Blood pressure corrected evoked flow responses Introduction Measurement of a visually evoked blood flow response in the posterior cerebral artery (PCA) is a well-known method to evaluate cerebral blood flow dynamics to changes in neuronal activity in humans 4, 14, 16, 19, 24, 27, 31, 32. For this, a visual stimulus is repetitively presented to a subject while cerebral blood flow velocity (CBFV) is measured using transcranial Doppler sonography in the PCA. The visual stimulus may consist of e.g. alternating checkerboards, reading a newspaper or watching a movie. Aligned to the start of each stimulus several responses are averaged to enhance signal to noise ratio. In this way transient effects of increased metabolic demand due to enhanced neuronal activity in the visual cortex can be evaluated. Another mechanism controlling cerebral blood flow is dynamic cerebral auto- regulation (dCA) 2, which continuously adjusts cerebral blood flow with respect to changes in cerebral perfusion pressure. Three different mechanisms are thought to contribute to cerebral autoregulation: metabolic, myogenic and neurogenic regulation 1. Metabolic regulation adjusts blood flow to both meta- bolic demand and oxygen supply. Myogenic regulation is the effect of transmural blood pressure changes on vascular smooth muscle tension to keep blood flow constant and neurogenic regulation is the control of the vascular smooth muscle tension through action of the autonomic nervous system. Because changes in arterial blood pressure (ABP), directly or via dCA may influ- ence cerebral blood flow velocity, one might expect that these effects are re- flected in evoked flow responses as well and therefore should be taken into account. Several investigators 6, 7, 12, 13, 25, 26 who studied evoked changes in CBFV also monitored concomitant changes in ABP. Their major conclusion was that the influence of blood pressure on evoked flow responses could be neglected due to the relatively small amplitude of these changes (one group 13 reported 2%). However, these investigators often evaluated ABP not on a beat-to-beat basis but time-averaged over periods of 30 to 60 s. Investigators looking at blood pressure on a beat-to-beat level 7, 15, 17, 23 showed significant effects on evoked flow responses. Moodyet al 15 showed a significant effect of beat-to-beat ABP on group-averaged evoked flow responses being an increase in ABP after stimu- lus start followed by a plateau after approximately 5 seconds. Azevedoet al 3, 4 also recorded blood pressure simultaneously with NVC testing but do not report an effect of blood pressure on NVC nor an effect of the stimulus on blood pres- sure. Using an autoregressive model Paneraiet al 17 recently showed that about 20% of CBFV variance could be explained by ABP in experiments while perform- ing a motor task. It could well be possible that visual stimuli also evoke transient 81
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