Remember, in hyperglycemic states, a"factitious hyponatremia" occurs
Remember, in hyperglycemic states, a"factitious hyponatremia" occurs: Excerpt from Avoiding Common Pediatric Errors
Author:
Esther Forrester, MD
What to Do - Make a Decision
To have a thorough understanding of fluid and electrolyte issues, one must
truly understand the body's composition of solutes and water and how equilibrium is maintained in normal circumstances. Water is the most plentiful
compound within the human body. Its percentage changes with age and
body composition. It accounts for 80% of body weight in severely preterm
infants, 70% in term infants, 65% in young children, and approximately
60% in older children and adolescents. There are two compartments for
body water: intracellular and extracellular compartments.
Intracellular water (ICW) accounts for approximately 40% of body
weight, or two thirds of total body water. Extracellular water (ECW) refers
to interstitial and intravascular fluid volumes, collectively. Interstitial fluid is
the water bathing the cells; serum is the water part of blood. ECW accounts
for approximately 20% to 25% of body weight, or one third of total body
water. Water movement between the interstitial and intravascular areas (i.e.,
water movement within ECW) is governed by starling forces. The equation
is as follows:
Fluid movement K*=[(Pc - Pi) - (IIc - IIi)]
where K= capillary filtration coefficient, Pc = capillary hydrostatic pressure,
Pi = interstitial hydrostatic pressure, IIc = capillary oncotic pressure, and
IIi = interstitial oncotic pressure.
The movement of fluid across a capillary membrane is governed by
the permeability of that membrane and the difference in hydrostatic and
oncoticpressuresoneachsideofthemembrane.Whennormalhomeostasisis
disrupted(i.e.,dehydration),thenetmovementoffluidisfromtheinterstitial
compartment into the intravascular compartment in the hopes of regaining
homeostasis and maintaining blood pressure. On the other hand, the net
movement of water is from the intravascular space into the interstitium
when there is intravascular volume overload or hypoalbuminemia.
Movement of water between fluid compartments (i.e., between the intracellular and extracellular space) occurs based on osmotic gradients. The
osmolality of each compartment is reflective of the amounts and types of
solutes in each space. Osmolality is defined as the number of milliosmoles of
solute per kilogram of water. Extracellular homeostasis is regulated by the
kidney.Intracellularhomeostasisisregulatedbyavarietyoftransportmechanisms.Waterflowsfreely(acrosssemipermeablecellmembrane)betweenthe
intracellular and extracellular compartments to equalize osmolality. Sodium
and chloride are the major extracellular solutes. Glucose and urea nitrogen also play a significant role in extracellular osmolality, and the following
equation estimates serum osmolality:
2 × Na+(mEq/L) + [BUN (mg/dL)]/2.8 + [Glucose (mg/dL)]/18
where BUN = blood urea nitrogen.
Based on the above equation, one can see how hypernatremia creates
hyperosmolality; however, hyponatremia is not synonymous with hypo-
osmolality (urea, glucose, or other osmoles contribute to serum osmolality).
Glucose does not cross cell membranes easily. Therefore, a high serum
glucose (remember, serum reflects the extracellular compartment) results
in an osmotic gradient with the movement of fluid from the intracellular
into the extracellular compartment. This leads to a lower serum sodium
concentration and is sometimes referred to as "factitious hyponatremia." In
hyperglycemic states, the movement of fluid results in a 1.6 mEq/L decrease
in levels of serum sodium for every 100 mg/dL elevation in glucose above
normal. The calculation to estimate the "true" serum sodium is:
[Na+] + [Glucose (mg/dL) - 100]/100 × 1.6
Similarly,severehyperlipidemiaandhypernatremiacanresultinapseudohyponatremia. Correction of the hyperglycemia, hyperlipidemia, and/or
hyperproteinemia results in a diffusion of water back into the intracellular compartment and normalization of serum sodium levels. Although the
intracellular and extracellular compartments are separate, they play off of
one another; if the extracellular osmolality increases, water will "exit" from
the intracellular space for example. Cell function is also affected and may
be compromised. Intracellular edema or contraction can result in deadly effects. For example, brain cells have an extremely low tolerance for swelling
or contraction. In diabetic ketoacidosis, there is movement of fluid from the
intracellular into the extracellular space (due to hyperglycemia), which can
lead to cerebral dysfunction. Thankfully, the body has protective mechanisms to protect and restore homeostasis. For example, see Figure 136.1,
which depicts how the body responds to volume loss.
Suggested Readings
Mortiz ML, Ayus JC. Disorders of water metabolism in children: hyponatremia and hyperna
tremia. Pediatr Rev. 2002;23:371–380.
Ruth JL. Wassner SJ. Body composition: salt and water. Pediatr Rev. 2006;27:181–187; quiz
188.
Pictures
Book Source Details
- Book Title: Avoiding Common Pediatric Errors
- Author(s): Anthony D Slonim MD, DrPH; Lisa Marcucci MD
- Year of Publication: 2008
- Copyright Details: Avoiding Common Pediatric Errors, Copyright © 2008 Lippincott Williams & Wilkins.
More About Hyperglycemic Hyperosmolar Nonketotic Syndrome
More Medical Textbooks Online about Hyperglycemic Hyperosmolar Nonketotic Syndrome
Review other book chapters online related to Hyperglycemic Hyperosmolar Nonketotic Syndrome:
Medical Books Excerpts
- Diabetes Mellitus
- "The 10-Minute Diagnosis Manual: Symptoms and Signs in the Time-Limited Encounter" (2000)
- [ read ]
Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Avoiding Common Pediatric Errors
Authors: Anthony D Slonim MD, DrPH; Lisa Marcucci MD
Publisher: Lippincott Williams & Wilkins
Copyright: 2008
ISBN: 0-7817-7489-6
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