Hypernatraemia

Question 1

Causes of hypernatraemia

Answer 1

• Dehydration
• Osmotic diuretics, e.g. Hyperosmolar non-ketotic diabetic coma (=HHS: Hyperosmolar Hyperglycaemic State)
• Diabetes insipidus
• Excess IV saline

Question 2

Hypernatraemia >>> rate of correction

Answer 2

No greater than 0.5 mmol/hour correction is appropriate

(no specific guidelines by NICE, but it is generally accepted)

Hyponatraemia should be corrected with great caution

Question 3

What does happen if hypernatraemia is corrected too rapidly?

Answer 3

• Risk of cerebral oedema
• Although brain tissue can lose sodium and potassium rapidly, lowering of other osmolytes (and importantly water) occurs at a slower rate >>> predisposing to cerebral oedema >>> resulting in seizures, coma and death

Question 4

If a patient has dehyration + hypernatraemia >>> how to calculate free water deficit?

Answer 4

Free water deficit (in litre) = (Patient sodium – 140)/ 140) x total body water (TBW)

• Here, assumed normal Na is 140mmol/L (as its the average)
• TBW for 72kg person is 40L average
• For any weight >>> TBW = 60% of the body weight