Chapter 2 Figure 5.Relationship between index of autoregulation (ARI) and phase during spontaneous breathing for method II. Recently, Czosnykaet al showed good correlation (R=0.62) between ARI and the time correlation based cerebral autoregulation method Mx 9. We calculated ARI from the step response of our transfer function. Figure 5 shows the correlation between ARI and phase that was not shown before, which is even better (R2=0.72). However, these correlations do not imply that dCA parameters repro- duce well. Our low ICC values for ARI are comparable to the findings of Brodie et al while our ICC’s for RAP show much higher values (ranging from 0.86 to 0.92 versus 0.70-0.75) 6. The reason for this difference is not clear. Brodie estimates RAP from the first harmonic of BP and CBFV 1, whereas we per- formed linear regression between BP and CBFV waves 21. The possible influ- ence of the analysis method with regard to this needs to be elaborated further. The high ICC values for reproducibility of RAP are promising and could open directions to a more robust parameter to estimate dCA. Although RAP in itself is not a measure of dCA, the relation of variations in RAP in relation with ABP changes may be worthwhile to investigate. Only in nine of the 19 subjects end-tidal pCO2 was recorded. Therefore, no results could be given for the whole group. As was shown by Reinhardet al 26, especially during paced breathing, where the subjects are likely to increase their tidal volumes influence of pCO2 can be seen. pCO2 drops after the start of paced 36
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