Potassium Clinical Chemistry
Introduction
Potassium is largely an intracellular ion. Plasma potassium levels are not always a good indicator of intracellular levels; in acidosis the exchange of H+ and K+ ions leads to the depletion of intracellular potassium and elevated plasma potassium. The converse occurs in alkalosis.
Aldosterone secretion promotes sodium retention and potassium excretion. Clinical features of potassium depletion include muscle weakness, ileus and cardiac arrhythmias, rhabdomyolysis and renal dysfunction. Hypokalaemia is of particular significance in the cat and is usually associated with CRF. In most cases, hyperkalaemia arises due to a diminished ability to excrete potassium. Potassium excess is therefore associated with hypoadrenocorticism and some forms of renal disease (especially post renal azotaemia). Marked hyperkalaemia is potentially life threatening causing bradycardia and cardiac arrest.
Small animals
Causes of hyperkalaemia
- Acute renal failure
- Hypoadrenocorticism
- Post renal azotaemia (urethral obstruction, urinary tract trauma)
- Pseudohyperkalaemia
EDTA contamination of serum
Thrombocytosis
Leucocytosis (leakage from cells)
Haemolysis (Akitas and Shibas)
- Massive tissue damage
- Metabolic acidosis (renal failure, certain types of diarrhoea)
- Peritoneal effusions
Causes of hypokalaemia
- Chronic renal failure (particularly cats)
- Diuretic therapy
- Vomiting and diarrhoea
- Hypokalaemic myopathy (hypokalaemic periodic paralysis) of Burmese kittens
- Insulin therapy
- Administration of potassium depleted fluids
- Excessive mineralocorticoid therapy
- Metabolic alkalosis for example gastric vomiting
Complementary tests
In small animals a Na:K ratio is an aid to the diagnosis of hypoadrenocorticism; a ratio <25:1 is supportive but an ACTH stimulation test is required for confirmation if the clinical signs are suggestive of Addison’s disease. The ratio can also be reduced by other factors increasing plasma potassium including renal disease.
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Equine
Potassium levels in the extracellular fluid are influenced most by renal function and do not always reflect potassium levels in the intracellular compartment. Potassium distribution depends on the acid-base status as it is exchanged for hydrogen ions across the cell membrane. Hyperkalaemia is a potential emergency due to induction of cardiac dysrhythmias.
Causes of hyperkalaemia
- Reduced extracellular fluid volume, hypovolaemia with renal shut down
- Metabolic acidosis
- Polyuric renal disease
- Post renal obstruction in foals
- Pseudohyperkalaemia
In vitro haemolysis
Prolonged storage
- Muscle damage
Rare causes of hyperkalaemia
- Anuric renal failure
- Urinary tract disruption
- Tissue necrosis
- Inherited hyperkalaemic periodic paralysis
Causes of hypokalaemia
- Prolonged anorexia
- Dietary deficiencies
- Gastrointestinal tract loss (lower bowel obstruction), diarrhoea
- Enterocolitis
- Profuse sweating
- Peritonitis
Rare causes of hypokalaemia
- Metabolic alkalosis
- Renal tubular acidosis
- Iatrogenic (diuretics, bicarbonate or insulin administration)
Complementary tests
Urine clearance ratios will assist interpretation of serum electrolyte and mineral levels see fractional electrolyte excretion (FE) values (%).
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