Czosnyka Marek, Richards Hugh K, Reinhard Matthias, Steiner Luzius A, Budohoski Karol, Smielewski Piotr, Pickard John D, Kasprowicz Magdalena
Academic Neurosurgical Unit, Addenbrooke's Hospital, Cambridge, UK.
Neurol Res. 2012 Jan;34(1):17-24. doi: 10.1179/1743132811Y.0000000040.
The cerebrovascular time constant (τ) describes the time to establish a change in cerebral blood volume after a step transient in arterial blood pressure (ABP). We studied the relationship between τ, ABP, intracranial pressure (ICP), and end-tidal carbon dioxide concentration (EtCO2).
Recordings from 46 anaesthetized, paralysed and ventilated New Zealand rabbits were analysed retrospectively. ABP was directly monitored in the femoral artery, transcranial Doppler (TCD) cerebral blood flow velocity (CBFV) from the basilar artery, and ICP using an intraparenchymal sensor. In nine animals end-tidal CO2 (EtCO2) was monitored continuously. ABP was decreased with injection of trimetophan (n = 11) or haemorrhage (n = 6) and increased by boluses of dopamine (n = 11). ICP was increased by infusion of normal saline into the lumbar cerebrospinal fluid space (n = 9). Changes in cerebral compliance (C(a)) were estimated as a ratio of the pulse amplitude of the cerebral arterial blood volume (CBV) and the pulse amplitude of ABP. Changes in cerebrovascular resistance (CVR) were expressed as mean ABP or cerebral perfusion pressure (CPP) divided by mean CBFV. Time constant τ was calculated as the product of CVR and C(a).
The time constant changed inversely to the direction of the change in ABP (during arterial hypo- and hypertension) and CPP (during intracranial hypertension). C(a) increased with decreasing CPP, while CVR decreased. During a decrease in CPP, changes in C(a) exceeded changes in CVR. In contrast, during hypercapnia, the decrease in CVR was more pronounced than the increase in C(a), resulting in a decrease in τ.
Cerebrovascular time constant τ is modulated by ABP, ICP, and EtCO2.
脑血管时间常数（τ）描述了动脉血压（ABP）出现阶跃瞬变后，脑血容量建立变化所需的时间。我们研究了τ、ABP、颅内压（ICP）和呼气末二氧化碳浓度（EtCO2）之间的关系。
对46只麻醉、瘫痪并接受机械通气的新西兰兔的记录进行回顾性分析。在股动脉直接监测ABP，通过经颅多普勒（TCD）监测基底动脉的脑血流速度（CBFV），并使用脑实质内传感器监测ICP。在9只动物中持续监测呼气末二氧化碳（EtCO2）。通过注射曲美芬（n = 11）或出血（n = 6）使ABP降低，通过注射多巴胺推注（n = 11）使ABP升高。通过向腰段脑脊液间隙输注生理盐水（n = 9）使ICP升高。脑顺应性（C(a)）的变化以脑动脉血容量（CBV）的脉搏振幅与ABP的脉搏振幅之比来估计。脑血管阻力（CVR）的变化表示为平均ABP或脑灌注压（CPP）除以平均CBFV。时间常数τ计算为CVR与C(a)的乘积。
时间常数的变化与ABP（在动脉低血压和高血压期间）和CPP（在颅内高压期间）变化的方向相反。C(a)随CPP降低而增加，而CVR降低。在CPP降低期间，C(a)的变化超过CVR的变化。相反，在高碳酸血症期间，CVR的降低比C(a)的增加更明显，导致τ降低。
脑血管时间常数τ受ABP、ICP和EtCO2调节。
