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。     

Cardiac output and stroke volume changes with endurance training: the HERITAGE Family Study

PURPOSE: The purpose of this study was to determine the magnitude of changes in cardiac output (Qc), stroke volume (SV), and arterial-mixed venous oxygen difference (a-vO2 diff) during submaximal exercise following a 20-wk endurance training program, with the primary focus on identifying differences in response by race, sex, and age. METHODS: The participants in this study (N = 631) were healthy and previously sedentary men (N = 277) and women (N = 354) of varying age (17-65 yr) and race (blacks, N = 217; whites, N = 414) who had completed the HERITAGE Family Study protocol. After baseline measurements, participants trained on cycle ergometers 3 d x wk(-1) for a total of 60 exercise sessions starting at the HR associated with 55% of maximal oxygen uptake (VO2max) for 30 min/session and building to the HR associated with 75% of VO2max for 50 min/session, which was maintained during the last 6 wk. HR, Qc (CO2 rebreathing), and SV (Qc/HR) were determined in duplicate at 50 W and at 60% of VO2max on two different days both before and after training. RESULTS: After training, there were significant decreases in HR and Qc, and significant increases in SV and a-vO2 diff at 50 W (except for no change in a-vO2 diff in black men). The changes in HR differed by sex and age, and the changes in SV, Qc, and a-vO2 diff differed by race. Qc decreased by 0.6 L x min(-1) at 50 W for the total sample, consistent with the decrease in VO2 at this power output. At 60% of VO2max HR decreased, and SV, Q, and a-VO2 diff increased. There were small differences in response by sex (HR and SV), race (HR), and age (HR and Qc). CONCLUSION: It is concluded that the cardiovascular systems of men and women, blacks and whites, and younger and older subjects are not limited in their ability to adapt to endurance training.     

Acute recovery profile of lung volumes and function after running 5 miles.

The purpose of this study was to characterize the acute changes and recovery profile of lung volumes and function subsequent to strenuous aerobic exercise. Eight experienced runners (X age = 25 yrs; wt = 73 kg; ht = 181 cm) completed three identical 5 mile runs. Determinations were made of forced vital capacity (FVC), residual volume (RV), closing capacity (CC), and pulmonary diffusion capacity (DLCO). Measurement of cardiac output (Q) and stroke volume (SV) occurred simultaneously with the 10 second DLCO breathhold maneuver. Measurements were obtained before and 5, 15 and 25 minutes after each run. FVC was reduced (-4.5%) 5 min post-run with a return to pre-run values by 15 min. CC (+16%) and RV (+18%) remained elevated for at least 30 min post-run. DLCO did not appear to be effected by the run. However, the single-breath DLCO breathhold maneuver consistently caused a fall in SV at rest and during recovery. The hypothesis has been forwarded that an increase in central blood volume post-run accounted for the acute reduction in FVC. The sustained elevation in RV resulted from early closure of the small airways possibly due to an increase in extra-vascular lung water.     

Inspiratory impedance effects on hemodynamic responses to orthostasis in normal subjects

BACKGROUND: Breathing through an impedance threshold device (ITD) might prove effective as a countermeasure against post-spaceflight orthostatic hypotension since it increased blood pressure (BP) and cardiac output in supine human subjects. OBJECTIVE: We tested the hypothesis that spontaneous breathing through an ITD would attenuate the reduction in stroke volume and BP during orthostasis in human subjects. METHODS: There were 19 volunteers (10 men, 9 women) who completed two 80 degrees head-up tilt (HUT) protocols with (active) and without (sham control) an ITD set to open at -7 cm H2O pressure. Heart rate (HR), stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP), and total peripheral resistance (TPR) were measured non-invasively during transition from supine to HUT. RESULTS: HUT caused significant elevation in HR and reductions in SV, CO, TPR, and MAP. Hemodynamic effects of HUT were similar for sham and active ITD. Further analysis revealed a subset (n = 11) of subjects who demonstrated a > 20% decrease in SV during HUT with the sham ITD. In this subset of subjects, the ITD attenuated (p = 0.004) the %deltaSV (-22.5 +/- 3.0%) during HUT compared with the sham ITD (%deltaSV = -37.4 +/- 2.6%). There was no statistical effect of ITD use in the subgroup who demonstrated < 20% reduction in SV (-16.6 +/- 0.4%). CONCLUSIONS: Use of an ITD may provide significant protection against orthostatic compromise in individuals with greater than 20% reductions in SV, such as astronauts returning from space.     

