Analgesia has evolved during the last years thanks to a better understanding of pain physiology and the knowledge of its negative consequences together with the availability of newer analgesic drugs and techniques. Alleviation of pain is mandatory not only for ethical reasons, but also therapeutic convenience; There is increasing evidence demonstrating the negative consequences of pain on recovery and morbidity. Since surgery is one of the most common sources of pain, adequate perioperative anaesthetic/analgesic techniques are of great importance.
Selection of analgesic and anaesthetic techniques
Appropriate selection of anaesthetic and analgesic techniques is required when pain is unavoidable, such as during surgery. Unlike 10-20 years ago, there is a wide selection of anaesthetics and analgesics that can efficaciously be given to animals. Similarly, an increasing body of data related to the proper use of these drugs has become available.
A common pitfall when providing anaesthesia to animals is the inadequacy of the selected technique to the severity of the procedure. The real analgesic properties of many anaesthetics are probably overestimated; drugs like propofol, thiopentone or even inhalant anaesthetics are poor analgesics, so, to avoid a painful response usually there are two alternatives: co-administration of an analgesic or high, and potentially dangerous, doses of anaesthetics. In general terms, analgesia-anaesthesia is mandatory in all surgical procedures but sedation or hypnosis may be considered in non-painful procedures. Balanced anaesthesia and analgesia is considered a safer alternative to the use of a single anaesthetic or analgesic to provide either anaesthesia or analgesia respectively. This is achieved by combining several drugs at lower doses and thus reducing the appearance of side effects. Common examples of balanced anaesthesia are a combination of a hypnotic like propofol or low doses of isoflurane with an opioid analgesic like fentanyl. This technique provides excellent analgesia without the deleterious effects of high doses of isoflurane when given alone to provide anaesthesia. Similarly, balanced analgesia can be obtained by giving an opioid together with a non-steroidal anti-inflammatory drug and a local anaesthetic technique in e.g., orthopaedic surgery.
Pre-emptive analgesia is another relatively recent technique that ensures the administration of analgesic techniques before the animal experiences pain. This requires the administration of effective analgesia since inadequate dosage or proper selection of analgesics to the severity of the procedure results in failure to produce analgesia. Early effective analgesia should be provided in any potentially painful procedure.
Analgesia
Due to the difficulty of determining when an animal is in pain, analgesia should be provided whenever a procedure or a condition is likely to cause pain without reliable pain assessment. In absence of other evidences, the anthropomorphic approach has been instituted: it is assumed that something that is painful in a human will also be painful in an animal. Possible exceptions or variations to this would be, for example, the response to laparotomy, where anatomical and physiological considerations may account for the difference. Nevertheless, individual assessment should be instituted whenever possible.
Analgesic techniques and drugs can be appropriately chosen from a growing selection to better fit the requirements of the clinical setting. The best practices should answer questions like why (severity), when (timing), where (route), or how (formulations). Newer non-steroidal antiinflammatory drugs (NSAIDs) are potent analgesics, especially when there is a marked inflammatory component (e.g., surgery). NSAIDs can be given intra-operatively when there are no specific contraindications: renal, gastrointestinal or coagulation impairment. Analgesic potency of NSAIDs may be increased by co-administration of opioids as a balanced analgesic technique. Partial agonist (buprenorphine) or agonist-antagonist opioids (butorphanol) should be avoided in animals experiencing, or expected to experience, severe pain since they may antagonise the analgesic potency of more potent opioids (pure agonists). Other analgesic drugs like alpha-2 receptor agonists (xylazine, medetomidine), ketamine or local anaesthetics can also be included in a balanced analgesic technique.
Analgesic drugs should be selected considering their potency in order to match the severity of the painful procedure. Potent opioids (morphine, pethidine, and the fentanyl family of drugs) are given in very painful procedures while less potent opioids (butorphanol, buprenorphine) can be administered in other procedures causing low to moderate pain. Newer NSAIDs may provide analgesia for severe pain in conscious animals (carprofen, meloxicam, ketorolac), but they are less used in the intraoperative period.
Appropriate dosage of analgesics, especially the newer formulations, should be based on objective criteria, i.e., individual pain assessment. However, extrapolation of data from other species should be avoided; cats usually require lower doses of opioids and NSAIDs on a mg/kg basis than dogs, and the administration of larger doses of NSAIDs, for instance, may induce toxicity, such as increased intra and postoperative bleeding.
Since analgesia should be provided before pain is experienced, opioids and NSAIDs can be given in the preoperative period and some of them can also provide sedation (opioids, but also alpha-2 agonists, and ketamine). Potent opioids with a relatively short duration of action can be given when painful stimulus is highest, i.e., during surgical manipulation, normally during the first period of the surgical procedure. To provide analgesia in the postoperative period drugs should be selected based on their duration of action but also on the severity of the procedure. Newer NSAIDs (carprofen, meloxicam) appear to provide effective analgesia for low to severe pain for up to 24 hours whilst low potency opioids like buprenorphine or butorphanol are only adequate to treat low to moderate pain despite their relatively long lasting action (6-12 hours).
Most analgesics are given by the IM, IV and SC routes. Other routes/techniques should be considered: local anaesthetics are highly effective while minimising the amount of other drugs to be used. Examples of this are the use of epidural analgesia-anaesthesia, or brachial anaesthesia for orthopaedic surgery of the hindlimb or forelimb, respectively. Local analgesia has gained much popularity in humans in the last ten years but is still underused in animals.
Newer formulations together with the knowledge of the mechanism of action of known drugs have introduced alternative ways to produce analgesia. Fentanyl, a short-acting opioid has been used mainly in the intraoperative period but a newer formulation in a patch administers the drug through intact skin to provide chronic analgesia for up to three days. EMLA cream produces anaesthesia of the skin when applied topically. Local analgesia can be provided with drugs other than local anaesthetics; ketamine, opioids like morphine or fentanyl, or alpha-2 agonists like xylazine may provide long-lasting analgesia (up to 24 hours) when given by the epidural route.
As stated previously, the use of balanced analgesia minimises common side effects observed with larger doses of a single analgesic. Use of analgesics usually reduces the dose of anaesthetics, which normally cause marked dose-dependent cardiovascular and respiratory depression as well as effects on other body systems and functions.