Invited CommentaryAcute hospital-induced hyponatremia in children: A physiologic approach
Section snippets
Maintenance therapy
Standard intravenous maintenance therapy7 is designed to replace ongoing physiological water losses when oral intake is suspended. These are net insensible (∼35 mL) and urinary (∼65 mL) water losses; they are indexed to metabolic rate expressed in 100 kcal/day increments. Consequently, the average allowance for water as maintenance therapy is 100 mL/100 kcal/dayb
Hypovolemia
Hypovolemia is defined as impaired circulation in which organ and tissue perfusion are reduced. Hypovolemia develops with: (1) a contraction of ECF as occurs from net loss of body fluid, including diarrhea and vomiting, renal or cerebral salt wasting, (2) extravasation of plasma into tissues as occurs with infection, burns, or trauma (“third space loss”), or (3) dislocation of blood volume into capacitance vessels as occurs with septic shock or loss of muscle tone. These conditions, acting
Brain injury
Brain injury because of hyponatremia is described as water intoxication. It results when retention of free water rapidly decreases serum sodium; brain volume increases because water rapidly equilibrates across cell membranes and solutes do so only slowly. Solutes of both cell and ECF are diluted; both cell and ECF volumes increase. Given the unique anatomic and physiologic features of the brain, these increases in brain volume cause intracranial pressure to rise. If swelling is sufficient,
Severe hypovolemia without hyponatremia
An extreme example of hypovolemia results in patients with severe burns from extensive extravisation of plasma into interstitial fluid, compromising circulation and perfusion. Plasma ADH and other vasoactive agents dramatically rise; urine flow decreases. For example, a 40% skin surface burn results in huge transfers of plasma fluid and albumin into both the burn site and normal interstitium. Current treatment is to give 80 mL/kg (2 mL/% skin surface burn) of isotonic saline or Ringer's
Gravity and hypovolemia
Gravity contributes to dislocation of blood volume. Normal young men standing upright with muscles fully relaxed develop syncope and hypotension within 15 minutes. Blood is sequestered in the lower limbs; plasma ADH increases dramatically. When the men lie down, eliminating the gravity effect, these signs abate.26 Children lying supine for extended periods lose muscle tone; this has a comparable effect, pooling blood dorsally. Anesthesia and sedation exaggerate this effect. Rapid ECF expansion
Protocol for avoiding hypovolemia
The first priority in planning fluid therapy for acutely ill children is to determine if hypovolemia is likely. If so, rapid rehydration/restoration therapy is indicated. If hypovolemia is marginal to moderate, 20 to 40 mL/kg of isotonic saline or Ringer's solution, given in 2-4 hours, should suffice. If hypovolemia is more severe, 40 to 80 mL/kg may be needed. Therapy is indexed to body weight, not to metabolic rate, because blood and ECF vary with body weight.
Surgical patients are subject to
Risks of using isotonic saline for maintenance therapy
The two proposals5., 6. to use isotonic saline as maintenance therapy, in some cases for extended periods, was recommended without describing any clinical experience. This, we believe, incurs unwarranted risks.27., 28. It is comparable to the use of hypotonic saline as initial or replacement therapy without clinical trials. Several potential problems can be anticipated. Giving isotonic saline at maintenance rates to hypovolemic patients unnecessarily delays the correcting of hypovolemia;
Summary
Children admitted with acute disease or for surgery incur a significant risk of hyponatremia if even subtle hypovolemia is present and is not corrected before they are given hypotonic saline as maintenance therapy. The risk is exaggerated if maintenance therapy is given in excess of recommendations. This risk is greatly reduced (1) by initially giving children with even subtle hypovolemia, 20 to 40 mL/kg of isotonic saline in 2-4 hours and (2) by correctly calculating maintenance therapy,
References (29)
- et al.
Intravenous fluids for seriously ill children: time to reconsider
Lancet
(2003) - et al.
Antidiuretic hormone excess
J Pediatr
(1979) - et al.
Risks of administration of hypotonic fluids for pediatric patients in ED and prehospital settings
Am J Emerg Med
(2000) - et al.
Dissociation of osmoregulation from plasma arginine vasopressin levels following thermal injury in childhood
Burns
(2000) - et al.
Endothelial and vascular smooth muscle function in sepsis
J Crit Care
(1994) - et al.
Intravenous fluids for seriously ill children
Lancet
(2004) - et al.
Hyponatremia associated with pneumonia or bacterial meningitis
Arch Dis Child
(1985) - et al.
Acute hyponatremia in children admitted to hospital: retrospective analysis of factors contributing to its development and evolution
BMJ
(2001) - et al.
Hyponatremia in premature babies and following surgery in older children
Acta Pediatr Scand
(1987) - et al.
Hyponatremia and death or permanent brain damage in healthy children
BMJ
(1992)