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.     

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.     

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。     

累加角度头高斜位下心率、血压、心率变异性与脑血流变化的特征

目的探讨累加角度头高斜位暴露下 ,正常人体血压、心率、脑血流及心率变异性 (HRV)变化特征 ,为建立飞行员晕厥检查的方法学提供生理依据。方法采用自行研制的多功能双向式医用倾斜床 ,结合先进的十二导联同步心电图描记方法和经颅多普勒脑血流成像技术进行累加角度 ( + 4 5°→ 60°→75°)头高斜位 (HUT)暴露下的心率、血压、HRV和脑血流速率的测量与分析。结果 1 )随着倾斜角度加大 ,暴露时程延长 ,HR逐渐增快 ,SBP逐渐下降 ,DBP逐渐升高 ,PP逐渐缩小 ,与暴露前比较 ,HR、PP和DBP相差非常显著 (P <0 .0 5 ) ;脑血流速率 (包括 :Vs、Vm、Vd)逐渐下降 ,无论与暴露前比 ,或倾斜角度之间比 ,差异十分显著 (P <0 .0 1 ) ;mRR和sdRR逐渐减小。 2 )生理指标与技术指标直线相关分析发现 :在 1min时 ,SBP、DBP、MAP、Vd、sdRR与倾斜角度呈明显正相关 (P <0 .0 1 ) ;在 2 0min时 ,HR和sdRR与倾斜角度呈明显正相关 (P <0 .0 1 ) ,与SBP、PP、Vs、mRR明显负相关 (P <0 .0 1 ) ;暴露时程与HR呈明显正相关 (P <0 .0 1 ) ,与SBP、Vm、Vd、mRR呈明显负相关 (P <0 .0 1 )。 3)生理指标间相关分析发现 :HR与SBP、Vs、Vm、Vd 呈明显负相关 (P <0 .0 1 ) ;SBP与Vd 和mRR呈明显正相关 (P <0 .0 1 ) ;Vs、Vm、Vd 与mRR和sdR     

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.     

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     

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.     

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.     

The aftereffects of dynamic exercise on ambulatory blood pressure.

PURPOSE: Despite the widespread belief that endurance exercise lowers blood pressure (BP), reports of the lack of such an effect are common when using ambulatory blood pressure (ABP) monitoring. This review examined the influence of dynamic exercise on ABP to elucidate possible reasons for conflicting reports. METHODS: Criteria for inclusion were dynamic exercise studies that involved ABP determinations after exercise. A total of 23 studies containing 34 groups of which 12 were normotensive and 22 hypertensive met these criteria. The groups primarily consisted of unmedicated, sedentary, overweight, non-Hispanic white men and women with a mean +/- SEM age of 43.9 +/- 1.5 yr. RESULTS: Baseline mean day systolic BP (SBP) was 137.3 +/- 2.1 mm Hg (95% confidence limits) (133.0, 141.5) and diastolic BP (DBP) was 86.7 +/- 1.5 mm Hg (83.7, 89.7). After exercise, day SBP (N = 34) was reduced an average of 3.2 +/- 1.0 (-5.2, -1.2) (P = 0.003) and DBP by 1.6 +/- 0.6 mm Hg (-2.9, -0.4) (P = 0.013); night SBP (N = 20) by 3.4 +/- 1.0 (-5.6, -1.2) (P = 0.005) and DBP by 3.0 +/- 1.4 mm Hg (-5.6, -0.4) (P = 0.025); and 24-h SBP (N = 17) by 3.2 +/- 0.8 (-4.9, -1.6) (P = 0.001) and DBP by 1.8 +/- 0.5 mm Hg (-2.9, -0.7) (P = 0.003). After detection of outlying cases using regression diagnostics, initial SBP accounted for 30% (P = 0.001) and 26% (P = 0.018) of the variance in the change in day and night SBP after exercise, respectively. Similarly, initial DBP explained 37% (P = 0.000) and 33% (P = 0.005) of the difference in day and night DBP after exercise. CONCLUSION: The ABP difference after exercise is a function of initial values such that groups with the highest baseline BP experience the greatest postexercise ABP reductions. The ABP dynamic exercise studies raise important methodological considerations that should be attended to in future work.     

