Blood pressure corrected evoked flow responses In the extended RGCA model as shown in figure 1, the output of the NVC model of equation 1 is added to the output of the model for dynamic cerebral autoregu- lation as described by Tieckset al 29 describing the transfer from blood pressu- re to blood flow velocity. K =0 2 2 relative CBFV % 1.5 K 2 K 1 3 0.5 K =1 0 2 -2 0 2 4 6 8 10 12 14 Time s Figure 2 An example step response of dCA model indicating the effect of parameters K2 and K3. The dynamic cerebral autoregulation transfer can be illustrated by considering the CBFV response to a stepwise increase in blood pressure (figure 2). This increase in blood pressure induces a proportional increase in blood flow velocity. The process of dynamic cerebral autoregulation subsequently adapts cerebrovas- cular resistance in order to compensate the initial passive increase in blood flow velocity. Previous studies showed, that this process of autoregulation can also be described by a second order control system 21, 29 of which the transfer function in the Laplace domain is given by equation 2. ð K Î H CA ? K3 μ ×1 / (T μ s) 2 - 2 μ2 μ D μ s -1Ì (2)ÏT³ K3 represents an overall proportional gain and K2 indicates till what extent CA is able to regulate blood flow velocity. A value for K2 of 0 means no active autore- gulation, a value of 1 fully working autoregulation. The time course of blood flow 85
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