Effective water clearance and tonicity balance: the excretion of water revisited.

OBJECTIVE: To demonstrate (1) that hyponatremia is usually due to an inappropriately low rate of excretion of electrolyte-free water and (2) that the measure "effective water clearance" (EWC) provides better information about renal defence of the body tonicity than does the classic measure free-water clearance, and to provide the rationale for calculating a "tonicity balance," which involves using water and sodium plus potassium intakes and their renal excretion to reveal the basis for changes in body tonicity.
DESIGN: Prospective study. PARTICIPANTS: Four normal subjects with no conditions affecting excretion, 10 patients with advanced congestive heart failure (CHF) and 5 patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). INTERVENTION: Normals and patients were administered a standard water load (20 mL per kg of body weight) during 45 minutes, and blood and urine samples were taken before, during and after the load was given. MAIN OUTCOME MEASURES: Urine and blood sodium and potassium concentrations, osmolar clearance, free-water clearance, electrolyte clearance and EWC.
RESULTS: The water load was excreted rapidly by normals, more slowly by patients with CHF, and not at all by patients with SIADH. The EWC was positive in normals and those with CHF, but negative in those with SIADH. In patients with CHF, the EWC, but not the free-water clearance, helped explain why hyponatremia was corrected after the water load was given.
CONCLUSIONS: In subjects with abnormal water excretion, the EWC provides the physiologic explanation for the renal role in variations in natremia. The authors propose a bedside evaluation of renal water and electrolyte handling that takes into consideration the role of urinary potassium in body tonicity. Changes in body tonicity can be explained by a "tonicity balance," a calculation in which the source and the net balance of sodium, potassium and water are considered.