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.     

Inspiratory resistance, cerebral blood flow velocity, and symptoms of acute hypotension

INTRODUCTION: Symptoms of orthostatic intolerance, e.g., following prolonged bed rest and microgravity exposure, are associated with reductions in cerebral blood flow. We tested the hypothesis that spontaneously breathing through an impedance threshold device (ITD) would attenuate the fall in cerebral blood flow velocity (CBFV) during a hypotensive orthostatic challenge and reduce the severity of reported symptoms. METHODS: While breathing through either an active ITD (-7 cm H2O inspiratory impedance) or a sham ITD (no impedance), 19 subjects performed a squat stand test (SST). Symptoms upon stand were recorded on a 5-point scale (1 = normal; 5 = faint) of subject-perceived rating (SPR). To address our hypothesis, only data from symptomatic subjects (SPR > 1 during the sham trial) were analyzed (N = 9). Mean arterial blood pressure (MAP) and mean CBFV were measured continuously throughout the SST and analyzed in time and frequency domains. RESULTS: Breathing with the active ITD during the SST reduced the severity of orthostatic symptoms in eight of the nine symptomatic subjects (sham ITD SPR, 1.9 +/- 0.1; active ITD SPR, 1.1 +/- 0.1), but there was no statistically distinguishable difference in the reduction of mean CBFV between the two trials (sham ITD, -39 +/- 3% vs. active ITD, -44 +/- 3%). High frequency oscillations in mean CBFV, however, were greater during the active ITD trial (7.8 +/- 2.6 cm x s(-2)) compared with the sham ITD trial (2.5 +/- 0.9 cm x s(-2)). CONCLUSIONS: Higher oscillations in CBFV while breathing with the active ITD may account for the reduction in symptom severity during orthostatic hypotension despite the same fall in absolute CBFV.     

Impairment of cardiovascular and vasomotor responses during tilt table simulation of "push-pull' maneuvers

BACKGROUND: Numerous studies have shown that tolerance to positive acceleration (+Gz) is impaired subsequent to an exposure of less than +1 Gz. HYPOTHESIS: Vasodilation induced by antecedent negative Gz (-Gz) exposure delays sympathetic vasoconstriction during subsequent +Gz, further reducing G-tolerance. METHODS: There were 20 subjects tested on an electronic tilt table, and exposed to the following randomized head-up tilt (HUT) and head-down tilt (HDT) conditions: +75 degrees HUT for 60 s, followed by transition to either 0 degrees (supine) HDT, or -25 degrees HDT, or -45 degrees HDT for 7 or 15 s at tilt rate of 45 degrees x s(-1). This was followed by HUT, divided into three periods: HUT1 (approximately 3-10 s), HUT2 (approximately 15-22 s), and HUT3 (approximately 27-35 s). Systolic blood pressure (SBP) was normalized to heart and head-levels. Stroke volume (SV) was estimated using impedance cardiography; forearm blood flow (FBF) estimated by venous occlusion plethysmography and forearm vascular resistance (FVR) was calculated from FBF and SBP. Total peripheral resistance (TPR) was estimated by MAP/(SV*HR). RESULTS: Heart-level SBP decreased significantly during HDT for both HDT durations (p < 0.01). SBP increased significantly at head-level during HDT (p < 0.001). During HUT1 heart and head-level SBP decreased for all conditions (p < 0.001), recovering to baseline levels by HUT2. TPR decreased significantly for all HDT conditions (p < 0.001), with this decrease related to the degree of HDT angle (p < 0.05). During HUT1, TPR remained depressed below baseline. At HUT2, TPR remained decreased for the -45 degrees/7-s condition only (p < 0.01). FBF decreased significantly during HDT (p < 0.02), with the magnitude related to the HDT angle. FBF remained elevated during HUT1 (p < 0.01). FVR decreased as a function of HDT angle during HDT (p < 0.001), with the decrease persisting into the HUT1 phase (p < 0.01). By the HUT2 and HUT3 periods, FVR were above baseline levels for the -45 degrees HDT condition (p < 0.01). CONCLUSION: These results confirm in humans the delayed recovery of peripheral vascular resistance observed in animal studies when -Gz precedes +Gz. Since SV recovered to baseline levels during the "pull" phase (HUT1-3), with TPR and forearm vascular resistance remaining depressed, baroreflex-mediated peripheral vascular control is delayed. This delay at higher subsequent +Gz levels is dangerous for the military pilot, since symptoms of G-intolerance due to delay in head-level BP recovery will ensue at lower absolute +Gz levels during push-pull type maneuvers.     