The effects of breathhold on lactate accumulation, PO2, PCO2 and pH of blood

This investigation is concerned with the effect of breathholding during exercise on the partial pressure of blood gasses and the energy supply mechanisms of the human body. Subjects (n = 15) included males and females. They were subjected to intermittent exercise periods of 10 s with 10-s resting periods for a duration of 12 min. This protocol was selected to simulate an underwater hockey game during which in-practice intervals of submersion are alternated with intervals on the surface, each of a 10-s duration. Two tests per subject were executed, i.e. one while breathing normally and another where the active period was executed apneically. Statistically significant differences were observed in the PCO2 (breathhold higher than breathing normally; p less than 0.005) and pH (breathhold lower than breathing normally; p less than 0.01) of arterial capillary blood during the exercise period. The lactate concentration of arterial blood did not differ significantly (p greater than 0.05). The PO2 of the breathhold test was slightly lower than that of the control test.     

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。     

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 body position on measurements of diffusion capacity after exercise

BACKGROUND: Pulmonary diffusing capacity for carbon monoxide (D1co), alveolar capillary membrane diffusing capacity (Dm), and pulmonary capillary blood volume (Vc) are all significantly reduced after exercise. OBJECTIVE: To investigate whether measurement position affects this impaired gas transfer. METHODS: Before and one, two, and four hours after incremental cycle ergometer exercise to fatigue, single breath D1co, Dm, and Vc measurements were obtained in 10 healthy men in a randomly assigned supine and upright seated position. RESULTS: After exercise, D1co, Dm, and Vc were significantly depressed compared with baseline in both positions. The supine position produced significantly higher values over time for D1co (5.22 (0.13) v. 4.66 (0.15) ml/min/mm Hg/l, p = 0.022) and Dm (6.78 (0.19) v. 6.03 (0.19) ml/min/mm Hg/l, p = 0.016), but there was no significant position effect for Vc. There was a similar pattern of change over time for D1co, Dm, and Vc in the two positions. CONCLUSIONS: The change in D1co after exercise appears to be primarily due to a decrease in Vc. Although the mechanism for the reduction in Vc cannot be determined from these data, passive relocation of blood to the periphery as the result of gravity can be discounted, suggesting that active vasoconstriction of the pulmonary vasculature and/or peripheral vasodilatation is occurring after exercise.     

Hypokinetic circulation in persons with paraplegia

INTRODUCTION: It is well established that hemodynamic dysfunction, resulting in diminished upper-extremity work capacity, occurs in persons with spinal cord injury (SCI) as compared with those who are nondisabled (ND). Although it has been shown that persons with paraplegia display higher values of heart rate (HR) with lower values of stroke volume (SV) during exercise, it is not resolved whether there is adequate compensation to produce similar values of cardiac output (.Q) as in ND. PURPOSE: This study examined central cardiovascular responses (HR, SV, and .Q) of 20 subjects with complete thoracic level SCI (T(4)-T(11)) and 20 sedentary ND subjects during matched levels of arm-crank (AC) exercise. METHODS: All subjects performed an incremental peak AC test to volitional exhaustion with continuous metabolic analysis and HR measurement via open circuit spirometry and 12-lead electrocardiography, respectively. Stroke volume was assessed using transthoracic impedance. RESULTS: Heart rate was higher for SCI (P< 0.05) with significantly lower values for SV and .Q at rest (approximately 25%). Peak responses were significantly higher for ND in all factors except HR. Although subpeak HRs at matched absolute workloads were significantly higher for SCI (12-20 beats.min (-1) ), SV and .Q were significantly lower (P< 0.05). CONCLUSIONS: The results of this study indicate that .Q is significantly lower in SCI than in ND during AC, despite significantly greater values of HR. These findings also suggest that the disparity in exercise values of .Q is related to differences exhibited at rest.     

