近似熵及其在心率变异分析中的应用

目的：介绍近似熵;并将之应用于心率变异分析,分析立位测试过程中心血管系统复杂性的变化。方法：对８名被试者进行立位耐力测试,记录心电信号并用近似熵进行心率变异分析。结果：与立位前平卧位相比,立位０￣５ｍｉｎ、５￣１０ｍｉｎ、１０￣１５ｍｉｎ和１５￣２０ｍｉｎ心脏Ｒ－Ｒ间期显著降低,而立位０￣５ｍｉｎ近似熵显著低于平卧位和立位１５￣２０ｍｉｎ,立位１５￣２０ｍｉｎ近似熵显著低于平卧位,说明立位测试过程中心血管系统复杂性降低,心血管系统的调节模式有改变。结论：近似熵应用于心率变异的分析是可行的。     

Some circulatory responses to exercise at different times of day

Circadian rhythms in heart rate were examined at rest, immediately pre-exercise, during submaximal and maximal exercise on a cycle ergometer, and during recovery post-exercise (N = 10). Observations were made under controlled conditions at 0300, 0900, 1500, and 2100 hours. A significant circadian rhythm was found for resting heart rate lying supine and sitting pre-exercise (P less than 0.05), peak values being measured at 1500 hours. The acrophase in the oral temperature rhythm at 1739 hours was not significantly out of phase with that of resting heart rate (P greater than 0.05). The rhythm in heart rate persisted during submaximal exercise (150 W) and at the maximal rate (P less than 0.05); the amplitude of the rhythm was attenuated at maximum. Ratings of perceived exertion at submaximal and maximal exercise intensities, and time of day (P greater than 0.05). The increment of 0.2 degrees C in oral temperature during exercise did not exhibit circadian variation (P greater than 0.05). A significant rhythm was found for recovery heart rates in minutes 2, 3, 4, and 5 post-exercise (P less than 0.05). Observations of systolic and diastolic pressures pre- and post-exercise were inconclusive. Therefore, the circadian rhythm in heart rate responses to exercise should be considered when a heart rate variable is used as a criterion in fitness testing or as an index of physiological strain.     

复杂度在立位期间心率变异分析中的应用

目的 介绍非线性指标复杂度,并将之应用于心率变异分析。分析立位测试过程中心血管系统复杂性的变化。方法 对8名被试者进行立位耐力测试。记录心电信号并用复杂度进行心率变异分析。结果 与立位前平卧位相比,立位0-5min,5-10min,10-15min和15-20min心脏R-R间期,R-R间期标准差和相临R-R间期差值的标准差（RMSSD）显著降低。立位0-5min,15-20min复杂度和近似熵显著低于平卧位。结论 立位期间心率变异降低。心血管系统复杂性降低。复杂度应用于心率变异的分析是可行的。     

Heart rate variability: response following a single bout of interval training

We investigated the effect of exercise on heart rate variability by analysing the heart rate power spectrum prior to, and 1 and 72 h following, an interval training session. Subjects initially performed a graded test to exhaustion to determine maximal oxygen uptake (VO(2) max) and the running speed at which VO(2) max was first attained (vVO(2) max). The training session was completed on a separate day and comprised six 800 m runs at 1 km x h (-1) below vVO(2) max. Prior to the training session (pre), 1 h following the training session (+ 1h), and 72 h following the training session (+ 72 h), subjects sat quietly in the laboratory for 20 min whilst breathing frequency was maintained at 12 breath x min (-1). Cardiac cycle R-R interval data were collected over the final 5 min of each 20 min period and analysed by means of autoregressive power spectral analysis to determine the high frequency (HF) and low frequency (LF) components of heart rate variability. Heart rate was higher, and the standard deviation of the R-R intervals was lower, at + 1 h than for pre or + 72 h (p < 0.05). The HF and the LF components of heart rate variability were also lower (p < 0.05) for + 1 h than for pre or + 72 h when the data were expressed in ms(2). However, no changes in the LF:HF ratio were observed, and the changes in the HF and LF components disappeared when the data were expressed as a fraction of the total power. Whilst these findings illustrate the importance of controlling the timing of exercise prior to the determination of heart rate variability, the time course of the post-exercise heart rate variability response remains to be quantified.     