Squat-stand test response following 10 consecutive episodes of head-up tilt

INTRODUCTION: The cardiovascular system is adaptable to multiple exposures to gravity over several days, and to repeated exposures in a single day. This study aimed to investigate if the cardiovascular adaptation observed following 10 +75 degrees head-up tilts (HUT) would improve the responses to the squat-stand test (SST). METHODS: There were 16 subjects who were randomly allocated into either a tilting group that underwent 10 +75 degrees HUTs in 70 min (Tilting) or a control group that underwent 70 min of rest (Control). Before and after HUT or rest, subjects performed a SST (SST1 and SST2, respectively). Heart rate (HR), BP [systolic (SBP), diastolic (DBP), mean (MAP)], total peripheral resistance (TPR), stroke volume (SV), and cardiac output (CO) were determined during both SSTs. The final 30 s of squat and the first 30 s of stand (divided into three 10-s blocks termed Stand10, Stand20, and Stand30) were compared between SST1 and SST2. RESULTS: In the Tilting condition, during the squat phase of SST2, the following were significantly elevated: SBP (131 +/- 9 to 140 +/- 7.2 mmHg) and MAP (94 +/- 8.7 to 105 +/- 10.2 mmHg); DBP (76 +/- 9.4 to 87 +/- 11.9 mmHg); TPR (1197 +/- 524.6 to 229 +/- 610.5 dyn x s(-1) x cm(-5)) and HR were significantly decreased (78 +/- 6.9 to 73 +/- 7.5 bpm) compared with SST1. At Stand10, DBP and MAP were significantly increased (59 +/- 9.5 to 69 +/- 15.7 mmHg and 74 +/- 8.9 to 84 +/- 14.7 mmHg, respectively); at Stand20, SBP was increased (121 +/- 17 to 128 +/- 22 mmHg); and at Stand30 the following were increased: DBP (64 +/- 8.8 to 75 +/- 16.1 mmHg); SBP (127 +/- 9.2 to 139 +/- 15.8 mmHg); and MAP (79 +/- 8 to 90 +/- 14.9 mmHg). There were no differences observed between SST1 and SST2 in the Control group. DISCUSSION: Cardiovascular responses to the SST can be improved with 10 consecutive +75 degrees HUTs. This is predominantly due to an increase in DBP, indicative of a change in vascular resistance.     

