BMJ  2006;332:853 (8 April), doi:10.1136/bmj.332.7545.853-a

Letter

Disorders of sodium balance

Iatrogenic hyponatraemia—water overload and not enough potassium

EDITOR—Reynolds et al dealt with the important, everyday clinical problem of disorders of sodium balance and clearly stated that most cases of hyponatraemia are iatrogenic.¹ However, they did not clarify the reasons for iatrogenic hyponatraemia: usually a combination of water overload and inadequate potassium replacement. Too many doctors believe that a normal 24 hour requirement for water is 3 l or more, so they prescribe that volume to patients receiving intravenous fluids. If the intravenous fluid used is saline, volume overload can occur rapidly; if it is dextrose solution, the dextrose is metabolised, leaving the water to dilute extracellular fluid.

More importantly, this practice is often accompanied by a failure to provide adequate potassium replacement. The nephron is dedicated to the reabsorption of sodium, but only by exchange excretion of either potassium or hydrogen. The potassium is leached from intracellular fluid to be replaced by sodium to maintain the balance between cations and anions. The hydrogen ions are generated by carbonic anhydrase, with the bicarbonate diffusing "back" into the plasma. The overall result is a decrease in plasma sodium and an increase in plasma bicarbonate, with the treatment being proper replacement of potassium.

Arguably, the real failure of the paper was to ignore (like too many clinicians) the basic physiology of water, sodium, and potassium balance. These balances are very closely inter-related and should have been set out at the start of the review. Should they be printed at the top of every fluid balance chart to remind prescribers of the relevant normal physiology?

Ninewells Hospital, Dundee DD1 9SY j.a.w.wildsmith{at}dundee.ac.uk

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Competing interests: None declared.

References

1. Reynolds RM, Padfield PL, Seckl JR. Disorders of sodium balance. BMJ 2006;332: 702-5. (25 March.)[Free Full Text]

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Relevant Article

Disorders of sodium balance

Rebecca M Reynolds, Paul L Padfield, and Jonathan R Seckl
BMJ 2006 332: 702-705. [Extract] [Full Text] [PDF]