Assessment of dCA during CPB and CBFV (70 mHz). The authors hypothesized CPB provokes a highly protec- tive mechanism, thereby reducing CBF fluctuations by deactivation of B waves, revealing monotonous UB waves. The 0.1 Hz frequency we induced showed very high ( 0.9) coherence between ABP and CBFV, reflecting a strong linear relation between variability in ABP and CBFV. dCA phase, which decreased from hypo- capnia to hypercapnia, showed to be independent on coherence since coherence was already high for each level of paCO2. Figure 5 Example measurement of five minute episodes of cerebral carbon dioxide reactivity testing by near-infrared spectroscopy (SctO2) at hypocapnia (30 mmHg), normocapnia (40 mmHg) and hypercapnia (50 mmHg). The sinusoidal varying ABP and CBFV signals also allow visual appreciation of phase shift between ABP and CBFV. Figure 1 shows a positive phase shift since CBFV waves are occurring earlier than ABP waves as indicated by the vertical lines. The actual value of this phase shift is hard to estimate due to the variation in individual wave shapes, but it can be reliably estimated through transfer function analysis. This is calculated offline, but can be implemented online as well. Group averaged results show a phase shift at 0.1 Hz of approximately 0.3 rad in normocapnia (figure 2) which is markedly reduced compared to phase angles of 0.9 rad under normal circumstances 15, 16. Reduced phase during CPB may partly be due to the effect of anaesthesia 12 although two studies showed that anaesthesia through propofol preserved cerebral autoregulation 38, 49. Propofol decreases systemic vascular resistance which can be troublesome in 113
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