Modelling the Cerebral Haemodynamic Response in the Physiological Range of PaCO

Objective: Arterial CO (PaCO) 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 PaCO.

Approach: CBF velocity (CBFV, transcranial Doppler), blood pressure (BP, Finometer) and end-tidal CO (EtCO, capnography) were measured in 45 healthy volunteers (19 male, mean age 37.5 years, range 21-71) at baseline, and in response to hypo- (-5 mm Hg and  -10 mm Hg below baseline) and hypercapnia (5% and 8% CO), applied in random order.

Main Results: CBFV, 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 PaCO 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 CBFV, ARI, HR, ABP, CrCP and RAP dependency of PaCO 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 PaCO.