Blood pressure responses to acute static and dynamic exercise in three racial groups

PURPOSE: African Americans have a higher incidence of hypertension than other racial groups. Furthermore, some research suggests that normotensive individuals who exhibit exaggerated blood pressure (BP) responses to exercise may be at risk for future hypertension. This study sought to determine whether normotensive African Americans exhibited exaggerated BP responses to static exercise or dynamic exercise relative to Caucasian Americans and Asian Americans. METHODS: Thirty normotensive subjects participated from each of the three racial groups (15 men and 15 women). Subjects held 30% of maximal voluntary contraction (right knee extension) for 3 min, and BP was recorded during the 3rd min. On a separate occasion, subjects cycled for six min at a power equivalent to 70% of VO2 reserve, and BP was recorded during the 6th min. RESULTS: Static exercise produced large, significant increases in both systolic and diastolic BP (35 +/- 1.5 and 29 +/- 1.3 mm Hg, respectively). Dynamic exercise produced large, significant increases in systolic BP (51 +/- 1.6 mm Hg) and moderate, yet significant, increases in diastolic BP (8 +/- 1.0 mm Hg). There were no significant differences between racial groups in BP response to either static exercise or dynamic exercise. However, during dynamic exercise, men had a higher systolic BP response than did women. CONCLUSIONS: African Americans who are normotensive at rest do not exhibit a greater BP response to static exercise or dynamic exercise than do Caucasian Americans or Asian Americans.     

Heart rate and blood pressure changes with endurance training: the HERITAGE Family Study

PURPOSE: The purpose of this study was to determine the magnitude of change in resting and exercise heart rate (HR) and blood pressure (BP), by race, sex, and age, after a 20-wk endurance training program in 507 healthy and previously sedentary subjects from the HERITAGE Family Study. METHODS: After baseline measurements, subjects exercised on cycle ergometers 3 d x wk(-1) for a total of 60 exercise sessions starting at 55% of VO2max for 30 min x session(-1) and building to 75% of VO2max for 50 min x session(-1) for the last 6 wk. HR and BP at rest and during exercise (50 W, 60% of VO2max maximal exercise) were each determined in duplicate on two different days both before and after training (resting values at 24-h and 72-h posttraining). RESULTS: After the period of training, there was a small decrease in resting HR (-2.7 to -4.6 beats x min(-1) across groups at 72-h posttraining), and small changes (i.e., < 3 mm Hg) in resting systolic (SBP), diastolic (DBP), and calculated mean BP (MBP), which varied by race, sex, and age. During exercise at the same absolute work rate (50 W), HR, SBP, DBP, and MBP were all significantly reduced, with greater reductions in HR in women compared with men, and greater reductions in BP in blacks and older subjects compared with whites and younger subjects, respectively. At the same relative work rate (60% VO2max), HR, DBP, and MBP were reduced, but SBP remained unchanged. Blacks had a greater reduction in DBP, but whites had a greater reduction in HR. Finally, at maximal exercise, there was a small decrease in HR, with men and whites decreasing more than women and blacks; an 8 mm Hg increase in SBP, with men increasing more than women; a 4 mm Hg decrease in DBP, with blacks decreasing more than whites; and no change in MBP. CONCLUSION: In conclusion, the reductions in resting HR and BP with training were generally small, but the reductions during exercise were substantial and clinically important, with the older and the black populations experiencing greater reductions.     

Cardiovascular responses of women to lower body negative pressure

Lower body negative pressure (LBNP) has provided a method for studying cardiovascular responses in men while simulating a return to the stresses of 1-G following space flight. In this study, we have monitored responses of women to the stresses provided by LBNP. There were 20 women, 23-43 years, each tested in the follicular and luteal phases of the menstrual cycle. Variables were recorded during supine control; at -30, -40, -50 mm Hg LBNP; immediately after pressure release; and after 5 min recovery. There were no significant differences in response to LBNP between the two menstrual phases. During LBNP calf circumference was enlarged; transthoracic impedance was increased; stroke volume, left ventricular ejection time, the Heather Index of contractility and systolic pressure were reduced; total peripheral resistance was elevated; and cardiac output fell despite a rise in heart rate. Differences in cardiovascular variables between 0 mm Hg LBNP and -50 mm Hg LBNP were generally similar to reported differences between supine and standing. The responses of these women to LBNP were qualitatively similar to those reported for the Apollo astronauts and other male subjects. These women appeared to compensate with a greater heart rate response; however, the net cardiovascular compensation as determined from arterial pressure appears to be similar in men and women.     

