Dogs & Anaesthesia

by Shane Jackson M.A., Vet MB., MRCVS



Anaesthesia is a state of unconsciousness brought about by the administration of one or more drugs to affect the nervous system so that awareness of stimuli from the environment, e.g. pain, and the ability to respond to those stimuli are reduced.  This subject can be quite frightening when one considers that over administration of anaesthetic agents can cause profound anaesthesia, and in extreme cases can lead to death.

There are, however, some very important reasons why anaesthetics are used in the veterinary field.  Reduction in the awareness of pain and immobilisation of the patient are obviously vital, but anaesthesia will also allow relaxation of the patient’s muscles and this can facilitate the surgery required.  By keeping an animal still, asepsis (germ-free conditions) can be maintained.  Anaesthesia is often essential when X-rays are taken, but also because the handlers can leave the vicinity and avoid exposure to X-rays.  Safety of handlers also has to be taken into account when dealing with aggressive animals.

Although each patient is treated as an individual, most will undergo the same anaesthetic technique.  The dog is given a pre-medicant, usually by injection, though occasionally by mouth.  The drug, usually acepromazine (ACP), has the effect of tranquillising the dog but it has two bonus effects as well.  First, it is an anti-emetic, that is, it reduces the chances of vomiting occurring.  Second, it reduces the amount of drug required to induce anaesthesia by about one third.  Dogs which have had ACP also seem to recover after anaesthesia in a much more controlled and smoother fashion.

After a suitable interval, when the premedicant has been absorbed, anaesthesia can be induced.  This can be done in a number of ways, but we usually rely on one of two drugs for the dog. For many years thiopentone, a barbiturate  has been the mainstay, but in the last two decades propofol has come on the market and is being used more frequently.  Both drugs are given by intravenous injection, and it is commonplace that the front leg is clipped to expose the required vein.  Once the drug has been given, the dog quietly sinks to the table, unconscious, in a matter of seconds.  A tube, called an endotracheal tube, is positioned via the mouth into the trachea (windpipe) and this ensures an airway is available at all times.  Should the dog vomit, the tube prevents vomitus from getting onto the lungs.

Both thiopentone and propofol are relatively short acting agents.  In other words, if nothing else were given, the dog would start to wake up in a few minutes after induction with thiopentone and in less time with propofol.  So in most cases, and anaesthetic gas, mixed with oxygen, is given directly into the lungs via the endotracheal tube.  The amount of gas can be varied according to the needs of the patient and the operation being performed.  When the surgery has been completed, the anaestheic gas is stopped and oxygen alone is given for a few minutes.  After that, the patient is allowed to breather room air but through the endotracheal tube. The tube is only removed when the ability to swallow and cough, which has been suppressed during anaesthesia, has returned. 

As mentioned above, each patient is an individual and each is assessed as to whether a particular regime or drug combination is safe for that animal.  Age, breed and size of the animal are taken into account, as well as underlying conditions, such as heart, liver or kidney disease. There are also some special cases. Diabetic dogs should be fully stabilised, if at all possible, before anaesthesia is contemplated, and the dogs which have epilepsy are not pre-medicated with ACP, but rather with valium.  When bitches require a caesarean, they are usually given propofol rather that thiopentone because there is far less adverse effect on the young while they are in the womb.  The bitch appears to rouse more quickly after propofol and is able to suckle the young sooner.

There is a combination of two drugs (ketamine and medetomidine) which can be administered by intramuscular injection to induce anaesthesia.

Giving injection intravenously can be hazardous to the anaesthetist if the patient is aggressive, so it would appear that the above drug combination is heaven sent, especially when the effects can be rapidly reversed with another drug, atipamezole.  However, medetomidine can have profound effects on the heart and the anaesthetic combination does not provide adequate pain relief for major surgery.  This form of anaesthesia is usually reserved for relatively minor techniques in a healthy animal.

From the above it can easily be seen that anaesthesia is a huge subject and that I have barely scraped on the surface.  Many more drugs than I have mentioned are available for use and all have their advantages and disadvantages.  What I have attempted to do is to give some insight into what happens, before, during and after anaesthesia.