VLF (1.3 vs 12.8 bpm 2/Hz, p < 0.01) and LF (1.6 vs 6.1 bpm 2/Hz, p < 0.01) bands, as a sign of increased sympathetic tone, whereas the median HF PSDs (parasympathetic tone) remained unchanged (1.8 bpm 2/Hz each, p = 0.8). Similarly, the IBS patients had increased VLF (3.04 vs 14.93 bpm 2/Hz, p < 0.01) and LF (2.8 vs 8.7 bpm 2/Hz, p < 0.01) PSDs on standing up, but the HF PSD was also raised (from 2.4 to 5.7 bpm 2/Hz, p = 0.04). On changing from standing to the deep-breathing mode, the normals had a significant increase in the HF (from 1.8 to 10.3 bpm 2/Hz, p < 0.001) and a significant reduction of the VLF (from 12.8 to 2.2 bpm 2/Hz, p < 0.01) PSDs. The reduction of the LF PSD was not significant (from 6.1 to 5.6 bpm 2/Hz, p = 0.6). In IBS, HF PSD remained constant (5.7 bpm 2/Hz each, p = 0.6), whereas the LF PSD increased from 8.7 to 24.2 bpm 2/Hz ( p < 0.0001). The VLF PSD was reduced (from 14.9 to 4.1 bpm 2/Hz, p < 0.0001). In IBS, the median sympathovagal outflow ratio was significantly lower in the standing position (1.4 vs 2.8, p < 0.02) and higher in the deep-breathing mode (7.33 vs 0.42, p < 0.0001) than normal. CONCLUSIONS: IBS patients have reduced sympathetic influence on the heart period in response to orthostatic stress and diminished parasympathetic modulation during deep breathing.

Dynamics of spectral components of heart rate variability during changes in autonomic balance.

Frequency domain analysis of heart rate variability (HRV) has been proposed as a semiquantitative method for assessing activities in the autonomic nervous system. We examined whether absolute powers, normalized powers, and the low frequency-to-high frequency ratio (LF/HF) derived from the HRV power spectrum could detect shifts in autonomic balance in a setting with low sympathetic nervous tone. Healthy subjects were examined for 3 h in the supine position during 1) control conditions (n = 12), 2) acute beta-blockade (n = 11), and 3) chronic beta-blockade (n = 10). Heart rate fell during the first 40 min of the control session (72 +/- 2 to 64 +/- 2 beats/min; P < 0. 005) and was even lower during acute and chronic beta-blockade (56 +/- 2 beats/min; P < 0.005). The powers of all spectral areas rose during the first 60 min in all three settings, more so with beta-blockade (P < 0.05). LF/HF was found to contain the same information as powers expressed in normalized units. LF/HF detected the shift in autonomic balance induced by beta-blockade but not the change induced by supine position. In conclusion, none of the investigated measures derived from power spectral analysis comprehensively and consistently described the changes in autonomic balance.

System response of the sinoatrial node during vagal stimulation

The SA node response to modulations in canine vagal tone was investigated by means of the heart rate variability power spectrum. A new algorithm that was developed for accurate power spectrum estimation of short R-R segments is described. The performance of the algorithm was assessed for ECG recordings obtained from a controlled experiment, in which a frequency modulated pulse train was applied to the vagal nerve after vagal transaction and blockade of the sympathetic system. The power spectrum calculated for 20-25 heartbeats showed conspicuous spectral peaks in accordance with the different modulating frequencies between 0.1 and 0.3 Hz. The presence of spurious peaks was negligible even when the analysed signal segment consisted of only 20-25 beats. These results imply that for a certain range of modulating frequencies the sinoatrial node responds linearly to fluctuations in the parasympathetic tone. System identification methods that include fitting a linear model to the heart rate variability signal and analysis of the residuals were used for confirming the hypothesis of linearity. For higher frequencies of the modulating signal, usually above 0.3 Hz, the system was found to deviate from linearity. Keywords: heart rate signal, power spectrum, point process

An estimate of fetal autonomic state by spectral analysis of human umbilical artery flow velocity waveforms.

Determination of gestational age-related fluctuations in heart rate in the umbilical artery of the early human fetus. Doppler velocity recordings from human umbilical artery were obtained, in a cross-sectional study design in 137 singleton pregnancies at 10-20 weeks of gestation. After exclusion criteria were applied, data on 117 normal pregnancies were available and subdivided into group I: 10-12 weeks (n = 49); group II: 13-16 weeks (n = 43); and group III: 17-20 weeks (n = 25). Blood flow velocity waveforms were reconstructed from Doppler audio signals. Variability in heart rate was calculated using Fast Fourier Transforms (FFT). Individual heart rate variability power spectra were subdivided into frequency bands. Fetal heart rate variability decreases at 10-20 weeks and demonstrates a shift to lower frequencies at 17-20 weeks. Fetal heart rate variability is related to gestational age and shows a shift to lower frequencies which may reflect autonomic functional development.

