Water imbalance is common among the critically ill. The excessive increases or decreases in body water can be lethal. There are numerous pathologic and iatrogenic causes for water imbalance, the most troublesome being disorders of antidiuretic hormone (ADH) secretion. ADH is also known as arginine vasopressin (AVP). The prime determinant of free water excretion is the regulation of urinary flow by circulating levels of arginine vasopressin (AVP) in plasma. Disorders of water metabolism can be divided into hyperosmolar disorders, in which there is a deficiency of body water relative to body solute, and hypo-osmolar disorders, in which there is an excess of body water relative to body solute. The classical hyperosmolar disorder is diabetes insipidus (DI), and the classical hypo-osmolar disorder is the syndrome of inappropriate antidiuretic hormone secretion (SIADH). DI can result from either inadequate AVP secretion (central or neurogenic DI) or inadequate renal response to AVP (nephrogenic DI). On the other hand, the dilutional hyponatraemia of SIADH develops due to persistent detectable or elevated plasma AVP concentrations.