体位、意识状态对高压氧治疗患者血压、心率的影响

目的 观察不同体位、意识状态对高压氧(hyperbaric oxygen,HBO)治疗患者血压、心率的影响.方法 HBO治疗患者共57例,按体位和意识状态分组,其中意识清醒组43例,意识障碍组14例,坐位组37例,卧位组20例.对行HBO治疗的57名患者分别于HBO治疗前、加压结束前、稳压25min、减压结束前、HBO治疗结束后lh测量血压及心率,并进行统计学分析.结果 意识清醒组患者仅在减压时收缩压、舒张压、平均动脉压[ (129.42±21.47)mm Hg、(81.79±12.65) mmnHg、(97.67±14.38)mm Hg]与治疗前[(121.72±16.31)mm Hg、(75.51±9.77) mm Hg、(90.91±9.64) mm Hg]比较升高,差异有统计学意义(P＜0.05);坐位组患者仅在减压时舒张压、平均动脉压[(81.30±12.36) mmHg、(97.11±13.99) mm Hg]与治疗前[(75.38±9.48)mm Hg、(91.14±9.51)mm Hg]比较升高(P＜0.05);其余时间点、其他组别患者、组间比较则变化不明显(P＞0.05).意识清醒组、坐位组患者从加压时开始,直到治疗结束后1h,心率较治疗前均明显减慢(P＜0.05);意识障碍组、卧位组患者从稳压时开始到减压前,心率较治疗前均明显减慢(P＜0.05),治疗结束后lh恢复到治疗前水平(P＞0.05).结论HBO治疗时,除高分压氧作用外,体位、意识状态可能是血压、心率的变化影响因素,应引起足够的重视.     

Increasing daily walking lowers blood pressure in postmenopausal women.

PURPOSE: The American College of Sports Medicine and the Centers for Disease Control and Prevention (ACSM-CDC) recommend 30 min of daily moderate-intensity physical activity for health; however, the effectiveness of this recommendation in lowering blood pressure (BP) in hypertensives is unclear. The present study tested the hypothesis that walking activity following the ACSM-CDC physical activity recommendation would lower BP in postmenopausal women with high BP. METHODS: Resting BP was measured in 24 postmenopausal women with borderline to stage 1 hypertension at baseline, 12 wk, and 24 wk. Fifteen women in the exercise (EX) group walked 3 km.d-1 above their daily lifestyle walking, whereas 9 women in the control (CON) group did not change their activity. Walking activity was self-measured with a pedometer in both groups. RESULTS: Resting systolic BP was reduced in the EX group after 12 wk by 6 mm Hg (P < 0.005) and was further reduced by 5 mm Hg at the end of 24 wk (P < 0.005). There was no change in diastolic BP with walking. The CON group experienced no change in BP at either 12 or 24 wk. Body mass was modestly reduced by 1.3 kg in the EX group after 24 wk (P < 0.05); however, it was not correlated with the change in BP. There were no changes in selected variables known to impact BP including percent body fat, fasting plasma insulin, or dietary intake. CONCLUSION: In conclusion, a 24-wk walking program meeting the ACSM-CDC physical activity recommendation is effective in lowering systolic BP in postmenopausal women with borderline to stage 1 hypertension.     

