Academic Neurosurgical Unit, Addenbrooke's Hospital, Cambridge, United Kingdom.
Ultrasound Med Biol. 2012 Jul;38(7):1129-37. doi: 10.1016/j.ultrasmedbio.2012.02.014.
The time constant of cerebral arterial bed (in brief time constant) is a product of brain arterial compliance (C(a)) and resistance (CVR). We tested the hypothesis that in normal subjects, changes in end-tidal CO(2) (EtCO(2)) affect the value of the time constant. C(a) and CVR were estimated using mathematical transformations of arterial pressure (ABP) and transcranial Doppler (TCD) cerebral blood flow velocity waveforms. Responses of the time constant to controlled changes in EtCO(2) were compared in 34 young volunteers. Hypercapnia shortened the time constant (0.22 s [0.17, 0.26] vs. 0.16 s [0.13, 0.20]; p = 0.000001), while hypocapnia lengthened the time constant (0.22 s [0.17, 0.26] vs. 0.23 s [0.19, 0.32]; p < 0.0032). The time constant was negatively correlated with changes in EtCO(2) (R(partial) = -0.68, p < 0.000001). This was associated with a decrease in CVR when EtCO(2) increased (R(partial) = -0.80, p < 0.000001) and C(a) remained independent of changes in EtCO(2). C(a) was negatively correlated with mean ABP (R(partial) = -0.68, p < 0.000001). In summary, the time constant shortens with increasing EtCO(2). Its potential role in cerebrovascular investigations needs further studies.
大脑动脉床的时间常数（简称时间常数）是脑动脉顺应性（C(a)）和阻力（CVR）的产物。我们检验了以下假说，即在正常受试者中，二氧化碳末端（EtCO(2)）的变化会影响时间常数的值。使用动脉压（ABP）和经颅多普勒（TCD）脑血流速度波形的数学变换来估计 C(a)和 CVR。在 34 名年轻志愿者中比较了时间常数对受控 EtCO(2)变化的反应。高碳酸血症缩短了时间常数（0.22 秒[0.17,0.26]比 0.16 秒[0.13,0.20]；p=0.000001），而低碳酸血症延长了时间常数（0.22 秒[0.17,0.26]比 0.23 秒[0.19,0.32]；p<0.0032）。时间常数与 EtCO(2)的变化呈负相关（R(partial)=-0.68，p<0.000001）。这与当 EtCO(2)增加时 CVR 降低（R(partial)=-0.80，p<0.000001）和 C(a)不受 EtCO(2)变化影响有关。C(a)与平均 ABP 呈负相关（R(partial)=-0.68，p<0.000001）。总之，随着 EtCO(2)的增加，时间常数缩短。它在脑血管研究中的潜在作用需要进一步研究。
