INTRODUCTION
When I graduated in 1964, analgesia and pain management of our veterinary patients was not part of the Queensland veterinary school curriculum. In the 1973 edition of Limb and Jones text on veterinary anaesthesia, the control of pain is not mentioned as one of the reasons given for having knowledge of anaesthesia. Anaesthesia was often used as a means of restraint, and horses were routinely castrated in Australia using succinylcholine chloride, which effectively paralyzed them but gave no pain relief. The term chemical restraint was often used synonym for anaesthesia.
The standard anaesthetic agent used in small animals was pentobarbital sodium, a product that has now been discontinued except as euthanasia solution, in my country. This agent was given intravenously to achieve a plane of surgical anaesthesia and was periodically topped up' with incremental doses to prolong anaesthesia for longer operations.
Recovery was prolonged and the animals often became hypothermic and shivered uncontrollably as they recovered. The shivering, paddling movements and vocalisation was considered a "normal" part of anaesthetic recovery. Very often the animal's cries were attributed to anaesthetic recovery 'excitement' rather than pain.
Since I graduated all of these techniques have been superseded and huge advances have been made in veterinary anaesthesia and analgesia the last 40 years. Unfortunately some individual veterinarians in Australia and other parts of the world have not advanced as rapidly and still practice in a "time warp". We owe it to our animal friends to try to change this and this is the purpose of this welfare seminar.
Anaesthesia is used for the dual purposes of sedation, relaxation and restraint of the animal while we perform the operation and to eliminate as far as possible the painful sensations inherent in surgical operations. We need to eliminate pain during the surgery and in the post-operative period. The analgesia and anaesthesia need to be complimentary.
In general practice, a protocol for combined anaesthesia and analgesia needs to be relatively simple and very safe.
Many of our patients are admitted and go home the same day, so the agents used must allow the animal to be relatively bright and ambulatory when it is discharged.
Similarly, analgesia must be prolonged to enable the animal to have a peaceful and pain free recovery period post-operatively. There are obvious exceptions to these rules, major trauma patients and patients undergoing prolonged pain from cancer are examples, but in deference to the other speakers at this seminar, I do not intend to describe our protocols for these animals.
Cosmetic concerns are also taken into account--owners would be unhappy with having all the de-sexing patients clipped and shaved for epidural opioid administration, so we have to develop acceptable alternative techniques for these patients.
Cost is also a consideration in routine general surgery, so the use of Fentanyl patches is considered an expensive means of providing analgesia, although it may be the method of choice to provide prolonged pain relief in some patients.
The key analgesic agents used in my practice are morphine and the associated opioids and NSAIDS. Somewhat ironically the opioids have been readily available for hundreds of years, but it is only in the last 50 years that they have been widely used in animals other than man.
In the seventeenth century, Descartes, the French philosopher, regarded animals as "machines", with no souls, minds or feeling. The strongly held theological beliefs of the time also dictated that animals did not have souls. Many other scientists of the period used animals in experiments that were obviously painful and used this Cartesian philosophy to justify their actions. This practice continued until quite recent times. Community and scientific attitudes to animal welfare have changed of recent times. Veterinary scientists have been part of this animal welfare protest and it is encouraging that our profession is now at the forefront of the move to promote effective analgesia as part of our daily therapeutic routines in practice. Experimental Ethics committees in research institutions now impose a higher standard of analgesia for experimental animals than is practiced in most veterinary practices.
Surgical analgesia
Surgical analgesia is an integral part of a technique of balanced anaesthesia. The analgesic agents are incorporated into our pre-anaesthetic technique to calm and relax the animal, reduce struggling and stress at induction and to reduce the amount of anaesthetic agents required to maintain a surgical plane of anaesthesia. The residual effects of the pre-medicant drugs and analgesics also can reduce anaesthetic recovery excitement and make the recovery a smooth transition from anaesthesia to calm sleep. The same analgesics can be used to prolong analgesia into the post-operative recovery period.
In clinical veterinary practice many of our patients are healthy animals. We routinely perform surgical procedures ranging from routine de-sexing to major orthopaedic and soft tissue surgeries on relatively healthy patients. We also perform major surgery, sometimes of an emergency nature, on animals that are suffering from serious illness and co-existent disease. The pre-medicant and anaesthetic protocols need to be flexible enough to take the conditions of all these patients into account. There is no such thing as an all encompassing protocol for all our patients.
Dog anaesthesia
Our current protocol for routine anaesthesia in healthy dogs is as follows:
Premed atropine (0.05mg/kg s/c)/ACP (0.05mg/kg s/c)/morphine (0.05 to 1mg/kg s/c) or buprenorphine (0.01mg/kg s/c). The premed is modified in animals are in shock to minimise hypovolaemia/hypotension.
Induction with IV Ketamine/Diazepam slowly to effect.
Dogs then intubated and anaesthesia maintained on Isoflurane/oxygen ± Nitrous oxide as required.
Analgesia with opioids/NSAIDS is continued post-operatively.
Surgical fluids are used routinely in most patients.
Cat anaesthesia
The history of cat anaesthesia in my practice is similar to dogs. From 1964 to the present we moved from Pentobarbital to thiopentone. Thiopentone has a narrow safety margin and carries the risk of tissue damage due to perivenous injection. Because of these problems, we moved to the use of SaffanTM1 (Alfaxalone/alphadolone) as soon as it became available in Australia. We used this product safely for many years, in spite of the relatively common allergic reactions to the Cremophor EL base of the original formulation of SaffanTM. To minimise allergic reactions we adopted a premedication protocol, using atropine 0.05mg/kg and acetylpromazine 0.05 mg per kg and an injectable antihistamine, chlorpheniramine 0.2mg/kg.
