Clinical History.
The dog was first presented for anal adenomas which were subsequently removed under general anaesthesia. A routine palpation of the abdomen whilst the dog was under anaesthesia revealed a mass which was diagnosed as a splenic tumour. As the owners had not given consent for this surgery the dog was booked in for removal of the spleen 2 weeks later, as the dog was not showing any clinical symptoms consistent with a splenic tumour. A blood sample to measure packed cell volume was taken pre-operatively and this was recorded at 0.47 L/L, which was normal. The owners were warned of the risks associated with general anaesthesia of an old, obese dog undergoing a splenectomy and were required to sign a consent form to allow general anaesthesia and surgery.

Pre-anaesthetic clinical examination and premedication.
The dog was admitted for general anaesthesia having been starved for 12 hours with access to water until 2 hours prior to premedication. A clinical pre-operative check revealed no thoracic or cardiovascular abnormalities. Pulse was 92 per minute, respiration 34 per minute and temperature recorded at 38.8 C. Mucous membranes (m/m) were pink and capillary refill time (CRT) was just under 1 second. Premedication was given with 0.6 mg of acepromazine (ACP C Vet) by subcutaneous route and 7 mg of morphine (Morphine Sulphate, Evans) by intramuscular route. A 20 guage intravenous catheter was placed aseptically into the right cephalic vein and an infusion of Hartmanns crystalloid was started at a rate of 10 mls/kg/hour. One hour later the premedication seemed to have had no effect. The dog was still excitable and quite aggressive, needing to be muzzled in order to carry out induction, having already bitten someone without warning whilst being taken out of the kennel.

Induction.
Induction was achieved with 110 mg of propofol (Rapinovet, Schering-Plough) by slow intravenous injection given to effect through the catheter, and a cuffed 12 mm endo- tracheal tube was used to intubate the dog. A semi-closed parallel Lack circuit with a 4L resevoir bag was used with flow rates of 4L nitrous oxide and 2L oxygen, and halothane was used as the inhalation agent. Additional analgesia was given after induction with 125 mg carprofen (Rimadyl, Pfizer) by subcutaneous route. Antibiotic cover was provided with 225 mg ampicillin (Norobrittin, Norbrook) by subcutaneous route.

Radiography.
A radiograph of the thorax in right lateral recumbency was taken to ensure that no metastases had occurred. The radiograph revealed no visible abnormalities and the dog was prepared for surgery and a total splenectomy was performed.

Maintenance of anaesthesia.
For a detailed account of general anaesthesia see attached anaesthetic record. Duration of anaesthesia was 150 minutes with the dog able to walk 40 minutes after the end of anaesthesia.

After induction the dog was placed in dorsal recumbency and it was noticed after a few minutes that the pulse oximeter reading had fallen to 82% (hypoxaemia) and that the mucous membranes (m/m) were beginning to appear red. This indicated that the dog was hypoventilating. Neither a Bains circuit or a ventilator was available so the nitrous oxide was turned off and the oxygen increased to 8 L. Intermittent positive pressure ventilation (IPPV) was carried out and the pulse oximeter rose to 97% and the m/m returned to pink. At the time of the first incision the heart rate rose to 158 bpm in response to the surgical stimulation. The halothane was increased to 2% and the nitrous oxide was put back on at the previous flow rates, and IPPV was continued for the duration of anaesthesia 5 times a minute to assist with the dog's spontaneous respiration. This kept the pulse oximeter readings above 90% (the level at which hypoxaemia occurs). The heart rate continued to be quite erratic throughout the anaesthetic, although the ECG showed normal complexes and rhythm. The halothane remained at 2% for most of the duration of anaesthesia. The respiration remained fairly constant around 30 to 35 per minute. Extubation took place 5 minutes after cessation of anaesthesia.

Post-operative recovery.
The dog was very excitable on recovery and only received 700 mls of the Hartmann's as it continually tied the giving set in knots. The morphine was repeated after 4 hours. A PCV taken the following morning recorded at 0.36 L/L and the dog had slightly pale m/m's. However, it was eating and temp was normal so it was sent home with 10 Rimadyl 50 mg tablets to be given one twice daily with food, and 10 Ampicillin capsules also to be given one twice daily to start immediately. A post-operative check two days later revealed a PCV of 0.37 L/L and the dog was very bright and eating well. Sutures were removed at 10 days and the PCV was last recorded at 0.42 L/L.