Spectral analysis of heart period variability in anorexia nervosa

The aim of this study was to investigate the response of autonomic cardiac control to postural change using spectral analysis, in patients with anorexia nervosa. Spectral components of total variability as well as of low and high frequencies were analyzed for 17 anorexic patients with mean body mass index (14.9 +/- 1.9) kg/m2 and for 9 healthy age-matched women with body mass index (20.3 +/- 1.7) kg/m2 , in supine and standing postures. During standing posture, increased heart rate in all subjects was accompanied by the decrease in total variability and high frequency spectral powers. In supine posture, anorexic patients demonstrated the reduced low frequency spectral power. Compared to control women, during standing posture anorexic patients showed higher heart rate, reduced total variability and high frequency spectral powers. Statistically significant correlation was noticed between body mass index and spectral power of low frequency in both supine and standing posture. Alterations in autonomic cardiac control induced by anorexia nervosa could be estimated by spectral analysis of heart period variability.     

非稳定周期轨道在立位心率变异分析中的应用

目的 研究立位心脏R－R间期信号的非稳定周期轨道的结构，进一步探讨心率变异（HRV）的动力学特征。方法 记录8名受试者平卧位5min和立位20min过程中的心电图，检测HRV信号的非稳定周期轨道。结果 立位时高周期数（周期2和周期3）的非稳定周期轨道出现率降低，HRV吸引子变得相对简单；非稳定周期1轨道位置随着体位和时间的改变而改变1。说明HRV的动力学特征有改变，心血管系统的调节功能有改变。结论：非稳定周期轨道可以刻划HRV的动力学性质，是分析HRV的潜在的方法。     

Heart rate variability after isocaloric exercise bouts of different intensities

PURPOSE: To examine cardiac autonomic modulation, via heart rate variability (HRV), after exercise bouts at 50% (LO) and 80% (HI) of VO2 reserve (VO2R). METHODS: Thirteen male volunteers (age: 25.7+/-3 yr) exercised on a treadmill at either LO or HI (randomly assigned) on separate days, expending 300 kcal during each bout. During each visit, electrocardiogram (ECG) recordings were obtained during a supine 5-min period before exercise (PRE) and during 30 min of postexercise supine recovery (POST). The last 25 min of POST was divided into five 5-min segments to closely examine postexercise HRV. ECG segments were analyzed for the standard deviation of RR intervals (SDNN), whereas total- (lnTP), low- (lnLF), and high-frequency (lnHF) power were obtained via spectral analysis. LF and HF were further normalized (LFNU and HFNU) to better quantify autonomic balance. RESULTS: There were main effects of time and intensity on SDNN such that it was lower than PRE until 15 min POST, and lower after HI. There were significant intensity x time interactions on lnTP, lnLF, lnHF, LFNU, HFNU, and lnLF/lnHF. Post hoc analysis revealed that lnTP and lnLF were lower than PRE up to 20 min POST after HI. lnHF was lower than PRE for 25 min POST for HI, and for 10 min POST for LO. LFNU and lnLF/lnHF were greater, whereas HFNU was lower, compared with PRE up to 15 min POST, and from 20-25 min POST for HI only. CONCLUSION: Exercise at 50% VO2R caused less of a shift in the cardiac autonomic balance, with a quicker restoration of vagal modulation than exercise at 80% VO2R.     

Ascending aortic blood flow dynamics following intense exercise

The purpose of this study was to compare and contrast aortic blood flow kinetics during recovery from intense aerobic (maximal oxygen uptake test) and anaerobic (Wingate anaerobic power test) exercise. Fifteen healthy male subjects (VO2max = 56.1 +/- 5.8 mk/kg/min) participated in this study. Beat-to-beat peak aortic blood flow velocity (pkV) and acceleration (pkA) measurements were obtained by placing a 3.0 MHz continuous-wave ultrasonic transducer on the suprasternal notch at rest and during recovery (immediately post-exercise, 2.5 min, and 5.0 min) following the two exercise conditions. Peak velocity and acceleration significantly increased (p less than 0.01) from rest to immediately post-exercise and remained elevated throughout the 5-min recovery period. No differences were observed between the aerobic and anaerobic tests. Stroke distance significantly declined (p less than 0.01) immediately following exercise and progressively rose during the 5-min recovery period. The results indicate that: 1) aortic blood flow kinetics remained elevated during short-term recovery, and 2) intense aerobic and anaerobic exercise exhibit similar post-exercise aortic blood flow kinetics.     

