Sensitivity to anaesthesia
At some time in their lives, most pets will need to undergo medical procedures that require them to be sedated and placed under anaesthesia. From teeth cleaning procedures, spaying and neutering, to surgical procedures for repairing fractures, removing obstructions from the digestive tract, repairing torn ligaments, etc.; these procedures cannot be done while the animal is conscious.
However, not all pets can tolerate anaesthetic drugs. There are a few dogs whose size, pedigree, and even rare genetic mutations can cause them to have adverse reactions to anaesthesia as well as the pre- and post-anaesthetic process. That being said, there is no way to predict which dogs will be sensitive to anaesthesia. Each veterinarian will assess and treat each individual animal based on their own unique make-up, age, and health status.
Can dogs be allergic to anaesthetic drugs?
Some dogs will have an allergic or anaphylactic reaction to anaesthetic drugs. Fortunately, the chance of this happening is only on in 100,000 – or 0.001% – and if it does, the allergic reaction could be as mild as some skin swelling or as severe as anaphylactic shock. Most allergic cases are mild and the risk of death is very, very low.
An allergic reaction is very different to anaesthetic sensitivity, which has a genetic component, and can also be related to the age and conformation (shape) of the dog.
Which dogs are sensitive to anaesthesia?
The dog breeds most commonly associated with sensitivity to anaesthesia are the sighthounds like greyhounds, saluki, Afghan hounds, whippets, borzoi, and deerhounds – with greyhounds getting the most press when it comes to anaesthesia sensitivity.
Sighthounds in general (and greyhounds in particular) are given special consideration when it comes to anaesthesia. Their lean physiology with low body fat gives them a high surface-area-to-volume ratio, which can cause hypothermia during anaesthesia as well as during the recovery period. They also have lower serum protein concentration in their bodies, and since anaesthetic drugs are protein-bound, this can increase the effects of the anaesthetic. These extremely deep-chested dogs are just built differently and it’s their conformation that affects how medications are redistributed in their bodies. Sighthounds also have lower amounts of the liver enzymes responsible for metabolising drugs and processing toxins. This results in these medications being expelled more slowly from their system, delaying the recovery from anaesthesia, which can be dangerous.
Does this mean greyhounds can’t go under anaesthetic?
No. Advancements in veterinary medicine as well as each individual dog’s physiology and health baseline mean that greyhounds can still be effectively treated under anaesthesia. Special protocols for delivering pre-medication and anaesthetic drugs have been developed for sighthounds and other dogs with anaesthesia sensitivity.
Are sighthounds the only dogs with anaesthesia sensitivity?
No. In 2020, scientists at Washington State University's College of Veterinary Medicine discovered that the same genetic mutation in sighthounds that caused a lower production of liver enzymes, was also present in other dog breeds like Labradors and golden retrievers. One in 50 golden retrievers, one in 300 Labradors and around one in 3,000 mixed breed dogs were seen to have this mutation. Lower levels of liver enzyme CYP2B11 has implications for the processing of all medications – not only anaesthetic drugs – so it’s important for these pet owners and their vets to take this into consideration for any treatment.
Herding breeds like border collies, rough and smooth collies, Australian shepherds, Shetland sheepdogs and Old English sheepdogs can be born with a genetic mutation for multi-drug resistance, or the MDR1 gene. The presence of this mutation results in the drugs accumulating in the central nervous system (including the brain). This has severe implications for anaesthetic drugs, which can cause over-sedation and compromise the dogs’ breathing. Pet owners with these breeds can ask the vet about testing for the MDR1 gene as a precaution against anaesthesia sensitivity and drug accumulation.
Dog breed size and anaesthetic sensitivity
There are implications for both toy breeds and giant breed dogs when it comes to anaesthesia sensitivity. For small dogs, it’s not so much the anaesthetic drugs that the dogs are sensitive to, but the increased risks of placing such small breeds under anaesthesia. Their tiny blood vessels are not as easy to access for the placement of the intravenous catheter, compared to larger dogs. They may also be more prone to hypothermia during anaesthesia, but there are measures that can be used to minimise heat loss and reduce the risk. Similarly, their small size makes them more difficult to monitor while under anaesthesia and challenges the accuracy of the readings.
Giant breeds have a different rate of drug metabolism compared to other size dogs, and they actually require lower dosages of anaesthetic drugs than small, medium and large breeds. This shows that it is important that each individual dog is treated with a drug protocol in general, and an anaesthetic protocol in particular, to suit their unique health profile.
Some giant breed dogs may be sensitive to anaesthesia simply because giant breeds reach their senior years faster than smaller dogs. Senior dogs may have a more difficult time processing and expelling medication because of impaired organ function in their ageing bodies.
Are brachycephalic dogs sensitive to anaesthesia?
Brachycephalic dogs – those with ‘flat’ faces, short airways, an elongated palate, and different laryngeal and tracheal structures – are not necessarily more sensitive to anaesthesia than other dogs. Anaesthesia is just a bit riskier for them because of their existing respiratory challenges. Their already compromised airways can narrow even more and cut off their air supply altogether, so brachycephalic dogs need to be intubated to ensure a consistent supply of air.
The moments before intubation and the period after extubation are critical when it comes to brachycephalic breeds. The vet needs to ensure the dog is breathing normally without the tube and there are no obstructions in the airway. These dogs can get into respiratory trouble very quickly, so this adds an extra facet to the usual risks of anaesthesia.
If you are concerned about your dog’s risks during anaesthesia, do not let this put you off having medical procedures like teeth-cleaning and spaying or neutering done. Speak to the vet about your concerns and they will be able to devise a protocol for safe anaesthesia based on your dog’s specific needs.