Assessment of dCA during CPB indexed pump 2 3 flow l/min/m 2.5 2 0 10 20 30 40 50 ABP mmHg 120 100 80 0 10 20 30 40 50 CBFV cm/s 40 20 0 10 20 30 40 50 Time s Figure 1 Effect of cyclic changing pump flow (upper panel) on arterial blood pressure (ABP, middle panel) and cerebral blood flow velocity (CBFV) in the middle cerebral arteries (lower panel) at paCO2 = 40 mmHg in a patient undergoing CPB. Results Because of technical problems the measurement at paCO2 = 30 mmHg could not be performed in a single patient. All other measurements were recorded without problems. Figure 1 shows an example of the signals recorded during the induced changes in pump flow at paCO2 = 40 mmHg clearly showing the stepwise in- crease and decrease of indexed pump flow. ABP and CBFV vary with the same 6/min frequency. Based on these ABP and CBFV wave-shaped signals dCA was evaluated using transfer function analysis. Table 2 shows group results for three paCO2 levels. Mean ABP and its standard deviation are not significantly different for paCO2 = 30 mmHg (hypocapnia) compared to 40 (normocapnia) and 50 mmHg (hypercapnia), whereas power spectral density at 0.1 Hz is significantly higher for hypercapnia compared to both hypocapnia (p<0.05) and normocapnia (p<0.01). Figure 2 shows power spectral density for ABP and CBFV. The ABP peak at 0.1 Hz increases with paCO2 which is not true for the CBFV peak. 109
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