It is now recognized that many of the principles of anaesthesia for animals with intracranial disease are relevant to animals with spinal disease. Arterial blood flow in the spinal cord is autoregulated in the same manner as cerebral blood flow to remain constant over a range of blood pressures. In addition, arterial CO2 has also been shown to be important in regulation of spinal cord blood flow. Considerations for sedating and anaesthetizing patients with spinal disease are summarized in Table Considerations for sedation and anaesthesia of patients with spinal disease:
|Pain||Disc hemiation, fracture, infection, inflammation||Analgesia ()|
|Muscle spasm||Muscle relaxants (benzodiazepines)|
|Mechanical instability||Fracture||Careful positioning of animal|
|Cervical malformation (wobbler)||Support neck during intubation|
|Atlantoaxial subluxation||Avoid agents that cause vomiting|
|Decreased spinal cord perfusion||Hypovolaemia: dehydration; blood loss||Correct fluid and electrolyte abnormalities before anaesthesia|
|Impaired autoregulation in diseased spinal cord: small decrease in blood pressure may cause ischaemia||Maintain normotension:|
|Use agents that minimally depress autoregulation|
|Lesions proximal to thoracolumbar junction cause autonomic imbalance and hypotension||Use agents that minimally depress cardiovascular function|
|Respiratory insufficiency / failure||Cervical spinal injury: loss of intercostal and diaphragmatic innervation||Pre-oxygenate|
|Monitor adequacy of ventilation:|
|Sternal recumbency may restrict diaphragm||Pulse oximetry|
|Retraction of trachea during cervical surgery can obstruct airway||Blood gas analysis|
|Provide assisted ventilation when necessary|
|Potent opioids used for intraoperative analgesia depress ventilation|
|Hypothermia||Autonomic imbalance: peripheral vasodilation increases heat loss Surgical exposure||Monitor temperature
Active warming: warm water or air blankets
|Surgical blood loss||Can be significant if venous sinus is damaged||Quantify blood loss: swabs, suction bottles
Transfusion: Blood loss >20% blood volume Haemoglobin concentration <8 g/dl
|Myelography||Contrast agent predisposes to seizure activity||Slow injection
Head elevation after cisternal injection
Avoid anaesthetic agents that decrease seizure threshold or have inherent epileptogenic activity
Premedication usually involves the use of opioids, with or without the use of agents with sedative or anxiolytic properties. Animals with spinal injury, particularly those that are paralysed, can be very anxious. In these cases, provided the animal is normovolaemic, low-dose acepromazine maleate (ACP) (0.01-0.02 mg/kg i.m.) in combination with opioids can be useful. Some practitioners avoid the use of ACP if myelography is to be performed because of concerns about post-myelo-graphic seizures.
Medetomidine may be useful for sedation and anxiolysis in extremely anxious or fractious animals without cardiac disease and that are normally hydrated. Intramuscular doses as low as 2-5 ug/kg can provide useful sedation when combined with other agents such as opioids or ACP. Medetomidine has the additional benefits of providing analgesia and skeletal muscle relaxation. Medetomidine is reported occasionally to cause vomiting and thus should be used carefully when cervical instability is suspected.
Benzodiazepines are generally poor sedatives in otherwise healthy dogs but provide useful sedation in cats when used in combination with other agents. Midazolam (0.2 mg/kg), in combination with opioids or ketamine, is useful for premedication of cats with spinal disease. Ketamine (3-5 mg/kg i.m.) may also be combined with ACP for premedication in normovolaemic cats with normal renal function. However, as intramuscular injections are painful and can provoke extreme agitation, they are best avoided in cats with unstable spinal disease.
Induction is performed with intravenous agents to ensure rapid, controlled induction with minimal struggling.
Selection of an appropriate induction agent is based on similar principles to those described for animals with intracranial disease. The characteristics of intravenous agents and suggested dose rates are described in Table Characteristics of intravenous sedatives and induction agents used in animals and recommendations for use in animals with central nervous system disease. To decrease the dose of the intravenous agent and minimize cardiovascular depression, concurrent administration of a potent opioid, such as fentanyl, or benzodiazepines can be used during induction.
Intubation should be performed carefully, with adequate support of the head and neck, particularly in animals with cervical injuries.
The animal should be placed on a flat surface or table with its head and neck flat and supported by the table. Intubation is facilitated by use of a laryngoscope (). Excessive neck extension should be avoided in dogs with caudal cervical lesions, while excessive neck flexion should be avoided in animals with atlantoaxial subluxation or other cervical fractures.
Maintenance is usually performed with inhalation agents. As autoregulation and CO2 responsiveness are maintained better with isoflurane than with halothane, isoflurane is the preferred maintenance agent. Nitrous oxide is reported to increase ICP and is avoided in patients with intracranial disease. Whetherthe same precautions are warranted in animals with spinal cord injury and compression is not known. Intraoperative analgesia can be provided by infusions of potent mu opioids such as fentanyl, alfentanil and remifentanil ().