Does endurance training affect orthostatic responses in healthy elderly men?

PURPOSE: To investigate the effects and time course of endurance training on the regulation of heart rate (HR), arterial pressure (AP), norepinephrine (NE), and plasma volume (PV) during orthostatic stress in healthy elderly men. METHODS: Thirty-one healthy men (65--75 yr) were randomly allocated into endurance training (N = 20, EX) and control (N = 11, CON) groups. The EX group cycled 3 d x wk(-1) for 30 min at 70% VO(2peak) for 12 wk x VO(2peak) was determined on an electronically braked cycle ergometer, before training and after 4, 8, and 12 wk of endurance training. The immediate (initial 30 s), early steady-state (1 min), and prolonged (5, 10, 15 min) beat-by-beat HR and AP responses during 90 degrees head-up tilt (HUT) were measured at least 3 d after each VO(2peak) test. Spontaneous baroreflex slopes were determined by application of linear regression to sequences of at least three cardiac cycles in which systolic blood pressure (SBP) and R-R interval changed in the same direction. Venous blood was collected during 90 degrees HUT and analyzed for changes in plasma NE concentrations, as well as hematocrit and hemoglobin to determine changes in PV. RESULTS: Endurance training significantly (P < 0.01) increased VO(2peak) (mL x kg(-1) x min(-1)) in EX by 10 +/- 2%. The immediate, early steady-state, and prolonged HR and AP responses and spontaneous baroreflex slopes during 90 degrees HUT were not significantly different (P > 0.05) between EX and CON groups before or after 4, 8, or 12 wk of endurance training. No significant differences (P > 0.05) were observed between EX and CON groups for peak changes in PV during orthostasis before (-15.0 +/- 1.4% vs -11.9 +/- 1.3%) or after 4 (-12.2 +/- 1.0% vs -12.7 +/- 1.4%), 8 (-13.7 +/- 1.2% vs -12.4 +/- 0.7%), and 12 wk (-10.8 +/- 1.6% vs -10.6 +/- 0.6%) of endurance training, suggesting a similar stimulus presented by 90 degrees HUT in both groups. Peak changes in NE concentrations during HUT were similar (P > 0.05) between EX and CON groups before (119 +/- 23 pg x mL(-1) vs 191 +/- 36 pg x mL(-1)) and after 4 (139 +/- 29 pg x mL(-1) vs 146 +/- 25 pg x mL(-1)), 8 (114 +/- 32 pg x mL(-1) vs 182 +/- 41 pg x mL(-1)), and 12 wk (143 +/- 35 pg x mL(-1) vs 206 +/- 42 pg.mL-1) of endurance training. CONCLUSIONS: These data indicate that in healthy elderly men, improvements in VO(2peak) can occur without compromising the regulation of HR, AP, NE, and PV during orthostatic stress.     

24h头低位卧床对心血管功能及立位应激反应的影响

目的 观察２４ｈ头低位卧床对心血管系统功能和头高位倾斜心血管反应变化的影响。方法健康男性学员６名,年龄２２～２３岁,进行了２４ｈ头低位卧床实验,卧床期间用阻法测量了心脏泵血功能指标,并观测了卧床前后头高位倾斜时的心血管反应和卧床期间尿量的变化。结果 在卧床期间受试者心率较卧床前（立位）显著减慢,第６、１２、１８ｈ的心率较卧床开始时（０ｈ）显著降低。ＳＶ和ＳＩ在卧床０、６、１２和２４ｈ较卧床前显著增     

