TeacherOfMedicine: Sodium chloride

Used to provide sodium and water intravenously in patients unable to take enough orally.
Used to expand circulating volume in states of circulatory compromise (including shock).
Used for reconstitution and dilution of drugs intended for administration by injection or infusion.
Used to treat hyponatraemia.

The normal sodium requirement for adults is about 1 mmol/kg/day.
Extracellular sodium concentrations are maintained at around 140 mmol/L by Na⁺/K⁺-ATPase, which pumps sodium out of cells in exchange for potassium.
As the main cation in extracellular fluid, sodium is the principal determinant of its osmolality.

Sodium chloride 0.9% contains 154 mmol/L sodium and is therefore roughly isotonic with ECF.
This means that ECF expands by approximately the same amount as the volume of sodium chloride 0.9% administered. About 20% of this remains in the intravascular space to expand circulating volume. About 80% of this will be lost into interstitial fluid.

Excessive administration of sodium chloride can cause a fall in cardiac output and precipitate heart failure by increasing left ventricular filling beyond the point of maximal contractility on the Starling curve.
Oedema may be caused by providing sodium more rapidly than the patient can excrete it. This is especially relevant in patients who have received large amounts of fluid.
Sodium chloride 0.9% contains 154 mmol/L of chloride, compared with about 100 mmol/L in ECF. The hyperchloraemia that may result from large-volume infusion can generate acidosis, due to increased urinary losses of bicarbonate.
Beware: rapid reversal of hyponatraemia with aggressive treatment can cause a severe disease: central pontine myelinolysis. Sodium restoration must be very slow and gradual. Specialist advice should be sought.

Fluid challenge volume should be reduced in patients with heart failure, due to the risk of worsening cardiac contractility.
In renal impairment, it is vital to monitor fluid balance closely to avoid overload.

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