Estimating normal and pathological dynamic responses in cerebral blood flow velocity to step changes in end-tidal pCO(2)

The regulation of cerebral blood flow (CBF) following changes in arterial b lood pressure (ABP) and end-tidal pCO(2) (EtCO2) are of clinical interest i n assessing cerebrovascular reserve capacity Linear finite-impulse-response modelling is applied to ABP, EtCO2 and CBF velocity (CBFV, from transcrani al Doppler measurements), which allows the CBFV response to ideal step chan ges in EtCO2 to be estimated from clinical data showing more sluggish, and additional random variations. The confounding effects of ABP changes provok ed by hypercapnia on the CBFV are also corrected for. Data from 56 patients suffering from stenosis of the carotid arteries (with normal or diminished cerebrovascular reactivity to EtCO2 changes - CVRCO2) were analysed. The r esults show the expected significant differences (p<0.05) between EtCO2 ste ps up and down, the significant contribution from ABP variation, and also d ifferences in the dynamic responses of patients with reduced CVRCO2 (p<0.01 after 10 s). For the latter the CBFV response appears exhausted after abou t 15 s, whereas for normals CBFV continues to increase. While dispersion of individual step responses remains large, the method gives encouraging resu lts for the non-invasive study of compromised haemodynamics in different pa tient groups.