Assessment of dCA during CPB 34. Regression analysis of the 0.1 Hz waves in our data to determine zero-flow pressure could be performed, but its relation with critical closing pressure is unclear. For phase angle the impairment of dCA with increasing paCO2 is much clearer. At 0.1 Hz, phase angle clearly decreases with increasing paCO2, whereas at 0.3 and 0.5 Hz, phase angle is close to zero for all levels of paCO2, indicating that dCA is not functioning at these higher frequencies. Also, the decrease in ARI with paCO2 confirms decreased dCA functioning. Impairment of dCA with hypercapnia was also shown by others using a thigh cuff deflation method as well as by transfer function analysis 3, 4, 22. dCA results, especially high coherence and worsened autoregulation at increas- ing levels of paCO2, demonstrate that our technique is feasible and permits dCA quantification during CPB. In this way testing of dCA may facilitate e.g. studies on the effect of different perfusion strategies or the effect of impaired dCA on post surgery stroke incidence. Recently, Ono et al 28 showed that impaired cerebral autoregulation during CPB, as evaluated by Mx, is associated with increased postoperative stroke risk. Their Mx measure for cerebral autoregula- tion is difficult to compare with dCA transfer function analysis parameters, since it concerns different frequencies. We analysed the ABP-CBFV relation at the induced 0.1 Hz frequency, whereas for Mx with a 10 second time resolution 30 consecutive points were used to calculate a cross correlation between ABP and CBFV or SctO2 and so, for Mx much lower frequencies (f < 0.05 Hz) are consid- ered. The effect of paCO2 on CBFV and SctO2 shows intact CO2R in the range between 30 and 50 mmHg during CPB although there is a clear CO2R asymmetry for hypocapnia compared to hypercapnia. Other investigators showed an exponen- tial relationship between end tidal CO2 and CBFV 20, 23. Since we controlled for arterial CO2 which is strongly correlated to end tidal CO2 8, 36 the relation- ship between arterial CO2 and CBFV can also be considered to be exponential. This explains why for hypocapnia CO2R is lower compared to CO2R for hyper- capnia. Limitations With TCD due to unknown blood vessel diameter no absolute flow but blood flow velocity is measured. Huberet al 19 showed via angiography that cerebral arteries with a diameter of at least 2.5 mm do not show changes in diameter with 115
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