Chapter 7 General Discussion Cerebral autoregulation is a homeostatic mechanism to compensate cerebral blood flow (CBF) for changes in cerebral perfusion pressure (CPP) 3, 27. Much research in the field of cerebral autoregulation has been aimed at noninvasively investigating this mechanism, eventually leading to a diagnostic tool. Using TCD, CBFV can be determined as a noninvasive measure for CBF assuming the diameter of the insonated vessel remains constant 2. CPP is the difference between arterial blood pressure (ABP) and intracranial pressure (ICP). Using finger arterial pressure measurement variations in ABP can be noninvasively measured. Efforts have been made to estimate ICP noninvasively 13, 31, 32, but since the accuracy is low these methods still have limited value 28. If intracranial pressure is not expected to be elevated as e.g. in head injury patients, ICP often is assumed to be constant and only variation in ABP is considered for dCA evaluation. In this thesis we have mainly been focusing on the univariate transfer function analysis of TCD recorded CBFV variations in relation to ABP variations. The aim of this thesis was to establish and improve clinical applicability of dynamic cerebral autoregulation evaluation. Our aim has been twofold, a methodological and a clinical one. First, the use of transfer function analysis for quantifying dynamic cerebral autoregulation has been evaluated. The influence of different signal analysis aspects has been studied as well as the influence of different ways of inducing blood pressure variations. Secondly, our aim was to evaluate clinical application of the developed dCA analysis. Dynamic cerebral autoregulation has been studied in several patient groups and the results have been analyzed in relation to disease state. In this chapter methodological aspects, clinical applications, limitations and future perspectives of dCA evaluation will be discussed. Methodological aspects that will be addressed are blood pressure stimuli for dCA evaluation, transfer function analysis results, reproducibility and multivariate analysis. Methodological aspects Blood pressure variation Transfer function analysis between ABP as input and CBFV as output is only reliable with sufficient variation of the input. Spontaneous fluctuations of ABP may occur due to autonomic variability of heart rate resulting in cardiac output changes and eventually blood pressure variation. These spontaneous ABP variations have the advantage not to depend on active patient 124
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