Tonicity balance: a tool to explain the basis for the dysnatremias

Derangements in plasma sodium concentration are best analyzed by carefully determining entries and output: for water, sodium and potassium, that is, b y calculating a tonicity balance. Five clinical hyponatremic examples are d iscussed: the beer drinker has a severe deficit in total body sodium; the e lderly female patient treated with a thiazide needs to be firstly repleted in potassium; the hypertensive transplanted patient with a multidrug treatm ent requires an increase of the urinary electrolyte-free water which has be en obtained by the oral administration of urea; the post-operative hyponatr emic cases (cases 4 and 5) are complex and involve a desalination phenomeno n. Close observation, repeated determinations of electrolytes in plasma, ur ine and entries, together with measurements of water input and output, will allow the tonicity balance of the patient to be understood and thus occasi onal tragedies such as observed in cases 4 and 5 to be prevented.