Changes in blood pressure, serum potassium and electrolytes with a combination of triamterene and a low dose of chlorthalidone

A controlled study was carried out in patients with mild to moderate hypertension to compare the efficacy and tolerability of chlorthalidone alone and chlorthalidone combined with the potassium-sparing diuretic triamterene. After a 4-week period on placebo, 129 patients were allocated at random to receive 1 tablet daily of either 25 mg chlorthalidone (67 patients) or 25 mg chlorthalidone plus 50 mg triamterene (62 patients) for 10 weeks. Ninety-one patients (48 who had received chlorthalidone alone and 43 the combination) entered the third part of the study. During this 6-week period there was a partial crossover, approximately half continuing with their existing medication and the other half with the alternative treatment. All patients were then treated for a final 2 weeks with placebo. The placebo periods were single-blind, the active treatment periods were double-blind. Patients were seen at regular intervals throughout the trial. At each visit, measurements were taken of blood pressure and pulse rate and routine haematological and biochemical tests made. The results showed that both treatments produced similar, clinically significant reductions in blood pressure within the first 4 weeks of active medication and by the end of the 10-week period 51% of those on chlorthalidone alone and 57% on the combination showed a decrease of at least 10 mmHg or to less than 90 mmHg in standing diastolic pressure. Serum potassium decreased with both therapies but was less with the combination than with chlorthalidone alone, and the incidence of a serum potassium less than 3.5 mmol/l was significantly less with the combination. After crossover, blood pressure control was maintained but serum potassium decreased in patients changed from the combination to chlorthalidone alone and increased in those changed from chlorthalidone alone to the combination. Few adverse effects were reported and were generally mild and similar in frequency with the two therapies.