Volume depletion in children

Children are more prone to volume depletion than adults for developmental, physiological, and social reasons. Children have a larger body surface area to mass ratio, a higher percentage of total body water, and faster metabolism, with concomitant increased need for water. Infants and young children depend on others to provide adequate fluid and nutrition.

Volume depletion results from an imbalance between intake of water and solutes, and their loss or redistribution, leading to a deficit in the extracellular fluid compartment. One of several causative conditions may exist.

• Gastrointestinal (GI) losses are the most commonly encountered. Decreased oral intake and excess fluid loss from vomiting, decreased bowel reabsorption, secretory diarrhoea, protein-losing enteropathies, GI bleed, or a combination of all of these factors may be present.

• Haemorrhage results in volume loss from the intravascular space. The volume of blood lost and the time course are particularly important in determining the signs, symptoms, and diagnostic findings.

• Renal losses are seen primarily in diabetic ketoacidosis, where electrolyte disturbances are universal and require meticulous assessment and monitoring. Other renal losses are from diabetes insipidus, diabetes mellitus, nephrotic syndrome, diuretic use, and primary renal tubule concentration defects.

• Skin losses are due to burns, heat stroke, fever, and heavy exercise, particularly when performed in conditions of high humidity and temperature.

• Extracellular space fluid sequestration: redistribution of intravascular fluid into the extracellular space as seen in conditions such as anaphylaxis, sepsis, small-bowel obstruction, congestive heart failure, hepatic failure, and hypoalbuminaemia.

Total body water comprises 2 compartments:

• Intracellular fluid

• Extracellular fluid.

The distribution of total body water in children differs from that in adults. Typically, newborns have about 75% total body water as a percentage of weight. Older children gradually approach the adult proportion of 60%. The reduction in total body water over time is primarily from the extracellular fluid compartment.[5]Carcillo JA, Wheeler DS, Kooy NW, et al. Shock: an overview. In: Wheeler DS, Wong HR, Shanley TP, eds. Pediatric critical care medicine: basic science and clinical evidence. London, UK: Springer-Verlag; 2007:274-298. The intravascular volume, of primary importance in haemodynamics, is about one third that of the total extracellular fluid compartment.

Normally, two-thirds of total body water is in the intracellular fluid compartment and one third in the extracellular fluid compartment (of which one quarter is plasma and one third is total blood volume). Sodium is found predominantly in the extracellular fluid compartment and is the primary solute that regulates the distribution of water between intracellular fluid and extracellular fluid compartments. If sodium is lost, the extracellular fluid compartment, and therefore intravascular volume, decreases. In precise terms, volume depletion, but not dehydration, involves loss of solutes, and therefore of extracellular fluid.

Conversely, dehydration means loss of water. Because cell membranes are for the most part freely permeable to water, dehydration affects the intracellular and extracellular compartments equally and results in symptoms of increased serum tonicity, such as thirst and altered mental status. In dehydration, water is lost in proportion to body water distribution. Plasma volume loss accounts for only one quarter of the extracellular space, which is one third of total body water. Therefore, only one twelfth of total losses in dehydration comes from intravascular plasma volume, explaining the relative preservation of intravascular volume and haemodynamic parameters.

Aetiological classification

• Gastrointestinal (GI) losses: inadequate intake, vomiting, diarrhoea, decreased bowel reabsorption, protein-losing enteropathies, GI bleeding, or any combination of these factors.

• Vascular losses: severe haemorrhage from any area of the body.

• Renal losses: impaired water or solute reabsorption (e.g., diabetes insipidus, diabetes mellitus, renal tubular defects, and diuretic use).

• Increased insensible losses: loss of skin integrity (e.g., burns), fever, hyperthermia, tachypnoea, and physiological states of high metabolic demand.

• Extracellular space fluid sequestration: redistribution of intravascular fluid into the extracellular space as seen in conditions such as anaphylaxis, sepsis, small-bowel obstruction, congestive heart failure, hepatic failure, and hypoalbuminaemia.