Anaesthesia and analgesia for companion animals
PRINCIPLES AND PRACTICE OF PAIN MANAGEMENT IN COMPANION ANIMALS
S Zaki
University of Sydney, Camperdown, NSW
Pain Management in our companion animals has come a long way in the last 15 years. Veterinary practitioners now recognise the importance of pain management as an integral part of the treatment and care they provide their animal patients. Despite this, many veterinarians are still unsure about the appropriateness and effectiveness of the analgesia they are currently prescribing.
There is little doubt that animals experience pain in the same way that humans do, even if they cannot verbally express the emotional component. Animals possess the neuroanatomical and neuro-pharmacologic components necessary for the transduction, transmission and perception of noxious stimuli. Although pain is a complex physiological, sensory and emotional experience, veterinarians are often limited in how they describe and quantify it. Animals are unable to use language to communicate what they feel and experience, so instead veterinarians rely on the physiological and behavioural responses that accompany the experience of pain.
Today the veterinary practitioner faces two major challenges:
- Developing an ability to accurately and consistently assess pain in animal patients, and
- Developing a sound working knowledge of the most appropriate drugs and techniques to use under different circumstances.
Pain assessment:
If you can’t diagnose a condition, how can you treat it?Deciding if an animal is in pain and assessing the severity of that pain continues to be a challenging problem for the veterinarian. Pain is one clinical sign that veterinarians sometimes choose to treat or ignore based on their clinical experience rather than on any scientific evidence. The idea that “if an animal looks like it is in pain then it probably is” forms the basis of many pain management decisions. Although this strategy limits our ability to detect pain in some animals, it does provide a starting point for pain assessment. Veterinarians need to remember however, that there are a number of complexities which can limit their ability to make accurate observations.
These include:
- Behavioural differences between species
- A lack of familiarity with an individual animal’s ‘normal’ behaviour
- Preconceived ideas about how an animal should behave when in pain
- Variability in pain tolerance between individuals.
- Difficulties associated with differentiating between stress/anxiety and pain
- Alterations in normal behaviour when animals are in a strange environment such as a clinical setting
- The effects of post-operative sedation/drowsiness in masking the signs of pain
The end result is that an animal in pain can miss out on receiving pain relief. For example, cats in general receive less pain relief than dogs. But do they feel less pain? There is certainly no anatomical, physiological or neurological difference between cats and dogs to support this, but there are significant behavioural differences between how cats and dogs display pain. And it so happens that dogs are more vocal and overt in demonstrating signs of pain compared to cats.
Analgesics
How much can we learn from the human experience?
Modern veterinary pain management is about using a ‘balanced multi-modal’ technique that achieves optimal pain control with minimal side effects. This means targeting the pain transmission pathway at multiple sites, using a number of appropriate drug combinations. There are numerous analgesic drugs and techniques available to the veterinarian today, many of which are also used to treat pain in people. Opioids, Non steroidal anti inflammatory drugs (NSAIDs) and local anaesthetics are the most widely used analgesics and form the foundations of most pain management protocols.
There are numerous opioids available in Australia however few are registered for veterinary use. Veterinarians prescribe opioids such as morphine and fentanyl for off-label use in cats and dogs based on the numerous publications which report on their efficacy and safety in these species. However, methadone, buprenorphine and butorphanol (all registered for veterinary use) remain the most commonly utilised opioids. Previously opioids were not given to cats because of the maniacal reactions they were said to produce. This excitation (opioid mania) is only seen when high doses are given (more than 10 times the clinical dose), and so is not a contraindication for their clinical use in cats.
NSAIDs are effective against pain associated with inflammation and tissue damage; however, they are not without their side effects. Traditionally NSAID’s have been used to manage chronic musculoskeletal pain, but they are now also gaining popularity for the treatment of acute and post surgical pain and inflammation. Currently carprofen and meloxicam are the only two NSAIDs registered for peri-operative use in cats and dogs. Although, most effective when used pre-operatively, care should be taken with candidate selection when NSAIDs are used peri-operatively. Because it is during this time of potential hypovolaemia and hypotension, that the ‘housekeeping’ prostaglandins play an important role in maintaining adequate renal blood flow.
Local anaesthetic techniques are used widely in large animal anaesthesia because they can alleviate the need for general anaesthesia. Until recently local anaesthetic techniques were often neglected as part of small animal anaesthetic and analgesic regimens. However, use of local anaesthetics contributes to a multi-modal analgesic technique and can often reduce the dose of other anaesthetic drugs required for maintenance of anaesthesia. Use of specific nerve blocks to prevent the relay of nociceptive information from the site of injury to the spinal cord can also provide pre-emptive analgesia, and may prevent or reduce the development of central sensitisation. Cats are considered to be more susceptible than dogs to systemic toxicity with the administration of local anaesthetic drugs, but problems are uncommon if appropriate doses are given.
Systemically administered lignocaine (IV) has been shown to be effective in the treatment of acute pain and post-operative pain, and hyperalgesia. Low continuous rate infusions of lignocaine are not associated with cardiovascular side effects and heart rate and blood pressure have been shown to be well maintained.
Ketamine is a drug that has been around for many years, and is widely used in veterinary medicine for sedation and injectable anaesthesia techniques. However recent interest in ketamine has been due to recognition of its profound analgesic properties, which are present at sub-anaesthetic doses. This allows ketamine to be used in low doses for analgesia, avoiding concurrent CNS side effects. Low doses of ketamine to provide analgesia are being adopted into small animal analgesic protocols where ketamine is used as an adjunctive analgesic in combination with opioids, NSAIDs, or other drugs. It is not recommended as a sole analgesic drug.
Alpha-2 adrenoreceptor agonists (?2 agonists) are recognised to be potent sedative, hypnotic and analgesic agents. These properties that make them useful adjuncts for anaesthesia in small animal practice. However despite widespread clinical use, concerns about the cardiovascular side effects of ?2 agonists have prevented their full adoption into veterinary practice.
In addition to the above analgesics, veterinarians continue to look towards pain management in people for alternative drugs and techniques to use in animal patients, where traditional methods are contraindicated or prove to be inadequate. As veterinarians become more confident with using these and other analgesic drugs safely and effectively, their ability to manage pain in companion animals will continue to improve.
Time of Presentation
Sunday 14 May 2006 - 1330-1500
References
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