CHAPTER 9: Fluids and Electrolytes

BODY COMPOSITION

• Total body water (TBW): 60% of total body weight (higher in newborns, up to 70%)

• Intracellular space: Two-third of TBW or 40% of total body weight

• Extracellular space: One-third of TBW or 20% of total body weight

• Interstitial space: 75% of extracellular fluid (ECF)

• Vascular space or plasma: 25% of ECF

• Predominant electrolytes in intracellular fluid compartment: Potassium and magnesium

• Predominant electrolytes in ECF compartment: Sodium, chloride, and bicarbonate

FLUID THERAPY

MAINTENANCE FLUID THERAPY

Maintenance fluid requirements can be estimated by the Holliday–Segar method. Daily water requirements are calculated based on body weight and the assumption that each kilocalorie of energy metabolized results in the net consumption of 1 mL of water (Table 9-1). Water requirements form the basis for the estimated needs for sodium and potassium (Table 9-2). This method is not recommended for premature infants or term infants younger than 2 weeks of age.

• While several studies argue strongly against the use of hypotonic maintenance fluids across all pediatric populations there are limitations in the evidence including the populations studied, study heterogeneity, and paucity of data on potential adverse events from an increased solute load

• It is equally likely that the administration of an inappropriately high fluid administration rate in the context of non-osmotic antidiuretic hormone release is responsible for most cases of iatrogenic hyponatremia

• The focus on tonicity of maintenance fluid without adequate study of the rate or volume clearly contributes to wide practice pattern variation

• Holliday–Segar maintenance therapy is based on the assumptions that all daily water losses occur as the result of either insensible or urine losses and that all homeostatic mechanisms are intact

• Insensible losses in the absence of conditions leading to increased fluid loss (e.g., fever, hyperventilation, prematurity and low birth weight, skin defects, burns) are usually 400–700 mL/m² body surface area (BSA) (higher in neonates, up to 1150 mL/m² BSA)

• Holliday–Segar method may not be appropriate in children with urine outputs that are abnormally high (e.g., adrenal failure, diuretic exposure) or low (e.g., hypovolemia, syndrome of inappropriate antidiuretic hormone (SIADH) secretion, renal failure, congestive heart failure, nephrotic syndrome, cirrhosis)

• Fever increases metabolic rate and therefore maintenance requirements go up (add about 10% for every °C increase > 38°C for the duration of the febrile episode)

• Burns: Increased needs based on percent body surface area involved

• Oligoanuria/anuria in a euvolemic/hypervolemic child (e.g., established kidney injury): Maintenance fluid should be prescribed by calculating insensible losses and replacing urine output every few hours

• Dextrose-containing intravenous fluids (IVFs) are used to supply a portion of the caloric needs, to prevent hypoglycemia and starvation ketosis

• Stock solutions containing 5% dextrose (D₅) are appropriate for most situations, but 10% dextrose (D₁₀) or higher is also available

• Dextrose concentrations above 12.5% are usually reserved for central catheters because the increased osmolality is irritating to peripheral veins