Perioperative Analgesia
World Small Animal Veterinary Association World Congress Proceedings, 2003
Sandra Forsyth, BVSc, DACVA
Institute of Veterinary, Animal and Biomedical Sciences, Massey University
New Zealand

Studies carried over the last few years investigating how often veterinarians administer peri-operative analgesia provide fairly dismal reading. Only about 50% of practitioners employed post-operative analgesia and many of those only gave it if they thought that their patient was "painful". With the ability of many small animals to mask their pain there are obviously many patients receiving inappropriate analgesic care. Why are we so reluctant to administer analgesia when on many occasions we fall over ourselves to give unwarranted antibiotics and vaccinations? There seems to be a number of reasons. For some veterinarians analgesia is not an area that has been given due consideration. Pain sensation and analgesia were not taught at vet school in their time and so this aspect of veterinary care is ignored. A small group believe that animals do not feel pain sufficiently to warrant analgesia and others believe that by providing analgesics animals will feel so good that they will leap around and damage the surgical site. This latter aspect is not a valid argument for withholding analgesia because analgesic agents obtund dulling aching pain but do not prevent the sharp, acute pain associated with stimulation of the damaged area. Others argue that providing analgesic drugs adds to the cost of a surgical procedure and so clients may be less willing to go ahead with surgery!!

"All surgery causes pain with some procedures causing more pain than others".

Why should we consider perioperative analgesia? As veterinarians our duty is to provide humane care of our patients and alleviating pain surely should be the most humane thing that we can do. In addition, provision of analgesia and reduction of stress improves the quality of anaesthetic induction and recovery. Appropriate preoperative analgesia reduces post-operative analgesic requirements and decreases chances of inducing a chronic pain syndrome (pathological long-lasting changes in pain pathways that are non-responsive to traditional analgesic techniques). It has been noted that inadequate analgesia during and after surgery can heighten pain sensation to a subsequent noxious stimulus that may occur months to perhaps years later. Nutritional intake and ambulation improve more rapidly so the patient can safely leave the hospital sooner. It has also been shown that metastases are less likely to occur in patients receiving adequate analgesia compared to those that received inadequate analgesia.

When it comes to pain prevention is better than cure because pain control becomes more difficult once the patient become aware of it. Prevention is achieved by providing analgesia before tissue damage occurs i.e., preemptive analgesia. Because much of the processing of nociceptive information occurs in the spinal cord rather than the brain, general anaesthetic agents that suppress cerebral function do not necessarily block nociceptive processing. Agents that act at the spinal cord level reduce input and decrease the potential for development of chronic pain syndromes.

Factors that enhance pain sensation include stress and anxiety and these should be reduced wherever possible. The effects of strange surroundings, unfamiliar voices, change in food or lack of food /water should be considered and reduced whenever possible. Fasting is necessary part of pre-anaesthetic management but keep the period to a minimum for the procedure being undertaken. Allow the animal to remain at home until the day of surgery, find out its usual food, provide care and attention prior to induction of anaesthesia.

Pre-medication

Premedication serves several purposes but importantly it aims to reduce stress and provide analgesia. Sedative such as ACP and α2 agonists are commonly administered in the pre-anaesthetic period and when given with an opioid provide excellent analgesia and stress reduction. However, ACP can produce profound vasodilation in the older or compromised patient and should be avoided in all but healthy patients. Similarly, the α2 agonists can induce profound cardiovascular depression even at minute doses (e.g., 1-2 µg/kg medetomidine in dogs) and so should be administered only to young fit, healthy patients. For the very young, the old and debilitated patients an opioid alone often provides adequate sedation and analgesia without noticeable cardiovascular depression. There is still debate about potential complications when NSAIDs are provided peri-operatively. Renal insufficiency, GI ulceration and decreased platelet function are possible and should be considered whenever these agents are used. Renal compromise is less likely if the patient is appropriately supported with parenteral fluids and monitored (HR, blood pressure, CRT, mucous membrane colour) during surgery. Hypotension and poor tissue perfusion must be avoided to use these drugs safely. Patients requiring emergency surgery MUST be normovolemic pre-op if they are to receive NSAIDs. Ketamine and tiletamine (Telazol®, Zolatil®) are N-methyl-D-asparate (NMDA) antagonists, a feature that provides them with analgesic activity via central inhibitory pathways. In man it has been shown that ketamine reduces hyperalgia and post-operative analgesic requirements. These agents are considered for sedation primarily in cats but occasionally are used in dogs.

Induction

A stress free induction should be the aim for both patient and clinician! If the patient is not satisfactorily sedated prior to induction consider adding in a further agent rather than increasing the dose of the original. Adding ketamine or tiletamine/zolazepam is often useful in cats whereas increasing the dose of the opioid component or adding a low dose of medetomidine (5 ugl/kg) may be useful in dogs.

