Poulin M J, Liang P J, Robbins P A
University Laboratory of Physiology, Oxford, United Kingdom.
J Appl Physiol (1985). 1996 Sep;81(3):1084-95. doi: 10.1152/jappl.1996.81.3.1084.
This study examined the dynamics of the cerebral blood flow response to hypoxia and hypercapnia in humans. Middle cerebral artery blood flow (MCAF) was assessed continuously using transcranial Doppler ultrasound. MCAF was calculated on a beat-by-beat basis as the product of the intensity-weighted mean velocity and the total power of the reflected signal. End-tidal PCO2 (PETCO2) and PO2 (PETO2) were controlled using a dynamic end-tidal forcing system. Six repeats of each of four protocols were administered to six subjects. The first was a control protocol with PETO2 held at 100 Torr and PETCO2 held 1-2 Torr above eucapnia throughout. The second was a hypoxic step protocol with PETO2 lowered from control values to 50 Torr for 20 min. The third was a hypercapnic step protocol with PETCO2 elevated from control by 7.5 Torr for 20 min. The fourth was a hypoxic-and-hypercapnic step protocol lasting 20 min. The total power of the Doppler signal remained relatively constant, suggesting that the cross-sectional area of the vessel changed little. After the initial transient in MCAF at the onset of the stimulus, no adaptation or progressive increase was observed over the remaining 20 min. A simple model consisting of a single pure delay, gain terms, time constants, and offsets for the on and off transients was fitted to the hypoxic and hypercapnic protocols. For hypercapnia, all the parameters for the onset were significantly different from the relief of the stimulus. The asymmetry was characterized by a slower on transient than off transient and also by a degree of undershoot after the relief of hypercapnia. Finally, the results from this study show that the cerebral blood flow response to hypoxia and hypercapnia in humans is much faster than has previously been thought.
本研究检测了人体大脑血流对缺氧和高碳酸血症的反应动态。使用经颅多普勒超声持续评估大脑中动脉血流（MCAF）。逐搏计算MCAF，其为强度加权平均速度与反射信号总功率的乘积。使用动态呼气末强制系统控制呼气末PCO2（PETCO2）和PO2（PETO2）。对6名受试者进行了4种方案中每种方案的6次重复实验。第一种是对照方案，PETO2维持在100托，PETCO2在整个过程中维持在比正常碳酸血症高1 - 2托。第二种是低氧阶梯方案，PETO2从对照值降至50托并持续20分钟。第三种是高碳酸血症阶梯方案，PETCO2比对照值升高7.5托并持续20分钟。第四种是持续20分钟的低氧 - 高碳酸血症阶梯方案。多普勒信号的总功率保持相对恒定，表明血管横截面积变化不大。在刺激开始时MCAF出现初始瞬变后，在剩余的20分钟内未观察到适应或逐渐增加。一个由单个纯延迟、增益项、时间常数以及开和关瞬变的偏移组成的简单模型被拟合到低氧和高碳酸血症方案中。对于高碳酸血症，刺激开始时的所有参数与刺激解除时显著不同。这种不对称性表现为开启瞬变比关闭瞬变更慢，并且在高碳酸血症解除后还有一定程度的下冲。最后，本研究结果表明，人体大脑血流对缺氧和高碳酸血症的反应比之前认为的要快得多。
