tailieunhanh - Chapter 046. Sodium and Water (Part 10)

Figure 46-2 Many causes of hypernatremia are associated with polyuria and a submaximal urine osmolality. The product of the urine volume and osmolality, ., the solute excretion rate, is helpful in determining the basis of the polyuria (see above). To maintain a steady state, total solute excretion must equal solute production. As stated above, individuals eating a normal diet generate ~600 mosmol/d. Therefore, daily solute excretion in excess of 750 mosmol defines an osmotic diuresis. | Chapter 046. Sodium and Water Part 10 Figure 46-2 Many causes of hypernatremia are associated with polyuria and a submaximal urine osmolality. The product of the urine volume and osmolality . the solute excretion rate is helpful in determining the basis of the polyuria see above . To maintain a steady state total solute excretion must equal solute production. As stated above individuals eating a normal diet generate 600 mosmol d. Therefore daily solute excretion in excess of 750 mosmol defines an osmotic diuresis. This can be confirmed by measuring the urine glucose and urea. In general both CDI and NDI present with polyuria and hypotonic urine urine osmolality 250 mosmol kg . The degree of hypernatremia is usually mild unless there is an associated thirst abnormality. The clinical history physical examination and pertinent laboratory data can often rule out causes of acquired NDI. CDI and NDI can generally be distinguished by administering the AVP analogue desmopressin 10 pg intranasally after careful water restriction. The urine osmolality should increase by at least 50 in CDI and will not change in NDI. Unfortunately the diagnosis may sometimes be difficult due to partial defects in AVP secretion and action. newpage Hypernatremia Treatment The therapeutic goals are to stop ongoing water loss by treating the underlying cause and to correct the water deficit. The ECF volume should be restored in hypovolemic patients. The quantity of water required to correct the deficit can be calculated from the following equation K ccd K u OSMu OSMpjTTKG K oœ K p K WWW. ECItMfOC. CüHt In hypernatremia due to water loss total body water is approximately 50 and 40 of lean body weight in men and women respectively. For example a 50kg woman with a plasma Na concentration of 160 mmol L has an estimated freewater deficit of L 160 - 140 140 x x 50 . As in hyponatremia rapid correction of hypernatremia is potentially dangerous. In this case a sudden decrease in osmolality .