Prone versus supine patient positioning during gated 99mTc-sestamibi SPECT: effect on left ventricular volumes, ejection fraction, and heart rate.

Gated myocardial perfusion SPECT allows assessment of left ventricular end-diastolic volume (EDV), left ventricular end-systolic volume (ESV), left ventricular stroke volume (SV), and left ventricular ejection fraction (LVEF). Acquiring images with the patient both prone and supine is an approved method of identifying and reducing artifacts. Yet prone positioning alters physiologic conditions. This study investigated how prone versus supine patient positioning during gated SPECT affects EDV, ESV, SV, LVEF, and heart rate. METHODS: Forty-eight patients scheduled for routine myocardial perfusion imaging were examined with gated (99m)Tc-sestamibi SPECT (at rest) while positioned prone and supine (consecutively, in random order). All parameters for both acquisitions were calculated using the commercially available QGS algorithm. RESULTS: Whereas EDV and SV were significantly lower (P < 0.0004) for prone acquisitions (EDV, 110.5 +/- 39.1 mL; SV, 55.9 +/- 13.3 mL) than for supine acquisitions (EDV, 116.9 +/- 36.2 mL; SV, 61.0 +/- 14.5 mL), ESV and LVEF did not differ significantly. Heart rate was significantly higher (P < 0.0001) during prone acquisitions (69.1 +/- 10.5 min(-1)) than during supine acquisitions (66.5 +/- 10.0 min(-1)). CONCLUSION: The observed position-dependent effect on EDV, SV, and heart rate might be explained by decreased arterial filling and increased sympathetic nerve activity. Hence, supine reference data should not be used to classify the results of prone acquisitions.     

Acute cardiovascular adaptation to 10 consecutive episodes of head-up tilt

BACKGROUND: The cardiovascular system is highly adaptable to sustained +Gz acceleration. Little is known as to whether the cardiovascular system can adapt to acute, repetitive +Gz exposures. This study tested the hypothesis that the cardiovascular system would adapt to repeated orthostatic challenges in a single session. METHODS: Over a 70-min period, 14 subjects were exposed to 10 +75 degrees head-up tilts (HUT). Each tilt involved a 5-min supine period followed by a 2-min HUT. Heart rate (HR), systolic BP (SBP), diastolic BP (DBP), mean arterial pressure (MAP), total peripheral resistance (TPR), stroke volume (SV), and cardiac output (CO) were determined non-invasively. Cardiovascular responses to HUT10 for the final 30 s of the supine period and the first 30 s of the tilt period were compared with those of HUT1. Integrated cardiac baroreflex sensitivity (BRS) was assessed using the Valsalva maneuver. RESULTS: MAP and DBP increased in both supine (MAP p = 0.009, DBP p = 0.002) and tilt periods (MAP p = 0.003, DBP p = 0.009) for HUT10 compared with HUT1. TPR increased during the tilt period only (p = 0.001) during HUT10 compared with HUT1. CO and SV were decreased during the supine period at HUT10 relative to HUT1; however, there were no differences in the tilt period at HUT10 for either CO or SV. There was no change in the response of BRS, HR, or SBP from HUT1 to HUT10. CONCLUSIONS: This study indicates that 10 repetitive HUTs can elicit changes in the cardiovascular responses to orthostasis, reflected by an increased vascular resistance.     

Potential benefits of maximal exercise just prior to return from weightlessness

The purpose of this study was to determine whether performance of a single maximal bout of exercise during weightlessness within hours of return to earth would enhance recovery of aerobic fitness and physical work capacities under a 1G environment. Ten healthy men (36-51 yr) underwent maximal supine exercise followed by upright maximal exercise before and after a 10-d bedrest period in the 6 degrees headdown position. A graded maximal supine cycle ergometer test was performed before and at the end of bedrest to simulate exercise during weightlessness. Following 3 h of resumption of the upright posture from the supine exercise test, a second maximal exercise test was performed on a treadmill to measure work capacity under conditions of 1G. Compared to before bedrest, peak VO2 decreased (p less than 0.05) by 8.7% and peak HR increased (p less than 0.05) by 5.6% in the supine cycle test at the end of bedrest. However, there were no significant changes in peak VO2 and peak HR in the upright treadmill test following bedrest. These data, based on a simulation, suggest that one bout of maximal leg exercise prior to return from 10 d of weightlessness may be adequate to restore preflight aerobic fitness and physical work capacity.     

