Fluid volume deficit (Hypovolemia)
Fluid volume deficit (FVD) occurs when loss of extracellular fluid volume exceeds the intake of fluid. It occurs when water and electrolytes are lost in the same proportion as they exist in normal body fluids, so that the ratio of serum electrolytes to water remains the same. Fluid volume deficit (hypovolemia) should not be confused with the term dehydration, which refers to loss of water alone with increased serum sodium levels. FVD may occur alone or in combination with other imbalances. Unless other imbalances are present concurrently, serum electrolyte concentrations remain essentially unchanged.
FVD results from loss of body fluids and occurs more rapidly when coupled with decreased fluid intake. FVD can develop from inadequate intake alone if the decreased intake is prolonged. Causes of FVD include abnormal fluid losses, such as those resulting from vomiting, diarrhea, GI suctioning, and sweating, and decreased intake, as in nausea or inability to gain access to fluids (Beck, 2000). Additional risk factors include diabetes insipidus, adrenal insufficiency, osmotic diuresis, hemorrhage, and coma. Third-space fluid shifts, or the movement of fluid from the vascular system to other body spaces (eg, with edema formation in burns or ascites with liver dysfunction), also produce FVD. Clinical Manifestations FVD can develop rapidly and can be mild, moderate, or severe, depending on the degree of fluid loss. Important characteristics of FVD include acute weight loss; decreased skin turgor; oliguria; concentrated urine; postural hypotension; a weak, rapid heart rate; flattened neck veins; increased temperature; decreased central venous pressure; cool, clammy skin related to peripheral vasoconstriction; thirst; anorexia; nausea; lassitude; muscle weakness; and cramps.
Assessment and Diagnostic Findings
Laboratory data useful in evaluating fluid volume status include BUN and its relation to the serum creatinine concentration. A volume-depleted patient has a BUN elevated out of proportion to the serum creatinine level (a ratio greater than 20:1). The cause of hypovolemia may be determined through the health history and physical examination. The BUN can be elevated due to dehydration or decreased renal perfusion and function. Also, the hematocrit level is greater than normal because the red blood cells become suspended in a decreased plasma volume. Serum electrolyte changes may also exist. Potassium and sodium levels can be reduced (hypokalemia, hyponatremia) or elevated (hyperkalemia, hypernatremia).
- Hypokalemia occurs with GI and renal losses.
- Hyperkalemia occurs with adrenal insufficiency.
- Hyponatremia occurs with increased thirst and ADH release.
- Hypernatremia results from increased insensible losses and diabetes insipidus.