Critical Fluid and Electrolytic Abnormalities in Clinical Practice
Marc A. Pohl
POINTS TO REMEMBER:
Loss of water from the body should be referred to as dehydration. Loss of salt and water from the body should be termed extracellular fluid volume contraction or volume contraction.
It is important to recognize the difference between dehydration and volume contraction: Normal saline, not dextrose 5% concentration in water (D5W), is the treatment for extracellular fluid (ECF) volume contraction. Fluid resuscitation for patients who are truly dehydrated should consist primarily of hypotonic fluids (e.g., D5W or D5W with normal saline).
The serum sodium concentration is regulated primarily by water balance, not by the total amount of sodium in the body.
Patients with congestive heart failure, nephrotic syndrome, and decompensated liver disease with ascites are examples of sodium excess states with increased ECF volume, but these patients may have normal, expanded, or contracted plasma volumes.
A useful approach to the patient with hyponatremia attempts to place the hyponatremic patient into one of three broad categories based on the history, physical examination, and basic laboratory tests:
Hyponatremia with hypovolemia (inadequate circulation):
With renal salt retention (urinary sodium concentration <10 to 15 mEq/L):
Gastrointestinal losses, profuse sweating.
With urinary sodium wasting (urinary sodium >20 mEq/L):
Adrenal insufficiency, diuretics, renal salt wasting, as in chronic renal failure or distal renal tubular acidosis.
Hyponatremia with edema (urinary sodium concentration usually <10 mEq/L):
Congestive heart failure, hepatic cirrhosis with ascites, nephrotic syndrome.
Hyponatremia without evidence of hypovolemia or edema:
Syndrome of inappropriate secretion of antidiuretic hormone (SIADH), reset osmostat, drugs.
SIADH is a relatively rare condition characterized by:
Hyponatremia with corresponding hypoosmolality of the serum and ECFs.
Continued renal excretion of sodium.
Absence of clinical evidence of fluid volume depletion or edema.
Normal renal function.
Normal adrenal and thyroid function.
Osmolality of the urine greater than that appropriate for the concomitant osmolality of the plasma or urine that is less than maximally dilute.
Most patients with SIADH have a low or low-normal serum uric acid level.
As a general principle, the treatment for hyponatremia depends on the underlying cause:
If there is contracted ECF volume, the depleted volume should be replenished with sodium and water, usually in the form of normal saline.
If an edematous state exists, water should be restricted; in most circumstances, both salt and water should be restricted. If congestive heart failure is the reason for the hyponatremia, water restriction, loop diuretics, and cardiotonic measures, such as the use of an angiotensin-converting enzyme (ACE) inhibitor, should alleviate the hyponatremia, especially if cardiac function improves.Related posts:
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