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Dynamic pressure‐flow relationships in the human cerebral circulation at rest with and without alpha1‐adrenoreceptor blockade

Dynamic pressure‐flow relationships observed within the cerebral circulation indicate that when the critical closing pressure (CCP; a functional index of cerebral vascular tone) is reached cerebral blood flow stops. The purpose of this study was to identify the influence of the alpha1‐adrenoreceptors on cerebral vascular tone at rest. This was accomplished by continuously measuring middle cerebral artery blood velocity (MCA V) and arterial pressure from the radial artery before and after Prazosin ingestion (1mg/20kg BW) in 6 healthy subjects. Subsequent calculations of the CCP were preformed for each condition by linear extrapolation from the steady‐state beat‐to‐beat waveforms of velocity and pressure. Prazosin decreased mean arterial pressure (Control: 92±4 mm Hg vs. Prazosin: 87±5 mm Hg; P=0.021), however, Prazosin had no effect on mean MCA V (Control: 61±5 cm/s vs. Prazosin: 58±5 cm/s; P=0.511) or cerebral vascular resistance index (Control: 1.72±0.14 mm Hg s/cm vs. Prazosin: 1.88±0.15 mm Hg s/cm; P=0.82). The correlation between the dynamic pressure‐flow relationship without Prazosin was r=0.96±0.01 while with Prazosin it was r=0.95±0.01. Prazosin did not significantly change CCP (Control:16.3±4.3 mm Hg vs Prazosin:14.4±3.1 mm Hg, P=0.32). These findings suggest that basal sympathetic activity has no effect on cerebral vascular tone at rest.