We now use an Australian made product called Afaxin CD-RTU2, which contains only Alfaxalone in a cyclodextrin base which can be used safely in both dogs and cats. Afaxin CD-RTU has no allergic response. It can be used IV and IM, and we intubate the cat and place them onto isoflurane/oxygen maintenance.
Once induced with Afaxin CD, cats are routinely intubated and placed onto oxygen. We routinely sprinkle a few drops of Lignocaine hydrochloride 2% onto the back of the pharynx and larynx approx 30 seconds prior to intubation to minimise laryngospasm.
Afaxin CD alone has a moderate anaesthetic effect for approx. 10 to 12 minutes and then wears off rapidly. We initially administer straight oxygen until we are sure that the cat is breathing well and then we introduce isoflurane gradually to maintain the surgical plane of anaesthesia. Respiration is not depressed and, provided the animal is left quiet and untouched, anaesthetic recovery is usually calm. Cats are extubated as soon as the swallowing reflex returns.
Analgesia in cats
Cats are not small dogs so some changes have to be made for the protocols to be used in cats.
For routine analgesia we premedicate cats with morphine (0.01mg per kg) every 4 hours or buprenorphine (0.005 to 0.01mg/kg) every 8 hours. Most routine minor surgery does not require additional doses.
Non-steroidal medication NSAIDS
The use of NSAIDs such as Meloxicam (0.1 to 0.2mg/kg), Carprofen (2 to 4 mg/kg) and Ketoprofen (2mg/kg) have been promoted for use as an analgesic for routine surgery such as de-sexings. These drugs can give prolonged relief from mild surgical pain and can be a useful adjunct to the use of opioid analgesics. The higher dose is usually used as a loading dose. However, they are not suitable for severe pain control and have no sedative effects. I routinely use Meloxicam or Carprofen, administered at the time of anaesthetic induction in surgeries such as Cruciate repairs and clean orthopaedic surgeries. Because NSAIDs can modify the response to infection I prefer not to use them in contaminated wounds where infection may be a risk. Because of the potential for renal damage NSAIDs should not be used if the animal is hypotensive or hypovolaemic. TolfedineTM3, Ketoprofen and Meloxicam may be safely used in cats.
Fluids during anaesthesia
To maintain blood pressure and tissue perfusion during anaesthesia we routinely give IV fluids at doses of 5 to 12 ml per kg bodyweight per hour. This dose is reduced to 3 ml per kg per hour following completion of surgery.
Ketamine
We use ketamine as our main induction agent in dogs. We always combine Ketamine hydrochloride with either diazepam or midazolam; because Ketamine can cause seizures in dogs and diazepam or midazolam effectively control these. Some anaesthetists recommend that the diazepam or midazolam be given before the ketamine, but in practice, we have seen few problems administering both agents simultaneously in the one syringe. Ketamine is one of the few agents that is miscible in injectable diazepam solutions; most other drugs cause precipitation of the diazepam. We use a mixture of Ketamine (100 mg/ml) and diazepam (2 mg/ml) and the dose used for induction in dogs is approx. 1 ml of the mixture per 10kg bodyweight. In sedated animals this dose is markedly reduced. We only use this mixture for induction as all our patients are intubated and maintained on isoflurane/oxygen/nitrous oxide. Therefore the animal only needs to be deep enough to pass the tracheal tube. Ketamine does have some residual analgesic effect
Ketamine either alone or in a Ketamine/ diazepam mixture can be used in cats, but we prefer to use Afaxin CD. It is important to realize that Ketamine does not work well as a visceral analgesic and in humans can give rise to severe hallucinations. In my opinion, it is not a drug that should be used as a sole anaesthetic agent.
Neuroleptic combinations
These are useful for providing sedation and analgesia for minor procedures or as a premedicant for general anaesthesia in difficult patients.
These three examples are useful in dogs:
Butorphanol 0.1mg/kg plus Acetyl-promazine 0.02mg/kg
Methadone or morphine 0.2mg/kg plus Acetylpromazine 0.02mg/kg
Medetomidine 0.01mg/kg plus Butorphanol 0.1mg/kg
All of these drug combinations will significantly potentiate anaesthetic induction and reduce the amount of anaesthetic agent required. Be careful using them.
Medetomidine and Xylazine are Alpha 2 agonists and these are dangerous drugs in dogs with anaemia, heart disease, renal failure. Two large studies4 showed a significantly higher risk of morbidity and mortality if these agents are used in anaesthesia. I prefer to avoid the use of medetomidine in dogs; however, Medetomidine alone is very useful for deep sedation in fractious cats and cats are more tolerant to its adverse effects. Doses of 0.5 to 0.75 ml per 5 kg cat can safely be used provided they are closely monitored. Medetomidine can be allowed to 'wear off' and this prolongs the analgesic effect. When the product is completely reversed by the antagonist, Atipamezole, there are no residual analgesic effects.
References
1. Glaxo
2. Jurox Pty., Ltd., 85 Gardiners Road, Rutherford, NSW 2320 Australia
3. VIRBAC
4. Clarke KW, Hall LK (1990) A survey of anaesthesia in small animal practice: AVA/ BSAVA reprt. J Assoc. Vet Anaesth; Vol 17: 4-10
5. Dyson RH, Maxie MG, Schurr (1998) Morbidity and mortality Associated with Anaesthetic Management in small Animal Practice in Ontario. J am Anim Hosp Assoc 34: 325-35