Surface monitoring of cerebral cortical blood flow.

Techniques of monitoring surface blood flow in the brain allow observation of dynamic "real-time" changes in cortical blood flow (CoBF). These techniques have evolved from pial window observations that have not been quantitative and frequently are unreliable. Surface monitoring does not require the development of a clearance curve so changes in flow are seen immediately. On the other hand, the local vascular geometry may affect these techniques; therefore, large surface vessels must be avoided and the probe must be of large enough size so that some averaging effect of the cortical capillary bed will be obtained. At the present time, the two techniques available for surface monitoring are thermal diffusion flowmetry (TDF) and laser-Doppler flowmetry (LDF). Thermal methods have been available longer and more experience has been obtained in experimental, operative, and postoperative monitoring of CoBF with these techniques. LDF was used in retina, gastric mucosa, and skin, and has only recently been applied to the cerebral cortex. In the operating theater, both techniques have demonstrated increased CoBF in normal brain after arteriovenous malformation resection and have demonstrated reduced CoBF in normal brain around brain tumors. Acute changes in CoBF with vascular manipulation during aneurysm surgery have been demonstrated with TDF. Postoperative monitoring of aneurysm patients has demonstrated the development of cerebral vasospasm with TDF as well as increased flow preceding the development of malignant cerebral edema in trauma patients. Artifacts occur in TDF with irrigation, loss of surface contact, and contact with large surface vessels. LDF has artifactual changes with movement, light, if large surface vessels come in view of the probe, and changes in hematocrit. Surface monitoring shows a great deal of promise in continuous evaluation of CoBF intraoperatively and postoperatively.