Cerebrovascular time constant: dependence on cerebral perfusion pressure and end-tidal carbon dioxide concentration

Objective: The cerebrovascular time constant (tau) 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 tau, ABP, intracranial pressure (ICP), and end-tidal carbon dioxide concentration (EtCO2). Method: 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 tau was calculated as the product of CVR and C-a. Results: 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 tau. Conclusion: Cerebrovascular time constant tau is modulated by ABP, ICP, and EtCO2.