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* please explain why SIADH is in euvolemic state?
  lala_usmle - 08/01/09 10:58
  I am just thinking why SIADH is in euvolemic state instead of hypervolemic state?
Anyone please explain it to me. Thanks very much!
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* Re:please explain why SIADH is in euvolemic state?
  drsab - 08/01/09 16:16
  The continued presence of ADH as in SIADH with water intake causes retention of ingested water. While a large fraction of this water is intracellular, the extracellular fraction causes volume expansion. Volume receptors are activated and peptides (eg, atrial natriuretic peptide from heart ) are secreted, which causes natriuresis with some degree of accompanying kaliuresis and diuresis.As a general rule ANP tends to antagonize the effect of ADH and AT-II. Thus, these patients are euvolemic or are slightly volume-expanded.

The plasma sodium concentration is the primary osmotic determinant of ADH release. However, in persons with SIADH, a non-physiologic secretion of ADH results in enhanced water reabsorption, leading to dilutional hyponatremia. Sodium excretion is intact, and the amount of sodium excreted in the urine varies with diet. Ingestion of water is an essential prerequisite to the development of dilutional hyponatremia; regardless of cause
(pitutary ,pulmonary)...... hyponatremia does not occur if water is restricted.
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