Clinical History.
The dog was presented with a subcutaneous mass just cranial to the right proximal humerus. A needle aspiration biopsy was taken and the mass proved to be a mastcell tumour. Complete surgical removal was considered to be impossible due to it's infiltrative nature. So a bone marrow biopsy was arranged, to check for metastases before the dog underwent radio therapy. The dog was prescribed 36 mg prednisolone once daily until the general anaesthetic. The owners were warned of the risks of general anaesthesia and signed a form giving permission for a general anaesthetic and surgery.

Pre-anaesthetic examination.
Bloods were taken for haematology and biochemistry profiles, the results of which were unremarkable. See anaesthetic record. Auscultation of the pulmonary and cardiovascular systems did not reveal any abnormalities. Temperature was 38.4 C, heart rate 96 per min and respiration 24 per min. The dog had been starved for 10 hours with access to water until 2 hours prior to induction.

Premedication.
Premedication consisted of 0.28 mg of glycopyrrolate (Robinul, Robins), 8.25 mg of butorphanol (Phoenix) and 55 mg diphenhydramine (Abbott) all by intramuscular route. The premedication worked extremely well and the dog was very quiet and sedate at the time of induction 20 minutes later. Prior to induction an 18 guage intravenous catheter was placed aseptically into the right cephalic vein.

Induction.
Induction was achieved using 13.8 mg diazepam (Elkins-Sinn), followed by titration of 129 mg of 2.5 % thiopentone sodium (Phoenix), both via the intravenous catheter. Intubation was performed with an 11 mm cuffed endotracheal tube and positioned by palpation of the uninflated cuff at the thoracic inlet. The tube was connected to a circle circuit with 2 L of oxygen. The cuff of the endotracheal tube was slowly inflated whilst the reservoir bag was very gently squeezed with the valve closed, and inflation was stopped when sounds of gas passing round the tube were no longer heard. An ECG monitor was connected to the dog, the electrodes being placed just proximal to the stifles and olecranons. An area just distal to the stopper pad on the palmar aspect of the left forelimb was clipped and a Doppler sensor taped on, after application of conducting gel, in order to monitor blood pressure. The dog was placed on a water heated pad for the duration of anaesthesia. See picture right.

Maintenance of general anaesthesia.
The valve on the circuit was opened and maintenance of anaesthesia was achieved using 1 L oxygen with isoflurane at 2% reducing to 1.5 % after 45 minutes. Lactated Ringers crystalloid was given at a rate of 10mls/kg/hr for the duration of anaesthesia. For a detailed account of anaesthesia see attached record sheet. The heart rate was recorded at between 70 and 75 bpm for the first 20 minutes, and then began to decrease steadily over the next 10 minutes to 50 bpm. The systolic blood pressure (BP) had only fallen slightly, to 115 mm Hg. A bolus of 0.137 mg of glycopyrrolate (Robinul, Robins) was given at this time by intravenous route. The ECG trace showed a second degree AV heart block for a duration of approximately 1.5 minutes, before returning to normal complexes. The heart rate peaked at 120 bpm 2 minutes after administration of the glycopyrrolate and the systolic BP rose to 160 mm Hg. The heart rate fell gradually over the next 20 mins and was recorded at 88 bpm on extubation. The systolic BP fell to 115 mm Hg and was recorded at 130 mm Hg at extubation. Respiration was spontaneous throughout anaesthesia, with good chest excursion, and recorded at between 20 - 30 per min. It was not considered necessary to ventilate the dog. The dog did not lose any temperature during the course of general anaesthesia. Duration of anaesthesia was 57 minutes.

Post-operative period.
The dog was extubated 5 minutes after the end of anaesthesia and the pulse at this time was 96 per min, respiration 36 per min and temperature 38.1 C. The bladder was reasonably full and was manually expressed before the dog was put into a recovery kennel. Sternal recumbency was achieved 10 minutes after extubation and recovery was quiet. The dog went home the following morning, to return in 3 days for a post-operative check and 10 days for sutures out and the biopsy results. This case was conducted in the University of Georgia and I was no longer present when the biopsy results were received.

Discussion.
The condition, age of the animal and the type of surgery being performed are all important when deciding on a suitable general anaesthetic procedure and the possible complications which could arise. All of this should be discussed prior to anaesthesia so that a fast, suitable course of action can be taken if a crisis occurs, and hopefully selecting the most suitable agents will reduce the risk of such crises occurring. An animal should always be weighed prior to anaesthesia, as correct dosing of drugs can be critical, especially in older animals which may not even need the full calculated dose. This was suspected in this case and a note made prior to induction to titrate the thiopentone slowly in order to avoid overdose. There are differing views as to when an animal should be classed as a geriatric, and there is also the 'real' age of the animal to be considered, ie whether the animal's general condition would lead to an assumption that it is older or younger than it's birth age. In this case the University of Georgia state that a dog of this breed over 7 years old is classed as an old dog and this dog's 'real' age would be even older due to it's condition. This would mean that it is more likely to suffer from hypotension, hypothermia and hypoventilation under anaesthesia and would probably need reduced anaesthetic doses.

