Wave reflection analysis of the human cerebral circulation during syncope

Up till now, the presence of wave reflection of pressure and flow waves was not considered in studies on the cerebral circulation. This study tested the hypothesis whether the typical changes in cerebral blood flow velocity (CBFV) seen in patients during vasovagal syncope can be explained by the emergence of a wave reflection site in the cerebrovascular vessels. Continuous recordings of peripheral blood pressure (ABP, by Finapres) and CBFV (by transcranial Doppler) of 20 control subjects and 10 patients with syncope during tilt table testing were analyzed. Wave reflection analysis (WRA) consisted of a multivariate regression analysis with CBFV as dependent variable and simultaneous ABP as well as delayed ABP (by systematically varied time lags) as independent variables. The time delay yielding the best prediction of CBFV was interpreted as the reflection time. A univariate regression analysis with only simultaneous ABP as independent variable served as control method. In patients and controls CBFV during supine position could be explained sufficiently (explained variance = 88-90%) by univariate regression without improvement by WRA. During syncope, multivariate regression improved the prediction of CBFV (explained variance = 58% with univariate and 77% with multivariate regression) in 9 of 10 patients. The mean reflection time was 160 ms. The results can be explained by a collapse of the distal bridging veins during systemic hypotension giving rise to a pressure wave moving backward with a resulting distortion of the flow wave. In particular, the WRA model could account for the characteristic changes in the diastolic flow shape during syncope.