Discussion.
Anaesthetic protocols should always be planned to suit the animal's condition at the time of surgery and the procedure to be performed. In this case the dog was not collapsed, as the spleen had not ruptured prior to surgery. However, a splenectomy can result in several complications under general anaesthesia and special consideration should be given to anaesthetic protocols to minimise the risks. In addition this dog was obese which also adds to the possible anaesthetic risk. Firstly, a pre-operative PCV was taken to be able to compare with post-operative PCV's, as anaemia can be a post-operative complication. Also, anaemia would preclude the use of nitrous oxide (if severe). Anaemia would also affect the accuracy of the pulse oximeter, as it measures the levels of oxygen saturation of the haemoglobin, not whether there are enough red cells to carry sufficient oxygen to the tissues. ACP was given as a premedication but it probably would have been wise not to use it for two reasons. Firstly, one side effect of ACP is to produce hypotension, which in itself could be one of the complications of removing a spleen which contains a large volume of blood. The second reason for not using ACP is that it is not usually an effective sedative for aggressive animals. In fact it can release the inhibtions that prevent a dog from biting, which is what appeared to happen in this case. However, as the dog did not give any indication of aggression prior to premedication and the owner made no mention of this behaviour this was totally unforeseen. One advantage of ACP is that it protects the heart from catecholamines. Choice of induction agent and maintenance agent is important for splenectomies as they are at risk from premature ventricular contractions (PVC) during general anaesthesia. PVC's can result from drugs, hypoventilation, hypoxaemia, pulmonary and myocardial contusions following trauma, and inadequate depth of anaesthesia.Thiopentone sensitises the myocardium to catecholamines which increases the risk of PVC's, and also depresses the respiratory and cardiovascular systems, so this agent should be avoided. Although propofol also depresses the respiratory and cardiovascular systems it does not sensitise the myocardium as much as thiopentone, and would be preferred to thiopentone. Diazepam (0.25 mg/kg) could be given intravenously before the propofol, which would reduce the amount of propofol needed for induction. Or ketamine (5 mg/kg) and diazepam (0.25 mg/kg) in the same syringe could have been given intravenously as an alternative induction agent. The advantage of using ketamine is that it does not produce as much cardiovascular depression. One third to one half of the dose should be given, followed by titration to effect after 30 seconds to allow intubation of the trachea. The disadvantage of using ketamine is that monitoring of depth of inhalation anaesthesia cannot be assessed using eye position, and monitoring instruments would be necessary to avoid overdose. Thiopentone (a barbiturate) also causes congestion of the spleen with blood, which would increase the risk of hypovolaemia and hypotension when the spleen was removed, so this agent is unsuitable for this procedure. A disadvantage of using propofol as an induction agent is that elimination of the drug can occur quite suddenly and consequently the depth of anaesthesia can change rapidly. This did occur with this dog just prior to the first incision and, as the record shows, increased respiration and pulse rate indicated that the depth of anaesthesia at this point was not adequate. The halothane was increased to take the dog back down to a suitable depth of anaesthesia. Halothane, which was used as the inhalation agent also sensitises the myocardium to catecholamines and it would have been better to use isoflurane which does not. However isoflurane is not available at this practice due to cost. Another problem encountered early in the anaesthetic was hypoventilation, caused by placing the animal in dorsal recumbency, which resulted in hypoxaemia, due to obesity. When an obese animal is placed on it's back the abdominal contents impede the diaphragm's movement. A Lack circuit is not designed for prolonged IPPV as rebreathing expired gases can occur. To try and overcome this the flow rate was turned up to 8 L. Neither a Bains circuit or ventilator was available at the time. The inspiratory phase was kept short (1 - 1.5 secs) as venous return is impeded during the inspiratory phase. The pulse oximeter rose to 97% and the m/m returned to pink. Nitrous oxide is an analgesic and is usually used at a ratio of 2:1 with oxygen. Ten minutes oxygen time must be used after cessation of nitrous oxide to avoid diffusion hypoxia. Turning off the nitrous oxide would have contributed to the dog showing a pain response to surgical stimulation, so the nitrous oxide was put back on at the previous flow rates and IPPV assistance continued 3 - 4 times a minute to assist the dog's spontaneous breathing. This kept the pulse oximeter reading above 90% for the duration of anaesthesia. Respiration rates remained quite fast throughout anaesthesia and this was attributed to obesity. Because of the risk of PVC's it was important to use the ECG to monitor the heart. Lignocaine at 1mg/kg was available to treat any PVC's if necessary, ie more than 2 in a row occuring or any PVC's in conjunction with low blood pressure. It would be necessary to ascertain the cause of the PVC's before treating with lignocaine, ie if caused by hypoxaemia or inadequate depth of anaesthesia then rectifying those problems should resolve the PVC's. Also, stopping the halothane and using isoflurane might rectify the problem. If neither of these work then a bolus of lignocaine can be tried. Close observation on depth of anaesthesia should be kept when using lignocaine (particularly if an infusion is used) as it is an anaesthetic and can lead to deepening of the anaesthetic level.

