Effects of Volatile Anesthetics on Cerebrocortical Laser Doppler Flow: Hyperemia, Autoregulation, Carbon Dioxide Response, Flow Oscillations, and Role of Nitric Oxide

Publisher Summary This chapter compares the direct effects of halothane and isoflurane on laser Doppler flowmetry (LDF), flow autoregulation, response to carbon dioxide, and spontaneous flow oscillations as measured by laser Doppler flowmetry in the parietal cerebral cortex of the rat. LDF is a novel and increasingly popular technique that can be used to monitor, in a localized manner, microvascular perfusion of various tissues, in particular, that of the brain. LDF monitors nondirectional flow of red blood cells in the microcirculation. Halothane and isoflurane produce dose-dependent increases in red blood cell flow in the rat cerebral cortex with isoflurane having a huge effect. Halothane but not isoflurane potentiates hypercapnic hyperemia of red cell flow. Neither anesthetic agent abolishes autoregulation as assessed by LDF. The present results obtained with LDF appear to be at variance with earlier studies that suggested that halothane causes greater vasodilatation in cerebral cortex than does isoflurane. An explanation of this apparent contradiction may be that LDF measures red blood cell flow in a small area of cortical tissue. Microvessel dilatation to isoflurane may result in increased capillary hematocrit and, consequently, a larger increase in red blood cell flow as measured by LDF than whole blood flow. In the presence of volatile anesthetic, phenylephrine infused to increase mean arterial blood pressure does not result in cerebral vasoconstriction. The chapter demonstrates that the known CO 2 response of cerebral blood flow is closely reflected by the similar response of LDF. However, Nitric oxide does not appear to be an obligatory mediator of halothane-induced hyperemia but may act in concert with other endothelium-derived relaxing factors.