Hyponatremia in congestive heart failure.

Hyponatremia has been identified as a risk factor for increased morbidity and mortality in patients with congestive heart failure (CHF) and other edematous disorders and can lead to severe neurologic derangements. Low cardiac output and blood pressure associated with CHF triggers a compensatory response by the body that activates several neurohormonal systems designed to preserve arterial blood volume and pressure. Hyponatremia in patients with CHF is primarily caused by increased activity of arginine vasopressin (AVP). AVP increases free-water reabsorption in the renal collecting ducts, increasing blood volume and diluting plasma sodium concentrations. Hyponatremia may also be triggered by diuretic therapy used in the management of symptoms of CHF. Hyponatremic disorders occur when the normal ratio of solutes to body water content is altered by parallel changes in serum sodium and osmolality. Hyponatremia is generally defined as a serum sodium ion concentration <135 to 136 mmol/L and can be broadly categorized into 2 types, dilutional or depletional. Dilutional hyponatremia is the most common form of hyponatremia and is caused by excess water retention. Depletional hyponatremia is usually hypovolemic, with an absolute deficiency of water but a relative excess of body water compared with sodium concentration.