There is evidence that pain therapy during the perioperative period results in less surgical stress, more rapid, smooth recovery and greater patient comfort with a shorter time spent in the clinic.

Pain management is more effective if given preoperatively (pre-emptive analgesia, before surgical stimulus starts) and by using multimodal pain therapy tailored to individual patients. It is therefore recommended to include analgesics as part of premedication or administer them while the animal is anaesthetised.

The majority of the analgesic agents used to treat perioperative pain are opioids, non-steroidal antiinflammatory drugs (NSAIDs), N-methyl-D-asparate (NMDA) antagonists, local analgesics and the alpha-2 adrenergic agonists. These different categories of drugs influence pain processing through different mechanisms, and can therefore be used in combination to maximise multimodal analgesia. Lower doses of each analgesic agent can be used, reducing the potential side effects and providing better quality analgesia.

The maintenance of anaesthesia can be carried out with injectable anaesthetic agents or volatile inhalational agents. None of these are likely to provide any significant analgesia. Additional analgesic techniques are often carried out if the surgical procedure is to be highly invasive or extensive. Most analgesics techniques will reduce the amounts of maintenance anaesthetic agents needed to keep the patient anaesthetised. Also, these additional techniques carried out perioperatively will complement the analgesic drugs given before the surgery and obtund the nociceptive stimulus during surgery.

Towards the end of the procedure the patient should receive an analgesic so that postoperative recovery is without pain.

Analgesics

Opioids

Opioid analgesics are considered to be the most efficacious drugs for pain relief. Within the UK, the purchase, storage and use of opioids is legally regulated. In the UK very few agents are licensed for use in small animals. The use of unlicensed opioids outside the cascade must be justified. Opioids exhibit a wide variety of clinical effects depending on the opioid, the dose administered and the animal species. This is due to their receptor specificity (mu, kappa, delta, sigma), potency and efficacy. Opioids can be administered through a variety of routes (intramuscular, intravenous, subcutaneous, intra-articular, epidural, transdermal).

Full mu agonists (e.g., morphine) are noted to produce the most profound analgesia and sedation. Their effects are dose-dependent and it is possible to reduce the intensity of most types of pain with them. In clinical use there is no upper limit to the analgesia available with the pure agonists. There is considerable individual variation in opioid requirements in dogs and cats. The goal is to achieve adequate plasma concentrations with analgesia. Some of their side effects are depression of the respiratory centre, decreased heart rate, vomiting and decrease the thermoregulation set point. Methadone, with similar analgesic properties to morphine, produces less sedation. Both morphine and methadone can be used in cats and dogs. Morphine can also be administered as a constant-rate infusion (CRI). Pethidine is short acting and only recommended for procedures associated with mild discomfort. Pethidine should not be administered intravenously because of histamine release. Fentanyl, which is a short-acting opioid similar to but more potent than morphine, can be administered intraoperatively in a continuous CRI and will produce significant reduction in the minimum alveolar concentration (MAC) requirement. This can be advantageous in patients with cardiac dysfunction allowing for the inhalational anaesthetic agent concentration to be reduced. Fentanyl can also be administered as a transdermal patch. A few adverse effects have been reported with the use of the patch. The rate of absorption dependent on the surface area of the patch that is exposed to the skin. Therapeutic plasma concentrations are reached within 24 hours in dogs and 12 hours in cats. There are some well known side effects to the use of fentanyl patches in dogs and cats. Sufentanil, alfentanil and remifentanil are short-acting mu agonists which are primarily administered by CRI to provide analgesia during general anaesthesia.

Partial agonists and mixed agonist-antagonists can be used. Buprenorphine (partial mu receptor agonists, kappa receptor antagonist) provides moderate analgesia. With increasing doses, analgesia may be reduced (bell-shaped response curve). It has a slow onset of action but a long duration of action. Butorphanol (partial kappa agonist, mu antagonist) provides excellent sedation but is a less effective analgesic when compared to some opioids. Duration of analgesia is variable depending on route of administration and degree of pain. Both are licensed for use in the dog and cat.

Opioids provide good analgesia for various types of procedure and should always be considered as the first line of pain relief. However more profound analgesia can be produced by combining opioids with analgesics from other drug groups as follows.

Ketamine

Ketamine (a NMDA antagonist) is routinely used as an anaesthetic agent in dogs and cats. Sub-anaesthetic doses of ketamine as an intravenous bolus followed by CRI have been shown to reduce the requirement of inhalational agents. The CRI should be titrated to the patient's response. It is also believed to reduce central sensitisation and wind-up. Ketamine can also be administered in combination with morphine alone or mixed with morphine and lidocaine as a CRI for perioperative and postoperative pain relief.

Alpha-2 Adrenoceptor Agonists

Medetomidine has potent analgesic properties mediated by alpha-2 adrenoceptor receptors centrally in the dorsal horn and the periphery. At low doses both sedative and analgesic effects are dose-dependent. Patient evaluation and selection is important because the alpha-2 adrenoceptor agonist will reduce cardiopulmonary function. Medetomidine can be used as a pre-anaesthetic agent in combination with an opioid as part of a balanced anaesthetic approach. For routine feline elective procedures, medetomidine, ketamine and butorphanol can be used. Medetomidine CRI can be used in severely painful or anxious patients to provide sedation and analgesia. Medetomidine can be added to a pre-existing analgesic CRI (ketamine, ketamine and morphine or ketamine, morphine, lidocaine). Medetomidine CRIs have the potential to cause severe bradycardia/hypotension; these patients should be monitored very closely. Intra-articular use of medetomidine is very effective. The combination of medetomidine, opioid and local analgesic can provide long-lasting analgesia when given into the joint after surgery. Dexmedetomidine is a new alpha-2 adrenoceptor agonist which has similar effects to that of medetomidine in the dog and cat.

Local Analgesics

Local analgesics are frequently used as part of balanced anaesthetic technique to produce analgesia by administering them at specific sites or directly on nerves. They are ideal for pre-emptive analgesia. The sensory blockade they provide reduces greatly the requirement for general anaesthetic agents. These techniques are relatively easy to perform. When administered as a CRI they are thought to inhibit the nociceptive surgical stimulus and also provides postoperative analgesia. In dogs lidocaine given as a bolus followed by a CRI has been used to control pain refractory to other analgesics.

Non-Traditional Analgesics

Drugs used to produce tranquillisation and muscle relaxation when administered in combination with analgesic and general anaesthetic agents produce adjunct analgesic effect.

Should Every Surgery Patient Get Opioids Perioperatively?

Why we use them:

 They provide analgesia during the perioperative period

 Opioids enhance sedation, even for procedures perceived as non-painful

 They produce minimal cardiopulmonary effects

 They reduce requirements of general anaesthetic agents

Some of the reasons not to use them:

 Depending on drugs available: vomiting may be contraindicated in some procedures

 Licensing and legislation: not a reason, an excuse

 Inadequate facilities, e.g., not able to ventilate, so do not use fentanyl, use alternative drug

 Ineffectiveness: chronic use may require other groups of drugs to be used

Finally, administer analgesics preoperatively. Use several classes of analgesic agents acting at different points of the pain pathways. Match doses and duration of action of the analgesic agents administered to the degree of expected tissue injury from surgery.