Comparative effect of diuretics on renal water excretion in hyponatraemic oedematous disorders

1. The effects of intravenous chlorthiazid and frusemide on urinary osmolality were compared in 19 hyponatraemic oedematous patients. 2. Frusemide (1 mg/kg) caused production of a dilute urine (urine/plasma osmolality ratio, Uosm./Posm., 1.64-0.84, P less than 0.01) whereas chlorthiazid (10 mg/Kg) did not (Uosm./Posm, 1.54-1.34, not significant. 3. The Osmolar clearance (Cosm.) was higher after frusemide than after chlorthiazid (11.45 vs 4.99 ml/min, P less than 0.01). When the doses of frusemide (0.25-0.5 mg/Kg) and chlorthiazid (20 mg/Kg) were chosen to give a similar Cosm. (7.25 vs 7.48 ml/min, not significant), the Uosm./Posm. was still lower after frusemide (2.20-1.00, P less than 0.001) than after chlorthiazid (1.75-1.26, P less than 0.01). 4. Exogenous vasopressin did not increase the low Uosm./Posm. after frusemide (1.00-1.00, not significant) but increased the ratio after chlorthiazid (1.34-1.68, p less than 0.01). 5. These results indicate that frusemide, but not chlorthiazid, lead to the excretion of a dilute urine in hyponatraemic oedematous patients. This dilution is not due to a greater solution excretion but is associated with a resistance to the action of vasopressin.