Use of acetylene breathing to determine cardiac output in young and older adults

PURPOSE: The aims of this investigation were: 1). to establish the day-to-day reproducibility of open-circuit acetylene breathing for measuring exercise cardiac output (Q(c)) in young and older adults; and 2). to compare estimates of Q(c) from open-circuit acetylene breathing with estimates of Q(c) from previously established closed-circuit acetylene rebreathing. METHODS: Twenty men (10 young: 28 +/- 1 yr; 10 older: 61 +/- 1 yr (mean +/- SE)) performed cycle ergometry exercise on 3 separate days. Q(c) was estimated using open-circuit acetylene breathing on 2 d, and closed-circuit acetylene rebreathing on 1 d. RESULTS: Open-circuit acetylene breathing was highly reproducible (young: standard error of measurement (SEM) = 1.52 L.min (-1) limits of agreement (LOA) = 0.2 +/- 4.2 L.min (-1), coefficient of variation 6% < CV < 8%, day 2 = (0.9 x day 1) + 2.4, r = 0.90, P< 0.001, r (2)= 0.82; older: SEM = 0.94 L.min (-1), LOA = 0.1 +/- 2.8 L.min (-1), 4% < CV < 10%, day 2 = (1.0 x day 1) + 0, r = 0.91, < 0.001, r(2) = 0.82). Estimates of Q(c) from open-circuit acetylene breathing demonstrated good agreement with closed-circuit acetylene rebreathing (young: SEM = 1.52 L.min (-1), LOA = 0.9 +/- 4.4 L.min (-1), 5% < CV < 10%, open-circuit = (1.0 x closed-circuit) + 1.5, r = 0.89, < 0.001, r (2) = 0.79; older: SEM = 1.13 L.min (-1), LOA = 0.1 +/- 3.2 L.min (-1), 5% < CV < 9%, open-circuit = (0.9 x closed-circuit) + 1.6, r = 0.88, < 0.001, r(2) = 0.78). CONCLUSION: These results demonstrate that open-circuit acetylene breathing provides reproducible measurements of Q(c) during exercise that demonstrate good agreement with values obtained from the acetylene rebreathing procedure in young and older healthy men.     

The effects of oral smokeless tobacco on the cardiorespiratory response to exercise.

The purpose of this investigation was to determine the effects of oral smokeless tobacco (OST) usage on oxygen uptake (VO2), cardiac output (Qc), stroke volume (SV), heart rate (HR), and plasma lactate concentration (Lc) during rest and exercise. Fifteen asymptomatic subjects were recruited from 18 to 33-yr-old male users of OST. Comparisons of the responses of VO2, Qc, SV, HR, and Lc were made between 2.5-g OST and placebo experimental conditions during rest and at 60% and 85% maximal VO2 treadmill exercise. Plasma nicotine concentrations (Nc) were determined by radioimmunoassay. There were significant increases in HR and Lc and a decrease in SV during rest and at 60% and 85% maximal exercise (P less than 0.05). Furthermore, there were no significant differences in maximal HR, Lc, and VO2 (P greater than 0.05). In conclusion, these data indicate that the increased Nc incurred by OST usage increases anaerobic energy production and produces an increased tachycardiac response to a given relative submaximal workload.     

Familial aggregation of stroke volume and cardiac output during submaximal exercise: the HERITAGE Family Study

