Dynamic cerebral autoregulation in CVR testing Except for 1 patient all were on statin treatment and about 50% of the patients were on some kind of antihypertensive treatment. Medications were not discon- tinued during the testing phase. The measurements were performed in the non-acute stroke phase, i.e. 6 weeks to 6 months post stroke (median 93, ranging from 50 to 190 days after stroke). Data analysis An automatic algorithm in the Task Force Monitor detected R-waves from the ECG and between every two R-waves the diastolic, systolic and mean values of the blood pressure waveform was determined. Correspondingly the peak- systolic, end-diastolic and mean CBFV values were determined. The beat-to-beat mean values of ABP and CBFV were exported to Matlab and resampled at 5 Hz using spline-interpolation 24. From the ABP the mean value was subtracted and the CBFVs were normalized with respect to the mean. This resulted in zero- mean signals suited for spectral analysis to estimate the transfer function. The transfer function was calculated by S ( f ) H ( f ) ? xy (1) S xx ( f ) whereSxx(f) is the auto spectrum of changes in arterial blood pressure and Sxy(f) is the cross-spectrum between the ABP- and CBFV-signals. The transfer function magnitude|H(f)| and phase spectrum (f) were derived from the real partHR(f) and imaginary partHI(f) of the complex transfer function as | H ( f ) |? H R ( f )_ 2 - H I ( f )_2 (2) H ( f ) H( f ) ? arctan I (3) H R ( f ) The squared coherence function 2(f) was estimated by | S ( f ) |2 i 2 ( f ) ? xy (4) S xx ( f )S yy ( f ) whereSyy(f) is the auto spectrum of changes in cerebral blood flow velocity. The squared coherence reflects the strength of the linear relationship between ABP and CBFV for each frequency on a scale from 0 to 1. For the estimation of the transfer function ensemble averaging was used with half overlapping windows with a length of 256 data points. With a 5 Hz sampling rate this resulted in 51.2 seconds long data epochs. The smoothness priors ap- proach 33 was used to remove the very low frequency trend. A hanning- window was applied on each data epoch. The spectrum was smoothed with a triangular window of 3 points wide corresponding to a bandwidth of 0.04 Hz. 47
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