Contribution of autonomic dysfunction to abnormal exercise blood pressure in type 2 diabetes mellitus

ObjectivesThe purpose of this study was to compare the presence and severity of autonomic dysfunction in type 2 diabetes mellitus patients, with and without exaggerated blood pressure responses to exercise.DesignWe performed a cross-sectional analysis of 98 patients with type 2 diabetes mellitus (aged 59 ± 9).MethodsBoth time (standard deviation of RR intervals, root-mean-square of successive RR interval differences) and frequency (total spectral power, high frequency, low frequency, very low frequency) domains of heart rate variability were analysed in a 5 min recording at rest and 20 min after a maximal treadmill test. An exaggerated blood pressure response to exercise was identified by peak blood pressure ≥190/105 mm Hg (women) or ≥210/105 mm Hg (men).ResultsEach group of either exaggerated exercise blood pressure response or normal blood pressure response consisted of 49 patients. At rest there were no significant differences between groups for all time and frequency domain parameters of heart rate variability. Post-exercise, there was a significant (p < 0.05) reduction in the SDNN, RMSSD and TP in the exaggerated exercise blood pressure group. Independent correlates (p < 0.01) of exercise systolic blood pressure included post-exercise TP, resting systolic blood pressure, cardiac autonomic neuropathy and beta-blockers (beta = ?0.28, adj. R² = 0.32, p < 0.001).ConclusionsReduced post-exercise heart rate variability in patients with type 2 diabetes mellitus, with an exaggerated exercise blood pressure response suggests preclinical autonomic dysfunction characterized by impaired vagal modulation.     

Aerobic training and cardiovascular responses at rest and during exercise in older men and women

PURPOSE: The effects of an intense 8-wk aerobic training program on cardiovascular responses at rest and during exercise, including heart rate variability (HRV) as an expression of autonomic modulation, were evaluated in subjects over 70 yr (mean: 73.9 +/- 3.5 yr). METHODS: Before and after training in 7 men and 8 women: a) heart rate (HR), blood pressures (BPs), pulse pressure (PP), and oxygen uptake were measured at rest, during, and after exhausting incremental exercise; b) HRV power spectra were calculated at rest in supine and sitting, and during and after two submaximal constant loads (5 min). Power in low-frequency (LF, 0.04-0.15 Hz) and high-frequency (HF, >0.15 Hz) bands were expressed as a percent of total power minus power < 0.04 Hz. RESULTS: After training: a) at rest HR and HRV parameters (in both body positions) were unchanged, whereas BPs decreased; b) peak cycle resistance and oxygen consumption increased by 25% and 18%, respectively, but no change in maximal HR and BPs were found; c) during submaximal loads HR was unchanged at the same metabolic demand, whereas SBP and DBP were lower than before at low loads whereas PP was unchanged. LF power decreased and HF increased at oxygen uptakes above about 0.7 L.min-1 similarly before and after training; and d) recovery of all parameters was similar to pretraining and complete after 10 min CONCLUSIONS: The increase in exercise capacity without changes in cardiovascular parameters suggests that 8 wk of aerobic training augmented peripheral gas exchange but not delivery to muscle. The lack of effect on HRV indicates that the improvements in aerobic power and cardiac autonomic modulation, at least in subjects over 70 yr, are dissociated. Moreover, the metabolic demand seems to be the main factor for the changes in HRV power spectra that occur during exercise.     

Heart rate variability during high-intensity field exercise in female distance runners

The purposes of this study were to demonstrate the transition of heart rate variability (HRV) during trials in the field and to examine the relationship between peak frequency of high-frequency band (HF) and stride frequency. Ten healthy long-distance college female runners (age 19-21 years) performed a 3000 m realistic time trial. The time-series power spectrum analysis by maximum entropy method was used to evaluate cardiac autonomic nervous activity during the race. Cross-correlation coefficients were calculated to estimate the degree of linear co-ordination between the central peak frequency of HF and stride frequency. Just after starting, the decrease in HF (0.15-1.00 Hz) and a transient increase of low-frequency band (LF)/HF were found. After that, the HF remained at a low level and LF/HF decreased sharply. These findings suggested that the parasympathetic activity was suppressed and sympathetic activity increased just after starting, and the sympathetic activity reached the saturated level according to continuation of high-intensity exercise. In spite of the significant decrease of HRV during trials, peak frequency of HF could be differentiated clearly. The cross-correlation coefficient of peak frequency of HF and stride frequency was from 0.703 to 0.868. This finding indicated that exercise rhythm reflected HRV during high-intensity running in the field.     