Familial aggregation of stroke volume (SV) and cardiac output (Qc by CO2 rebreathing) at 50 Watts (W) and 60 % of maximal oxygen uptake (VO2max) as well as their changes in response to a 20-week endurance exercise training program was assessed in 99 Caucasian families who participated in the HERITAGE Family Study. In order to interpret familial influences independent of effects of age, sex, and body size (indexed by body surface area here), SV and Qc levels were adjusted for these primary parameters prior to genetic analysis within four sex-by-generation groups (the responses to training were additionally adjusted for their baseline values). Maximal heritabilities for baseline SV, Qc, and their changes in response to training during the two stages of submaximal exercise were estimated using a familial correlation model. At 50W, maximal heritabilities reached 41% and 42% for baseline SV and Qc, respectively, and were 29% and 38% for the respective responses to training. At 60% of VO2max, maximal heritabilities reached 46 % for baseline SV and Qc, and were 24% and 30% for the respective responses to training. Generally there were no meaningful differences between the maximal heritabilities at 50 W and 60% of VO2max. However, the maximal heritabilities for the baseline were slightly higher than the estimates for the changes in response to training. Based upon results arising from these non-obese, non-hypertensive, and sedentary families, we found that SV and Qc at 50 W and 60% of VO2max as well as their changes in response to the 20-week endurance exercise training were moderately heritable. Not only genetic determinants but also familial non-genetic factors might attribute to the observed patterns of familial aggregation of SV and Qc during submaximal exercise in the present study.     

Pulmonary blood flow distribution in erect man in air and during breathhold diving

We used intravenously administered 99mTc-labelled macroaggregates and a gamma camera attached to a computer for measuring distribution of pulmonary blood flow per unit lung volume in eight healthy subjects sitting erect in air and also during breathhold diving to 1 or 10 m of depth. We measured distribution of perfusion in the supine position and substituted regional lung volume with regional perfusion in the supine for calculating regional perfusion per lung volume erect in air and during diving. The perfusion per unit lung increased rectilinearly down the lung in subjects below 30 years of age but decreased in the lowermost regions in older subjects. This decrease showed a strong correlation to closing capacity. An age-related decrease in transpulmonary pressure may influence both basal perfusion and closing capacity. During submersion, perfusion became equal in all regions with the exception of the lung apex which became hyperperfused. Close to the diaphragm, small inconsistent changes were noted. Redistribution was the same at surface (1 m of depth) with the lung volume being close to total lung capacity and at 10 m of depth when lung volume was compressed to functional residual capacity. During breathhold diving, high intrapulmonary blood volume and pressure became more important for blood flow distribution than gravity or lung volume, while differences in regional hypoxic vasoconstriction and in transpulmonary pressure seem to explain interindividual variation.     

Real-time MR imaging of aortic flow: influence of breathing on left ventricular stroke volume in chronic obstructive pulmonary disease

PURPOSE: To assess real-time changes of left ventricular stroke volume (SV) in relation to the breathing pattern in healthy subjects and in patients with chronic obstructive pulmonary disease (COPD). MATERIALS AND METHODS: Real-time magnetic resonance (MR) imaging flow measurements were performed in the ascending aorta of 10 healthy volunteers and nine patients with severe COPD. Breathing maneuvers were registered with an abdominal pressure belt, which was synchronized to the electrocardiographic signal and the flow measurement. Healthy subjects performed normal breathing, deep breathing, and the Valsalva maneuver. Patients with COPD performed spontaneous breathing. Paired two-tailed Student t tests were used in healthy volunteers to assess significant SV differences between normal breathing and deep breathing or the Valsalva maneuver. The results of measurements in the patients with COPD were compared with the results during normal breathing in healthy subjects with the unpaired two-tailed Student t test. RESULTS: In healthy subjects, SV decreased during inspiration and increased during expiration (r2 = 0.78, P <.05). When compared with the SV during normal breathing, mean SV did not change during deep breathing but declined during the Valsalva maneuver (P <.05). The difference between minimal and maximal SVs (ie, SV range) increased because of deep breathing or the Valsalva maneuver (P <.05). In normal and deep breathing, velocity of SV elevation and velocity of SV decrease were equal (which resulted in a ratio of 1), whereas during the Valsalva maneuver, this value increased (P <.05). Spontaneous breathing in COPD resulted in SV changes (P <.05) similar to those observed in healthy subjects who performed the Valsalva maneuver. CONCLUSION: Real-time MR imaging of aortic flow reveals physiologic flow alterations, which are dependent on variations in the breathing pattern.     

Comparison and reproducibility of ADC measurements in breathhold, respiratory triggered, and free-breathing diffusion-weighted MR imaging of the liver

Purpose

To compare and determine the reproducibility of apparent diffusion coefficient (ADC) measurements of the normal liver parenchyma in breathhold, respiratory triggered, and free‐breathing diffusion‐weighted magnetic resonance imaging (DWI).