下体负压旋转床模拟航空推拉效应对心血管功能的影响

目的探讨下体负压旋转床模拟航空推拉效应的效果 ,观察推拉动作对心血管功能的影响。方法 8名被试者在下体负压旋转床上进行“直立位 (HUT ,+1Gz) -倒立位 -直立位 +下体负压 (LBNP ,- 5 0mmHg) 1 0min”的模拟推拉效应试验及单纯直立位 +下体负压 (- 5 0mmHg) 1 0min的对照试验 ,用阻抗法测量了试验过程中心率 (HR)、血压 (BP)、基础阻抗 (Z0 )、每搏心输出量 (SV)、心输出量 (CO)及总外周阻力 (TPR)等心血管功能指标的变化。结果在模拟推拉效应试验中 ,有 3名被试者没有完成直立位 +LB NP作用 1 0min的试验 ,出现晕厥前症状 ,8名被试者平均耐受时间为 8.99± 1 .47min。而对照试验时 ,被试者均完成了 1 0min的直立 +LBNP试验。模拟推拉效应试验时 ,在倒立位 ,被试者HR、Z0 较直立位对照值降低 ,SV和CO较直立位对照值升高 ,均有显著性意义 ;直立位 +LBNP过程中 ,HR、Z0 、TPR较对照和倒立位值显著增高 ,SV和CO较对照和倒立位值显著降低 ,SBP在HUT即刻较对照值显著性增高 ,在HUT +LBNP过程中显著性降低。在对照试验时 ,上述指标呈现出相同的变化 ,但增高或降低的百分比 (% )低于模拟推拉效应试验 (HR除外 )。结论倒立位后 ,再直立 +LBNP作用 ,心血管功能下降程度大于单纯直立 +LBNP作用 ,下体负压旋转床     

Characteristics of the reactions to active orthostatic and water loading tests in persons with varying resistance to +Gz loads

Thirty-seven healthy male test subjects, aged 19-21, with different +Gz acceleration tolerance were examined. Their blood pressure (BP) and heart rate (HR) during 5-min tilt tests and 2% water loading tests were measured 2-3 weeks prior to centrifugation. Quantitative evaluation of orthostatic tolerance using an orthostatic index and BP and HR responses to tilt tests before and after water loading revealed specific features of cardiovascular regulation in the subjects with high and low +Gz acceleration tolerance. The negative predictive indicators include: decreased BP, HR and cardiac index in the supine position in combination with high orthostatic tolerance, as well as decreased orthostatic tolerance in combination with a lower function of vasoconstrictor mechanisms in the upright position and a lower sensitivity of carotid sinus reflexes to blood volume changes during tilt and water loading tests. When examining test subjects with high +Gz tolerance, preference should be given to those who can well tolerate tilt tests and show moderately high BP and HR in the supine position, as well as to those who exhibit a noticeable increment of diastolic BP during 5-min tilt tests.     

The effect of low-level normobaric hypoxia on orthostatic responses

BACKGROUND: Altitude-induced hypoxia is believed to decrease tolerance to orthostasis. This preliminary investigation assesses the interaction between low-level normobaric hypoxia and the cardiovascular responses to orthostasis. HYPOTHESIS: We hypothesize that low-level hypoxia at simulated altitudes up to 3350 m adversely affects cardiovascular responses to orthostasis. METHODS: Six women and eight men underwent + 75 degrees head-up tilt (HUT) at 3 simulated altitudes (115 m (377 ft), 1680 m (5500 ft) and 3350 m (11,000 ft)). Minute ventilation (VE), tidal volume (VT), breathing frequency (f), heart rate (HR), and arterial blood pressure (BP) were measured non-invasively. Ventilatory parameters (VE, VT, and f) were measured during the control period to assess the respiratory response to hypoxia. Cardiovascular variables (HR and BP) during HUT were compared and analyzed by calculating the deviation from control (pre-tilt) values. RESULTS: Respiratory data showed no significant hypoxic ventilatory response. The average HR deviation significantly decreased (p < 0.05) with HUT from 13 +/- 2 bpm (115 m) to 9 +/- 2 bpm (3350 m). The pattern of mean arterial BP deviation with HUT was similar across the three altitudes (-6.6 +/- 2.9 mm Hg at 115 m to -6.8 +/- 2.4 mm Hg at 3350 m), with no significant differences. CONCLUSIONS: Changes in HR and arterial BP are fundamental aspects of the cardiovascular response to orthostatic stress. Although there were no significant changes in the BP response to orthostasis with hypoxia, the HR changes seen in this study suggest that the ability to modulate HR under orthostatic stress is reduced with exposure to low-level normobaric hypoxia.     

