Anesthetizing neurologic patients (Proceedings)

Whether you are faced with anesthetizing a patient with the potential of increased intracranial pressure (ICP) or a patient with a spinal cord injury, there are special anesthetic considerations that should be made. Each patient should be thoroughly evaluated and an appropriate anesthetic protocol should be formulated.

Patients with increased intracranial pressure

Intracranial disease or brain injury can interfere with control of cerebral circulation. This can predispose the patient to ischaemic damage of the brain. Normal ICP ranges between 0-10 mmHg, but it is not often or easily measured in veterinary medicine. Clinical signs of ICP include depression, papillary changes, cardiovascular abnormalities, and changes in the respiratory pattern. Patients with severe increased ICP will exhibit a Cushing's Triad, also referred to as a Cushing's Reflex or Cushing's Response. The Cushing's Triad causes hypertension, bradycardia, and often respiratory disturbances. The patient tries to maintain perfusion by increasing blood pressure. The patient becomes hypertensive and the heart rate slows (sometimes as low as 20 to 30 beats per minute) to compensate. Ventricular arrhythmias can also occur. The concomitant bradycardia is not treated due to the often severe hypertension. Giving an anticholinergic will often not only increase heart rate, but also increase blood pressure.

It is important for the anesthetist to try and decrease (or avoid increasing) ICP whenever possible. Common ways include the following:

     • Avoiding neck leads when walking patient prior to anesthetizing

     • Avoid making patient vomit

     • Slightly elevating the head using a towel or sandbag

     • Avoid occluding the jugular vein, performing jugular venipuncture, and placing jugular catheters

     • Place lidocaine on tracheal opening to avoid coughing which increases ICP

     • Provide Intermittent Positive Pressure Ventilation (IPPV) via hand bagging or mechanical ventilator

          o Keep ETCO2 low normal – around 35mmHg

     • Mannitol bolus over 20 minutes

          o Decreases blood viscosity by increasing blood volume. This results in vasoconstriction and increased oxygen delivery to the brain. Boluses are more effective than a CRI.

Pre-anesthetic considerations for patients with potential ICP

It is ideal to perform a complete physical examination, run a complete blood count, biochemistry panel, and urinalysis. Chest radiographs and an abdominal ultrasound should also be performed prior to anesthetic induction. This will provide a thorough work-up and help rule out other potential medical problems.

Formulating a "safe" anesthetic plan is essential. If a painful procedure is going to be performed, full mu opioids need to be given to provide proper analgesia. Methadone is a good choice for patients with ICP because this drug rarely causes vomiting. Vomiting will increase ICP. Morphine almost always causes vomiting so this drug should be avoided when possible. Giving an antiemetic drug prior to administration of an opioid can help decrease vomiting. If a non-painful procedure will be performed (diagnostics such as MRI or CT) butorphanol is often a good choice for pre-medication. Butorphanol provides good sedation and does not usually cause vomiting. Mental status should also be taken into account prior to pre-medication. Patients who are obtunded usually require much smaller doses than those who are mentally appropriate.

Anesthetic considerations for patients with potential ICP

Prior to induction, patients should be pre-oxygenated to help prevent hypoxemia. Drugs such as propofol, thiopental (not on the market at this time), and opioids such as fentanyl are appropriate induction agents. These drugs are often mixed with a benzodiazepine such as diazepam or midazolam Benzodiazepines work synergistically with these drugs often decreasing the overall amount of induction agent needed to induce anesthesia. After anesthetic induction, lidocaine can be placed on the tracheal opening to help prevent coughing. Lidocaine takes about 60 seconds to become effective therefore the oxygen mask should be placed over the nose and mouth during this time. ETCO2 and IPPV should be employed immediately after intubation. The ETCO2 should be maintained at 35mmHg. ETCO2 is somewhat of a controversial subject. Some anesthesiologists believe that ETCO2 should be maintained between 30 and 33mmHg, but hyperventilation can cause ischemia to the brain. Hypoventilation can cause an increase in ICP.

