## Page 14

# proefschrift gommer inhoud met kaft.indd

Chapter 1
Different from evoking transient changes in ABP, periodic changes were induced
either through controlled breathing at a frequency of 6/min 11 or through
squat-stand maneuvers alternating 10 seconds of standing with 10 seconds of
squatting 7. These periodic changes allowed quantification of phase difference
between ABP oscillations and oscillations in CBFV even in the time domain. But
in general Fourier analysis is used for quantifying this phase shift. Giller 13 was
the first to use Fourier analysis for thequantification of dynamic cerebral auto-
regulation. However, he only evaluated coherence and gain and did not address
phase. Apart from evoked changes in ABP also spontaneous variation in ABP can
be used to evaluate dynamic cerebral autoregulation through transfer function
analysis using the Fourier transform 8, 20, 29. This method allows in particular
monitoring purposes, since no patient cooperation is needed.
Fourier analysis
Fourier analysis is a method for mathematically describing time series, e.g. an
ABP recording, in terms of amplitude and frequency instead of amplitude and
time. Any time series can be expressed as a sum of sine waves of specific frequen-
cies 6. Each sine wave of specific frequency is expressed by amplitude and phase
or phase angle, expressed as part of a 360 degrees circle. A full circle of 360
degrees can also be expressed as 2 radians. Comparing the Fourier analysis of
two time series, e.g. CBFV and ABP, for each frequency the amplitude ratio is
called gain and the phase difference is called phase shift. Graphs of gain and
phase plotted against frequency are called gain and phase spectra. The correlation
of two time series at a specific frequency can be calculated by the statistical
correlation and is called coherence. A coherence of 1 means perfect correlation,
whereas a coherence of zero indicates no correlation at that frequency. The
graph of coherence against frequency is called coherence spectrum.
Measurement means for recording cerebral autoregulation
To assess cerebral autoregulation at least recording of blood pressure and a
measure for CBF are required. Blood pressure can be recorded invasively via an
arterial line, but also reliable noninvasive means are available nowadays. For
continuous recording of CBF with possibility to use bedside calculation for
monitoring purposes TCD is the best choice 28.
12

proefschrift gommer inhoud met kaft.indd

To see the actual publication please follow the link above