splashing

Image courtesy of Pixabay, Public Domain Pictures

Fluid therapy is likely one of the most common interventions performed in pediatrics. Until recently, fluid therapy wasn’t given much thought, “reflecting the long held notion that fluid therapy is straightforward and of little consequence to the patient” (Osteermann, 2012). This post will be the first in a likely 3-part series that looks at fluids and acid-base in the care of pediatric patients. 

Case:

Julius is a 2 year-old boy, who presents to the ED with an acute diarrheal illness, which started 3 days prior (his older brother had a similar illness 1 week prior, and is now well). Initially, Julius was drinking well, however mom notes that over the last day he has stopped drinking and doesn’t appear to be making wet diapers. On your exam, you note him to be fatigued, with dry mucus membranes and vitals are significant for mild tachypnea and moderate tachycardia.  You recognize that he is hypovolemic and want to start fluids. What should you use? (NOTE: in this context, many would make the argument for NG fluids, however in the context of the post we are going to assume that this is not possible).

 

Why do we use fluids?

water-faucet

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  • Fluids are used for 2 main reasons (Davidson et al, 2013):
    • Maintain intravascular volume (“Fill the Tank”)
    • Maintain water and electrolyte homeostasis (e.g. hypo- vs. hypernatremia)

 

Flashback to Med School: Fluid Compartments (Davidson et al, 2013):

  • Remember that “Total Body Water” (TBW) is about 60% of lean body weight
    • Note that neonates generally have much higher TBW (~75% of body weight) and TBW decreases with age

fluid-compartments

Adapted from Davidson et al, 2013

Tonicity vs Osmolality (Khurana, 2013)

salt

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  • Osmolality (Osm) = moles of solute/kg of solvent
    • Depends on number of solute particles, not the specific type of particles
      • E.g. A 1 molar solution of NaCl has an osmotic concentration of 2 Osm, as NaCl will disociate into equal parts Na+ and Cl-
      • The osmolality of human intra- and extracellular fluid is 290 milliosmoles per kg (mOsm/kg)
        • This is largely determined by sodium, chloride, and bicarbonate (and to a lesser degree, glucose and urea).
  • Tonicity
    • Describes the movement of water between 2 compartments between a semi-permeable membrane (osmotic gradient)
      • In human physiology, everything is compared with that of human plasma
    • How does this apply to fluids we infuse?
      • Water will ALWAYS travel along its concentration gradient, from areas of low Osmolality to high Osmolality
        • Hypotonic fluids will result in the net influx of water into cells
        • Hypertonic fluids will draw fluids out of cells
    • As tonicity describes movement of water, it is only influenced by substances that cannot cross membrane
      • Substances that can freely cross membranes are called “ineffective osmoles” (e.g. dextrose, urea)

As such, osmolality does not equal tonicity

  • For example, the fluid D5 1/2NS is both hyperosmolar (owing to the dextrose) and hypotonic (again owing to the dextrose).

What is in the fluid we use?

 Common Fluid Choices and Their “Ingredients”

Human Body

Normal Saline (Isotonic)

Lactated Ringer’s (Isotonic)

D5 and 0.45% NaCl (Hypotonic)

Sodium 140 meq/L 154 130 77
Potassium 4 meq/L 0 4 0
Calcium 9 mg/dl 0 2.7 0
Chloride 102 meq/L 154 109 77
Lactate 0 0 28 0
Osmolality (mOsm) 298 308 273 406

 

How does one choose a fluid?

  • To choose a fluid, you must answer the question: What am I treating?
    • As mentioned earlier, this generally falls along the lines of: do I need to restore intravascular volume and/or do I need to provide daily requirements of water and electrolytes?
  • This question will be explored further on our next post, please stay tuned!

Conclusions

  • Fluid therapy is common in pediatrics, and should be approached like any other medication: Understand indications and any contraindications
  • Fluids can be classified by their osmolality (#moles solute/weight of solvent) and by tonicity (which describes the movement of water between a selectively permeable membrane)
    • Osmolality and Tonicity are related, but not equivalent. This is due to presence of “ineffective osmoles,” which are solutes than can freely cross membranes and therefore do not influence the movement of water
      • Hypotonic fluids will result in the net influx of water into cells
      • Hypertonic fluids will draw fluids out of cells
      • There will be no net movement of water with isotonic fluids

Faculty Reviewer: Lee Polikoff, MD

Sources

http://emcrit.org/pulmcrit/three-myths-about-plasmalyte-normosol-and-lr/ (for table with various [ ]’s

Davidson D et al. “Fluid Management in Adults and Children: Core Curriculum 2014.” Am J Kidney Dis. 2013; 63(4)700-

Edelson JB et al. “Intravenous Fluid Management in the Pediatric Hospital Setting: Is Isotonic Fluid the Right Approach for all Patients.” Current Treatment Options in Pediatrics. 2015; 1:90-99.

Khurana, Indu. Textbook of Human Physiology for Dental Students, 2nd Ed. Elsevier. 2013. p18.

Ostermann M. “The importance of fluid therapy: No longer an innocent bystander.” Monitor. 2012;19(6).