TREATING THE SYNDROME OF INAPPROPRIATE ADH SECRETION WITH ISOTONIC SALINE

It has been widely accepted that there is little use for saline treatm ent in the syndrome of inappropriate secretion of ADH (SIADH). However , having observed that most SIADH patients increased their plasma sodi um (PNa) after 2 I isotonic saline over 24 h, we investigated whether urine osmolality or the sum of urinary sodium and potassium (UNa + K) predicted this response, in 17 consecutive patients with chronic SIADH . The initial measure of urinary sodium plus potassium (UNa+K t(0)) wa s weakly correlated to the change in PNa (DPNa) after infusion (r = - 0.51; p< 0.05), while initial urine osmolality (UOSM t(0)) was a much better predictor (y= - 0.024x+ 12.90; r = - 0.81; p<0.001). The lack o f predictive value for UNa+K t(0) was probably because urine electroly te concentrations were not maximal for the corresponding initial UOSM. This reflects differences in salt intake between the patients. The th eoretical maximal value for UNa for K t(0) (fh max UNa+K t(0)) for a g iven UOSM t(0), was as good a predictor as UOSM t(0) (fh max UNa + K v s. DPNa: r=-0.81; p<0.001). A theoretical model describing the effect of 2 I isotonic saline infusion on DPNa as a function of UNa+K, produc ed values comparable to those observed in our patients. Only 6/17 pati ents, those with UOSM>530 mOsm/kg, had their hyponatraemia aggravated by 2 I isotonic saline. Many SIADH patients have lower UOSM; in most s uch patients, 2 1 of isotonic saline will improve PNa.