21天头低位(-6°)卧床前后下体负压和头高位倾斜暴露时的心血管反应

目的探讨２１天头低位（ＨＤＴ）卧床对人体立位应激下心血管反应的影响,以及比较下体负压（ＬＢＮＰ）和头高位倾斜（ＨＵＴ）两种立位应激下心血管反应的差异。方法６名受试者在２１天ＨＤＴ前、后分别进行ＬＢＮＰ（－４．００ｋＰａ／３ｍｉｎ、－６．６７ｋＰａ／３ｍｉｎ及－９．３３ｋＰａ／３ｍｉｎ）和ＨＵＴ试验（３０°／３ｍｉｎ、４５°／３ｍｉｎ、６０°／３ｍｉｎ及７５°／３ｍｉｎ）,以比较两种检测方法的血压（ＢＰ）和心率（ＨＲ）的变化情况。结果与ＬＢＮＰ（或ＨＵＴ）前相比：①ＬＢＮＰ（ＨＵＴ）时ＨＲ显著增加（Ｐ＜０．０１）,ＳＢＰ显著降低（Ｐ＜０．０５）;卧床后相应的变化量增加。②ＬＢＮＰ时ＤＢＰ降低（卧床后达显著,Ｐ＜０．０５）;而ＨＵＴ时ＤＢＰ增加（卧床后达显著,Ｐ＜０．０５）。③ＬＢＮＰ时ＭＡＰ均显著降低（Ｐ＜０．０５）;而ＨＵＴ时在卧床前无变化（Ｐ＞０．０５）,在卧床后显著增加（Ｐ＜０．０５）。结论ＬＢＮＰ和ＨＵＴ引起的ＣＶＳ反应并不相同。笔者认为,ＨＵＴ更能促进心血管系统对立位应激的调节作用。     

Cardiovascular effects of the breathhold used in determining pulmonary diffusing capacity.

BACKGROUND: The single-breath technique for determination of the diffusion capacity of the lung for CO (DlCO) requires a 10-s breathhold at total lung capacity. The assumption has been that this breathhold does not alter the components of DlCO, i.e., the diffusion capacity of the membrane (Dm) and the pulmonary capillary blood volume (Qc), and therefore measurement of these variables during breathhold represents these variables as they exist during normal breathing. The purpose of this study was to determine the effect of the 10-s breathhold on cardiac output (Q) and Qc while supine and standing. The hypothesis was that the standing posture would have a greater influence on Q and Qc during the breathhold than would the supine posture. METHODS: Twelve male subjects participated. Q, stroke volume (SV), heart rate (HR), BP (MAP), and total peripheral resistance (TPR) were determined before and during the 10-s breathhold determination of DlCO, Qc, and Dm. RESULTS: Results while supine were compared with those while standing. DlCO was reduced on standing, due mainly to a reduction in Qc. SV and Q decreased significantly during the 10-s breathhold in both postures. Both SV and Q decreased more when standing (-53% and -49.5%, respectively) than when supine (-40.5% and -36.5%, respectively). Thus, the 10-s breathhold caused significant reductions in Q and therefore may alter the measurement of DlCO and Qc. CONCLUSIONS: The greater decline in Q during the measurement of DlCO when standing would suggest that the DlCO and Qc values while breathing might be underestimated in the upright posture.     

Hemodynamic responses to upright tilt at sea level and high altitude

Hemodynamic responses to upright tilt were studied in eight young men at sea level (SL); after 1 h at 4,300 m simulated altitude (SA); and at 18 h, 66 h and 114 h during residence at 4,300 m (HA). Heart rate (HR), stroke volume (SV), cardiac output (CO), calf blood flow (CBF), blood pressure (BP) and total peripheral resistance (TPR) were obtained during supine rest and after 13 min of 60 degrees head-up tilt using an impedance monitor and electrosphygmomanometer. SL to HA changes in blood volume (BV) were calculated from hematocrit and hemoglobin values. Plasma norepinephrine (Nor) was measured at SL and after 18 h and 66 h of HA. Supine HR, TPR and BP were increased while SV, CO and CBF were reduced SL to HA (p less than 0.05). HR and BP in the upright position were increased SL to HA (p less than 0.05). The responses to tilt (delta supine to upright) were unaltered SL vs SA. With prolonged exposure, SV, CO, TPR and CBF responses to tilt were reduced (p less than 0.05). The reduced responses to tilt at HA were associated with a 10% decline in BV (p less than 0.01) and a 40% increase in Nor (p less than 0.05). It was concluded that the reduction in SV during tilt at SL and SA was compensated for by increases in HR and TPR in order to maintain BP. After 18 h HA, BP in the upright position was maintained only by an increase in HR.     

