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Disorders of Potassium Regulation (Hyperkalaemia (Aetiology (Metabolic…
Disorders of Potassium Regulation
Potassium Physiology
Most dietary intake of K+ is excreted through the kidneys into urine
Aldosterone stimulates secretion of K+ by the kidney
K+ is charged is and is required to maintain the resting potential of cells in the body
Hyperkalaemia
Definition
Serum K+ >5.5 mmol/L
A serum K+ >6.5 mmol/L is a medical emergency
= Excessive K+ due to excessive consumption at a fast rate e.g. via IV fluids
Aetiology
Metabolic acidosis or diabetic ketoacidosis
Rhabdomyolysis (death of muscle fibres, releasing K+, caused by trauma crush injury)
Drugs e.g. ACE inhibitors (blocks binding of aldosterone to receptor), NSAIDs, diuretics such as spironolactone
Addison's disease
AKI (lowers filtration rate so more K+ is in blood) or oliguric renal failure (a very small amount of urine is produced)
Massive blood transfusion
Pathophysiology
When K+ levels in the blood rise, the difference in electrical potential between the cardiac myocytes and the outside decreases
The threshold for an action potential is significantly decreased
The amount of K+ in the blood determines the excitability of nerve and muscle cells
Abnormal heart rhythms occur that lead to ventricular fibrillation and cardiac arrest
Symptoms
Weakness
Fast irregular pulse
Chest pain
Fatigue
Light headedness
Investigations
ECG shows tall T wave, small or no P wave, wide QRS complex
Results can be artefactual e.g. haemolysis from vigorous venepuncture or thrombocytothaemia
Serum K+ is >5.5 mmol/L
Treatment
Urgent
Insulin - this drives K+ into cells
Accompany glucose with insulin to avoid hypoglycaemia
IV calcium gluconate - reduces excitability of cardiomyocytes and decreases VF risk
Non urgent
Review medications to look for drugs that cause hyperkalaemia
Give polystyrene sulphonate resin - binds to K+ in the gut to decrease uptake
Treat underlying cause
Hypokalaemia
Definition
Serum K+ <3.5 mmol/L
Serum K+ <2.5 mmol/L requires urgent treatment
Aetiology
Vomiting and diarrhoea
Pyloric stenosis
Increased aldosterone secretion - this is caused by liver failure, Cushing's, Conn's, nephrotic syndrome
Intestinal fistula
Loop and thiazide diuretics
Pathophysiology
Increased leakage from the ICF causes hyperpolarisation of the myocyte membrane
Myocyte excitability is decreased
Low serum K+ causes a water concentration gradient out of the cell
High K+ secretion into cells so low plasma K+
Signs
Cramps
Tetany - intermittent muscle spasms
Hypotonia
Palpitations
Muscle weakness
Light headedness
Constipation
Investigations
ECG: No or small T wave, long PR and QT, ST depression, U wave
Serum K+ <3.5 mmol/L
Treatment
Severe hypokalaemia: IV K+
Do not give K+ to patients with oliguirc renal failure
Mild hypokalaemia: oral K+