Interaction of cardiac and muscle mechanical afferents on baroreflex control of the sinus node during dynamic exercise

The effects of cardiopulmonary baroreceptors and muscle mechanoreceptors stimulation on cardiac baroreflex sensitivity (BRS), and heart rate variability (HRV) were evaluated by measuring continuously and non‐invasively systolic blood pressure (SBP) and pulse interval (PI) during upright and supine passive cycling. BRS and HRV were evaluated with the cross‐correlation method (xBRS) and in the frequency domain, respectively. At rest, the shift from upright to supine posture enhanced xBRS from 16.4±12.1 to 23.4±12.9 ms/mmHg, and the high frequency (HF, 0.15–0.4 Hz) power of HRV from 48.9±18.6 to 55.1±14.7 normalized units (NU), while it attenuated the low‐frequency (LF, 0.04–0.15 Hz) power from 51.1±18.6 to 44.9±14.7 NU (P<0.05), respectively. During both upright and supine passive exercise, xBRS and the HF power were attenuated (10.0±8.0 and 12.5±9.0 ms/mmHg; 41.1±21.2 and 41.5±12.7 NU, respectively; P<0.05) and the LF power increased (58.8±21.2 and 58.5±12.7 NU, P<0.05), compared with rest. The effect of mechanoreflex activation overrides that of the cardiopulmonary baroreceptors loading resulting in decreased cardiac vagal outflow and reduced BRS during supine passive exercise.     

Heart rate and blood pressure variability during heavy training and overtraining in the female athlete

We investigated heavy training- and overtraining-induced changes in heart rate and blood pressure variability during supine rest and in response to head-up tilt in female endurance athletes. Nine young female experimental athletes (ETG) increased their training volume at the intensity of 70-90% of maximal oxygen uptake (VO2max) by 125% and training volume at the intensity of < 70% of VO2max by 100% during 6-9 weeks. The corresponding increases in 6 female control athletes were 5% and 10%. The VO2max of the ETG and the control athletes did not change, but it decreased from 53.0 +/- 2.2 ml x kg(-1) x min(-1) to 50.2 +/- 2.3 ml x kg(-1) x min(-1) (mean+/-SEM, p < 0.01) in five overtrained experimental athletes. In the ETG, low-frequency power of R-R interval (RRI) variability during supine rest increased from 6 +/- 1 ms2 x 10(2) to 9 +/- 2 ms2 x 10(2) (p < 0.05). The 30/15 index (= RRI(max 30)/RRI(min 15), where RRI(max 30) denotes the longest RRI close to the 30th RRI and RRI(min 15) denotes the shortest RRI close to the 15th RRI after assuming upright position in the head-up tilt test), decreased as a result of training (analysis of variance, p = 0.05). In the ETG, changes in VO2max were related to the changes in total power of RRI variability during standing (r = 0.74, p < 0.05). Heart rate response to prolonged standing after head-up tilt was either accentuated or attenuated in the overtrained athletes as compared to the normal training state. We conclude that heavy training could increase cardiac sympathetic modulation during supine rest and attenuated biphasic baroreflex-mediated response appearing just after shifting to an upright position. Heavy-training-/overtraining-induced decrease in maximal aerobic power was related to decreased heart rate variability during standing. Physiological responses to overtraining were individual.     

Effect of aerobic training on orthostatic tolerance, circulatory response, and heart rate dynamics.

