Assessment of dCA during CPB TCD based CO2R is significantly higher (p<0.01) for the hypercapnic change (5.0 ± 2 %/mmHg) compared to the hypocapnic change (2.5 ± 1 %/mmHg). 1 Coherence 0.5 0 0 0.1 0.2 0.3 0.4 0.5 Gain %/mmHg 4 2 0 0 0.1 0.2 0.3 0.4 0.5 1.5 Phase rad pCO = 30 mmHg 1 2 pCO = 40 mmHg 0.5 2 pCO2 = 50 mmHg 0 0 0.1 0.2 0.3 0.4 0.5 Frequency Hz Figure 3 Group averaged transfer function analysis showing averaged coherence (upper panel), gain (middle panel) and phase (lower panel) during cardiopulmonary bypass at three different p aCO2 levels. Error bars represent ± 1 standard deviation at 0.1 Hz. Figure 5 shows NIRS CO2R measurements for a typical patient for the three conditions. It shows that cerebral oxygenation increases with the paCO2 level. Similarly to TCD based CO2R, NIRS based CO2R is calculated and is also signifi- cantly higher (p<0.05) for the hypercapnic change (0.45 ± 0.3 %/mmHg) com- pared to the hypocapnic change (0.25 ± 0.3 %/mmHg). Discussion In this study we demonstrated the feasibility to induce arterial blood pressure variations for dCA evaluation by changing CPB pump flow at 6 cycles/min. The induced stepwise changes in pump flow by repetitively switching between three levels of indexed pump flow resulted in a staircase shaped pump flow pattern. 111
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