Potassium disorders

Question 1

Concentration of potassium in ICF and ECF

Answer 1

ICF 140 mmol/L

ECF 4 mmol/L

Question 2

Potassium balance

Answer 2

Potassium intake - 30-100 mmol/day

Renal excretion - 20-100 mmol/day

Around 5 mmol/day in faeces

Question 3

ECG changes seen with hyperkalaemia

Answer 3

Peaked T wave and widening QRS complex

Question 4

Categories of casues of hyperkalaemia test result

Answer 4

• Reduced excretion
• Redistribution
• Increased intake
• Pseudohyperkalaemia

Question 5

Causes of hyperkalaemia due to reduced excretion

Answer 5

• Renal failure
• ACE inhibitors
• Potassium sparing diuretics
• Addison’s disease

Question 6

Causes of hyperkalaemia due to redistribution

Answer 6

Acidosis

Question 7

Causes of hyperkalaemia due to increased intake

Answer 7

• Unlikely unless there is also impaired renal excretion
• Over-supplementation in IV fluids or TPN
• Blood transfusion

Question 8

Pseudohyperkalaemia

Answer 8

• Haemolysis
• Delayed centrifugation
• Sample contamination
• Increased platelet or white cell count

Question 9

Evaluation of hyperkalaemia

Answer 9

1. Is there an obvious cause?
   
   • Renal failure - creatinine and urea ↑
   • Haemolysis - red pink discolouration of serum
2. Medication
   
   • Spironalactone, ACE inhibitors adn ARBs all disrupt renin-angiotensin-aldosterone axis
   • Some intravenous drugs are given as potassium salts
3. Hypoaldosteronism
   
   • combination of ↑K+ and ↓Na+ is the classis Addisonian picture
   • if patient is hypotensive this diagnosis must be excluded urgently
4. Redistribution
   
   • K+ can be released from other tissues eg muscle (rhabdomyolysis), tumour cells (tumour lysis syndrome)
     
     • measure CK, urate
   • acidosis
     
     • check bicarbonate as evidence of ↑H+
5. Suspect pseudohyperkalaemia when persistent unexplained ↑K+

Question 10

Hypokalaemia ECG changes

Answer 10

Flattened T wave, U waves are prominant in all leads.

Question 11

Categories of causes of hypokalaemia test result

Answer 11

• Gastrointestinal loss
• Renal loss
• Redistribution

Question 12

Causes of hypokalaemia due to gastrointestinal loss

Answer 12

• Vomiting
• Diarrhoea
• Laxative abuse
• Villous adenoma
• Intestinal fistula

Question 13

Causes of hypokalamia due to renal loss

Answer 13

• Diuretics
• Magnesium depletion
• Cushing’s syndrome
• Conn’s syndrome
• Bartter’s or Gitelman’s syndrome
• Renal tubular acidosis

Question 14

Causes of hypokalaemia due to redistribution

Answer 14

• Insulin (high dose)
• Beta 2 agonists, eg salbutamol
• Metabolic alkalosis

Question 15

Evaluation of hypokalaemia

Answer 15

1. Obvious cause?
   
   • Vomiting/diarrhoea - [K+] in gut fluid is similar to blood
   • Loop or thiazide diuretics - promote urinary K+ loss
2. Biochemical evidence of redistribution?
   
   • Metabolic alkalosis - raised bicarbonate
   • Redistributive actions of insulin - low phosphate and glucose
3. Urinary potassium loss?
   
   • Low [Mg2+] affects tubular handling of potassium
   • Conn’s and Cushing’s are associated with increased mineralocorticoid activity
4. Other drugs?
   
   • Beta agonists like salbutamol or dobutamine
5. Consoder rarer causes
   
   • Hypokalaemic periodic paralysis
   • Bartter’s/Gitelman’s

Question 16

Management of hypokalaemia

Answer 16

• Correction of underlying cause
• Oral replacement in mild hypokalaemia
• Intravenous replacement if severe/marked clinical features/unresponsive to oral therapy
• Do not give IV potassium faster than 20mmol/hour
• Use ECG monitoring if giving IV potassium faster than 10mmol/hour