Improvement in functional capacity with spironolactone masks the treatment effect on exercise blood pressure

ObjectivesA hypertensive response to submaximal exercise is associated with cardiovascular disease but this relationship is influenced by functional capacity. Spironolactone improves functional capacity, which could mask treatment effects on exercise blood pressure. This study sought to examine this hypothesis.DesignRetrospective analysis of a randomized clinical trial.Methods102 participants (54 ± 9 years; 52% male) with a hypertensive response to maximal exercise (systolic BP ≥210 mm Hg men; ≥190 mm Hg women) were randomized to 3-month spironolactone 25 mg daily (n = 53) or placebo (n = 49). Submaximal exercise blood pressure was measured during low-intensity cycling (50, 60 or 70% age-predicted maximal heart rate). Functional capacity was measured as maximal oxygen capacity obtained during a maximal treadmill exercise test, and (resting) aortic stiffness by carotid-to-femoral pulse wave velocity.ResultsSpironolactone improved submaximal exercise systolic blood pressure vs. placebo (?4 ± 16 vs. 2 ± 15 mm Hg, p = 0.045, Cohen's d = 0.42), and had a small (but non-statistically significant) improvement in functional capacity (0.64 ± 5.10 vs. ?1.43 ± 5.04 ml/kg/min, p = 0.06, Cohen's d = 0.4). When treatment effects were expressed as the change in submaximal exercise systolic blood pressure relative to the change in functional capacity, a larger effect size was observed (?0.3 ± 1.1 vs. 0.3 ± 1.1 mm Hg/ml·kg·min^?1, p = 0.01, Cohen's d = 0.58), but was not explained by improved aortic stiffness.ConclusionsSpironolactone reduces submaximal exercise blood pressure, but this treatment effect may be hidden by improved functional capacity and a non-fixed workload. This highlights the most clinically relevant exercise blood pressure is at a low intensity and fixed workload where the influence of fitness on exercise blood pressure is removed, and the effects of therapy can be appreciated.     

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.     

Effect of exercise on recovery blood pressure in normotensive and hypertensive subjects

The effects of dynamic exercise on the acute recovery blood pressure (BP) were studied in normotensive and hypertensive subjects. Three groups [eight normotensives, age 19 to 29 yr (N1); eight normotensives, age 35 to 62 yr (N2); and eight hypertensives, age 44 to 57 yr (H)] were tested over three separate sessions. The first two sessions were for familiarization with the protocol and test procedures. Resting systolic BP decreased (P less than 0.01) in all groups from sessions 1 to 3: N1 = 126 to 121 mm Hg; N2 = 127 to 120; H = 155 to 142. Resting diastolic BP decreased (P less than 0.05) in the N1 and H groups from 77 to 73 and 98 to 95 mm Hg, respectively. On the third day, each subject followed the protocol of Wilcox et al. (8) of 15 min of seated rest, five 10-min periods of treadmill walking with a 3-min rest between each period, and 60 min of seated recovery. Exercise was performed at 67% of estimated maximal heart rate. In all three groups, significant (P less than 0.05) reductions in both systolic BP (N1 = -12 +/- 1; N2 = -10 +/- 2; H = -12 +/- 3) and diastolic BP (N1 = -5 +/- 2; N2 = -5 +/- 1; H = -7 +/- 2) occurred from pre-exercise rest to post-exercise rest. Systolic BP remained lower following 60-min recovery (P less than 0.02), while diastolic BP returned to pre-exercise levels in all three groups. No between-group differences were observed in the magnitude of reduction of BP post-exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular response to lower body negative pressure (LBNP) following endurance training

Eleven sedentary male volunteers were assigned to either an exercise (E) group (n = 6; endurance exercise for 12 weeks) or a control (C) group (n = 5; no exercise). After training, E significantly increased (p less than 0.01) their VO2max (pretraining: 37.0 +/- 2.3; posttraining: 44.6 +/- 2.5), whereas C showed no significant change. Heart rate (HR), arterial blood pressure (BP) and forearm blood flow (FBF) were measured both pre- and posttraining at rest and during 2 levels of LBNP: -10 mm Hg and -40 mm Hg. Both C and E had similar decreases in systolic BP and similar increases in HR and diastolic BP during LBNP when comparing the pre- and posttraining periods. In both groups, FBF significantly decreased during -40 mm Hg of LBNP in the pretraining period. However, after training, E had a significantly attenuated (p less than 0.05) decrease in FBF at -40 mm Hg (pretraining: -45.0 +/- 3.7%; posttraining: -29.8 +/- 3.1%). In C, there was no difference in the response of FBF to -40 mm Hg of LBNP comparing pretraining and posttraining. These findings indicate that endurance exercise training decreases the forearm vasoconstrictor response to high levels of LBNP.     

