Experimental isovolemic haemodilution-induced augmentation of carotid blood flow and oxygen transport through graded carotid stenoses.

The effect of haemodilution without volume expansion (isovolemic haemodilution) was assessed with respect to blood flow and oxygen transport across stenotic lesions of progressive severity in the dog carotid artery. As the mean haematocrit (Hct) was reduced from 40 +/- 1% (+/- SEM) to 32 +/- 0% (p less than 0.001), reductions in vascular resistance were significant across the 90% (p less than 0.001) and 95% (p less than 0.0003) relative carotid stenoses. Isovolemic haemodilution reduced fresh blood viscosity significantly by 27 +/- 3% (p less than 0.001) and 42 +/- 4% (p less than 0.001) at the low shear rates of 10 sec-1 and 1 sec-1 which are typical of low-flow states. Following a 20% reduction in Hct 30 to 35% increase (p less 0.001) in carotid blood flow occurred at non-critical degrees of stenosis while a mean 83% increase (p less than 0.001) occurred at the highly critical 95% relative stenosis. Oxygen transport after a 22% decrease in blood haemoglobin was significantly increased by 28% (p less than 0.01) at the 95% relative stenosis level. These data provide a physiologic rationale for the beneficial effects of haemodilution in acute cerebral ischaemia, cerebral vasospasm and cerebral revascularization or carotid endarterectomy.

Physiological neuroendocrinology of peptides, steroids and other hormones in cerebrospinal fluid.

Cerebrospinal fluid acts as a conduit in neuroendocrine regulation. Valid assessment of normal cerebrospinal fluid levels of peptides, steroids and other hormones requires clarification of reference concentrations in control patients and normal volunteers. Awareness of factors which may alter neuronal activity and, in turn, the relative composition of cerebrospinal fluid constituents is essential to the accurate sampling and hormonal analysis of cerebrospinal fluid.

Augmentation of cerebral blood flow induced by hemodilution in stroke patients after superficial temporal-middle cerebral arterial bypass operation.

Experimental hemodilutional therapy has been shown to raise collateral perfusion to acutely ischemic brain regions distal to occluded internal carotid (ICA) and middle cerebral (MCA) arteries and to reduce infarct size. Superficial temporal (STA)-MCA anastomosis surgically establishes additional regional collateralization, and this bypass angiographically enlarges over time. Despite bypass patency verification by Doppler recording made at the edge of the craniectomy, the microsurgical STA-MCA anastomosis in 11 stroke patients did not produce early changes in cerebral perfusion parameters in the MCA territory of either hemisphere as determined by 133xenon inhalation. Therefore, hemodilution was initiated in an effort to increase cerebral perfusion during this marginal period when the STA was beginning to dilate progressively. Incremental venesections with equal intravenous volume replacement with 5% human serum albumin caused a significant lowering of the hematocrit from 40 +/- 1 to 33 +/- 1%. This isovolemic hemodilutional therapy resulted in significant mean regional cerebral blood flow (rCBF) elevations of 23 +/- 5% (SE) in the bypassed MCA territory and of 25 +/- 6% in the opposite MCA region. The mean gray flow (F1) in the involved and homologous MCA regions significantly increased 27 +/- 8% and 30 +/- 11%, respectively. Similarly, the initial slope index (ISI2) significantly rose by 17 +/- 5% in the bypassed MCA territory and by 18 +/- 6% in the homologous region. These data objectively support the premise that reductions in hematocrit without intravascular volume expansion augment cerebral blood flow, probably by reducing blood viscosity.(ABSTRACT TRUNCATED AT 250 WORDS)

Rheology of the cerebral circulation.

Current concepts of brain perfusion focus on the importance of rheological factors in the determination of cerebral blood flow. Blood viscosity, a primary determinant of blood flow, increases as the shear rate (velocity gradient) decreases, thereby impeding cerebral perfusion. Hematocrit, erythrocyte aggregation, erythrocyte flexibility, platelet aggregation, and plasma viscosity differentially influence blood flow through the conductance vessels and microcirculation of the brain. In addition, the microcirculation is also affected by the Fahraeus effect, the inversion phenomenon, and the screening effect. Knowledge of these factors affecting blood flow provides a rationale for experimental and clinical rheological therapies in the treatment or prevention of acute focal cerebral ischemia.