Cardiovascular training effects in fighter pilots induced by occupational high G exposure

INTRODUCTION: A fundamental difference in the cardiovascular response to acceleration between a group of fighter pilots (FP) and a group of non-pilots (NP) has been demonstrated previously. This study investigated the longitudinal effects of repetitive occupational +Gz exposure on the cardiovascular response to acceleration. METHODS: There were 6 FP and 6 NP subjects who underwent rapid +75 degrees head-up tilt (HUT) on two separate occasions. The FP group were tested after a non-flying period of 5 wk (Test 1), and tested again after a period of repetitive exposure to high +Gz missions (Test 2). The NP group did not fly at all between Test 1 and Test 2. Mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), and total peripheral resistance (TPR) were all determined non-invasively. SV was determined using impedance cardiography and calculated via the Kubicek equation. For each variable, resting values and the response to tilt for both HUT tests within and between each group were compared. RESULTS: In the FP group, resting MAP was higher (86 mmHg) in Test 2 compared with Test 1 (78 mmHg). Between groups, FP resting MAP was only different from the NP resting MAP in Test 2. The FP HR response to HUT increased significantly between the two tests. CONCLUSIONS: These findings suggest a +Gz-induced cardiovascular training effect in the FP group. Repetitive exposure to +Gz results in an increased resting MAP and an elevated HR response to tilt, which may provide benefits to operational fighter pilots.     

Is there a disassociation of maximal oxygen consumption and maximal cardiac output?

PURPOSE: The purpose of the present study was to determine whether maximal cardiac output (Q) is affected by the duration of the maximal exercise test. METHODS: Nine healthy men (N = 6) and women (N = 3) performed three separate maximal treadmill exercise tests, separated by at least 24 h, and underwent a body composition assessment by hydrostatic weighing, all within a 2-wk period. A baseline maximal exercise test was performed to establish VO(2max). The second and third tests, assigned randomly, were designed to elicit the subjects' predetermined VO(2max) in either 6 or 12 min. Heart rate (HR), blood pressure (BP), minutes of ventilation, and oxygen consumption (VO(2)) were measured during all tests. At the end of the 6- and 12-min tests, Q was measured using an acetylene rebreathing technique. Stroke volume (SV), mean arterial pressure (MAP), total peripheral resistance (TPR), and arteriovenous O(2) difference were calculated using standard equations. RESULTS: Repeated-measures ANOVA indicated that there were no significant differences in HR and VO(2max) between the baseline, 6-min, and 12-min tests. Paired t-tests revealed significantly greater Q (25.1 +/- 5.6 vs 23.7 +/- 5.2 L.min-1) and SV (138.3 +/- 31.5 vs 130.5 +/- 31.2 mL) in the 6- versus 12-min tests, respectively. There were no significant differences in systolic BP, diastolic BP, MAP, TPR, or arteriovenous O(2) difference. CONCLUSIONS: Despite there being no difference in VO(2max) between the two tests, the 6-min maximal exercise test resulted in a significantly greater Q than the 12-min test, because of a significantly greater SV. Thus, there was a disassociation between VO(2) and Q during maximal exercise.     

