ICM - Electrolytes

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)