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Reproducibility of dynamic cerebral autoregulation
the recordings the Physiocal procedure, an intermittently occurring calibration
routine, was switched off. To measure cerebral blood flow velocity (CBFV)
simultaneously in the right and left middle cerebral artery a Transcranial Dop-
plersonography device (DWL Multi-Dop® X4, Sipplingen, Germany) was used.
The two 2 MHz-probes were held in position by a special head frame.
Data analysis
ECG, ABP and right and left CBFV signals were recorded on a DAT-tape (TEAC-
recorder) and thereafter offline digitised and stored on a PC with an in-house
made acquisition and analysis software package (AFOnew version 2.33, IDEE
Maastricht®, the Netherlands). The sampling frequency for each signal was set to
250 Hz. An automatic algorithm detected the ECG R-waves and determined the
subsequent diastolic, mean and systolic blood pressure and corresponding end-
diastolic, mean and peak-systolic CBFV. Artefacts were removed manually by
linear interpolation. The beat-to-beat values of ABP and CBFV were resampled
with 5 Hz using spline-interpolation 22. From the ABP the overall mean value
was subtracted and the CBFVs were normalised 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 autospectrum of changes in arterial blood pressure andSxy(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 autospectrum 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.
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