Modelling the cerebral haemodynamic response in the physiological range of PaCO2.

Objective Arterial CO2 (PaCO2) has a strong effect on cerebral blood flow (CBF), but its influence on CBF regulatory mechanisms and circulatory systemic variables has not been fully described over the entire physiological range of PaCO2. Approach CBF velocity (CBV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO2 (EtCO2, capnography) were measured in 45 healthy volunteers (19 male, mean age 37.5 years, range 21-71) at baseline, and in response to hypo- (-5mm Hg and -10mm Hg below baseline) and hypercapnia (5% and 8% CO2), applied in random order. Main Results CBV, cerebral dynamic autoregulation index (ARI), heart rate (HR), arterial blood pressure (ABP), critical closing pressure (CrCP) and resistance-area product (RAP) changed significantly (all p<0.0001) for hypo- and hyper-capnia. These parameters were shown to follow a logistic curve relationship representing a 'dose-response' curve for the effects of PaCO2 on the cerebral and systemic circulations. The four logistic model parameters describing each 'dose-response' curve were specific to each of the modelled variables (ANOVA p<0.0001). Significance The ability to model the CBV, ARI, HR, ABP, CrCP and RAP dependency of PaCO2 over its entire physiological range is a powerful tool for physiological and clinical studies, including the need to perform adjustments in disease populations with differing values of baseline PaCO2. .

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