头低位暴露后直立位下体负压作用的耐力变化和心血管反应

目的观察头低位后直立位下体负压暴露的耐力和心血管反应。方法8名被试者在下体负压倾斜床上进行“直立位-倒立位30s-直立位联合下体负压(-60mmHg)致晕厥前症状”的模拟推拉效应试验及单纯直立位 下体负压(-60mmHg)致晕厥前症状的对照试验,用阻抗法测量了试验过程中心血管功能指标的变化。结果在模拟推拉效应试验中,8名被试者出现晕厥前症状的平均耐受时间为4.5±2.4min,显著低于对照试验时的8.4±2.1min(P<0.01)。在倒立位,被试者HR较直立位基础值降低,SV和CO较直立位基础值升高,均有显著性意义。和基础值相比,直立位下体负压时HR增加的百分比显著低于对照试验(P<0.05),而直立位下体负压时SV和CO降低的百分比均显著高于对照试验时SV和CO降低的百分比(P<0.05)。在直立位下体负压暴露时,对照试验的PP和基础值比较显著降低(P<0.05),TPR比基础值显著增加(P<0.05)。结论倒立位后,再进行直立位下体负压作用,出现晕厥前症状的平均耐受时间缩短,心血管反应降低。     

Cerebral blood flow response and its association with symptoms during orthostatic hypotension

INTRODUCTION: The preservation of cerebral blood flow with orthostatic hypotension (e.g., following prolonged bed rest or microgravity exposure) is vital for the attenuation of symptoms and the maintenance of consciousness. We tested the hypothesis that decreasing mean arterial pressure (MAP) by > 30% is associated with compromised cerebral autoregulation and orthostatic symptoms during a squat-stand test (SST). METHODS: There were 19 subjects who performed an SST. MAP and middle cerebral artery blood flow velocity (CBFV) were recorded continuously. Subjects were divided retrospectively into those who reported: (1) at least one orthostatic symptom (Sx; n=9); or (2) no orthostatic symptoms (NSx; n=10). Cerebral autoregulation was assessed via the calculation of time to nadir and time to recovery for MAP and CBFV and linear regression analysis of the dynamic changes in MAP and CBFV (within 10 s of standing). RESULTS: On standing, MAP decreased by 37 +/- 2% (NSx) and 42 +/- 4% (Sx) (p = 0.100). CBFV fell by 6% more in the Sx group than in the NSx group (NSx, -33 +/- 1% vs. Sx, -39 +/- 3%, p = 0.032). Cerebral autoregulation remained intact in both groups as indicated by: (1) a faster time to nadir for CBFV compared with MAP; (2) a faster time to recovery for CBFV compared with MAP; and 3) a poor correlation between CBFV and MAP responses on standing (NSx R2 = 0.43; Sx R2 = 0.60). CONCLUSION: Lower cerebral blood flow during severe hypotension may account for the reporting of orthostatic symptoms, despite the maintenance of cerebral autoregulation.     

Inspiratory impedance threshold device effects on hypotension in heat-stroked swine

INTRODUCTION: Heat stroke is still an epidemiologically relevant disease with overall mortality rates as high as 15-33%. In heat stroke, hypotension is caused by relative central hypovolemia. We performed a feasibility study to determine if an inspiratory impedance threshold valve (ITDTM) set to open at -12 cm H2O would reverse systemic hypotension by enhancing venous return in hyperthermic, hypotensive, but spontaneously breathing animals. METHODS: Seven anesthetized pigs weighing 30 +/- 2 kg were warmed with a heating device until a mean rectal temperature of 44 degrees C was reached, and mean arterial blood pressure (MAP) was < or = 60 mmHg. The animals were then treated with the ITD. An intravenous bolus of 200 cc of 4 degrees C normal saline was delivered 20 min after the ITD was placed, and external cooling was started. RESULTS: Heat stroke criteria were achieved within 105 +/- 15 min. MAP had decreased from 105 +/- 5 to 57 +/- 5 mmHg, and respiratory rates had increased from 33 +/- 2 to 101 +/- 13 breaths/min. Addition of the ITD significantly improved MAP to 85 +/- 4 mmHg, and reduced respiratory rate to 54 +/- 6 breaths/min within 2 min. The effect was sustained until fluid replacement and external cooling were delivered 20 min later. At that point, MAP returned to baseline within 30 min, and 6/7 animals survived for an additional 30 min. CONCLUSIONS: Use of an inspiratory impedance threshold device resulted in an immediate rise in blood pressure in animals in heat stroke and preserved blood pressure for at least 20 min prior to cooling and fluid replacement.     

