Management of Fluid and Electrolyte problems in Children

If blood proteins or lipids are raised, the measured plasma [Na⁺] may be lower than the actual [Na⁺] in the aqueous phase, depending on the laboratory method used:

• If the lab use a Na-selective electrode or a conductance method, which measures the ratio of sodium to water in the plasma, the result will not be affected.
• However if a method such as flame photometry is used, which measures the [Na⁺] per volume of plasma, a "factitious" hyponatraemia will be recorded.

Thirst is stimulated by a rise in the plasma [Na⁺] of 2 mmol/l. For hypernatraemia to develop, this thirst response must fail.

To assess the cause of the hypernatraemia ask:

• What is the ECF volume?
• Has the body weight changed?
• Is the thirst response to hypernatraemia normal?
• Is the renal response to hypernatraemia normal?

What is the ECF volume?
Gain of sodium leads to ECF expansion. All other causes of hypernatraemia are due primarily to water loss.

Has the body weight changed?
Rarely fluid moves from the ECF to the ICF e.g. following a convulsion or rhabdomyolysis. Hypernatraemia then occurs with no change in body weight.

Is the thirst response to hypernatraemia normal?
A 2% increase in plasma tonicity stimulates thirst. Failure to take on EFW may occur in a baby who does not have control over access to fluids. The absence of thirst suggests a CNS lesion.

Is the renal response to hypernatraemia normal?
The appropriate response is a low volume of concentrated urine (> 1000 mOsm/kg H₂O). A failure to produce such a response suggests an ADH or renal problem.

Causes

Hypernatraemia due to water loss
Non-renal water loss - Hypotonic solutions may be lost through the skin, respiratory or GI tracts.
Renal water loss - Usually polyuria - diabetes insipidus or an osmotic diuresis.

Hypernatraemia due to sodium gain

• Use of replacement solutions containing more sodium than in the fluids being lost i.e. urine.
• Salt poisoning
• Ingestion of sea water
• Dialysis error

Treatment