Electrolyte disturbances

Question 1

What is the normal range of sodium and how is it maintained?

Answer 1

Normal range (serum): 135-145mmol/L
Principle extracellular cation.
Daily requirement of 1-2mmol/kg/day
Majority reabsorbed in the kidney - 90% PCT, 20% thick ascending limb, 5% DCT, 3% collecting duct)
Reabsorption is influenced by:
- RAAS
- ADH
- thirst
- beta-adrenoceptor stimulation in PCT

Question 2

What is hyponatraemia and how can it be classified?

Answer 2

Hyponatraemia is serum sodium level < 135mmol/L
Hyponatraemia can be classified according to:
- osmolality and volume status when trying to determine underlying cause
- or severity when determining emergency management and correction.

Question 3

How would you determine the cause of hyponatraemia?

Answer 3

In a patient with a proven hyponatraemia:

• By assessing the patient’s volume status and checking serum osmolality along with urinary sodium.
• Hyperosmolar states - due to osmotic extracelliular effects e.g. mannitol, hyperglycaemia.
• Normosmolar states - due to pseudohyponatraemia (e.g. hyperlipidaemia or hyperproteinaemia)
• Hyposmolar states - assess volume status:
• > hypovolaemia - low urine sodium (extra-renal fluid losses), high urine sodium (diuretics)
• > normovolaemia - low urine sodium (psychogenic polydipsia/low solute intake (alcohol), high urine sodium (SIADH, hypothyroid, low cortisol)
• > hypervolaemia - low urine sodium (cirrhosis, heart failure), high urine sodium (renal failure).

Question 4

How is severity of hyponatraemia classified?

Answer 4

By severity of serum sodium deficit and by presence of associated symptoms and signs.
Severe hyponatraemia would be serum sodium <120mmol/L with associated symptoms:
- nausea and vomiting
- confusion
- seizures
- coma

Question 5

How is hyponatraemia managed?

Answer 5

• Depends upon stability of patient, severity and underlying cause and duration.
• Establishing duration is often problematic.
• If patient has severe symptomatic hypernatraemia this should be treated with IV hypertonic saline to achieve a serum sodium of >120mmol/L and then correction at no more than 10mmol/L over 24h.
  Further treatment will then be guided by treating the underlying cause.

Question 6

What is SIADH and what can cause it?

Answer 6

Syndrome of inappropriate diuretic hormone.
Usually excessive ADH production to physiological homeostasis results in free water reabsorption and retention out of balance with sodium retention - leading to hyponatraemia and relative concentration of urinary sodium.
It can be caused by:
- Drugs (hypoglycaemic agents, antidepressants, antipsychotics, chemotherapeutic agents)
- Malignancy (Lung, brain, pancreas)
- CNS disorders (infection, trauma, infarction, haemorrhage)
- Pulmonary disorders (pneumonia)
- Pain

Question 7

How is SIADH treated?

Answer 7

• fluid restriction
• if symptomatic, hypertonic saline administration
• demeclocycline
• tolvaptan (ADH receptor antagonist)