Zhang R, Zuckerman J H, Pawelczyk J A, Levine B D
Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75231, USA.
J Appl Physiol (1985). 1997 Dec;83(6):2139-45. doi: 10.1152/jappl.1997.83.6.2139.
Our aim was to determine whether the adaptation to simulated microgravity (microG) impairs regulation of cerebral blood flow (CBF) during orthostatic stress and contributes to orthostatic intolerance. Twelve healthy subjects (aged 24 +/- 5 yr) underwent 2 wk of -6 degrees head-down-tilt (HDT) bed rest to simulate hemodynamic changes that occur when humans are exposed to microG. CBF velocity in the middle cerebral artery (transcranial Doppler), blood pressure, cardiac output (acetylene rebreathing), and forearm blood flow were measured at each level of a ramped protocol of lower body negative pressure (LBNP; -15, -30, and -40 mmHg x 5 min, -50 mmHg x 3 min, then -10 mmHg every 3 min to presyncope) before and after bed rest. Orthostatic tolerance was assessed by using the cumulative stress index (CSI; mmHg x minutes) for the LBNP protocol. After bed rest, each individual's orthostatic tolerance was reduced, with the group CSI decreased by 24% associated with greater decreases in cardiac output and greater increases in systemic vascular resistance at each level of LBNP. Before bed rest, mean CBF velocity decreased by 14, 10, and 45% at -40 mmHg, -50 mmHg, and maximal LBNP, respectively. After bed rest, mean velocity decreased by 16% at -30 mmHg and by 21, 35, and 39% at -40 mmHg, -50 mmHg, and maximal LBNP, respectively. Compared with pre-bed rest, post-bed-rest mean velocity was less by 11, 10, and 21% at -30, -40, and -50 mmHg, respectively. However, there was no significant difference at maximal LBNP. We conclude that cerebral autoregulation during orthostatic stress is impaired by adaptation to simulated microG as evidenced by an earlier and greater fall in CBF velocity during LBNP. We speculate that impairment of cerebral autoregulation may contribute to the reduced orthostatic tolerance after bed rest.
我们的目的是确定对模拟微重力（microG）的适应是否会损害直立应激期间的脑血流（CBF）调节，并导致直立不耐受。12名健康受试者（年龄24±5岁）接受了为期2周的-6度头低位倾斜（HDT）卧床休息，以模拟人类暴露于微重力时发生的血流动力学变化。在卧床休息前后，通过下肢负压（LBNP；-15、-30和-40 mmHg×5分钟，-50 mmHg×3分钟，然后每3分钟-10 mmHg直至前驱晕厥）的斜坡方案的每个水平，测量大脑中动脉的CBF速度（经颅多普勒）、血压、心输出量（乙炔再呼吸）和前臂血流量。通过使用LBNP方案的累积应激指数（CSI；mmHg×分钟）评估直立耐受性。卧床休息后，每个人的直立耐受性降低，LBNP各水平下，组CSI降低24%，同时心输出量下降幅度更大，全身血管阻力增加幅度更大。卧床休息前，在-40 mmHg、-50 mmHg和最大LBNP时，平均CBF速度分别下降14%、10%和45%。卧床休息后，在-30 mmHg时平均速度下降16%，在-40 mmHg、-50 mmHg和最大LBNP时分别下降21%、35%和39%。与卧床休息前相比，卧床休息后在-30 mmHg、-40 mmHg和-50 mmHg时的平均速度分别降低了11%、10%和21%。然而，在最大LBNP时没有显著差异。我们得出结论，直立应激期间的脑自动调节因对模拟微重力的适应而受损，这在LBNP期间CBF速度更早、更大幅度的下降中得到证明。我们推测，脑自动调节受损可能导致卧床休息后直立耐受性降低。
