Summary model extends the RG model with a CBFV contributing component being the output of a dCA model driven by blood pressure as input. Both models were evaluated for mean and systolic CBFV VEFR responses. The model-to-data fit errors from the RGCA mean and systolic blood pressure corrected model were significantly lower compared to the RG model: mean 0.8 % ± 0.6 vs. 2.4 % ± 2.8 p<0.001 systolic 1.5 % ± 1.2 vs. 2.2 % ± 2.6 p<0.001. The confidence bounds of all RGCA estimated NVC model parameters were significantly (p<0.005) narrowed. In conclusion, blood pressure correction of NVC responses by including cerebral autoregulation in model fitting of averaged VEFR responses results in significantly lower fit errors and by that in more reliable model parameter estimation. Blood pressure correction is more effective when mean instead of systolic CBFV responses are used. Measurement and quantification of NVC should include beat-to-beat blood pressure measurement. In chapter 6 a technique is introduced to evaluate dCA during normothermic nonpulsatile cardiopulmonary bypass (CPB). The technique uses continuous recording of invasive arterial blood pressure, middle cerebral artery blood flow velocity through transcranial Doppler sonography, in-line arterial carbon dioxide and pump flow measurement. dCA during CPB is estimated by TFA based on the response to blood pressure variation induced by cyclic 6/min changes of indexed pump flow from 2.0 to 2.4 up to 2.8 l/min/m2. In 37 patients during CPB we tested the feasibility of our technique to estimate dCA at hypocapnia (paCO2 = 30 mmHg), normocapnia (paCO2 = 40 mmHg), and hypercapnia (paCO2 = 50 mmHg). dCA phase decreased significantly (p<0.01) with increasing paCO2 from hypocapnia (0.58 ± 0.3 rad) to normocapnia (0.31 ± 0.2 rad) and hypercapnia (0.1 ± 0.1 rad). Also gain decreased (p<0.01) with increasing paCO2 from hypocapnia (3.4 ± 2 %/mmHg) to normocapnia (2.2 ± 0.9 %/mmHg) and hypercapnia (1.4 ± 0.4 %/mmHg). Studying dynamic cerebral autoregulation during cardiopulmonary bypass is feasible by changing pump flow in a 6/min rate. Hypercapnia results in impaired dCA. Ongoing and future studies are expected to clarify optimal perfusion strategies for various patient populations scheduled for operations involving cardiopulmonary bypass. In chapter 7 the main findings and shortcomings of this thesis are discussed as well as their implications for future research. 140
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