Chapter 6 Due to compliance of the arterial vascular bed a more smoothed almost sinusoi- dal variation is induced in arterial blood pressure. The resulting cerebral blood flow velocity shows a similar sinusoidal shape. The resemblance of ABP and CBFV signals to “pure” sine waves is confirmed by their power spectral densities which show a clear peak at 0.1 Hz and an almost hundredfold lower peak at the third harmonic at 0.3 Hz. The width of the peak at 0.1 Hz is partly due to the three-point triangular spectral smoothing that was applied 17. Since the evoked flow changes are square wave shaped only odd harmonics (3rd, 5th, etc.) are ex- pected. For transfer function analysis of dCA the induced variations are well suited, as shown in figure 3. 3.5 pCO = 30 mmHg 2 3 pCO = 40 mmHg 2 pCO = 50 mmHg step response %/mmHg 2 2.5 2 1.5 1 0.5 0 0 5 10 15 t s Figure 4 Group averaged step responses during cardiopulmonary bypass at three different paCO2 levels. Error bars represent the largest ± 1 standard deviation. To evaluate dCA during CPB sufficient blood pressure variability combined with coherent CBFV variability is necessary for reliable transfer function estimation. Little is known about spontaneous ABP variability during CPB. Nicoletet al 26 hypothesized that in absence of cardio respiratory variability during CPB ex- treme situations in physiologic control of CBF should be revealed. During extra- corporeal circulation they found no significant coherence between ABP and CBFV, possibly due to different dominant frequencies occurring in ABP (9 mHz) 112
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