三种加压呼吸水平下不同胸部对抗压时的心血管反应

目的 探讨加压呼吸时胸部对抗压对心血管的影响。方法 观察7名18－20岁健康男性青年穿两种背心抗荷裤，在30、50、70mmHg3种加压水平，持续3min加压呼吸的心血管反应。结果 所有被试者均完成试验。在加压值为50、70mmHg持续3min时，与各自对照组比，HR、MAP及TPR明显提高，SV明显降低，Q－Z间期在各加压值加压呼吸时无明显变化。两种装备在70mmHg加压呼吸时ΔSV，ΔTPR有明显性差异（P＜0．05）。结论 使用这两种装备进行加压呼吸均可造成MAP、TPR和HR明显增加，SV明显减少。加压呼吸对心肌收缩功能并无影响。在加压呼吸条件下，胸部对抗压与面罩腔内压力相等，对提高SV有明显效果。     

Baroreflex control of heart rate during heating in subjects with low orthostatic tolerance

BACKGROUND: Heat stress induces a reduction of orthostatic tolerance. The cardiovascular responses, including the cardiac baroreflex response to heat stress, were examined to test the hypothesis that subjects with orthostatically low tolerance demonstrate an impaired baroreflex control of heart rate (HR) during heat stress. METHODS: There were 44 healthy young volunteers who underwent whole body heat stress produced by a hot-water-perfused suit during supine rest for 45 min and 75 degrees head-up tilt (HUT) for 6 min. Esophageal temperature, HR, arterial pressure, and skin blood flow in the forearm and palm were measured continuously throughout the experiment. The sensitivity of the arterial baroreflex control of HR was calculated from the spontaneous changes in beat-to-beat arterial pressure and HR. RESULTS: The HUT was uneventful for 22 volunteers (higher tolerance group), but 22 volunteers (lower tolerance group) reached presyncope after 195 +/- 19 s. Esophageal temperature, HR, arterial pressure, and skin blood flow changed similarly in the two groups during heating. In the preheating condition, the sensitivity of the baroreflex control of HR did not differ significantly between the two groups. Heating did not alter the sensitivity of baroreflex control of HR in the higher tolerance group, but decreased it significantly in the lower tolerance group. Heating increased the number of heartbeats used for analysis of the baroreflex sensitivity in the higher tolerance group, but did not change it in the lower tolerance group. CONCLUSIONS: These results suggest that the impairment of vagal baroreflex control of HR during heat exposure aggravates the orthostatic intolerance in heat-stressed humans.     

Left ventricular volumes and hemodynamic responses to postexercise ischemia in healthy humans

PURPOSE: The purpose of this study was to determine the cardiac mechanisms involved in cardiovascular adjustments during postexercise circulatory occlusion (OCCL). METHOD: Heart rate (HR), mean arterial pressure (MAP), left ventricular end-diastolic (EDV) and end-systolic volumes (ESV), stroke volume (SV), cardiac output (CO), and total peripheral vascular resistance (total peripheral resistance (TPR)) were assessed in nine healthy volunteers during rest and static exercise at 30% of maximum voluntary contraction followed by either OCCL for 3 min or non-OCCL in a randomized crossover protocol. RESULTS: During handgrip, HR (+20%; P < 0.001), CO (+11%; P = 0.003), MAP (+18%; P = 0.001), and TPR (+6%; P = 0.004) increased, SV (-8%; P = 0.001) and EDV (-5%; P < 0.001) decreased, while ESV did not change (P > 0.05). These responses were similar between conditions (P > 0.05). During OCCL, HR, SV, and CO returned to baseline, whereas MAP (+19%; P < 0.001) and TPR (+9%; P = 0.004) remained elevated. EDV (+12%; P < 0.001) and ESV (+23%; P < 0.001) increased in parallel above resting values. CONCLUSION: Activation of muscle metaboreceptors during OCCL increased MAP by elevating TPR. Despite the higher afterload and increased ESV, CO and SV were kept similar to resting values because EDV also increased, implying the involvement of the Frank-Starling mechanism.     

