WATER

The full-term neonate is 80% water; this figure falls to about 60% by 12 months of age, and then remains almost constant throughout childhood. The intravenous (i.v.) fluid requirements for children in hospital are shown in Tables 99.1 and 99.2. The widely used formula for an active child shown in Table 99.1 was published in 1957,⁶ and some of the measurements of energy expenditure were made almost 100 years ago. The formula is a compromise between the requirements of a fully active child and the much lower requirements at basal metabolic rate – for example, the estimate of 100 ml/kg per day for an active 10-kg child in hospital is double the 50 ml/kg per day needed for a sick child at basal state (Table 99.1).^6,7

Table 99.1 Approximate intravenous fluid requirements for children (ml/kg per day)

Table 99.2 Approximate intravenous fluid requirements for children (ml per hour)

Table 99.3 shows that very ill children in intensive care often need much less water than the ‘standard’ amounts that are often given. For example, if a 15-kg child (‘standard’ maintenance fluid of 50 ml/hour,⁶Table 99.2) sustains a head injury and has evidence of high levels of ADH (maintenance fluid × 0.7),⁸ is ventilated with humidified gas (maintenance × 0.75),⁹ is paralysed (basal state, maintenance × 0.7),⁶ and is maintained at a rectal temperature of 36°C (maintenance – 12%),¹ then his actual full maintenance fluid requirement is (50 × 0.7 × 0.75 × 0.7) – 12% = 16 ml/hour. Even less water should be given initially if the child is overhydrated.

Table 99.3 Modifications to the fluid intakes for active children shown in Table 99.2

ADH, antidiuretic hormone; IPPV, intermittent positive pressure ventilation.

In very small children, all fluid administered has to be taken into account, including the volume of drugs (bicarbonate, dextrose and antibiotics) and ‘flushes’ used to clear i.v. lines (after blood sampling or administration of drugs).

The estimates of water requirements in Tables 99.1–99.3 are only approximate, and water balance must be monitored closely in any child in intensive care. Unfortunately, regular, accurate weighing of very sick children is often impractical, and hydration has to be assessed using:

In a child with oliguria following a severe ischaemic or hypoxic insult (such as birth asphyxia, near drowning or cardiac arrest), it may be helpful to measure the urine sodium concentration.¹⁰ In oliguria due to acute tubular necrosis, where restriction of fluid intake may be necessary, the urine sodium is usually more than 40 mmol/l. In oliguria due to hypovolaemia, the urine sodium is usually less than 20 mmol/l.

SODIUM^11,12

Sodium is predominantly an extracellular ion, so total body sodium is well represented by the serum concentration. It must be interpreted in the context of both total hydration and the relative amounts of water and sodium. Acute changes in serum sodium are usually due to changes in body water rather than changes in body sodium.

In the first 1–2 days of life, small preterm babies often have poor urine output and high transcutaneous fluid losses. They are therefore prone to hypernatraemia and hyperkalaemia, and such infants should usually be given 5% or 10% dextrose without sodium or potassium. From 2 days of age, 2–4 mmol/kg per day of sodium and potassium will usually be sufficient, but much higher intakes of sodium are needed in some preterm neonates due to their impaired renal conservation of sodium.

Hyponatraemia may be due to:¹¹

Acute hyponatraemia causes cerebral oedema, with a grave risk of cerebral herniation and death or severe brain damage.¹³ Hyponatraemia due to sodium deficit should be corrected by careful administration of sodium (Table 99.4). However, hyponatraemia is usually due to water excess rather than sodium deficiency, and this should be treated with restriction of water intake. If symptomatic, it can be corrected by careful administration of 3% sodium at 0.5 ml/kg per hour (Table 99.4), and furosemide 0.5 mg/kg i.v. if there are high ADH levels. Hyponatraemia must be corrected slowly: the serum sodium should increase by no more than 8 mmol/l each 24 hours, and even less in patients with long-standing hyponatraemia.¹¹

Table 99.4 Doses and formulae in paediatric fluid and electrolyte therapy

Hypernatraemia may be due to:¹²

With hypernatraemic dehydration, shock should be treated with boluses of 10 ml/kg of 0.9% saline. The water deficit should then be corrected very slowly using 0.9% saline so that the serum sodium falls no faster than 0.5 mmol/l per hour to prevent cerebral oedema.¹² If salt ingestion causes severe acute hypernatraemia without dehydration, peritoneal dialysis or haemofiltration may be indicated.

POTASSIUM¹⁴

Potassium is predominantly an intracellular ion, so total body potassium is poorly represented by the serum concentration. The concentration of potassium in the serum depends on pH as well as the total body potassium (which is usually about 50 mmol/kg). A child may have hypokalaemia without a deficit of total body potassium in the presence of alkalosis and, conversely, there may be a large deficit of potassium without hypokalaemia in the presence of acidosis. Potassium should rarely be infused at more than 0.3 mmol/kg per hour, and it should not normally be given to a patient with severe oliguria or anuria.