There are many accepted anesthetic protocols that can be used in patients with increased ICP. The two most common and well accepted methods include the use of sevoflurane and the use of total intravenous anesthesia (TIVA). Sevoflurane seems to be safe when administered at lower percentages (usually 2% or lower). Common drugs used for TIVA include the use of fentanyl and propofol as a CRI. TIVA can also be used in conjunction with low percentages of sevoflurane. Common doses for a propofol CRI are 0.1 to 0.4 mg/kg/min and for a fentanyl CRI are 0.5 to 0.7 mcg/kg/min in dogs and 0.3 to 0.5 mcg/kg/min in cats. A fentanyl loading dose of 5-10 mcg/kg is given to dogs while cats generally receive a 5 mcg/kg loading dose. Adding a CRI to your anesthetic protocol can minimize the amount of gas inhalant administered.

Extensive monitoring should be used whenever possible on patients with increased ICP. Common monitoring equipment includes: ECG, SPO2, ETCO2, rectal or esophageal temperature, Doppler with sphygmomanometer and blood pressure cuff or non-invasive blood pressure unit with cuff, arterial catheter for blood gas measurements and invasive blood pressure monitoring, and an external heating device. A secondary IV catheter should be placed if needed.

Fluid therapy should be administered during any anesthetic procedure. An anesthetic fluid rate of 10 ml/kg/hour is usually given unless the patient has an underlying medical condition preventing the use of maintenance fluids.

Post-operative care of patients with increased ICP

Post-operative pain management should be given when a painful procedure has been performed. Patients undergoing diagnostics such as CT or MRI do not need post-operative analgesia, but those undergoing procedures such as a brain biopsy, craniotomy, or any other painful procedure should get analgesics as needed. Common post-operative pain medications include single injections of full mu opioids given as needed or a fentanyl CRI administered at an analgesic dose (0.05 to 0.3 mcg/kg/min). Drugs such as buprenorphine or butorphanol can be given as needed for mildly painful procedures.

Recovery may be prolonged in patients with brain disease or head trauma. It is vital to keep recovering patients warm and maintain IPPV as needed until the patient is breathing on its own. Drug doses may need to be given at lower doses due to patient mentation.

Monitoring during MRI and CT

Patient monitoring can be very difficult for diagnostic procedures such as CT and especially MRI. Accessibility to and keeping patients warm is a major problem. CT scans are relatively short, therefore the anesthetist can run in and out to check vitals. It is also ideal to have a window or video camera to help monitor the patient during the periods when the CT machine is scanning. During a CT scan, warming devices such a forced air blankets (Bair Huggers) and most types of warm water circulating blankets can be used. Normal monitoring equipment can also be used as well.

MRI procedures are much longer than CT scans. The periods of scanning can last several minutes at a time and several sequences need to be performed. Special MRI compatible equipment is a must due to the magnets of the MRI machine. Heating patients can become an issue in the MRI unit. We have adapted a Bair Hugger blanket by attaching an extra-long hose to the unit. This is safe for the MRI unit and provides heat the patient. Microwavable heating pads such as the brand "Simply Cozy" work well for small patients and also do not interfere with the imaging.

Anesthesia for the seizure patient

Prior to anesthesia, any patient with seizures should have a full medical work-up including a complete blood count, biochemistry panel, urinalysis, chest radiographs, and an abdominal ultrasound. This will help rule out any underlying medical problems. Dealing with an epileptic or seizure patient can be difficult. Seizure medication should be continued even if the patient is undergoing an anesthetic/surgical procedure. Most common pre-medications can be used for patients with seizures. The type of drug chosen will depend on the procedure being performed. If the procedure is painful, a pure mu opioid should be chosen. If a simple diagnostic procedure is going to be performed a full mu opioid, buprenorphine or butorphanol can be used. It is often advised that acepromazine not be given to patients with epilepsy because it can lower the seizure threshold. This is somewhat controversial, but should probably be avoided when possible.