Premedication consisted of butorphanol (a partial agonist opioid), for both the sedative and analgesic components. Butorphanol has an onset of 10 minutes after intramuscular injection and lasts for 1 - 1.5 hours in the dog. In the US it is widely used as an analgesic but in this country the general opinion is that despite being an excellent sedative and anti-tussant, it's use as an analgesic is very limited as it appears to have very little analgesic effect. The author must agree with this opinion and perhaps a more potent opioid such as pethidine or morphine could have been used. Carprofen (a non steroidal anti-inflammatory drug) could not be used as it is contraindicated for use in conjunction with corticosteroids (prednisolone). There are differing views as to whether parasympathetic antagonists should be routinely used in premedication for several reasons. Firstly, in some cases it does dry up the salivary secretions and in others it makes them viscous and even more likely to block the endo- tracheal tube. It is contraindicated for use in animals with pre-existing tachycardia. Cats that have been given atropine often seem agitated and the pupillary dilation seems to upset them or cause them to panic. Atropine is said to prevent acid reflux from the stomach which can cause oesophagitis. The anticholinergic used in this case was glycopyrrolate as this is reported to be favourable for use in older animals. It has less occular effects than atropine and causes less tachycardia. Bradycardia (below 60 beats per min) did occur during the general anaesthesia, thought to be the result of the depressant effects of the anaesthetic agents, and glycopyrrolate was given intravenously to treat this. After adminstration the ECG trace showed heart block for approximately 1.5 minutes. This is 'normal' after giving the anticholinergic and should only be temporary. For a copy of the ECG trace showing the heart block (see below). After this initial heart block the heart rate did peak at 120 bpm and then gradually fell. It is important to ascertain that the cause of the bradycardia is not hypoventilation before giving glycopyrrolate and assisted ventilation should be under- taken first.

Diphenhydramine (an anti-histamine) was also included in the premedication, as mast cell tumours release histamines (which lower the blood pressure) when manipulated.This would appear to have been successful, as although the anaesthetic agents caused bradycardia, the systolic blood pressue did not fall below 110 mm Hg (80 mm Hg being considered as hypotension). Diazepam (a benzodiazepine) was given at induction prior to thiopentone in order to reduce the amount thiopentone needed. This is important in ill or elderly animals as thiopentone produces both respiratory and cardiovascular depression, whereas diazepam has little effect. It was very successful in this case, as 330 mg was the calculated dose of thiopentone needed and only 129 mg was given in order to allow intubation. Diazepam can also be used in this way with other induction agents.

Isoflurane was used as the maintenance agent as recovery from this agent is faster than halothane, as much less is retained in the body to be metabolised by the liver. It is very expensive and therefore often used in conjunction with a closed circuit, as low flow rates reduce the amount vapourised and wasted via the scavenging system. In this case the flow rate through the circuit was 1L per minute (minute volume), but it is possible to reduce the minute volume in a circle circuit to 500 mls, but it is advisable never to use as a totally closed circuit.

Old animals have difficulty in maintaining body temperature during general anaesthesia so the dog was placed on a water heated pad. This proved successful and the dog did not lose any temperature during the anaesthetic. If this had been a problem a warm air blower was available to blow warm air via a hose placed under a towel which covers the animal. Towels heated in the microwave are also used to pack around the animal if, say, abdominal surgery is performed. Hypothermia will delay recovery from anaesthesia and will increase the potency of anaesthetic drugs during anaesthesia, due to slowed metabolism of drugs. After long anaesthetics, where possible, bladders are manually expressed before recovery from anaesthesia. This ensures that the animal does not soil itself with urine during recovery when it is still recumbent. This is especially important if dressings have been used. Failure to express a bladder will be reported to the ICU staff, so that they can take appropriate measures to avoid this problem.

Hartmann's is the crystalloid of choice for general operative fluids and 10mls/kg/hr is the accepted rate of infusion. Intra-operative fluids are important to maintain good renal perfusion, particularly in older animals whose renal function is likely to impaired. Monitoring instruments are very useful in anaesthesia, as long as the anaesthetist is able to interpret them and also accepts that instruments can give inaccurate readings due to poor connections etc. Observation is still the most important skill of a good anaesthetist.

This anaesthetic was well managed and, as the other cases also show, changes in an animal's status can occur very quickly and one person should be dedicated only to monitoring the anaesthetic and not required to do anything else. Unfortunately this does not always happen in general practice, due to lack of staff.

References:
Veterinary Anaesthesia, Hall & Clarke, Saunders, 9th Ed 1991
Manual of Anaesthesia, BSAVA 1992
Prof C Trim, BSAVA Congress 1999
Prof C Trim 1999 - visit to University of Georgia, USA