No abstract

Circadian rhythms, as well as the reversed natural body rhythms of the rat compared with humans, should be considered when extrapolating data to human or other animal studies. Temporal rhythms may also provide information concerning the cascading disease processes associated with cerebral ischemia.

Laser-Doppler flowmetry provides a continuous measurement of blood flow without violating the natural state of circulation. The linearity of the laser-Doppler and hydrogen clearance methods of blood flow measurement were compared using a protocol that produced changes in cerebral blood flow that might be experienced in a neurosurgical setting. Cerebral blood flow was measured in both hemispheres of 12 adult cats during the snaring of one common carotid artery, the intracisternal injection of 5 mg of 5-hydroxytryptamine creatinine sulfate, and hypervolemic hemodilution, which produced a 25% reduction in blood hematocrit. The percentage of baseline laser-Doppler flowmetry and hydrogen clearance flows showed an acceptable degree of correlation (R2 = 0.762) over the range of cerebral blood flows measured. More rigorous analysis using Bland and Altman's difference against mean test showed that 10 minutes after hemodilution, the two methods displayed a level of variation outside the limits of agreement (-21.85 to 22.03%). Laser-Doppler flowmetry provided a noninvasive and continuous measure of blood flow, increasing the ability to observe instantaneous changes in cerebral microcirculation. However, laser-Doppler flowmetry did not record absolute blood flow, was affected by cerebral tissue shrinkage, and did not accurately measure flow under conditions of changed blood hematocrit.

SUMMARY Cerebral autoregulation can be duplicated in vitro using the large middle cerebral arteries from a calf. The limits of autoregulation were between 50 and 150 mm Hg. Excessively high pressures may lead to the appearance of the "sausage-or bead-string" response followed by forced dilation. These results suggest the existence of an intrinsic myogenic mechanism responsive to intraluminal pressure changes.Stroke, Vol 12, No 5, 1981CEREBRAL autoregulation is defined as the maintenance of a relatively constant blood flow in the face of moderate changes in perfusion pressure. In vivo it is seen as vasoconstriction after increased intraluminal pressure and vasodilation in response to pressure decreases.1 ' 2 The normal range of autoregulation in cerebral vascular systems is between 45 and 170 mm Hg, 3 but the range can be altered by pathological factors such as hypertension. 4 Excessively high pressures may lead to the appearance of the "sausage-or beadstring" phenomenon 6 and then forced dilation, 6 concurrent with the loss of autoregulation. 7 In the absence of autoregulation, cerebral blood flow is passively dependent upon perfusion pressures.Previous in vitro studies with cerebral arteries have described the effects of metabolic, ionic and pH changes that may mimic those occurring in vivo during normal and abnormal conditions. 8 -9 This report describes an in vitro method for the study of autoregulation in the cerebrovascular system. As such, it is the first quantitative study of autoregulation in vitro in a cerebral vessel. The vascular responses to pressure duplicated those found in vivo which suggests the existence of an intrinsic myogenic mechanism responsive to intraluminal pressure changes. MethodsWithin 15 minutes after death, middle cerebral arteries were collected from a calf and placed in modified Krebs solution. Modified Krebs solution had the following mM composition: NaCl 118.8, KC1 4.7, KH 2 PO 4 1.2, CaCl, 1.2, MgSO« 1.2, NaHCO s 14.9, dextrose 5.6. Osmolarity of the solution was 276 milliosmoles, and pH was 7.4. Sections of white matter were removed along with each vessel to preserve intact branching. The vessels were threaded onto a glass rod in Krebs solution maintained at 37°C and equilibrated with 12% oxygen and 5% carbon dioxide. After the brain tissue was gently teased away from the artery, branches were tied off with 10-0 suture. The vessel was then cannulated between 2 glass tubes in a whole mount apparatus similar to that used by Farrar.10 After cannulation, the artery was stretched longitudinally (about 15%) between the glass rods to its approximate in vivo length. The vessel was perfused, both extraluminally and intraluminally, with oxygenated Krebs solution (37 C C) with a peristaltic perfusion pump (Harvard Apparatus). Extraluminal