Anesthetic Management for Neurosurgical Procedure in Sheep

Presenter: Vince Mendenhall, DVM, PhD, Preclinical Consulting LLC

Abstract: Documentation regarding complications after laminectomy in sheep is scarce but not so in humans. Prior to improvements in anesthetic management, neurologic deficits following laminectomy ranged from ~52% of a minor nature to ~11% major, mostly due to swelling. Before similar methods were adopted in sheep, this rate was higher likely due to their tendency to “third space”. A laminectomy has a higher rate of potential complications simply due to swelling and the sensitivity of the spinal cord to even slight external pressure. The goal must be to decrease the bulk of the spinal cord during surgery to allow more room to work, and also to decrease swelling. This can be done through anesthetic management alone combined with delicate techniques.

Third spacing (swelling/edema) occurs when fluid moves from the intravascular space to the extracellular space (ECS). This occurs as a result of tissue damage (surgery) that increases capillary permeability and decreases plasma proteins. This situation is exacerbated by the use of inhalation anesthetics that naturally cause vasodilation and by the administration of crystalloid IV fluids that by dilution further decrease plasma proteins. One cannot eliminate inhalation anesthetics, but IV crystalloid fluids should not be given if the total protein is < 6 g/dL. Four mg/kg of dexamethasone administered preoperatively will decrease CNS edema and inflammation by improving microcirculation and stabilizing lysosomal membranes.

Swelling can also be reduced with Mannitol (1 g/kg IV over 30 minutes) to decrease CNS pressure. Hyperventilation lowers CNS blood flow and pressure by decreasing PaCO2 which induces arterial vasoconstriction mediated by increasing pH in the ECS. There is ~2% decrease in CNS blood flow for every 1 mmHg decrease in PaCO2. Thus, hyperventilation provides relaxation in the surgical field by significantly decreasing CNS bulk. Operating conditions are best improved with a PaCO2 of ~25 +/- 2 mmHg. This level reduces CNS pressure by ~5 mmHg and the risk of swelling by ~14%. Care should be taken to not go less than this because a PaCO2 of <20 mmHg induces cerebral ischemia and cardiac arrhythmias.

In emergency situations, 1.5 mLs/kg of 5% hypertonic saline may be given as a bolus, while monitoring Na levels to insure that they are 6 g/dL.

In combination with the pre- and postoperative regimens as described by my colleagues, the incidence of minor neurological deficits postoperatively since I’ve instituted this anesthetic regimen 3 years ago is now at <~2% in over 150 cases, all of which resolved within 7 days after surgery with a decreased incidence of issues altogether.