Blood pressure responses to different arm positions during vertical head-down ankle suspension

This study compares the effects of different arm positions on blood pressure during vertical head-down ankle suspension. Eighteen healthy volunteers (10 men and 8 women, aged 20-30 years) were placed first in the upright sitting position for 4 min, and second in the head-down suspension position for 4 min. In the latter position, the arms were first alongside the head and then raised alongside the body at the end of the 4th min. Blood pressure was measured non-invasively during the last 30 s of the 4th min of upright sitting and head-down suspension with the arms alongside the head, and then immediately following the movement of the arms alongside the body. In relation to the upright sitting position, head-down suspension with the arms dangling alongside the head resulted in a significant increase in brachial artery blood pressure (121/78 mm Hg vs. 146/91 mm Hg, respectively). This finding may be explained on the basis of a variable hydrostatic component. Following the immediate movement of the arms to alongside the body while still suspended, there was a significant decrease in blood pressure to 124/79 mm Hg. No significant change in blood pressure occurred from upright sitting to full head-down suspension with the arms alongside the body (121/78 mm Hg vs. 124/79 mm Hg, respectively). Based on previous disparate reports of pressure during head-down suspension, the findings of the present study indicate a need to standardize the method of blood pressure measurement during this procedure.     

Time course of blood pressure changes immediately after maximal exercise

AIM: The aim of this study was to investigate the effect of exhaustive exercise on the time course of arterial blood pressure (BP) and heart rate (HR) during upright resting (inactive) and loadless pedaling (active) recovery from a bicycle exercise to exhaustion. METHODS: The subjects were 11 healthy normotensive males. Systolic, diastolic and mean BP, and HR were recorded every 20 s for the initial 6 min of the recovery period. RESULTS: The time course of all BP measures during inactive and active recovery was characterized by a marked and sudden drop during the initial 20-s period, followed by a quick rise. This was followed by a gradual decline till the end of the recovery period. The time course of HR recovery, on the other hand, exhibited a smooth decline without the initial drop. With active recovery, the initial drop of diastolic and mean BP was less than the inactive recovery. After the 20 s period, the diastolic BP and HR were kept slightly higher with the active recovery than the inactive recovery. CONCLUSIONS: A sudden drop of the BP occurred at the initial recovery period of postcycle exercise to exhaustion though HR did not show such a change. The initial BP drop could be attenuated by the actively pedaling the cycle without load.     

Cardiovascular response during upright isometric dead lift in young, older, and elderly healthy men

Isometric exercises raise the mean arterial blood pressure and impose serious stress on left ventricular function. It may be more dangerous for older than for younger people. The effect of aging on blood pressure response was studied in 75 subjects, 25 normal young men aged 25 +/- 4 years, 25 older men aged 51 +/- 3.2 years, and 25 elderly men aged 68 +/- 4 years, during upright isometric dead lift at 30% maximal voluntary effort for 3 min. The heart rate, systolic blood pressure, and diastolic blood pressure were measured during the 3rd min of dead lift contraction. Tension exerted was significantly lower in the older group (33 +/- 4 kg) and lowest in the elderly group (26.9 +/- 6 kg) compared with the young group (43 +/- 8 kg). All groups showed similar and significant increases in heart rate, systolic and diastolic blood pressure during dead lift (p less than 0.01). These data indicate a similar pattern of blood pressure response to acute pressure overload in young, older, and elderly normal men.     

Aftereffects of exercise and relaxation on blood pressure.

