Читать книгу Disorders of Fluid and Electrolyte Metabolism - Группа авторов - Страница 32

The kidneys have an extraordinary capacity to excrete large amounts of water but there is a limit to this capacity, and when this limit is exceeded patients can develop hyponatremia. The volume of ingested water that will culminate in the development of hyponatremia can be estimated from urinary osmolality and the solute excretion rate by the following calculation:

where UOsm is the urine osmolality (in mOsm/L), USL is the urine solute load (in mOsm/day), and V is the 24 h urine volume (in L).

Thus, the daily urine volume is:

The lowest achievable UOsm is approximately 50 mOsm/L. Given the rather fixed nature of this variable, the daily solute intake becomes a critical factor in determining the daily urine output. Western diet usually contains 600–900 mOsm of solute per day. Assuming an intake at the 900 mOsm level and replacing in equation 4, we have:

Thus, in an individual with normal diluting ability (as reflected in the very low urinary osmolality) and adequate solute intake (as reflected in the excretion of 900 mOsm/day), only an intake of fluids that exceeds 18 L over 24 h would result in the development of hyponatremia.