Perioperative hyponatremia has been recognized as a serious in-hospital complication for many years. Because the kidney responds to changes in extracellular fluid tonicity by adjusting water excretion, a defect in any of several key elements of water excretion can lead to water retention and hyponatremia. Most cases of hyponatremia are caused by impaired renal water excretion in the presence of continued water intake.
For the kidney to excrete excess free water and thereby protect the extracellular fluid against hyponatremia, there must be an adequate glomerular filtration rate (GFR), adequate delivery of glomerular filtrate to the diluting segments of the distal nephron, intact tubular diluting mechanisms, and appropriate inhibition of antidiuretic hormone (ADH) synthesis and release. Virtually all of the clinical disorders producing hyponatremia are based on abnormalities of these few mechanisms of water regulation. Finding the reason for impaired renal water excretion is the key to diagnosing the cause of hyponatremia. Impaired renal water excretion may be caused by impaired GFR (renal failure), impaired water delivery to the diluting segments of the distal nephron because of increased proximal reabsorption (decreased extracellular fluid volume and edematous states), impaired renal diluting mechanism (thiazide diuretics), the syndrome of inappropriate ADH (SI-ADH) due to a variety of causes including the perioperative state, and hypothyroidism or adrenal insufficiency.
Any of the states that impair water excretion can produce hyponatremia in a patient with an initially normal serum sodium concentration if sufficient free water is supplied. Therefore, a patient who has one of the conditions listed above, including the perioperative state, may be considered “water intolerant” even if the serum sodium is normal. Such a patient is at risk for developing severe hyponatremia if given hypotonic IV fluids or a large oral water load. An understanding of the basic mechanisms leading to impaired water excretion and “water intolerance” is therefore an important key to avoiding perioperative hyponatremia.