Reproducibility of dynamic cerebral autoregulation clude the major frequency range of dCA. The choice of the 0.04-0.16 Hz fre- quency interval does not introduce new problems of standardization since in this paper all methodologies and protocols are mutually compared using this fixed interval. Furthermore, the graphical presentation of our results as a function of frequency allows estimation for other frequency bands. Paced breathing analysis was only possible for combinations I and II (table 1) because of the limited 5 min duration of these recordings. ARI was calculated from the first 7 seconds of the step response function using a previously described method 16. The step response to calculate the ARI is obtained by integrating the impulse response, being the inverse Fourier trans- form of the estimated transfer function. From the ABP and CBFV waveforms the critical closing pressure and RAP were determined per beat using linear regres- sion 21. Before regression, individual ABP and CBFV beat waveforms were aligned for maximum correlation. RAP (mmHgs/cm) is the reciprocal slope of the regression line. Coherence criterion To allow reliable interpretation of dCA results coherence levels needs to exceed a minimum level. This minimum level, above which coherence differs significantly from zero, depends on the degrees of freedom of the spectral estimation proce- dure (computation in appendix A). Computation based on the degrees of freedom resulted in a minimum needed squared coherence level of 0.06 for the 15-minute spontaneous breathing episodes and of 0.2 for the 5-minute 6/min breathing periods. Multimodal pressure flow analysis For multimodal pressure flow analysis a recently improved method was used including ensemble empirical mode decomposition (EEMD) 14, 20. To extract spontaneous oscillations in ABP and CBFV empirical mode decomposition was used to decompose ABP and CBFV signals into intrinsic modes. For the EEMD this decomposition was repeated 100 times with added white noise having an amplitude of 10% of the standard deviation of the signal. The resulting 100 reali- zations of intrinsic mode functions were averaged per mode. Only component 3 and 4 were used for the analysis, since these were the only components with frequencies in the range from 0.04-0.16 Hz. To compare the values for phase and gain with the results from the transfer function analysis averages were taken over the whole measurement course. A recently modified transfer function analysis (MTFA) also was applied 15. This method evaluates gain and phase with transfer 25

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