BACKGROUND: This study was performed to investigate the effects of aerobic training on orthostatic tolerance and to quantify the post-training changes in cardiovascular response and heart rate variability (HRV). METHODS: Tolerance and circulatory responses to two types of lower body negative pressure (LBNP) were examined and compared in a group of healthy male students before and after 6 mo of aerobic training, and the results were further compared with a group of athletes (runners). Changes in HRV associated with training were analyzed by conventional and time-varying autoregressive spectral analysis, as well as by approximate entropy measurement (ApEn)--a statistic quantifying heart rate "complexity" derived from non-linear dynamics. RESULTS: After aerobic training, there was an initial transient hypotension during the supine -50 mmHg LBNP testing and a significant decrease in tolerance to upright graded LBNP in most of the student-subjects. Moreover, after training, there was a significant decrease in ApEn value of the HRV time series during both supine control and LBNP testing, and the rate of cardiac vagal withdrawal and sympathetic activation during the onset of LBNP was faster than that before training. CONCLUSIONS: The present study has provided further evidence that certain types of aerobic training may affect orthostatic tolerance and may be associated with a loss of complexity of HRV during supine resting and orthostatic stress.     

Reliability of heart rate variability measures at rest and during light exercise in children

OBJECTIVES: To investigate the reliability of heart rate variability (HRV) measures at rest and during light exercise in children. METHODS: Short term (five minute) HRV was assessed in 12 children (11-12 years of age). HRV measures were collected at rest with the children supine, breathing at 12 breaths/min, and during exercise on a cycle ergometer while exercising at 25% of peak oxygen uptake. Both resting and exercise data were collected twice from each child. RESULTS: Intraclass correlation coefficients were low to moderate for most measures with wide confidence intervals for each variable in both resting and exercise conditions. Random variation (typical error) within repeated measurements ranged from 31% to 187%. CONCLUSIONS: These preliminary findings suggest that HRV measures are unreliable at rest and during light exercise in children aged 11-12 years. Tighter control of extraneous influences is recommended.     

Post-exercise palpation of pulse rates: its applicability to habitual exercisers

Despite the increased popularity of heart rate (HR) monitors, endurance-trained adults as well as habitual exercisers often use pulse rate palpation to periodically monitor exercise intensity. However, due to the rapid recovery of HR following exercise bouts, post-exercise palpation of pulse rates may underestimate exercise HR. To test this hypothesis, we studied 20 young physically active adults performing two sets of exercise for 5 min at 70% and 85% of maximal HR on the treadmill; one with carotid and another with radial pulse count. Post-exercise palpation of pulse rate was lower (P < 0.01) than the actual HR during exercise, underestimating exercise HR by 20-27 bpm (beats per min). Even when ECG tracings of HR were analyzed immediately after exercise (0-15 s), a significant underestimation of exercise HR (7-9 bpm) still persisted (P < 0.05). Following exercise, pulse rate obtained by carotid palpation at both intensities and radial palpation at the lower intensity was no different from the corresponding HR measured with ECG. In the radial artery trial at the higher exercise intensity, pulse rate following exercise was lower (10 bpm; P < 0.05) than ECG-derived HR. Arterial stiffness, which is closely associated with arterial baroreflex sensitivity, was not significantly related to the changes in HR with carotid palpation. We concluded that post-exercise pulse palpations may not be appropriate as an indicator of exercise intensity in habitual exercisers.     

Lower-body negative-pressure exercise and bed-rest-mediated orthostatic intolerance

PURPOSE: Supine, moderate exercise is ineffective in maintaining orthostatic tolerance after bed rest (BR). Our purpose was to test the hypothesis that adding an orthostatic stress during exercise would maintain orthostatic function after BR. METHODS: Seven healthy men completed duplicate 15-d 6 degrees head-down tilt BR using a crossover design. During one BR, subjects did not exercise (CON). During another BR, subjects exercised for 40 min.d(-1) on a supine treadmill against 50-60 mm Hg LBNP (EX). Exercise training consisted of an interval exercise protocol of 2- to 3-min intervals alternating between 41 and 65% (.)VO(2max). Before and after BR, an LBNP tolerance test was performed in which the LBNP chamber was decompressed in 10-mm Hg stages every 3 min until presyncope. RESULTS: LBNP tolerance, as assessed by the cumulative stress index (CSI) decreased after BR in both the CON (830 +/- 144, pre-BR vs 524 +/- 56 mm Hg.min, post-BR) and the EX (949 +/- 118 pre-BR vs 560 +/- 44 mm Hg.min, post-BR) conditions. However, subtolerance (0 to -50 mm Hg LBNP) heart rates were lower and systolic blood pressures were better maintained after BR in the EX condition compared with CON. CONCLUSION: Moderate exercise performed against LBNP simulating an upright 1-g environment failed to protect orthostatic tolerance after 15 d of BR.     