Materials and Methods

Eleven healthy volunteers underwent three series of DWI. Each DWI series consisted of one breathhold, one respiratory triggered, and two free‐breathing (thick and thin slice acquisition) scans of the liver, at b‐values of 0 and 500 s/mm². ADCs of the liver parenchyma were compared by using nonparametric tests. Reproducibility was assessed by the Bland–Altman method.

Results

Mean ADCs (in 10^?3 mm²/sec) in respiratory triggered DWI (2.07–2.27) were significantly higher than mean ADCs in breathhold DWI (1.57–1.62), thick slice free‐breathing DWI (1.62–1.65), and thin slice free‐breathing DWI (1.57–1.66) (P < 0.005). Ranges of mean difference in ADC measurement ± limits of agreement between two scans were ?0.02–0.05 ± 0.16–0.24 in breathhold DWI, ?0.14–0.20 ± 0.59–0.60 in respiratory triggered DWI, ?0.03–0.03 ± 0.20–0.29 in thick slice free‐breathing DWI, and ?0.01–0.09 ± 0.21–0.29 in thin slice free‐breathing DWI.

Conclusion

ADC measurements of the normal liver parenchyma in respiratory triggered DWI are significantly higher and less reproducible than in breathhold and free‐breathing DWI. J. Magn. Reson. Imaging 2008;28:1141–1148. © 2008 Wiley‐Liss, Inc.

     

The role of the right ventricle during hypobaric hypoxic exercise: insights from patients after the Fontan operation

OBJECTIVES: The principal objective of this study was to examine the importance of the right ventricle for maximal systemic oxygen transport during exercise at high altitude by studying patients after the Fontan operation. BACKGROUND: High-altitude-induced hypoxia causes a reduction in maximal oxygen uptake. Normal right ventricular pump function may be critical to sustain cardiac output in the face of hypoxic pulmonary vasoconstriction. We hypothesized that patients after the Fontan operation, who lack a functional subpulmonary ventricle, would have a limited exercise capacity at altitude, with an inability to increase cardiac output. METHODS: We measured oxygen uptake (VO2, Douglas bag), cardiac output (Qc, C2H2 rebreathing), heart rate (HR) (ECG), blood pressure (BP) (cuff), and O2 Sat (pulse oximetry) in 11 patients aged 14.5+/-5.2 yr (mean +/- SD) at 4.7+/-1.6 yr after surgery. Data were obtained at rest, at three submaximal steady state workrates, and at peak exercise on a cycle ergometer. All tests were performed at sea level (SL) and at simulated altitude (ALT) of 3048 m (10,000 ft, 522 torr) in a hypobaric chamber. RESULTS: At SL, resting O2 sat was 92.6+/-4%. At ALT, O2 sat decreased to 88.2+/-4.6% (P < 0.05) at rest and decreased further to 80+/-6.3% (P < 0.05) with peak exercise. At SL, VO2 increased from 5.1+/-0.9 mL x kg(-1) x min(-1) at rest to 23.5+/-5.3 mL x kg(-1) x min(-1) at peak exercise and CI (Qc x m(-2)) increased from 3.3+/-0.7 L x m(-2) to 6.2+/-1.2 L x m(-2). VO2 peak, 17.8+/-4 mL x kg(-1) x min(-1) (P < 0.05), and CI peak, 5.0+/-1.5 L x m(-2) (P < 0.05), were both decreased at ALT. Remarkably, the relationship between Qc and VO2 was normal during submaximal exercise at both SL and ALT. However at ALT, stroke volume index (SVI, SV x m(-2)) decreased from 37.7+/-8.6 mL x min(-1) x m2 at rest, to 31.3+/-8.6 mL x min(-1) x m2 at peak exercise (P < 0.05), whereas it did not fall during sea level exercise. CONCLUSIONS: During submaximal exercise at altitude, right ventricular contractile function is not necessary to increase cardiac output appropriately for oxygen uptake. However, normal right ventricular pump function may be necessary to achieve maximal cardiac output during exercise with acute high altitude exposure.     

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。