potassium

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	Potassium
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General
Hyperkalaemia
Hypokalaemia

General (return to top)

Potassium is the major intracellular cation present at concentrations of about 150 mmol/L. Total body potassium is about 3000 mmol of which 98% is intracellular. An average diet includes about 100mmol of potassium per day. Extracellular potassium is about 4 mmol/L. 60mmol/day is secreted into the gastro-intestinal tract, of which about 90% is resorbed (ie fecal losses are about 10-20mmol/day).

About 800 mmol of potassium is filtered at the glomerulus each day, of which nearly all is resorbed in the proximal tubule and Loop of Henle. Potassium excretion is controlled in the distal tubule and collecting ducts under the influence of aldosterone and other mineralocorticoids involving exchange with sodium or hydrogen ions. The obligatory daily urinary potassium loss is about 20-30 mmol with the average urinary excretion matching daily intake minus gastrointestinal losses.

Changes in plasma potassium may be caused by changes in potassium intake or excretion, or by exchange with intracellular compartments. This may occur by cell lysis, pH changes and other factors such as insulin or digoxin.

Hyperkalaemia (return to top)

Hyperkalaemia is often multifactorial, with renal failure and increased intake the most common causes. There are many causes of artefactual hyperkalaemia and if there is any doubt a repeat sample should be collected.

HYPERKALAEMIA - CAUSES
Artefact	Drip arm, EDTA contamination Difficult collection, prolonged fist-clenching Thrombocytosis (only with serum - gold top tube) Leukocytosis Delayed separation from red cells Haemolysis during collection
Increased Intake	Oral (eg fruit juices) Intravenous (old blood, drugs eg potassium penicillin, therapy)
Decreased output	Renal failure Hyponatraemia Addisons Disease or other mineralocorticoid deficiency Drugs (spironolactone, triamterene, amiloride, captopril, enalapril, heparin, NSAIDS Hypoxia
Redistribution	Acidosis Tissue damage (eg tumour lysis syndrome) Insulin deficiency Digoxin overdose Intavascular haemolysis

EFFECTS: Hyperkalaemia initially causes membrane excitability due to partial membrane depolarisation, then as level of serum potassium increases it may cause muscle weakness, arrythmias, and eventually cardiac arrest. The ECG changes are: peaked T waves, decreased p waves, QRS widening, ST-T slurring, Sine wave, arrest
Due to the prevalence of artefactual hyperkalaemia elevated levels should be checked, particularly if the finding is contrary to clinical expectations. With markedly elevated levels an ECG can be used to rapidly confirm the presence of hyperkalaemia.

Hypokalaemia (return to top)

Hypokalaemia is most commonly due to reduced intake or increased renal losses. The effect of drugs, especially diuretics, should always be considered.

HYPOKALAEMIA - CAUSES
Artefact	Drip arm
Reduced Intake	"Tea and toast" diet Inadequate intravenous supply
Increased Output - Renal (spot Urine K > 20 mmol/L)	Drugs: especially diuretics, amphoterocin, carbenicillin Mineralo-corticoid excess: eg aldosteronism (primary or secondary), Cushings syndrome, Bartters syndrome Osmotic diuresis Renal tubular acidosis
Increased Output - GIT	Diarhhoea and Vomitting Resonium Fistulae
Redistribution	Alkalosis Glucose/insulin Catecholamines Salbutamol Hypokalaemic periodic paralysis Thyrotoxic periodic paralysis

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For further information please contact Dr Graham Jones on 8382-9100

gjones@stvincents.com.au

Last updated 16/5/05