Anesthesia for neurologic disease and procedures (Proceedings)

Emergency back surgery

The primary concerns for anesthesia for dogs with spinal neurologic disease are the need to prevent pulmonary aspiration of gastric fluid if food has not been withheld, to maintain a low intracranial pressure (ICP) and therefore a low spinal cord pressure, and the provision for pain management. Myelography is often associated with acute changes in arterial blood pressure. Positioning for dorsal decompression surgery is often responsible for hypoventilation.

The risk of gastric reflux is increased in patients scheduled for emergency anesthesia and surgery, brachycephalic breeds, old patients, overweight patients, presence of megaesophagus, and with the administration of mu-agonist opioids. Induction of anesthesia in patients at increased risk for regurgitation should be performed with the patient in an upright position until the endotracheal tube is inserted and the cuff inflated.

Increased arterial carbon dioxide concentration has the greatest effect for increasing ICP. Adequacy of ventilation should be monitored and controlled ventilation started in patients that are hypoventilating. Ketamine increases ICP and should be avoided in these patients. Isoflurane and sevoflurane cause vasodilation and increased ICP but this can be somewhat offset by controlling ventilation and maintaining PaCO2 at low normal values. Inhalation agents may be avoided in patients with preexisting increased ICP, head trauma, large intracranial masses and for surgical craniotomy for tumor excision.

Myelogram

Blood pressure should be monitored constantly as multiple position changes occur during radiology and myelography that can have a significant adverse impact on cardiovascular function and arterial pressure. Increased ICP should be avoided by appropriate choice of anesthetics and use of controlled ventilation when necessary. Artificial ventilation should be employed at least during placement of the spinal needle and injection of contrast agent as an increase in spinal pressure modifies the test injection for evaluating correct needle placement. Occasionally injection of iohexol is followed immediately by a moderate or severe decrease in blood pressure. Iohexol may be responsible for seizures during recovery from anesthesia. Maintaining balanced electrolyte infusion at 10 ml/kg/h during anesthesia will promote urine formation and elimination of the iohexol. If the patient does not go to surgery immediately following the myelogram, it is advisable to maintain anesthesia for approximately 45 minutes after iohexol injection to allow time for the agent to be eliminated. The dog should be positioned in a head-up position during this time to promote caudal movement of the iohexol and out of the brain. A rapid return to consciousness should be avoided. Diazepam, 0.25 mg/kg, IV should be administered if facial twitches are observed during recovery from anesthesia. Sometimes the patient may have a smooth recovery from anesthesia and yet seizure 2-3 hours later.

Dorsal decompression surgery

Prone positioning the dog or cat may limit thoracic and abdominal excursion, particularly if the surgeons leans on the patient during surgery. Insertion of a pad under the pelvis may help to lift the abdomen off the table and ease ventilation. The head should be padded so that it is level with the spine; lower head increases ICP. Separation of the muscles for access to the surgical site is a major stimulus and adequate analgesia is essential. Mu receptor opioids such as morphine, hydromorphone, oxymorphone, methadone, or fentanyl provide the best analgesia. A continuous infusion of fentanyl, 6 micrograms/kg/h, is a useful supplement to opioid premedication. A morphine epidural is useful for lumbosacral decompression but needle placement may leave a hematoma.

Ventral cervical decompression

This surgical procedure may cause airway obstruction, blood loss, dysrhythmias, and air embolism. A long endotracheal tube should be selected so that the tip of the tube can extend past the surgical site when the trachea is retracted. Retraction can cause airway obstruction by compressing the lumen of the tube or by pressing the wall of the trachea over the end of the tube. Partial or complete obstruction can easily be detected if the patient is connected to a capnogram by a decreased and altered waveform or no detection of CO2. The intravenous catheter must be easily accessible after the drapes are applied because laceration of a sinus can cause a moderate blood loss. A catheter in the back leg is a viable precaution. The blood loss should be estimated and 2 times the volume lost replaced with balanced electrolyte solution in addition to the maintenance fluid volume. If the vascular laceration is large and the dog is breathing spontaneously, air can be entrained into the vascular system and that occasionally causes cardiac arrest. Immediate replacement of spontaneous ventilation with controlled ventilation in the event of vascular laceration should prevent air embolism.

