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Fluid management is essential in caring for the critically ill child and can be challenging, given the alteration of normal fluid and electrolyte physiology in critical illness.
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Fluid and electrolyte derangements are common in critically ill children and either can be related to the patient's underlying disease or can be a consequence of therapy. Estimating fluid and electrolyte needs in the critically ill child requires an understanding of normal basal metabolism as well as knowledge of fluid homeostasis. Furthermore, the derangements that occur in critical illness must be appreciated in order to adapt fluid and electrolyte management appropriately. It is important to note that many of the principles that guide fluid and electrolyte management in children are based on normal physiologic principles, which can be severely altered in critical illness.
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Body Water Distribution and the Intracellular and Extracellular Compartments
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Total body water (TBW) accounts for 50% to 80% of body weight and varies by gender and age.1-4 Water is largely distributed into the intracellular fluid (IFC) compartment and the extracellular fluid (ECF) compartment. The ICF compartment comprises approximately two-thirds of TBW, or 40% of total body weight, while the ECF compartment comprises approximately one-third of TBW, or 20% of total body weight. The ECF is further divided into interstitial fluid and plasma.2-4 The ECF decreases over time in infants, reaching 30% at 1 year of age, and approaching 20%, or adult values, in early childhood. The ICF volume remains relatively constant throughout infancy to adulthood.1-2
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The ICF and ECF space each have a primary solute that is generally restricted to that compartment by a semipermeable membrane. This primary solute helps regulate the osmolality of the compartment and the movement of water between spaces. The primary solute in the ICF compartment is potassium (K+), while the primary solute in the ECF is sodium (Na+). Potassium (K+) and magnesium (Mg+) are the major cations in the ICF, while proteins and organic phosphates are the major anions. The primary cation in the ECF is sodium (Na+), while the major anions are (Cl) and Display Formula.1-2
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Lastly, TBW content is inversely correlated with adipose tissue content and decreases with age. Total body water accounts for 75% of body weight in the term infant. By 6 months of age, TBW content decreases to 65%, and by 1 year of age, TBW approaches 60% of body weight.1-4 On average, males have slightly higher TBW (as a percentage of body weight) than females, which is attributed to the slightly higher percentage of adipose tissue in females. Understanding the relationship between TBW content and weight and the electrolyte components in each space are important when calculating fluid deficits.