The Influence of Intravascular Volume Expansion on Cerebral Blood Flow and Blood Volume in Normal Rats

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Abstract
Intravascular volume expansion can increase cerebral blood flow (CBF) in patients with cerebral ischemia. However, the changes in CBF produced by volume expansion in the normal brain remain the subject of debate, and the changes on cerebral blood volume (CBV; one determinant of intracranial pressure) have not been described. The effects of acute nondilutional volume expansion on cardiac output (CO), CBF, and CBV in normal rats were examined. Normocarbic, normothermic halothane/N2O-anesthetized rats were given 0, 6, 12, 24, or 36 ml of a blood-hetastarch mixture (n = 6 for each group). When volume loading was completed, 3H-nicotine was used to measure CBF, while 99mTc-labeled red cells and 14C-dextran were used to measure cerebral red cell and plasma volumes. Animals were killed by microwave irradiation (8KW x 770 msec). In different animals, cardiac output (CO) was measured by thermodilution during the infusion of 36 ml of blood-hetastarch. Central venous pressure (CVP) increased from 3.9 +/- 0.7 mmHg (mean +/- SD) in control animals to 13.0 +/- 2.0 mmHg in rats given 36 ml of blood. Cardiac output increased to 138 +/- 11% of control. There were no changes in arterial Hct or mean arterial pressure. There were no significant changes in CBF (which varied from 127 +/- 22 ml.100 g-1.min-1 in controls to 102 +/- 14 after the infusion of 36 ml). However, CBV increased in a linear fashion with increasing CVP, from 2.96 +/- 0.57 ml/100 g in control conditions to 4.47 +/- 0.83 ml/100 g after the infusion of 36 ml of blood/hetastarch. In normal brain, CBF does not change during volume expansion, in spite of an increase in CO. This contradicts earlier studies, the results of which indicate an independent role for cardiac output in CBF control. Cerebral blood volume increased in proportion to CVP, suggesting simple passive venous distention. Whether this could increase ICP and compromise cerebral perfusion remains unclear.

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