OBJECTIVE: To study the acute aftereffects of exercise and relaxation, performed alone and in combination, on blood pressure (BP) measured at baseline and during stressful conditions. DESIGN: Clinical trial with comparison of groups and repeated measures in each group. SETTING: Exercise Hemodynamic Laboratory, University of S?o Paulo, Brazil. PARTICIPANTS: Fourteen normotensive (NT) and 16 essential hypertensive (HT) subjects. INTERVENTIONS: Four random experimental sessions: relaxation (RX-20 min); exercise [EX-cycle ergometer, 53 min, 50% peak oxygen uptake (VO2peak)]; exercise plus relaxation (EX+RX); and control (C-73 min rest). Measures were taken before and after interventions at baseline and during Stroop color test. MAIN OUTCOME MEASURES: Auscultatory and plesthysmographic BPs. RESULTS: Systolic and diastolic BPs decreased significantly after all the interventions. The decreases in both BPs were significantly greater after the EX+RX session, and were also greater in the HT (EX+RX session, -10+/-1/-7+/-1 and -15+/-2/-8+/-1 mm Hg for the NT and HT, respectively). During mental stress, systolic BP increased significantly and similarly after all the experimental sessions. Diastolic BP also increased significantly during stress; however, the increase was significantly greater after the RX session. At the end of the mental stress, diastolic BP was significantly lower after the EX (74+/-3 mm Hg) and EX+RX (72+/-3 mm Hg) sessions than after the C (79+/-3 mm Hg) and RX (78+/-3 mm Hg) sessions. CONCLUSIONS: In NT and HT subjects, a single bout of exercise or relaxation has hypotensive effects, further enhanced by their combination, and greater in the HT. Moreover, exercise performed alone or in combination with relaxation decreases systolic and diastolic BPs during mental stress.     

Effects of posture on left ventricular diastolic filling during exercise

BACKGROUND: Measuring the transmitral flow velocity with Doppler echocardiography is a useful method for evaluating left ventricular diastolic function. However, there are few data regarding the effect of posture during exercise on transmitral flow velocity. METHODS: The transmitral flow velocity with pulsed-wave Doppler echocardiography was measured during supine and upright bicycle ergometer exercise in 10 normal young men without cardiac disease (26.7 +/- 5.5 yr). RESULTS: The ratio of the early rapid filling wave to the atrial filling wave (E/A) was gradually decreased with increasing exercise intensity. At rest and during recovery, the E/A ratio was significantly higher (P < 0.01) in the supine position than in the upright position. This difference was caused mainly by the higher E wave in the supine position. However, E wave and E/A ratio did not differ between the upright and supine position during exercise. CONCLUSION: Although measurement of left ventricular filling is completely noninvasive and clinically useful for evaluating diastolic function, it was found that the E/A ratio was profoundly influenced by posture and exercise intensity. These results suggest that the potential influences of posture and exercise intensity on the filling velocities should be taken into account when interpreting diastolic function by Doppler echocardiography.     

Effects of repeated Valsalva maneuver straining on cardiac and vasoconstrictive baroreflex responses

INTRODUCTION: We hypothesized that repeated respiratory straining maneuvers (repeated SM) designed to elevate arterial BPs (arterial baroreceptor loading) would acutely increase baroreflex responses. METHODS: We tested this hypothesis by measuring cardiac baroreflex responses to carotid baroreceptor stimulation (neck pressures), and changes in heart rate and diastolic BP after reductions in BP induced by a 15-s Valsalva maneuver in 10 female and 10 male subjects at 1, 3, 6, and 24 h after performing repeated SM. Baroreflex responses were also measured in each subject at 1, 3, 6, and 24 h at the same time on a separate day without repeated SM (control) in a randomized, counter-balanced cross-over experimental design. RESULTS: There was no statistical difference in carotid-cardiac and peripheral vascular baroreflex responses measured across time following repeated SM compared with the control condition. Integrated cardiac baroreflex response (deltaHR/ deltaSBP) measured during performance of a Valsalva maneuver was increased by approximately 50% to 1.1 +/- 0.2 bpm x mm Hg(-1) at 1 h and 1.0 +/- 0.1 bpm x mm Hg(-1) at 3 h following repeated SM compared with the control condition (0.7 +/- 0.1 bpm x mm Hg(-1) at both 1 and 3 h, respectively). However, integrated cardiac baroreflex response after repeated SM returned to control levels at 6 and 24 h after training. These responses did not differ between men and women. CONCLUSIONS: Our results are consistent with the notion that arterial baroreceptor loading induced by repeated SM increased aortic, but not carotid, cardiac baroreflex responses for as long as 3 h after repeated SM. We conclude that repeated SM increases cardiac baroreflex responsiveness which may provide patients, astronauts, and high-performance aircraft pilots with protection from development of orthostatic hypotension.     