IPPV is recommended during anaesthesia in the spinal patient for several reasons:
- • The detrimental effects of inhalation agents, such as isoflurane, on spinal blood flow regulation can be minimized by maintaining normocapnia
- • Surgical access requires that the animal is positioned in sternal recumbency, which can interfere with diaphragmatic excursions and impair ventilation
- • The doses of opioid agonists recommended for intraoperative use produce marked respiratory depression, necessitating IPPV.
During diagnostic imaging and surgery in animals with spinal disease, non-invasive monitoring of cardio-pulmonary function with electrocardiography, oscillometric blood pressure measurement, capnography and pulse oximetry is generally adequate. In animals where cardiopulmonary dysfunction (e.g. cranial cervical surgery; trauma involving multiple organ systems) or excessive blood loss is expected, invasive monitoring is recommended. Monitoring techniques have been discussed in detail above.
Parenteral administration of opioid agents provides the most effective pain relief in animals with spinal disease.
Due to the severity of pain in most animals with spinal disease, analgesia is best achieved with opioid analgesics (). Although vomiting is less likely in animals in pain, the use of morphine is generally avoided in animals with cervical injury where violent movements associated with vomiting can cause further injury to an unstable lesion. In addition, recumbent animals that vomit may not be able to clear vomitus from their pharynx and mouth, predisposing to airway obstruction and subsequent aspiration.
Parenteral administration is recommended in the immediate perioperative period. In addition, skin patches that deliver fentanyl transcutaneously may provide a useful adjunct to perioperative analgesia (). As there is a delay of approximately 12-24 hours in attainment of peak concentrations, the patches should be placed the day before surgery where possible. There is marked individual variation in the serum concentrations achieved; thus, while useful, fentanyl patches should not be relied on as the sole method of providing analgesia.
Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to decrease inflammatory pain. Cyclooxygenase inhibitors, carprofen and meloxicam and the mixed lipoxygenase and cyclooxgenase inhibitor tepoxalin are registered for perioperative use as these agents do not interfere with platelet function. Concurrent administration of NSAIDs and corticosteroids is contraindicated due to increased risk of gastrointestinal ulceration and haemorrhage; thus the use of NSAIDs is best delayed until it has been decided whether the patient will need corticosteroid therapy. To reduce the incidence of gastrointestinal ulceration in animals receiving steroids or NSAIDs, concurrent administration of gastrointestinal protectants is recommended.
Muscle relaxants, such as benzodiazepines (diazepam 0.25 mg/kg orally q8h), may provide useful adjuncts to pain management in patients with stable spinal injury by alleviating muscle spasms commonly observed in animals with spinal disease. Skeletal muscle relaxation may be detrimental in animals with unstable spinal lesions as it reduces the splinting effects of the epaxial muscle.
Fluid balance must be maintained, and intravenous fluid therapy is essential during anaesthesia in all cases. In hypovolaemic animals, volume deficit should be replaced before anaesthesia. As blood loss can be surprisingly high during spinal surgery, the amount of blood on swabs and in suction bottles should be monitored throughout anaesthesia. Blood transfusion is indicated when blood loss is >20% of the circulating blood volume or haemoglobin concentration is <8 g/dl. Smaller amounts of blood loss associated with hypotension can be managed with administration of colloids such as etherified starch (maximum dose 20 ml/kg/24 h).
Heat loss can be a problem, particularly when spinal cord injury causes sympathetic nervous system dysfunction and peripheral vasodilation. Temperature should be monitored during anaesthesia. Warming can be provided during anaesthesia by use of warm waterbeds or air blowers. Heat and moisture exchange devices () can be placed between the endotracheal tube and circuit, to decrease heat and moisture loss from the patient.
It is essential that administration of analgesia is continued during the initial recovery period into the postoperative period to ensure a calm, pain-free recovery. In some cases use of low-dose sedatives, such as ACP (0.01-0.02 mg/kg Liu), may be required in extremely stressed or agitated animals. Analgesia in the immediate postoperative period can be provided by incremental administration of mu agonists or continuous-rate infusions. Morphine and fentanyl have both been used postoperatively as continuous-rate infusions in humans and animals ().
Urinary catheters can be important in recumbent animals and in any animal where bladder function may be altered by spinal injury. They are particularly useful in trauma patients with extensive soft-tissue bruising. Catheters should be inserted using a sterile technique and attached to closed collection systems to prevent UTI. Antibiotics should not be used empirically in these patients as their use will encourage development of resistant UTIs. Catheter systems should be used for as short a time as is necessary. (For management of indwelling catheters see above.) Frequent intermittent manual bladder expression or catheterization is an alternative option but is not always possible. Use of drugs to relax the internal and external urethral sphincters can facilitate manual expression ().
For recumbent animals, padded bedding is essential to prevent pressure sores. Non-slip matting and padded kennels also help to reduce injury in ataxic or weak animals that may be unstable when trying to walk.