Fluid and electrolyte management in children.

The body is comprised of solids and water; the proportion of water changes according to age. Total body water (TBW) is divided between the intracellular fluid (ICF) and the extracellular fluid (ECF), separated by cell membrane. The ECF can be further subdivided into water within the intravascular space (IVS) and the interstitial space (ISS), separated by capillary membrane. The biggest change in water content takes place during intra-uterine gestation and the first 3 years of life (Table 1). At birth, a higher percentage of water is in the ECF, unlike older children and adults where the higher proportion is intracellular. Despite having a similar osmolality (290–320 mosmoles), the electrolyte content of the ECF and ICF are very different. The ECF contains a high concentration of sodium, bicarbonate and chloride, with a low concentration of potassium, calcium and magnesium. In contrast, the ICF has a high concentration of potassium and magnesium and a low concentration of sodium and bicarbonate. Molecular movement takes place between the various fluid compartments by one of three mechanisms: (i) simple diffusion through the lipid membrane (oxygen, carbon dioxide); (ii) movement through protein channels (sodium, potassium, calcium); and (iii) facilitated diffusion through transmembrane carrier proteins (glucose, amino acids). Water transport between the ECF and ICF is by osmosis; it will move according to the number of osmotically active particles on each side of the membrane. Water movement between the ISS and the IVS also depends on hydrostatic forces within the capillary which forces water at the arterial end out into the ISS. The high protein content of the IVS exerts a colloid osmotic pressure which pulls fluid back into the IVS at the capillary venous end. The normal functioning of this system depends on the structural integrity of the capillary membrane and the removal of protein from the ISS by the lymphatic system. The ECF volume is controlled by manipulation of its major cation, sodium. The sensors in this system are carotid baroreceptors, atrial stretch receptors and the juxtaglomerular apparatus adjacent to afferent renal arterioles. A reduction in ECF volume causes non-osmotic ADH release, stimulation of the sympathetic nervous system to cause vasoconstriction, release of atrial natriuretic peptide and activation of the renin-angiotensin-aldosterone system. Control of osmolality is by varying water intake and excretion. The sensors concerned are osmoreceptors found in the hypothalamus. A rise in ECF osmolality triggers the sensation of thirst and causes release of ADH, which increases water re-absorption at the renal collecting ducts. Young children and debilitated patients may not be able to respond to the sensation of thirst. At birth, the glomerular filtration rate (GFR) is only 25–30% that of the adult. However, by 4 weeks of age, the kidney achieves 90% maturity. The neonatal kidney has a poor concentrating ability and cannot excrete or conserve sodium as well as an older child, causing an increased obligatory water loss. Neonates can increase their urine Fluid and electrolyte management in children