Cationexchange resins

Cation-exchange resins are used to treat hyper-kalaemia by acclerating potassium loss through the gut, especially in the context of poor urine output or prior to dialysis (the most effective means of treating hyperkalaemia). The resins consists of aggregations of big insoluble molecules carrying fixed negative charges, which loosely bind positively charged ions (cations); these readily exchange with cations in the fluid environment to an extent that depends on their affinity for the resin and their concentration. Resins loaded with sodium or calcium exchange these cations preferentially with potassium cations in the intestine (about 1 mmol of potassium per gram of resin); the freed cations (calcium or sodium) are absorbed and the resin plus bound potassium is passed in the faeces. The resin does not merely prevent absorption of ingested potassium, but it also takes up the potassium normally secreted into the intestine and ordinarily reabsorbed.

In hyperkalaemia, oral administration or retention enemas of a polystyrene sulphonate resin may be used. A sodium phase resin (Resonium A) should obviously not be used in patients with renal or cardiac failure as sodium overload may result. A calcium phase resin (Calcium Resonium) may cause hypercalcaemia and should be avoided in predisposed patients, e.g. those with multiple myeloma, metastatic carcinoma, hyperparathyroidism and sarcoidosis. Enemas should be retained for as long as possible, although patients rarely manage for as long as necessary (at least 9h) to exchange potassium at all available sites on the resin.

Alteration of urine pH by drugs is sometimes desirable. The most common reason is in the treatment of poisoning (a fuller account is given on p. 155). A summary of the main indications appears below.

Alkalinisation of urine

• increases the elimination of salicylate, phenobarbitone and chlorophenoxy herbicides, e.g. 2,4-D, MCPA

• reduces irritation of an inflamed urinary tract

• discourages the growth of certain organisms, e.g. Escherichia coli.

The urine can be made alkaline by sodium bicarbonate i.v., or by potassium citrate by mouth. Sodium overload may exacerbate cardiac failure, and sodium or potassium excess are dangerous when renal function is impaired.

Acidification of urine

• is used as a test for renal tubular acidosis

• increases elimination of amphetamine, methylene dioxymethamphetamine (MDMA or 'Ecstasy'), dexfenfluramine, quinine and phencyclidine, although it is very rarely needed.

Oral NH4C1, taken with food to avoid vomiting, acidifies the urine. It should not be given to patients with impaired renal or hepatic function. Other means include arginine HC1, ascorbic acid and CaCl2 by mouth.

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