Propofol, alphaxalone (Saffan®, Alfaxan®, Alfaxan-CD®), thiopental, etomidate, inhalant agents and the benzodiazepines do not provide any additional analgesia whereas ketamine and the opioids do. A cyclohexamine induction (ketamine or tiletamine) following an opioid (or combination) premed should provide additional analgesia.

Maintenance

Potent inhalational maintenance agents (isoflurane, halothane, sevoflurane) provide little analgesia other than preventing the animal from being conscious of nociceptive input. Balanced anesthetic techniques that include an opioid provide greater "stress" relief and analgesia. The disadvantages to using opioids may include prolonged recovery (drug and dose dependent) and hypoventilation necessitating assisted ventilation. Nitrous oxide is an excellent analgesic that obtunds "wind-up" of nociceptive information in the spinal cord. The mechanism of this action appears to be related to its ability to induce release of endogenous opioids or catecholamines. It is rapidly expired at the end of surgery and so doesn't provide post-operative analgesia.

Local anesthetic techniques are most effective at preventing transmission of nociceptive information. The LA can be placed peripherally into tissue or joints or more centrally into the epidural or subarachnoid space. Agents such as bupivacaine with long duration of action provide intra-op as well as post-operative analgesia and are the usual choice for surgical procedures. However, because motor blockade occurs in addition to sensory blockade if the animal needs proprioception immediately after surgery a shorter acting agent such as lidocaine should be chosen. In this case analgesia will also be short lived but the consequences of spinal wind-up will not be present. Blood pressure should be monitored when using spinal or epidural anesthesia techniques because sympathetic blockade can result in profound hypotension. Opioids can also be placed in the spinal column and joints to provide intra-operative and post-op analgesia for up to 12-24 hours after surgery.

Joints--0.5% bupivacine 0.2-0.25 ml/kg elbow and 0.2-0.4 ml/kg shoulder and stifle. Total dose should not exceed 2 mg/kg if multiple joints.

Epidural opioids--morphine has a long duration of action because it is hydrophilic. 0.1 mg/kg of 1 mg/ml

Recovery

As surgery finishes an analgesic should be given so that its peak effect occurs before the animal wakes up. This should be balanced with the prolonged time to recovery that can occur if the agent is given too early. Excitement during recovery is not uncommon and we need to determine if there is a pain component to this. Additional analgesia should be provided-preferably an agent that can be given IV and has a short duration to onset. Only once you can be sure that further analgesia is not required should you consider sedating the animal. Studies have shown that for a particular surgery where tissue trauma has been similar requirements for analgesics in man can vary by as much as 40 fold-we should expect no less in our patients. For sedation ACP (0.1 mg/kg IV) can be useful in dogs although some animals require 1 or 2 additional doses before they settle. Onset is slow and you need to wait about 10 minutes before deciding that an animal requires further drug. If the animal becomes more excitable after giving the short acting IV opioid then it is possible that this is due to a direct effect of the opioid. Consider partially reversing the animal with a mixed agonist such as nalbuphine (0.03-0.1 mg/kg IV) given slowly and to effect so that excitement may be reduced without reversing the analgesic effect. Cats do not respond as well as dogs to ACP administration so provided that cardiovascular function is normal medetomidine (2-5 ug/kg IV) can be administered. If excessive sedation or other problem occurs the drug can be reversed with atipamezole.

Over the next 24 hours opioids can be continued or the animal can receive a NSAID provided that hypotension has not occurred during surgery, intra-op fluids are given and the animal had normal pre-op hepatic and renal function. There appears to be a synergistic effect between the opioids and NSAIDs and animals are often pain free after receiving this combination.

Morphine infusions are very useful for extremely painful animals but result in sedation and maintenance of the IV line is necessary. Fentanyl patches can be placed but because of their prolonged time to onset they either need to be placed the day before surgery or require concurrent parenteral opioids until they are effective.

The least rational approach is to "wait and see" if the animal exhibits signs of pain. The animal's pain can be much more difficult to control with higher doses of drugs needed. Also if the animal doesn't exhibit signs that we associate with pain then they go unrecognised and their pain continues.

Summary

Management of pain begins with good patient handling to reduce stress and anxiety. Appropriate drugs pre- and intra-op can reduce spinal "wind up "ensuring easier post-op management and also provide analgesia during the procedure itself. Post-operatively analgesia should be considered for at least the first 12-24 hours in all patients and longer for more invasive procedures such as orthopedic surgery.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Sandra Forsyth, BVSc, DACVA
Institute of Veterinary, Animal and Biomedical Sciences
Massey University
New Zealand


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