HYPOVOLEMIA CAUSES DIFFERENT EFFECTS ON HEART RATE AND BLOOD PRESSURE VARIABILITIES IN DOGS

S134 Spectral analysis of heart rate variability (HRV) and blood pressure variability (BPV) resulted in a characteristic power spectrum with two main regions: a low frequency peak (<0.15Hz) and a high frequency peak centered at the respiratory frequency. The shape of HRV and BPV power spectra may be markedly altered by hypovolemia, since hypovolemia modifies autonomic nervous control of the cardiovascular system. The aim of the present study was to evaluate the effects of hypovolemia on HRV and BPV simultaneously in dogs. METHODS: Seven dogs weighing 13-16kg were anesthetized with thiamylal(10mg/kg), maintained with alpha-chloralose(15mg/kg/hr) and vecuronium(0.1mg/kg/hr) and mechanically ventilated with room air at 14 breaths/min. Catheters were placed in the femoral artery for measurement of arterial blood pressure and in the left femoral artery for removing blood. Splenectomy was done. We induced hypovolemia by removing blood in graded stages (0%, 10%, 20%, 30%, 40% of the estimated blood volume; EBV). After blood pressure was stabilized in each stage, heart rate and mean blood pressure were continuously recorded for 10 min. The areas of the spectral peaks within each measurement were calculated as follows: LF (0.04-0.15 Hz) and HF (0.15-0.4 Hz). The data were analyzed using repeated measurement of ANOVA followed by Fisher PLSD test. P < 0.05 was considered statistically significant. RESULTS: HR did not change during hypovolemia while mean BP decreased significantly after 30% of EBV hemorrhage. LF of HRV significantly increased after 20% of EBV hemorrhage, but HF of HRV did not change. Both LF and HF of BPV increased significantly in proportion to cumulative withdrawal of the blood. DISCUSSION: LF of HRV and BPV during hypovolemia increased and HF of HRV did not changed, indicating the shift of the autonomic balance towards sympathetic dominant. An excellent quantitative correlation between the HF and LF of BPV and the degree of hypovolemia was shown during graded hemorrhage. Even small degree of hypovolemia was detected by changes in LFs of HRV and BPV and HF of BPV. HF of HRV was not useful to detect hypovolemia. Our study suggests that the spectral analysis of HRV and BPV during graded hemorrhage shows different characteristics in the quantitative evaluations of autonomic nervous system during hypovolemia. (Table 1)Table 1: Power spectrum of HRV and BPV during graded hemorrhage

Alterations in spectral characteristics of heart rate variability as a correlate of cardiac autonomic dysfunction after esophagectomy or pulmonary resection.

Both esophagectomy and pulmonary resection are associated with postoperative cardiac complications, partly because of autonomic perturbations involving the heart. This study was undertaken to determine whether heart rate variability (HRV), employed as an index of cardiac autonomic function, changes in patients undergoing esophagectomy or pulmonary resection. Electrocardiographic RR intervals were measured in 20 esophagectomized patients, 10 undergoing right and 10 undergoing left pulmonary resection on the preoperative day as baseline data and on postoperative days 1, 3, 5, 7, 14, and 30. Instantaneous heart rate was calculated every 250 ms from 416-s data of RR intervals. Power spectra of HRV for 128 s were computed using a fast Fourier transform and normalized by squared mean heart rate. The average ten sets of normalized HRV power were obtained by integrating the following power spectral bands: the low-, (0.06-0.10 Hz), high- (0.15-0.40 Hz), and total-frequency regions (0.01-0.40 Hz). In the esophagectomy group, mean low-, high-, and total-frequency HRV power decreased after surgery to 17%, 6%, and 15% of their preoperative values, respectively, and these indexes remained suppressed for up to 30 days. After right pulmonary resection, low- and total-frequency HRV power decreased through 30 and 7 postoperative days, respectively. In the left pulmonary resection group, HRV remained unchanged. In the esophagectomy group, mean (+/- SEM) heart rate increased from 78 (+/- 3) bpm to more than 90 bpm throughout the study, and body temperature from 36.5 (+/- 0.1) degrees C to more than 37.0 degrees C through 14 postoperative days. Heart rate and body temperature remained increased for 3 days after pulmonary surgery. Mean arterial pressure remained unchanged in the three surgical groups. Reductions indicate HRV after esophagectomy or right pulmonary resection indicate a substantial and prolonged surgical injury to the autonomic nervous control of pulse rate.

