Chapter 99 Paediatric fluid and electrolyte therapy
Children need a much higher intake of water and electrolytes per kilogram of body weight than adults; this makes children more susceptible to dehydration if they have abnormal losses of water or a reduced intake. On the other hand, an inability to excrete a water load, due to immature kidneys (in neonates) or high levels of antidiuretic hormone (ADH), means that children can easily be given too much water intravenously. There are good reviews of fluid and electrolyte therapy in paediatric and neonatal intensive care.1–5
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)
| Active child in hospital | |
| < 10 kg | 100 ml/kg per day |
| 10–20 kg | 1000 ml + (50 ml/kg per day for each kg over 10 kg) |
| > 20 kg | 1500 ml + (20 ml/kg per day for each kg over 20 kg) |
| Sick, but not intubated | |
| < 10 kg | 50 ml/kg per day |
| 10–20 kg | 500 ml + (30 ml/kg per day for each kg over 10 kg) |
| > 20 kg | 800 ml + (20 ml/kg per day for each kg over 20 kg) |
| Intubated, with humidified inspired gases | |
| < 10 kg | 35 ml/kg per day |
| 10–20 kg | 350 ml + (20 ml/kg per day for each kg over 10 kg) |
| > 20 kg | 550 ml + (12.5 ml/kg per day for each kg over 20 kg) |
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
| Decrease | Adjustment |
|---|---|
| Humidified inspired air | × 0.75 |
| Basal state (e.g. paralysed) | × 0.7 |
| High ADH (IPPV, brain injury) | × 0.7 |
| Hypothermia | − 12% per °C |
| High room humidity | × 0.7 |
| Renal failure | × 0.3 (+ urine output) |
| Increase | |
|---|---|
| Full activity + oral feeds | × 1.5 |
| Fever | + 12% per °C |
| Room temperature > 31°C | + 30% per °C |
| Hyperventilation | × 1.2 |
| Neonate preterm (1–1.5 kg) | × 1.2 |
| radiant heater | × 1.5 |
| phototherapy | × 1.5 |
| Burns first day | + 4% per 1% area burnt |
| subsequently | + 2% per 1% area burnt |
ADH, antidiuretic hormone; IPPV, intermittent positive pressure ventilation.
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.
SODIUM11,12
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
| Albumin 20% | undiluted: 2–4 ml/kg 5% in 5% dextrose or saline: 10–20 ml/kg |
| Bicarbonate (number of mmol of deficit) | under 5 kg: base excess × wt(kg) × 0.5 (give half of this) over 5 kg: base excess × wt(kg) × 0.3 (give half of this) |
| Blood volume | 85 ml/kg in neonate 70 ml/kg in older children |
| Calcium | chloride 10% (0.7 mmol/ml Ca++): maximum 0.2 ml/kg i.v. stat, requirement 1.5 ml/kg per day gluconate 10% (0.22 mmol/ml Ca++): maximum 0.5 ml/kg i.v. stat, requirement 5 ml/kg per day |
| Dextrose | for hypoglycaemia: 1 ml/kg 50% dextrose i.v. in neonate: 4 mg/kg per min (2.4 ml/kg per hour 10% dextrose) day 1, increasing to 8 mg/kg per min (up to 12 mg/kg per min with hypoglycaemia) for hyperkalaemia: 0.1 U/kg insulin and 2 ml/kg 50% dextrose i.v. stat |
| Magnesium | chloride 0.48 g/5 ml (1 mmol/ml Mg++): 0.4 mmol (0.4 ml)/kg per dose slow i.v. 12-hourly sulphate 50% (2 mmol/ml Mg++): 0.4 mmol (0.2 ml)/kg per dose slow i.v. 12-hourly |
| Mannitol | 0.25–0.5 g/kg per dose i.v. (1–2 ml/kg of 25%) 2-hourly, provided serum osmolality < 330 mosm/kg |
| Packed cells | 10 ml/kg raises Hb 3 g%, 1 ml/kg raises PCV 1%. |
| Potassium | maximum 0.3 mmol/kg per hour, requirement 2–4 mmol/kg per day, 1 g KCl = 13.3 mmol K+. Hyperkalaemia: see dextrose |
| Sodium | depletion: 3% saline at 0.5 ml/kg per hour (maximum Na rise 8 mmol/l in 24 hours); requirement 2–6 mmol/kg per day, 1 g NaCl = 17.1 mmol/Na+ |
| Urine | minimum acceptable is 0.5–1.0 ml/kg per hour |
PCV, packed cell volume.
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.
