Hyponatremia in the setting of ECF volume expansion is usually associated with edematous states, such as congestive heart failure, hepatic cirrhosis, and the nephrotic syndrome. These disorders all have in common a decreased effective circulating arterial volume, leading to increased thirst and increased AVP levels. Additional factors impairing the excretion of solute-free water include a reduced GFR, decreased delivery of ultrafiltrate to the diluting site (due to increased proximal fractional reabsorption of Na+ and water), and diuretic therapy. The degree of hyponatremia often correlates with the severity of the underlying condition and is an important prognostic factor. Oliguric acute. | Chapter 046. Sodium and Water Part 6 Hyponatremia in the setting of ECF volume expansion is usually associated with edematous states such as congestive heart failure hepatic cirrhosis and the nephrotic syndrome. These disorders all have in common a decreased effective circulating arterial volume leading to increased thirst and increased AVP levels. Additional factors impairing the excretion of solute-free water include a reduced GFR decreased delivery of ultrafiltrate to the diluting site due to increased proximal fractional reabsorption of Na and water and diuretic therapy. The degree of hyponatremia often correlates with the severity of the underlying condition and is an important prognostic factor. Oliguric acute and chronic renal failure may be associated with hyponatremia if water intake exceeds the ability to excrete equivalent volumes. Hyponatremia in the absence of ECF volume contraction decreased effective circulating arterial volume or renal insufficiency is usually due to increased AVP secretion resulting in impaired water excretion. Ingestion or administration of water is also required since high levels of AVP alone are usually insufficient to produce hyponatremia. This disorder commonly termed the syndrome of inappropriate antidiuretic hormone secretion SIADH is the most common cause of normovolemic hyponatremia and is due to the nonphysiologic release of AVP from the posterior pituitary or an ectopic source Chap. 334 . Renal free-water excretion is impaired while the regulation of Na balance is unaffected. The most common causes of SIADH include neuropsychiatric and pulmonary diseases malignant tumors major surgery postoperative pain and pharmacologic agents. Severe pain and nausea are physiologic stimuli of AVP secretion these stimuli are inappropriate in the absence of hypovolemia or hyperosmolality. The pattern of AVP secretion can be used to classify SIADH into four subtypes 1 erratic autonomous AVP secretion ectopic production 2 normal regulation