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。     

The chemoreflex: adult versus child comparison

PURPOSE: Compare the influence of the chemoreflex on heart rate (HR) and blood pressure (BP) responses between children and adults. METHODS: Seventy children (35 boys and 35 girls) and 70 adults (35 men and 35 women) performed 30% maximal voluntary contraction static handgrip exercise (SHG) for 3 min, followed by 4 min of muscle ischemia and then 6 min of recovery, while HR and BP were measured. RESULTS: During 3 min of SHG, systolic BP (SBP) increased significantly more in men versus boys (23+/-9 vs 18+/-9%) and women versus girls (21+/-10 vs 15+/-7%), respectively. Diastolic BP (DBP) increased similarly during SHG in adults versus children. During SHG, HR increased similarly in males but significantly more in girls versus women (16+/-10 vs 11+/-9%), respectively. During occlusion, HR fell to baseline and remained through recovery in adults and children. Both SBP and DBP decreased similarly during the first minute of occlusion but remained significantly above baseline in adults and children. During the first minute of recovery, BP decreased to baseline in adults. In children, DBP decreased but remained significantly above baseline through recovery. SBP decreased in children during minute 1 of recovery yet remained significantly above baseline for 1-2 min in children. CONCLUSIONS: The muscle chemoreflex influence on HR and BP is similar in children and adults as evident in the return of HR to baseline, with a similar concomitant maintenance of BP above baseline during occlusion. When occlusion is released, BP returns to baseline in adults, whereas in children DBP remains significantly above baseline through recovery and SBP for 1-2 min. The differing BP response may be explained by a more active arterial chemoreflex in children.     

Model of depressed myocardium shows orthostatic intolerance with or without reduced blood volume

INTRODUCTION: The development of orthostatic hypotension (OH) is complex and multi-factorial. Previous simulation work indicates that myocardial contractility depression (MCD) may increase OH when there is a total blood volume decrease. This paper hypothesized that MCD increased OH in both humans with and without decrease in blood volume. METHODS: A model, which was previously used to reproduce cardiovascular response to lower body negative pressure (LBNP), hypovolemia, and MCD, was modified by incorporating the physiologic mechanism of plasma filtration into the interstitium during LBNP. The model was evaluated by human experimental results. Using the model, HR and BP response to LBNP were simulated at conditions of 10%, 20%, and 30% MCD. Additionally, HR and BP response to LBNP were simulated at conditions of 10% and 20% MCD with a 12% decrease in blood volume. RESULTS: Simulation results indicate that the increments of HR and decrements of systolic BP (SBP) and mean arterial pressure (MAP) rise with the increases of MCD. Specifically, simulation results indicate that about 30% MCD would cause OH (HR: 117 bpm; SBP: 92 mmHg; MAP: 78 mmHg). It also indicates that about 20% MCD would cause OH (HR: 134 bpm, SBP: 84 mmHg, MAP: 73 mmHg) with a 12% decrease in total blood volume. CONCLUSION: It is suggested that MCD increases OH whether or not there is a total blood decrease, and further suggested that MCD induced by both spaceflight and heart disease may increase OH.     

