free water deficit calculation
- related: Nephrology
- tags: #literature #nephrology
Free water deficit is calculated by taking into account the patient’s total body water (as a function of sex and weight) and plasma Na+:
For this patient, the deficit is [(172/140) – 1] × 81 × 0.6 = 11.1 L. The degree of free water deficit is often underestimated, leading to slower-than-desired correction.
Hypernatremia is seen commonly in the ICU, often in patients who cannot access free water because of environmental, neurologic, or psychiatric reasons. Alternatively, those with diabetes insipidus (neurogenic or nephrogenic) may lose free water excessively as can those with sustained osmotic diuresis. Hypernatremia signals a decrease in total body water in relation to total body sodium. Although often seen in association with hypovolemia, hypernatremia can also be seen in euvolemic and hypervolemic states.
There are no randomized clinical trials to guide how rapidly or fully the water deficit should be corrected. For adults with chronic hypernatremia (defined as present for longer than 48 h), a rate of correction not to exceed 12 mEq/L/day (12 mmol/L/day) is often recommended out of concern for cerebral edema. However, cerebral edema due to correction of chronic hypernatremia has never been reported in adults and seems isolated to pediatric patients. Moreover, very slow correction of chronic hypernatremia has been associated with excess mortality. Therefore, acute hypernatremia should be corrected rapidly. For most patients, hypernatremia can be treated with enteral or IV fluids containing free water, although desmopressin may be needed in those with deficiency of the antidiuretic hormone. Although calculations of free water deficit provide a guide to therapy, they should not be relied on alone. Rather, plasma [Na+] should be measured periodically and treatment modified accordingly.1