Dynamic cerebral autoregulation in CVR testing versible, they concluded that the latter dose was the optimal choice for clinical testing. For standardized test conditions these authors strictly recommended weight-adjusted administration. In our study we applied 15 mg/kg. A higher dose of ACZ would probably have further increased CBFV and further impaired cerebral autoregulation, but also wouldhave increased adverse effects (only one of the patients tested in this study reported slight light-headedness) and is there- fore not recommendable. So, CVR testing using ACZ administration with a dose of 15 mg/kg is not capable of determining the CVR to its full extent and should not be interpreted as such. We not only observed a decrease of the phase angle after ACZ infusion but also a statistically significant (p<0.005) increase of the coherence in the autoregulation frequency band. Increased coherence indicates that the relation between ABP and CBFV becomes more linear. In other words, the waveform of the CBFV signal resembles more the waveform of the ABP signal. This is typical for increased autoregulation impairment since a well-functioning autoregulation prevents blood pressure variations being transferred to the CBFV. Increased coherence can be interpreted as worsened autoregulation 10, a finding which is in line with our LF related findings. dCA versus CVR CVR and dCA parameters show poor correlation. In figure 2 LF is plotted versus CVR for the pre-and post-ACZ infusion period. Although not significant, there seems to be a negative linear relation between CVR and LF post ACZ infusion. The regression lines in this graph show that for subjects with low CVR the effect on the phase angle in the two stages is minimal whereas for high CVR the de- crease in phase angle after acetazolamide infusion is more pronounced. Plotting (relative) phase changes against CVR did not show this more markedly. DCA gain is hardly influenced by the infusion of ACZ. The negative correlation between phase angle and CVR suggests that increased cerebrovascular flow due to diameter increase of the arterioles decreases dCA performance. A comparison with a rubber hose comes up, where increasing the diameter diminishes the elasticity of the hose and by that the degree too which the diameter can be varied. The exact mechanism by which ACZ increases cerebral blood flow remains unclear 12, 17, 35. But, regardless of the cause of vasodilatation, at higher levels of cerebral blood flow the plateau in the static autoregulation curve is less wide (less autoregulated) 26. Subjects with low CVR have only a small increase in CBFV and therefore the static autoregulation curve is not “flow” shifted and narrowed much. As a result, autoregulation remains intact. For subjects with high 53
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