Power spectral analysis of heart-rate variability reflects the level of cardiac autonomic activity in rabbits

Power spectral analysis of heart rate (HR[ variability was tested in conscious rabbits to assess the reliability of this method for assessing cardiac autonomic function in normal rabbits under resting conditions. Evaluation of power spectrum was performed in 5 rabbits under normal resting conditions and after sympathetic, parasympathetic and combined sympathetic plus parasympathetic blockade. Rabbits were randomly assigned to undergo sympathetic (propranolol[ or parasympathetic (methscopolamine[ blockade at the initial step followed by combined blockade. The power spectrum of heart-rate variability in rabbits was presented as one broad spectral component at frequencies mainly between 0 and 0.5 Hz. This component was considerably modulated by both sympathetic and parasympathetic influences with substantial overlap of sympathetic- and parasympathetic-related components of the spectrogram. Nevertheless, it was clearly shown that power of heart-rate variability at frequencies from 0.4373 Hz to 0.5625 Hz was determined only by parasympathetic influences, and sympathetic modulation of HR was presented mainly at frequencies from 0.0625 Hz to 0.1875 Hz. Spectral subcomponent analysis of the power spectrum of HR variability may be useful to follow changes in cardiac autonomic function in rabbits.

The power spectral analysis of heart rate variability in athletes during dynamic exercise—Part II

In this study, the effects of long-term physical training on autonomic function in athletes and the response of the autonomic nervous system to dynamic exercise were investigated in nonathletes and athletes with power spectral analysis of heart rate variability (HRV). This study was performed on 13 healthy subjects (5 athletes and 8 non athletes). Electrocardiographic (ECG) signals were continuously recorded during (1) 15 min of rest in a sitting position on a bicycle ergometer, (2) the dynamic exercise test to the point of exhaustion, and (3) a 15 min postexercise period. After the recorded ECG signals were sampled at 500 samples/s, the instantaneous HRV signal was constructed from the detected R peaks and then resampled at 4 Hz in order to obtain an evenly spaced time series applicable to power spectral analysis. After linear trends were removed by the robust locally weighted regression algorithm, the power spectrum of HRV was estimated for contiguous records of 512 samples by Burg's maximum entrophy method. HRV was quantified by determining the spectral area (power) in two frequency bands, low-frequency power (LF power: 0.05-0.15 Hz) and high-frequency power (HF power: 0.15-0.8 Hz), and their ratio. The comparison between athletes and nonathlete was performed in terms of the above-mentioned parameters. Although both groups showed similar trends in heart rate (HR) at all stages of protocols, HR in athletes was significantly lower than that in nonathletes during rest and postexercise. In athletes and nonathletes, LF and HF powers gradually decreased with exercise. As recovery progressed, they continued to increase gradually, but remained below resting level. During rest and postexercise, HF power in athletes was significantly (p < 0.05) higher than than in nonathletes. Also, the recovery of HR and HF powers during early recovery (PO1) was more rapid in athletes than in nonathletes. Both groups showed an attenuation of LF and HF powers during dynamic exercise. It is likely that, in athletes, the lower HR during rest and the more rapid recovery of HR postexercise was due to a high level of HF power, indicating that vagal activity was enhanced by the adaptive changes in neural regulation produced by long-term physical training.

The power spectral analysis of heart rate variability in athletes during dynamic exercise—Part I

In this study, the effects of long-term physical training on autonomic function in athletes and the response of the autonomic nervous system to dynamic exercise were investigated in nonathletes and athletes with power spectral analysis of heart rate variability (HRV). This study was performed on 13 healthy subjects (5 athletes and 8 non athletes). Electrocardiographic (ECG) signals were continuously recorded during (1) 15 min of rest in a sitting position on a bicycle ergometer, (2) the dynamic exercise test to the point of exhaustion, and (3) a 15 min postexercise period. After the recorded ECG signals were sampled at 500 samples/s, the instantaneous HRV signal was constructed from the detected R peaks and then resampled at 4 Hz in order to obtain an evenly spaced time series applicable to power spectral analysis. After linear trends were removed by the robust locally weighted regression algorithm, the power spectrum of HRV was estimated for contiguous records of 512 samples by Burg's maximum entrophy method. HRV was quantified by determining the spectral area (power) in two frequency bands, low-frequency power (LF power: 0.05-0.15 Hz) and high-frequency power (HF power: 0.15-0.8 Hz), and their ratio. The comparison between athletes and nonathlete was performed in terms of the above-mentioned parameters. Although both groups showed similar trends in heart rate (HR) at all stages of protocols, HR in athletes was significantly lower than that in nonathletes during rest and postexercise. In athletes and nonathletes, LF and HF powers gradually decreased with exercise. As recovery progressed, they continued to increase gradually, but remained below resting level. During rest and postexercise, HF power in athletes was significantly (p < 0.05) higher than than in nonathletes. Also, the recovery of HR and HF powers during early recovery (PO1) was more rapid in athletes than in nonathletes. Both groups showed an attenuation of LF and HF powers during dynamic exercise. It is likely that, in athletes, the lower HR during rest and the more rapid recovery of HR postexercise was due to a high level of HF power, indicating that vagal activity was enhanced by the adaptive changes in neural regulation produced by long-term physical training.