Common induction drugs for these patients include propofol and thiopental (not currently available). Using a benzodiazepine will often decrease the overall amount of drug required to induce anesthesia. Either isoflurane or sevoflurane are appropriate inhalants for maintenance anesthesia. TIVA or partial TIVA anesthesia with a propofol or fentanyl is CRI is also appropriate for these patients. These patients should be intubated and appropriate monitoring equipement should be placed. At the minimum, temperature and blood pressure should be monitored. Advanced monitoring equipment should be used based on the needs of the patient. For example, if the patient has underlying heart disease and ECG should be placed.

Post-operative pain management should be given when necessary. Common post-operative drugs include full mu opioids, buprenorphine, and butorphanol. The type of drug chosen should be appropriate for the type of pain the patient is experiencing. Severely painful procedures should not receive buprenorphine or butorphanol.

Anesthesia for patients requiring spinal surgery

Spinal cord injury due to intervertebral disc disease, trauma, and neoplasia is very common. These patients can present to the clinic in extreme pain and should be treated with full mu opioids as part of their pre-medication regime. Common full mu opioids include methadone, morphine, hydromorphone, and oxymorphone. Prior to anesthesia, any patient with spinal injuries/disease should have a full medical work-up including a complete blood count, biochemistry panel, urinalysis, chest radiographs, and an abdominal ultrasound. This will help rule out any underlying medical problems. Some of these diagnostics may need to be performed with pain medications on board due to the pain of manipulation for radiographs and/or ultrasound.

**If a myelogram is going to be performed as part of the pre-surgical diagnostics, the patients head should remain elevated after the contrast has been injected. Possible seizures can occur post-contrast and on recovery.

Common induction agents for these patients include propofol +/- benzodiazepine, thiopental (not currently available) +/- benzodiazepine, and ketamine + benzodiazepine. Patients are intubated and generally maintained on either sevoflurane or isoflurane inhalant. Appropriate monitoring equipement should be placed just after induction. At the minimum, temperature and blood pressure should be monitored. Advanced monitoring equipment should be used based on the needs of the patient. For example, if the patient has underlying heart disease an ECG should be placed.

Many spinal procedures are painful and patients often benefit from balanced or "multi-modal" anesthesia techniques. Balanced techniques include the use of epidural anesthesia or analgesia and analgesic CRIs of fentanyl and/or ketamine. Please see the proceedings for "Pain Management Techniques" to read more about CRIs, epidural, and how to properly calculate them.

Post-operative pain management is imperative for these patients. Patients are often given a single dose of a full mu opioid and repeated as needed or they can be kept on a fentanyl CRI as needed for pain. If steroids have not been given and the patient does not any sign of renal or liver disease, the use of a non-steroidal anti-inflammatory (NSAID) should be considered. Urinary catheters should be placed when possible to help the patient during the recovery process as they are often recumbent for a few days.

Anesthesia for patients undergoing CSF sampling

The CSF tap for most patients is fairly short (usually less than one hour) and is generally not very painful. Patients are often pre-medicated with full mu opioids or even butorphanol. Induction agents often include propofol or thiopental (not currently available) with or without a benzodiazepine. Patients are always intubated and at a minimum, temperature, blood pressure, and ETCO2 should be monitored. Additional monitoring can be added if necessary. IPPV should be provided if needed. Post-operative analgesics are not generally necessary.

General considerations for any patient under anesthesia

     • All patients should have an IV catheter placed prior to anesthetic induction

     • All patients should be intubated during the procedure regardless of time under anesthesia

     • Management of hypotension in any of these patients is generally maintained by decreasing the percentage of inhalant, fluid boluses of a crystalloid or colloid (when safe), and the use of a positive inotrope (when appropriate).

     • General fluid therapy is given at 10 ml/kg/hr unless contraindicated.

     • At the minimum, basic monitoring equipment is used regardless of time under anesthesia (temp, blood pressure, heat support)

          o Advanced monitoring should be used when indicated – ECG, ETCO2, SPO2

     • Post-operative pain management should be used when needed