Cerebrospinal fluid collection

Collection of cerebrospinal fluid (CSF) has the potential for causing cerebellar herniation if the ICP is high. Several manipulations of the anesthetic protocol can decrease ICP and decrease the risk of herniation. Ketamine and Telazol should be avoided. Dogs with meningitis have a moderate to severe decreased requirement for anesthetic drugs and very little agent is needed to induce anesthesia in some dogs.The blood pressure should be less than systolic pressure of 145 mmHg before needle placement. Controlled ventilation is advisable throughout needle placement and CSF collection to ensure low PaCO2. If capnography is available, spontaneous ventilation can be allowed if the end-tidal CO2 remains less than 42 mm Hg. The patient should be observed for airway obstruction by kinking the endotracheal tube when the head and neck are positioned for a cisternal tap.

Upper airway exam for laryngeal paralysis

The danger from anesthesia for dogs with laryngeal paralysis is exacerbation of the obstruction and creation of hypoxia. Furthermore, anesthesia must interfere as little as possible with laryngeal function to facilitate a diagnosis. Administration of morphine or hydromorphone may induce vomiting which can result in aspiration pneumonia. Acepromazine may cause pharyngeal relaxation and worsen obstruction. Thiopental is the best of the induction drugs for viewing laryngeal function; propofol is less easy to evaluate, especially when it induces apnea (Jackson et al 2004). Ketamine should not be used to assess laryngeal function for laryngeal paralysis.

Anesthetic protocols that satisfy these requirements are as follows: Placement of an IV catheter followed by premedication with butorphanol, 0.2 mg/kg, IV with or without midazolam, 0.2 mg/kg. Pre-oxygenation is applied for several minutes before induction of anesthesia with thiopental sufficient only to open the jaws allow a view of the larynx. After confirmation of presence or absence of paralysis and if the intention is to proceed to surgery, anesthesia is induced completely with midazolam or diazepam, if not already given, and more thiopental to facilitate endotracheal intubation. Propofol may be used as an alternative to thiopental but given in small increments to avoid apnea and interference with laryngeal assessment.

Doxapram, 1 mg/kg, has been administered to dogs to increase rate and depth of breathing and was deemed a useful aid to assessment of laryngeal function (Tobias et al 2004). However, the increase in negative airway pressure from exaggerated breathing can result in airway obstruction from laryngeal closure and tracheal intubation may be required.

Transnasal laryngoscopy in sedated dogs is an alternative to general anesthesia (Radlinsky et al 2004). The protocol that we currently use is placement of an IV catheter and, when all is ready for endoscopy, IV administration of butorphanol, 0.2 mg/kg, and midazolam, 0.2 mg/kg, IV. A small amount of lidocaine is instilled into the nostril with the dog's nose elevated. The endoscope is then gently inserted through the nostril and passed into the pharynx. After observation of laryngeal movement, the scope is moved to lightly touched the larynx and the resultant response noted.

Myesthenia Gravis

Muscle of ventilation are usually impaired resulting in hypoventilation during anesthesia. Controlled ventilation may be necessary and a pulse oximeter should be used in recovery to monitor oxygenation.

Drugs and dosages

References

Jackson AM, Tobias K, Long C, et al. Effects of various agents on laryngeal motion during laryngoscopy in normal dogs. Vet Surg 2004;33:102-106.

Tobias KM, Jackson AM, Harvey RC. Effects of doxapram Hcl on laryngeal function of normal dogs and dogs with naturally occurring laryngeal paralysis. Vet Anaesth Analg 2004;31:258-263.

Radlinsky MG, Mason DE, Hodgson D. Transnasal laryngoscopy for the diagnosis of laryngeal paralysis in dogs. J Am Anim Hosp Assoc 2004;40:211-215.