Spectral analysis of heart rate variability during exercise in trained subjects

PURPOSE: To investigate the effects of strenuous exercise on heart rate variability (HRV). METHODS: We evaluated the effects of exercise intensity and duration on HRV indices in 14 healthy trained subjects. Each subject exercised for 3, 6, and 9 min at 60 and 70% of the power achieved at maximal oxygen consumption (PVO2(max)) and for 3 and 6 min (or 3 min twice) at 80% of PVO2(max). The electrocardiogram RR intervals were recorded then processed by fast(FFT) and short-time (STFT) Fourier transform for determination of low-frequency (LF, 0.045-0.15 Hz) and high-frequency (HF, 0.15-1.0 Hz) components. RESULTS: The LF and HF components expressed as absolute power (ms2) decreased significantly at the onset of exercise (P < 0.05). However, with increasing exercise intensity, the HF component expressed as normalized units (n.u.) (reflecting parasympathetic modulation) increased significantly, whereas the LF component (n.u.) and LF/HF ratio (both reflecting sympathetic modulation) decreased significantly (P < 0.05). STFT showed that increasing exercise intensity was associated with a shift in HF peak frequency related to an increase in respiratory rate and a marked decrease in LF power (ms2). Moreover, HFn.u. rose (r = 0.918, P < 0.01) and LFms2 fell as minute ventilation increased (r = 0.906, P < 0.01). CONCLUSIONS: Parasympathetic respiratory control and nonautonomic mechanisms may influence the HF-peak shift during strenuous exercise. HRV and the usual indexes of sympathetic activity do not accurately reflect changes in autonomic modulation during exhaustive exercise.     

Maximal exercise as a countermeasure to orthostatic intolerance after spaceflight

Previous investigators have suggested that maximal exercise performed 24 h before the end of bed rest, a spaceflight analog, restores prebed rest plasma volume, baroreflex responses, and orthostatic tolerance. PURPOSE: In this case report, we examined the effect of a similar exercise protocol 24 h before a Shuttle landing on the orthostatic responses of four crewmembers (EX) after spaceflights of 8-14 d. Four additional crewmembers (CON) served as controls and did not perform exercise during the final day of the flight. METHODS: Each crewmember performed a 10-min stand test approximately 10 d before launch (L-10) and within 1-2 h of landing (R+0). Cardiac stroke volume was measured (Doppler ultrasound) supine and during each min of standing for three EX and three CON subjects. RESULTS: Preflight, all crewmembers completed the stand test and each group had similar heart rate and blood pressure responses. Postflight, all subjects also completed the 10-min stand test. Each group had similarly elevated supine and standing heart rates, elevated diastolic and mean arterial blood pressures, and reduced pulse pressures compared to L-10. However, postflight cardiac output, mean +/- SEM, (EX: 4.5+/-0.6 L x min(-1); CON: 3.1+/-0.3 L x min(-1)) and stroke volume (EX: 43+/-7 mL x beat; CON: 30+/-6 mL x beat) were higher after 10 min standing in the EX subjects compared to CON subjects. CONCLUSIONS: For these four crewmembers, maximal exercise performed 24 h before landing may have helped maintain stroke volume but did not maintain heart rate and blood pressure responses during standing compared to preflight.     

Overall increase in heart rate variability after the Square-Wave Endurance Exercise Test training

Objectives. – We studied the influence of the “Square-Wave Endurance Exercise Test” (SWEET) training program on heart rate (HR) variability during supine rest, 60° upright position and submaximal constant exercise.Methods. – Beat-by-beat HR was recorded during 10 min in the three conditions in 14 healthy women. Before and after 6 weeks of training (45 min, three times a week; n = 7) or daily life (control group, n = 7), time- and frequency-domain analyses of HR variability were performed (standard deviation of interbeat interval, root mean square of successive differences, total-, low-, high-frequency power of HR variability).Results. – After training, HR decreased during supine rest, 60° upright and submaximal exercise, whereas time- and frequency-domain analyses of HR variability indices increased (P < 0.05). No significant changes were observed in the control group.Conclusion. – Six weeks of training by SWEET elicited an increase in heart rate variability (HRV) not only at rest, but also during orthostatism and submaximal exercise. It can, therefore, provide an useful training form in a large population of healthy or diseased people.