Liver Disease Applied Physiology

The liver is the largest organ in the body receiving 30% of cardiac output. The majority of nutritional, haematological and detoxification metabolism occurs in the liver, including the breakdown or excretion of many anaesthetic drugs. Other physiological roles of the liver include the manufacture of proteins, lipoproteins and carbohydrates which includes the clotting factors and the proteins to which most anaesthetic drugs are bound. Globulins are not produced in the liver. The liver also acts as a store for vitamins, minerals and carbohydrates. In the presence of significant liver disease, all these processes become disturbed.

Different conditions cause differing patterns of dysfunction. Excessive red cell turnover causes jaundice by overloading the pathways of haem breakdown even though other liver functions may remain relatively normal. Obstructive and cholestatic jaundice causes major disruption to metabolic pathways and to the absorption of fats and fat soluble vitamins, causing further problems. Hepatocellular dysfunction may be toxic or infective in origin but will result in the same picture of unconjugated bilirubinaemia, fat malabsorption and metabolic disturbance. In the end stage of hepatic failure, virtually all the body's metabolic processes are disturbed. Clinical features include clotting failure, coma from ammonia toxicity because of disturbed protein metabolism, hypoglycaemia because of poor glycogen metabolism, water overload and major electrolyte imbalance. There is usually portal hypertension that causes the formation of collateral circulation including oesophageal varices. If these bleed then the sudden high protein meal provided by enteral haemoglobin may precipitate hepatic coma in an otherwise compensated patient, a desperate situation which may be irretrievable. Anaesthesia in liver failure is, therefore, not for the inexperienced.

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