third space losses - bowel obstruction;pancreatitis; muscle trauma;burns
sweat losses e.g. endurance exercise
volume depletion is generally diagnosed clinically from the history, physical examination, and laboratory results
clinical signs of volume depletion include
orthostatic decreases in blood pressure and increases in pulse rate, dry mucus membranes, decreased skin turgor
if signs of volume depletion and hyponatraemia
then should be considered hypovolaemic hyponatraemia unless there are alternative explanations for these findings (1)
elevations of urea, creatinine, urea–creatinine ratio, and uric acid level indicate possible volume depletion
however these findings are neither sensitive nor specific, and they can be affected by other factors (eg, dietary protein intake, use of glucocorticoids).
urine sodium excretion is generally more helpful
spot urine [Na+] should be <30 mmol/L in patients with hypovolaemic hyponatraemia unless the kidney is the site of sodium loss
if cllinical assessment is equivocal
a trial of volume expansion can be a useful diagnostic tool (also will be therapeutic if volume depletion is the cause of the hyponatraemia)
a 0.5 to 1 L infusion of isotonic (0.9%) NaCl, patients with hypovolaemic hyponatraemia will begin to correct their hyponatraemia without developing signs of volume overload
in contrast, if SIADH
urine [Na+] will increase but the serum [Na+] will remain unchanged or decrease as the administered water is retained and the sodium load excreted in a smaller volume of concentrated urine (1)
Reference:
(1) hyponatraemia Treatment Guidelines 2007: Expert Panel Recommendations The American Journal of Medicine 2007; 120 (11);S1:S1-S21.
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