Due to methodologic difficulties, few investigations have been made on the blood flow velocity in the cerebral microcirculation. Using a newly developed video camera method, we simultaneously measured the blood flow velocity and diameter of pial arteries during hemorrhagic hypotension, after blood pressure recovery, and during CO2 inhalation in cats. When the mean arterial blood pressure was lowered from 129.7 ± 6.6 to 71.5 ±4.1 mm Hg, the blood flow velocity inevitably decreased from 36.6 ± 5.3 to 27.0 ± 3.9 mm/sec (p<0.001). The calculated blood flow rate[π X (diameter/2)2 × flow velocity] was preserved in cases with concomitant vasodilation. Conversely, the blood flow velocity increased from 25.3 ± 5.1 to 31.0 ± 5.4 mm/sec (p < 0.001) after mean arterial blood pressure recovery from 67.1 ± 3.7 to 129.8 ± 5.8 mm Hg. The blood flow rate was again preserved in vessels with a vasoconstrictive response. Each pial artery apparently dilated or constricted in proportion to the decrease or Increase in flow velocity during blood pressure changes, maintaining a constant cerebral blood flow. This indicated the importance of the pial arteries in the mechanisms of cerebral blood flow autoregulation. During 5% CO2 inhalation, the blood flow velocity increased markedly from 25.4 ± 4.6 to 37.2 ± 10.0 mm/sec (p <0.05), while the pial artery diameter (85.0 ± 13.7 μm) increased by 9.6 ± 1.5% (p<0.01). The increased flow velocity might be attributable to preferential dilatation of small arterioles or intraparenchymal vessels during hypercapnia. Not only the pial arterial dilatation but also the increase in blood flow velocity may contribute to the cerebral blood flow increase during CO2 inhalation.
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