Blood pressure and sympathetic activity in normotensive aviators during short-haul fixed-wings flights

BACKGROUND: In order to evaluate the impact of in-flight stress on BP and sympathetic activity during a short fixed-wing flight, we have studied 18 healthy and normotensive commercial captain aviators, aged 37.5 +/- 4.9 yr. METHODS: The protocol consisted of a 120-min flight period, divided into segments of pre-flight, take off, mid-cruise, approach and landing, and a 120-min control period. In both flight and control periods, all subjects underwent BP monitoring, heart rate recording and urine collection for catecholamines. RESULTS: Systolic and diastolic BP (SBP/DBP) were higher during the flight, as a whole, when compared with the control period (134 +/- 11 vs. 121 +/- 8 mmHg, p < 0.05 and 84 +/- 8 vs. 76 +/- 9 mmHg, p < 0.05, respectively). During the flight period, SBP increased in the pre-flight, take-off, approach and landing segments, whereas DBP increased in the take-off, mid-cruise, approach and landing segments. Heart rate did not change in any flight segment. Urinary catecholamines increased during the flight period in comparison to control period 0.20 (0.10 1.8) mg x mg-1 creatinine vs. 0.10 (0.10-1.0) mg x mg-1 creatinine; p < 0.05). CONCLUSION: We conclude that inflight stress increases BP of the normotensive aviators by sympathetic activation during short-haul flights.     

下体负压对抗21d头低位卧床后立位耐力不良的研究

目的观察ＬＢＮＰ对２１ｄＨＤＴ－６°卧床模拟失重所致立位耐力不良的对抗效果。方法１２名健康男性青年志愿者,进行２１ｄＨＤＴ－６°卧床实验。随机分为对照组和下体负压组,每组６人。与对照组不同,下体负压组在最后一周,每天进行１ｈ、－４．０ｋＰａ的下体负压锻炼。结果卧床前,１２名受试者顺利通过７５°、２０ｍｉｎ立位耐力检查。卧床第１０ｄ立位耐力检查时,对照组有５人、ＬＢＮＰ组有４人出现晕厥前或晕厥症状,两组平均耐受时间均低于卧床前（Ｐ＜０．０５）;第２１ｄ时,对照组有５人未通过,平均耐受时间较卧床前显著降低（Ｐ＜０．０５）;而ＬＢＮＰ组有１人未通过,平均耐受时间显著高于对照组（Ｐ＜０．０５）。结论２１ｄ头低位卧床后立位耐力显著降低。下体负压能够有效对抗头低位卧床导致的立位耐力降低。     

The effect of caffeine on the cardiovascular responses to head-up tilt

BACKGROUND: Both caffeine and orthostasis have known cardiovascular effects. The possible interaction between these factors remains unknown. This study aimed to determine the effect of caffeine consumption on cardiovascular responses to head-up tilt. METHODS: Sixteen subjects underwent three +75 degree head-up tilts: i) control, ii) acute, after a dose of 5 mg x kg(-1) body mass of caffeine or placebo, and iii) chronic, following 7 d of caffeine or placebo consumption at a daily dose of 5 mg x kg(-1) body mass. Heart rate (HR), systolic BP (SBP), diastolic BP (DBP), and mean arterial pressure (MAP) were measured using a Portapres BP monitor. RESULTS: The overall pattern of heart rate (HR) response in both caffeine and placebo groups showed a significant increase in HR after tilting for each tilt. Acute caffeine consumption significantly decreased resting HR (p < 0.05). After chronic consumption, resting HR was no longer significantly different. In the control condition, the overall pattern of response to tilt for SBP, DBP, and MAP showed no significant differences in either group. An acute dose of caffeine caused a significant fall (p < 0.05) in all BP variables in response to tilt. This effect was also seen after chronic caffeine consumption in SBP and MAP, but not in DBP. CONCLUSIONS: The results of this study indicate that both acute and chronic consumption of caffeine can lead to impaired cardiovascular function after exposure to an orthostatic challenge. This impaired function, reflected in a decreased resting HR and an inability to maintain MAP, is potentially due to impaired baroreflex function.