旋转床模拟推拉动作的心血管效应

目的　使用旋转床模拟推拉动作 ,观察推拉动作时人体的生理反应 ,探讨推拉效应的发生机理。　方法　 11名受试者在旋转床上经受“直立位 ( 1GZ)→倒立位 ( - 1GZ)→直立位 ( 1GZ)”的模拟推拉动作 ,测定心率 ( HR)、血压 ( BP)、基础阻抗 ( Z0 )、每搏心输出量 ( SV)及总外周阻力( TPR)等生理指标。　结果　在倒立位时 HR、平均动脉压 ( MAP)、舒张压 ( DBP)、Z0 及 TPR较对照降低均有显著性意义 [HR分别为 ( 6 0 .8± 7.9)次 / min与 ( 87.8± 7.4)次 / min,P<0 .0 1;MAP分别为( 83.8± 8.2 ) m m Hg与 ( 91.8± 15 .0 ) mm Hg,P<0 .0 5 ;DBP分别为 ( 6 7.3± 8.3) mm Hg与 ( 77.0±15 .6 ) m m Hg,P<0 .0 5 ;Z0 分别为 ( 2 7.0± 2 .8) Ω 与 ( 2 9.1± 3.3) Ω,P<0 .0 1;TPR分别为 ( 73.15±12 .12 ) k Pa· s· L- 1 与 ( 113.3± 36 .4) k Pa· s· L- 1 ,P<0 .0 1],SV升高有显著性意义 [分别为 ( 15 5 .0± 2 6 .1) m l与 ( 78.7± 19.5 ) ml,P<0 .0 1],SBP有降低趋势 ,但未达显著水平。在再次直立位时 ,HR、MAP、DBP、Z0 及 TPR较对照降低均有显著性意义 [HR分别为 ( 81.9± 8.9)次 / min与 ( 87.8± 7.4)次 / m in,P<0 .0 5 ;MAP分别为 ( 88.1± 13.6 ) mm Hg与 ( 91.8± 15 .0 ) m m Hg,P<0     

Cardiovascular responses to head-up tilt after an endurance exercise program

The cardiovascular responses to 10 min of orthostasis were assessed before and after an aerobic exercise program. Five men and five women (18-25 years old) exercised for 7 weeks, four times per week, for 50 min per session at 70% of maximal heart rate (HR). Before and after the exercise program, maximal aerobic power (VO2max) was determined, and HR, systolic (SBP), diastolic (DBP), and pulse (PP) blood pressures were measured each minute during 5 min of supine rest, 10 min of foot-supported 70 degree head-up tilt (HUT), and 5 min of supine rest. Orthostatic tolerance was not determined. Calf compliance was measured in five of the subjects before and after the program as the change in leg volume at occluding pressures of 20, 40, 60, 80, and 100 mm Hg. Following the program, VO2max increased by 8.7% (p = 0.012), while decreases were noted in resting HR (9.4%, p = 0.041), SBP (5.0%, p less than 0.0005), and DBP (14.2%, p less than 0.0005). Despite a greater HR increase during HUT (7.1 beat.min-1, p = 0.034), SBP decreased by 3.4 mm Hg during HUT after the exercise program (p = 0.008). No differences were noted in the changes in DBP, MAP, or PP upon tilting (p greater than 0.05). After the program, the amount of fluid pooled in the calf at high occluding pressures (80 and 100 mm Hg) increased by 0.96 +/- 0.24 and 1.10 +/- 0.33 ml.100 ml tissue-1 (X +/- S.E.M., p = 0.017 and p = 0.028, respectively). We suggest that control of blood pressure during 10 min of orthostasis may be altered by endurance exercise training.     

