Hemorheology of the cerebral circulation in stroke.

ARTERIAL STENOSIS, thrombosis, embolization and vasospasm often produce a reduction in regional brain perfusion to levels which result in focal cerebral ischemia.The metabolic consequences of this ischemic insult produce neurologic deficits which may be reversible or irreversible, depending Upon both the depth and duration of the ischemia. 1Therefore, the study of factors influencing cerebral blood flow in the normal and pathological state has assumed a prominent position in the field of cerebrovascular disease.Both surgical and medical modalities have been proposed as therapeutic options for the prevention, reversal and amelioration of ischemic neurologic deficits.Carotid endarterectomy and extracranial-to-intracranial arterial bypass have become routine surgical procedures for patients with selected forms of cerebrovascular disease.Unfortunately, in instances of acute focal cerebral ischemia, the efficacy of these procedures performed as emergencies is limited by the time requirements 2 for radiological assessment, patient preparation, and execution of these operations.Nevertheless, if the treatment of ischemic stroke is to be successful, early reperfusion of the ischemic brain regions is imperative.As opposed to surgical revascularization, medical manipulation of blood viscosity offers an effective and rapid means of increasing perfusion to acutely ischemic cerebral regions. 1 In addition, such therapy may serve as an adjunct to the surgical treatment of cerebrovascular disease, 4 as well as the chronic management of patients with reduced cerebral perfusion.Awareness of hemorheologic parameters influencing cerebral perfusion allows a rational approach to the clinical treatment of focal cerebral ischemia. Basic PrinciplesHemorheologic studies are primarily concerned with blood viscosity.Viscosity represents a measure of internal friction, which, in laminar flow systems, correlates shear stress to share rate. 5The term shear rate is almost synonymous with velocity gradient.Blood flow in any given cerebral vessel has a range of velocities with that of axial flow being greater than that of peripheral flow.If the geometry of that vessel is constant, shear rate is directly proportional to velocity of blood flow.The lowest shear rates occur in the venules of the microcirculation.