Anesthetic Considerations for Horses with Neurologic Disease

Abstract

149Neurologic disease can present a variety of challenges for the anesthetist. The unique anatomical features of the brain limit its ability to respond to inflammatory, traumatic, and neoplastic challenges. Horses presenting with spinal cord or peripheral neuropathies can be challenging to the anesthetist due to the increased danger of a large, ataxic patient. The goal of neurologic anesthesia is to maintain adequate nourishment and blood delivery to the brain, minimize the development of seizures, minimize the effects of ataxia on the patient's safety, and prevent further deterioration of neurologic status.An understanding of the anatomy and physiology of the nervous system and how it relates to anesthesia is important in understanding how best to maximize patient safety and minimize morbidity and mortality.The brain is housed entirely within a bony cavity. This rigid structure does not allow for an increase in volume when faced with a traumatic, inflammatory, or neoplastic insult. Minor changes in cerebral blood flow (CBF), cerebrospinal fluid (CSF), or tissue volume (i.e., mass) can quickly result in a moderateto-severe increase in intracranial pressure (ICP). Elevated ICP can cause regional and global cerebral ischemia and hypoxia due to a reduction in cerebral perfusion pressure (CPP) and CBF, and ultimately can lead to potentially fatal displacement of brain and neuronal tissue, including brainstem herniation. 1 CBF is directly proportional to CPP and indirectly proportional to cerebral vascular resistance (CVR). Of these components, only CBF can be easily influenced by the anesthetist and thus is of particular interest.In a normal horse, CBF is under metabolic (chemical), myogenic (autoregulation), and neurogenic factors. 2 This control ensures consistent, high flow of blood to the brain over a range of arterial blood pressures. The mean arterial pressure (MAP) range within which CBF remains under autoregulatory control has not been defined in horses; however, in humans, the CBF remains consistent over 70-140 mm Hg, and animals may have better regulation of flow at lower arterial pressures. 2 Horses, due to their large size, undergo large orthostatic shifts when positioned in lateral or dorsal recumbency for general anesthesia. 3 These fluid shifts can alter MAP and thus have an effect on CPP.Chemical or metabolic factors that influence CBF include cerebral metabolic rate (CMR), the partial pressure of arterial carbon dioxide (P a CO 2 ), the partial pressure of arterial oxygen (P a O 2 ), and temperature. An increase in CMR will lead to an increase in CBF. 2 Anesthetics, both injectable and inhaled, can alter CMR and therefore influence CBF. Likewise, an increase in the temperature of the brain causes an increase in CBF.There is a directly proportionate relationship between CBF and P a CO 2 . These changes are due to intra-and extracellular pH alterations. Hypocapnia and alkalemia lead to a decrease in CBF due to vasoconstriction, resulting in an increase in CVR. Conversely, hypercapnia and acidemia resul...