A thorough physical examination is an integral part of the pre-anaesthetic preparation of every patient. Not only does the examination provide the opportunity to detect concurrent disease or abnormalities, it also allows assessment of the behaviour and temperament of the animal, factors that influence practical aspects of anaesthetic management. Sometimes it is possible to correct abnormalities found on pre-anaesthetic examination before induction of anaesthesia (e.g., electrolyte disturbances) and for elective procedures patients should be stabilised before anaesthesia. A comprehensive description of the impact of every medical finding on choice of anaesthesia protocol is beyond the scope of this abstract; therefore, it will focus on the anaesthetic considerations associated with common medical conditions and biochemical and haematological abnormalities seen frequently on preoperative blood tests.

Normovolaemic Anaemia

Diminished oxygenation due to anaemia and inadequate oxygen-carrying capacity of the blood can have serious implications during anaesthesia, primarily due to ischaemic effects on the myocardium and brain. The physiological limit of oxygen transport is not known in humans or animals, either awake or during anaesthesia. Chronic anaemia is better tolerated than acute anaemia because of increases in diphosphoglycerate levels in red blood cells, so that, as in people, cardiac output does not change until the haemoglobin concentration falls below 70 g/l (7 g/dl). Acute anaemia is accompanied by increases in cardiac output in order to maintain adequate oxygen delivery to vital organs. Increased cardiac output is accompanied by increased myocardial work, which in the face of a reduced blood oxygen content may lead to myocardial hypoxia. Decisions regarding perioperative blood transfusions can be difficult and require case-based clinical judgment. No fixed haemoglobin concentration is recommended as a transfusion trigger in humans, although current guidelines suggest that a blood transfusion is rarely indicated if haemoglobin concentration is above 100 g/l (10 g/dl) and is almost always necessary if the concentration is below 60 g/l (6 g/dl). Final decision-making about whether a transfusion is indicated depends on the severity of clinical signs, risks of further blood loss and the acuteness of anaemia.

Effects of Normovolaemic Anaemia on Anaesthetic Management

Most anaesthetic agents cause cardiovascular depression, which will impair compensatory increases in cardiac output to maintain oxygen delivery in the face of anaemia. Remember that provision of 100% oxygen during anaesthesia will not significantly improve oxygen delivery in anaemic animals and that pulse oximetry readings do not represent oxygen delivery when the oxygen-carrying capacity of the blood is reduced. The anaesthetic protocol should be geared towards a balanced technique to minimise deterioration in oxygen delivery. Avoid drugs with significant cardiovascular effects (e.g., medetomidine, which will increase myocardial work, acepromazine which may cause splenic sequestration of red blood cells). Severe opioid-induced bradycardias will also compromise cardiac output and oxygen delivery; therefore, concurrent administration of atropine with potent opioids is sensible. Pre-oxygenation prior to induction of anaesthesia will reduce the risk of hypoxia if induction of anaesthesia is associated with a period of apnoea before intubation. Fluid therapy must be administered cautiously in order to avoid dilution causing exacerbation of the anaemia.

Hypoalbuminaemia

Hypoalbuminaemia (serum albumin concentration <15 g/l) alters Starling's forces and favours loss of fluid from the vascular space, hypovolaemia and decreased systemic perfusion pressure. It may also encourage the development of peripheral oedema and ascites due to loss of fluid from the vascular system into the third space. Many endogenous and exogenous substances, including drugs such as thiopental and NSAIDs are bound to albumin, such that hypoalbuminaemia can lead to an increased unbound free fraction of the drug following administration compared to normal animals.

Effects of Hypoalbuminaemia on Anaesthetic Management

There is the potential for overdose following administration of highly protein-bound drugs to hypoalbuminaemic animals. The clinical relevance of this is unknown, but it can be avoided by dosing hypoalbuminaemic animals slowly and to effect with protein-bound induction agents such as thiopental and not giving two highly protein-bound drugs immediately after each other. Some patients may require administration of albumin or synthetic colloids to facilitate resolution of oedema and expansion of the intravascular compartment. Maintenance of a normal blood pressure during anaesthesia can be difficult in hypoalbuminaemic animals unless fluid therapy includes natural or synthetic colloids.

