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
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