Estimating normal and pathological dynamic responses in cerebral blood flow velocity to step changes in end-tidal pCO2

Abstract

The regulation of cerebral blood flow (CBF) following changes in arterial blood pressure (ABP) and end-tidal pCO2 (EtCO2) are of clinical interest in assessing cerebrovascular reserve capacity. Linear finite-impulse-response modelling is applied to ABP, EtCO2 and CBF velocity (CBFV, from transcranial Doppler measurements), which allows the CBFV response to ideal step changes in EtCO2 to be estimated from clinical data showing more sluggish, and additional random variations. The confounding effects of ABP changes provoked 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 results show the expected significant differences (p < 0.05) between EtCO2 steps up and down, the significant contribution from ABP variation, and also differences in the dynamic responses of patients with reduced CVRCO2 (p < 0.01 after 10 s). For the latter the CBFV response appears exhausted after about 15 s, whereas for normals CBFV continues to increase. While dispersion of individual step responses remains large, the method gives encouraging results for the non-invasive study of compromised haemodynamics in different patient groups.

Two-breath CO2 test detects altered dynamic cerebrovascular autoregulation and CO2 responsiveness with changes in arterial Pco2

“…It was not clear from their results whether the elevation of arterial blood pressure under hypercapnia influenced the cerebrovascular response. In contrast, Simpson et al (29) found no effect of PCO 2 on the computed step response of MFV to a theoretical step increase or decrease in BP MCA with different levels of arterial PCO 2 . These apparently contradictory results are consistent with the present results and further point to the greater sensitivity of CVRi than of MFV in detecting an effect on autoregulation (8).…”

Cerebral Autoregulation: From Models to Clinical Applications

“…Impulse/step responses like the ones shown in Fig. 1 can also be estimated directly in the time domain, using MA or ARMA models (moving average/autoregressive) (Edwards et al 2001;Liu et al 2003;Panerai et al 2000Panerai et al , 2001Simpson et al 2000Simpson et al , 2001 or linear differential equation models, like the one proposed by Tiecks et al (1995). The Aaslid-Tiecks model uses a 2nd order ODE whose gain (K), damping factor (D) and time constant (T) are combined to produce 10 template CBFV step response curves.…”

Transcranial Doppler for evaluation of cerebral autoregulation

“…In addition to the assessment of CA that will be discussed later, other facets of vasomotor function, such as cerebrovascular reactivity to carbon dioxide, have benefitted from the availability of TCD which has in fact become the predominant tool for studies in this area [59,98,103,132,153,154]. A growing application is the use of functional TCD (fTCD) to quantify the cerebrovascular response to cognitive and sensorimotor stimulation [1,32,64,131,138].…”