Liver Disease

Biochemical abnormalities that are suggestive of liver disease (elevated alanine aminotransferase (ALT), alkaline phosphatase (ALP), serum bile acids) are a common finding on preoperative blood tests, particularly in older dogs. Impaired liver function can influence anaesthesia protocol in several ways because of the many functions carried out by the liver. The magnitude of these effects are determined by the severity of the liver disease, but the following principles should be applied when anaesthetising animals with liver dysfunction:

 Diseases of the liver can impair the function of liver microsomal enzymes leading to a prolonged duration of drug action. Choose drugs that are short acting (e.g., propofol) and maintain anaesthesia with an inhalant agent (that is minimally metabolised) to avoid a prolonged recovery due to the repeated administration of injectable agents.

 Avoid administration of halothane to animals with liver disease. Halothane undergoes a greater degree of hepatic metabolism than other modern inhalants such as sevoflurane or isoflurane; therefore the potential for a prolonged recovery is greater.

 Medetomidine reduces liver blood flow. This is not of significance in animals with normal liver function, but medetomidine may have the potential to critically decrease liver blood flow in animals that already have altered liver function and therefore should be avoided.

 Hypoglycaemia can occur in animals with liver disease due to an impaired ability of the liver to synthesise glucose. Monitor blood glucose concentration perioperatively and supplement with glucose intravenously if necessary.

 The liver is the production site for most clotting factors; therefore animals with marked liver dysfunction may be at an increased risk of haemorrhage during surgery. Blood clotting function should be assessed before surgery in animals with moderate to severe liver dysfunction and appropriate therapy initiated to improve blood clotting before surgery.

 The metabolism of NSAIDs will be prolonged when compared to normal patients. The patient should be carefully assessed before NSAID administration and, if given, the prolonged metabolism should be considered when recommending a dosing interval.

 Patients with chronic liver disease have decreased hepatic blood flow caused by increased vascular resistance in the portal vein. As a consequence hepatic blood flow and oxygen delivery to the liver are more dependent on hepatic arterial blood flow than is the case in normal patients. Use of a balanced anaesthesia technique to maintain cardiovascular stability during anaesthesia can reduce the risk of a further deterioration in liver function after anaesthesia due to hypoxia of liver tissue.

Chronic Compensated Renal Failure

Biochemical evidence of renal dysfunction is a common finding in older cats presented for anaesthesia. These animals compensate for the renal dysfunction by drinking more to maintain glomerular filtration rate. Anaesthesia prevents this compensatory mechanism; therefore appropriate fluid therapy must be initiated in the perioperative period to prevent a deterioration in renal function after anaesthesia. Chronic renal failure is often associated with other physiological changes including weight loss, uraemia, electrolyte abnormalities, anaemia, hypertension and hypoalbuminaemia. Careful assessment of the patient is required before anaesthesia, and management to reduce the magnitude of, or correct, these changes should be initiated to stabilise the animal before the start of anaesthesia. For elective procedures hypokalaemia can be corrected with oral potassium supplementation, and uraemia can be reduced by ensuring an adequate preload is maintained with intravenous fluids.

The most important anaesthetic consideration for animals with compensated chronic renal failure is fluid therapy. Establishing intravenous access with a catheter allows maintenance of intravenous fluid therapy throughout anaesthesia and into the recovery period until the animal is once again able to drink and self-regulate fluid balance. Although any balanced electrolyte solution can be used to maintain renal function, hypernatraemia is common in patients receiving lactated Ringer's or Hartmann's.

Use of half-strength NaCl solution (0.45%), supplemented with potassium to avoid hypokalaemia is a suitable alternative. A major goal during anaesthesia should be to maintain renal blood flow to prevent exacerbation of renal damage due to hypoxia. Premedication with drugs with minimal cardiovascular effects such as benzodiazepines and opioids, or a low dose of acepromazine, combined with use of a balanced anaesthesia technique to avoid requirement for high concentrations of inhalant agents is recommended. Monitoring blood pressure during anaesthesia combined with adjustment of fluid therapy and depth of anaesthesia to correct hypotension helps to reduce the risk of renal ischaemia. Many chronic renal failure patients are very thin so protection against hypothermia by increasing environmental temperature, using insulation and active warming can assist in the maintenance of normothermia. Perioperative administration of NSAIDs is contraindicated in animals with chronic renal failure due to the role of prostaglandins and leukotrienes in autoregulation of renal blood flow and glomerular filtration pressure. Analgesia can be provided using opioids and regional anaesthesia techniques using local anaesthetics if appropriate.