Cirovic S, Walsh C, Fraser W D
Defence and Civil Institute of Environmental Medicine, North York, Ont., Canada.
Aviat Space Environ Med. 2000 May;71(5):514-21.
When the human body is exposed to a high gravitational load, the blood supply to the brain is reduced and loss of consciousness may occur. Our goal is to identify the principal mechanical causes of reduced blood supply to the brain during high +Gz.
We have developed a mathematical model to investigate the influence of Gz on the cerebral circulation. Blood flow is modeled using a one-dimensional flow approximation, in which the cross-sectional area of elastic vessels is determined as a non-linear function of the transmural (blood minus external) pressure. The intracranial vessels are subjected to cerebrospinal fluid pressure (PCSF) which is determined from the condition that the cranial volume is conserved.
For a constant pressure difference of 100 mm Hg applied to the arterial and venous ends of the model, blood flow is diminished for +Gz. At approximately +5 G, the blood flow predicted by the model is insufficient to maintain normal functioning of the brain. PCSF is approximately equal to the blood pressure in the large intracranial veins for all values of Gz. Extracranial arteries and the intracranial vessels do not collapse, even when Gz is substantially higher than normal. However, the extracranial veins are collapsed even for moderate +Gz.
Even if cardiac output is maintained at normal levels, cerebral perfusion will fall because of the increasing resistance of the cerebral flow circuit. This increase is largely due to the collapse of the extracranial veins, which begins at moderate Gz and becomes dominant at a Gz of approximately 4.5.
当人体暴露于高重力负荷时，脑部血液供应会减少，可能会出现意识丧失。我们的目标是确定在高 +Gz 期间脑部血液供应减少的主要机械原因。
我们开发了一个数学模型来研究 Gz 对脑循环的影响。使用一维血流近似对血流进行建模，其中弹性血管的横截面积被确定为跨壁（血液减去外部）压力的非线性函数。颅内血管承受脑脊液压力（PCSF），该压力由颅内容积守恒的条件确定。
对于在模型的动脉端和静脉端施加的 100 mmHg 的恒定压差，+Gz 时血流减少。在大约 +5 G 时，模型预测的血流不足以维持大脑的正常功能。对于所有 Gz 值，PCSF 大约等于颅内大静脉中的血压。颅外动脉和颅内血管不会塌陷，即使 Gz 远高于正常水平。然而，即使对于中等 +Gz，颅外静脉也会塌陷。
即使心输出量维持在正常水平，由于脑血流回路阻力增加，脑灌注仍会下降。这种增加主要是由于颅外静脉的塌陷，其在中等 Gz 时开始，并在大约 4.5 的 Gz 时占主导地位。