It might have been better to give the Rimadyl at the same time as the premedication as this would have ensured that it was effective at the time of the first incision. Research shows that Rimadyl has a synergistic effect when given in conjunction with morphine (a pure opioid acting on the mu receptors). Rimadyl is the only Non-Steroidal Anti-Inflammatory analgesic considered safe to use peri-operatively due to the fact that it does not inhibit the body's production of prostaglandins which play a role in protection of the gut and maintenance of renal function. Capillary refill time increased from 1 second to 1.5 seconds during ligation of the spleen and Haemaccel was substituted for Hartmann's to support the circulation volume, as the tumour was very friable and started to rupture.

Haemaccel is a gelatin used as a plasma expander which will remain in the circulation for about 5 hours. Hartmann's crystalloid was used at an initial rate of 10mls/kg/hr as this is the recommended fluid and rate of infusion for general intra-operative use. The rate of infusion of the Haemaccel was speeded up at this point and after 20 minutes the CRT returned to 1 second. Haemorrhage was not severe in this case, but if it had been it would have been better to give whole blood.If blood loss is to replaced by Hartmanns, three times the amount of blood lost should be given. However, for this procedure it is really important to provide intravenous fluids as it will be expected that removing a spleen which contains a fairly large amount of blood will have an effect on the cardiovascular system. Peripheral pulses, ie lingual and carpal were present throughout. This would indicate that, together with pink m/m's and reasonable CRT, that blood pressure remained reasonable during anaesthesia. Ideally it would be best to be able to monitor blood pressure with either direct measurement via an arterial catheter or indirectly with a cuff and Doppler or Dynamap type of measurement. None of these are available at this practice.

Temperature loss was minimal (1.2C) despite the age, duration of anaesthesia and an open abdomen. This was mainly due to having a heated operating table and using warmed fluids. This would have contributed to the quick recovery. Analgesia was suitable for this procedure and, although morphine is reported to delay recovery, it didn't in this case which was actually a disadvantage as fluid infusion had to be abandoned after recovery due to excitability of the dog.

Overall, this was a successful anaesthetic in view of the possible complications which could have occurred. The two features of the anaesthetic which were least successful were the inadequate depth of anaesthesia at the time of the first incision and management of the hypoventilation. Unfortunately some limitations are set by the equipment available, and although not ideal the pulse oximeter readings were kept above 90% (the level at which hypoxaemia occurs) for the duration of the anaesthetic after 1 minute below 90%. All complications were discussed prior to induction and relevant treatment protocols also prepared in order that the surgeon would not have to concentrate on surgical and anaesthetic emergencies simultaneously.

References:
Small Animal Surgery, T Fossum. Mosby. p 450 1997
Anasethesia of the Critical Patient, C Trim. BSAVA Congress 1999
Principles of Anaesthesia. Cont Ed Course. BSAVA March 1999 L Hughes & R McMurphy