Electrolytes - Do they Really Matter?

Electrolyte evaluation is extremely important in hospitalised ill patients as abnormalities may contribute to clinical signs of weakness, inappetence and neuroogical abnormalities. The electrolytes common evaluated include sodium, potassium and chloride. Some machines may provide information on ionised calcium and magnesium.

Sodium

Sodium is tightly controlled by the body as its extracellular concentration is closely liked to water and cell size. It follows therefore that changes in water will result in changes in sodium and typically pure water loss (e.g. hyperthermia or diabetes insipidus) will cause an increase in blood sodium concentration and severe cellular dehydration whereas water gain (as seen in heart disease) will result in hyponatraemia. Whatever the cause if sodium is more than 5mmol/l outside of the reference range attempts to correct it should be made extremely slowly (no more than a 0.5mmol/l/hour change). Rapid changes in sodium concentration result in cellular swelling or dehydration. This is most pronounced in the brain where cell swelling results in cerebral oedema, increased intracranial pressure and resultant deterioration in mental status and cellular dehydration results in myelinolysis. This has been reported in a handful of cases of hypoadrenocorticism where sodium concentrations were increased rapidly resulting in excess movement of water out of the cells as extracellular sodium concentrations increased. Management of sodium abnormalities can be extremely challenging and require frequent monitoring and fluid changes. In some cases a number of fluids may be needed at any one time. Clinical signs of sodium abnormalities are typically only seen when changes are severe and include mental obtundation, weakness, seizures and coma.

Potassium

Potassium is the most abundant intracellular cation with approximately 95% of total body potassium within the cells. Hence hypokalaemia typically represents a MASSIVE total body deficit of potassium. Extracellular potassium levels are tightly maintained and the difference between intracellular and extracellular potassium is vital for maintenance of the resting membrane potential. Abnormalities in potassium therefore result in effects on excitable tissues- most obviously in the heart and skeletal muscles.
Abnormalities in potassium are the most commonly seen electrolyte abnormality in small animal practice. Hypokalaemia is extremely common in animals that have been inappetent for a period of time, particularly if there are concurrent increased losses e.g. in polyuria or diarrhoea. Clinical signs of hypokalaemia are typically nonspecific but may contribute to signs of weakness, lethargy, anorexia and ileus particularly in cats. Ventroflexion of the neck of cats is typically only seen with severe hypokalaemia. Extremely severe hypokalaemia can lead to paralysis of respiratory muscles, hypoventilation and death. Correction of hypokalaemia should be initiated as soon as practically possible. Severe hypokalaemia should prompt the use of maximal potassium supplementation in fluids (0.5mmol/kg/hr). These highly supplemented solutions should be carefully marked to avoid potential overdosing. On no account should potassium be infused neat. A table for supplementation is reproduced in these notes. [...]