Influence of weight training status on hemodynamic adjustments to isometric actions

BACKGROUND: The purpose of this study was to investigate the hemodynamic adjustments to fatiguing isometric handgrip (IHG) performed at 20 and 60% of maximal voluntary contraction (MVC) in 10 weight-trained (WT, 4.8+/-1.6 yrs) and 8 untrained (UT) men. METHODS: Hemodynamic measures were recorded at rest, during sustained IHG to fatigue, and during recovery. Blood pressures and heart rates (HR) were measured by auscultation and electrocardiography, respectively. Stroke volume (SV) was assessed by impedance cardiography. Mean arterial pressure (MAP), cardiac output (CO) and total peripheral resistance (TPR) were calculated. RESULTS: Between group comparisons in peak hemodynamic adjustments (fatigue value-resting value) to IHG were analyzed across intensities by MANOVA and follow-up univariate tests. The peak adjustments in MAP, HR, and SV during IHG at 20 and 60% MVC were dependent on intensity (60%>20% MVC) but were not significantly different (p>0.05) between groups. However, the intensity-dependent, peak adjustments in CO (WT=1.17+/-1.2 L x min-1 vs UT= -0.40+/-1.7 L x min-1, p=0.005) and TPR (WT=4.4+/-6.1 PRU vs UT=10.2+/-8.3 PRU, p=0.02) were significantly different between groups across intensities of IHG. CONCLUSIONS: The results suggest that weight training does not significantly influence the pressor response but may significantly modify the adjustments in total-body circulation and vascular resistance during fatiguing IHG performed at 20 and 60% MVC.     

Stroke volume during orthostatic challenge: comparison of two non-invasive methods

BACKGROUND: Real time non-invasive determination of stroke volume (SV) is important to astronaut orthostatic testing. We compared simultaneous estimates of SV calculated from peripheral pulse waveforms with a more conventional non-invasive technique. METHODS: Ten men and nine women completed 12-min protocols. The relative change (% delta) in beat-to-beat SV was estimated non-invasively from changes in pulse waveforms measured by application of infrared finger photoplethysmography (IFP) and thoracic impedance cardiography (TIC). The % deltaSV values were calculated from continuous measurements in the supine posture and over the first 10 s (T1), second 10 s (T2), and 3 min (T3) of 80 degrees head-up tilt (HUT). RESULTS: Average % deltaSV measured by IFP at T1 (-11.7 +/- 3.7%) was statistically less than the average % deltaSV measured by TIC at T1 (-21.7 +/- 3.1%), while average % deltaSV measured by IFP at T2 (-16.2 +/- 3.9%) and T3 (-19.1 +/- 3.8%) were not statistically distinguishable from the average % deltaSV measured by TIC at T2 (-21.8 +/- 2.5%), and T3 (-22.6 +/- 2.9%). Correlation coefficients (r2) between IFP and TIC were 0.117 (T1), 0.387 (T2), and 0.718 (T3). CONCLUSION: IFP provides beat-to-beat (real-time) assessment of % deltaSV after 20 s of transition to an orthostatic challenge that is comparable to TIC. IFP technology flown during space missions can be used to assess physiological status and countermeasure effectiveness for orthostatic problems that may arise in astronauts after spaceflight. While the peripherally measured IFP response is delayed, the ease of implementing this monitor in the field is advantageous.     

The effect of baroreflex adaptation on the dynamic cardiovascular response to head-up tilt

BACKGROUND: Baroreflex adaptation to repetitive +Gz has been reported previously. The underlying mechanism may involve different responses of stroke volume (SV) and total peripheral resistance (TPR) to +Gz. HYPOTHESIS: The previously observed enhanced mean arterial pressure (MAP) regulation in fighter pilots (FP) is mediated by increases in SV and/or TPR. METHODS: There were 8 pilots and 12 non-pilots who underwent head-up tilt. SV was determined using impedance cardiography. RESULTS: MAP increased significantly in FP, due to heart rate (HR) and TPR increasing more and SV decreasing less. CONCLUSION: Baroreflex adaptation results in better performance of HR, SV and TPR in response to +Gz.