Spectral analysis of heart rate variability during quiet sleep in normal human fetuses between 36 and 40 weeks of gestation

Respiratory sinus arrhythmia (RSA) is a clinical manifestation of the parasympathetic nervous system which can be identified in the high-frequency region of the heart rate variability (HRV) power spectrum. The purpose of this study was to determine the relative contribution of RSA to overall HRV for human fetuses in quiet sleep. The study population consisted of 13 normal human fetuses between 36 and 40 weeks of gestation for whom data were collected during spontaneous breathing and normally occurring apneic periods. Fetal breathing was monitored continuously using real-time sonography. The fetal electrocardiogram was captured transabdominally in 3-min blocks at a rate of 833 Hz and fetal R-waves were extracted from the raw signal using adaptive signal processing techniques. Fetal behavioral state was determined at the beginning and end of each 3-min data collection period. The fetal R-wave interbeat intervals (IBIs) were converted to equally-spaced, time-based data, and linear detrending of the time series was accomplished by subtracting the mean heart period from each weighted IBI. Total power (TP, 0.0–2.5 Hz) was divided into RSA (0.4–1.0 Hz), high-frequency (HF, 0.2–2.5 Hz), low-frequency (LF, 0.04–0.2 Hz), and very-low-frequency (VLF 0.0–0.04 Hz) regions, and the power densities were summed to determine the absolute power for each frequency component. A total of 81 3 min blocks (mean per subject 6.3, range 2–14) were available for analysis. Eleven (85%) of 13 fetuses demonstrated at HF peak during fetal breathing, and RSA accounted for approximately 20% of the TP. Although HF amplitude was reduced in the absence of fetal respiratory activity, RSA did not invariably disappear during periods of apnea, as the HF peak was recognized in seven (54%) fetuses when no respiratory activity was observed. Total power was approximately evenly distributed between the VLF, LF, and HF regions, and this distribution was independent of whether the fetus was breathing or not. However, the absolute power was significantly greater ( P < 0.05) during breathing as compared to non-breathing for TP and for the VLF, LF, HF, and RSA components. We conclude that RSA is common in normal human fetuses and is a significant factor contributing to HRV.

POWER SPECTRAL ANALYSIS OF HEART RATE VARIABILITY AFTER SPINAL ANAESTHESIA

Power spectral analysis of heart rate (HR) variability is a useful tool with which to assess cardiac autonomic activity. The low frequency bands have been considered as both sympathetically and parasympathetically mediated components, while the high frequency bands have been assumed to be the parasympathetically mediated respiratory components. It has been anticipated that spinal anaesthesia to the thoracic level may modulate cardiac autonomic activity to reduce HR and arterial pressure by blocking cardiac sympathetic activity. In order to quantify the alterations in cardiac autonomic activity, we have analysed the power spectra of HR variability for 30 min after subarachnoid administration of hyperbaric amethocaine. Using 256-s R-R interval data obtained from continuously recorded ECG, low frequency (Lo: 0.04-0.15 Hz) and high frequency (Hi: 0.15-0.40 Hz) band widths were integrated and their serial alterations were computed by shifting subjected R-R intervals at 60-s intervals. After the subarachnoid injection, arterial pressure, HR and Lo decreased and Hi and the Hi:Lo ratio increased. These changes were observed within 15-20 min. Ventilatory frequency did not change throughout the study. These findings suggest that the decrease in HR and arterial pressure after subarachnoid administration of hyperbaric amethocaine reflect decreased sympathetic activity and increased parasympathetic activity in the cardiac autonomic nervous system.

Time-variant power spectrum analysis for the detection of transient episodes in HRV signal.

A time-variant algorithm of autoregressive (AR) identification is introduced and applied to the heart rate variability (HRV) signal. The power spectrum is calculated from the AR coefficients derived from each single RR interval considered. Time-variant AR coefficients are determined through adaptive parametric identification with a forgetting factor which obtains weighed values on a running temporal window of 50 preceding measurements. Power spectrum density (PSD) is hence obtained at each cardiac cycle, making it possible to follow the dynamics of the spectral parameters on a beat-by-beat basis. These parameters are mainly the LF (low frequency) and the HF (high frequency) powers, and their ratio LF/HF. These together account for the balanced sympatho-vagal control mechanism affecting the heart rate. This method is applied to subjects suffering from transient ischemic attacks. The time variant spectral parameters suggest an early activation of LF component in the HRV power spectrum. It precedes by approximately 1.5-2 min the tachycardia and the ST displacement, generally indicative of the onset of an ischemic episode. The results suggest an arousal of sympathetic system before the acute attack.