The role of exercise training in the treatment of hypertension: an update

Hypertension is a very prevalent cardiovascular (CV) disease risk factor in developed countries. All current treatment guidelines emphasise the role of nonpharmacological interventions, including physical activity, in the treatment of hypertension. Since our most recent review of the effects of exercise training on patients with hypertension, 15 studies have been published in the English literature. These results continue to indicate that exercise training decreases blood pressure (BP) in approximately 75% of individuals with hypertension, with systolic and diastolic BP reductions averaging approximately 11 and 8mm Hg, respectively. Women may reduce BP more with exercise training than men, and middle-aged people with hypertension may obtain greater benefits than young or older people. Low to moderate intensity training appears to be as, if not more, beneficial as higher intensity training for reducing BP in individuals with hypertension. BP reductions are rapidly evident although, at least for systolic BP, there is a tendency for greater reductions with more prolonged training. However, sustained BP reductions are evident during the 24 hours following a single bout of exercise in patients with hypertension. Asian and Pacific Island patients with hypertension reduce BP, especially systolic BP, more and more consistently than Caucasian patients. The minimal data also indicate that African-American patients reduce BP with exercise training. Some evidence indicates that common genetic variations may identify individuals with hypertension likely to reduce BP with exercise training. Patients with hypertension also improve plasma lipoprotein-lipid profiles and improve insulin sensitivity to the same degree as normotensive individuals with exercise training. Some evidence also indicates that exercise training in hypertensive patients may result in regression of pathological left ventricular hypertrophy. These results continue to support the recommendation that exercise training is an important initial or adjunctive step that is highly efficacious in the treatment of individuals with mild to moderate elevations in BP.     

Hemodynamic responses to seated and supine lower body negative pressure: comparison with +Gz acceleration.

Hemodynamic changes between upright and supine lower body negative pressure (LBNP) to levels of -70 mm Hg were compared in 8 subjects (5 males, 3 females) and correlated with their findings during simulated Shuttle reentry acceleration with a slow onset rate of 0.002 G/s (1,020 s to peak +2 Gz) and during gradual onset exposures (0.03 G/s) to +3 Gz and +4 Gz. Six of the 8 subjects were able to tolerate 2 min at peak +2 Gz, 2-5 min at +3 Gz, and 1-2 min at +4 Gz. Heart rate (HR) at any given level of upright LBNP regularly exceeded supine levels. HR change at -50 mm Hg in upright subjects (+47.7 bpm from 74.1 +/- 1.9 (M +/- S.E.) bpm, control) was 2.6 times greater than in supine subjects (+18.3 bpm from 64.8 +/- 2.8 bpm, control). HR values at -40 mm Hg supine (73.7 +/- 2.6) matched seated upright pre-LBNP control levels (74.1 +/- 1.9 bpm), while values at -70 mm Hg supine (102.5 +/- 4.4 bpm) were not significantly different from those at -40 mm Hg upright (103.1 +/- 4.0 bpm). Peak HR during +3 Gz (145.8 +/- 7.7 bpm) and +4 Gz (152.3 +/- 6.5 bpm) significantly exceeded recorded supine and upright LBNP levels, whereas values at +2 Gz (104.8 +/- 5.5 bpm) closely matched those at -40 mm Hg upright (103.1 +/- 4.0 bpm) and -70 mm Hg supine (102.5 +/- 4.4 bpm). Supine LBNP HR changes in this relatively small group of subjects closely matched those previously reported in the literature.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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。