💧 Paediatric Fluid Resuscitation & Electrolytes
Weight-based fluid calculations, dehydration assessment, DKA management, and electrolyte disturbances — the essential framework for the MRCS exam and clinical practice.
Why Children Are Different
Children are not small adults. Their physiology demands a fundamentally different approach to fluid management. The key differences driving every calculation are:
The Three Physiological Differences That Matter Most
1. Higher metabolic rate — per unit body weight, children consume more oxygen and produce more CO₂. They therefore require proportionally more water for metabolic reactions and insensible losses (breathing, skin).
2. Greater surface area-to-weight ratio — a neonate has 3× the body surface area per kg of an adult. This massively amplifies insensible fluid losses through the skin and respiratory tract.
3. Immature renal concentrating ability — neonates cannot concentrate urine as effectively as adults. They need more water to excrete the same solute load. This means they are more vulnerable to both dehydration AND fluid overload.
Body Water Distribution
Total body water as a proportion of body weight decreases with age:
| Age Group | Total Body Water (% body weight) | Extracellular Fluid (%) | Intracellular Fluid (%) |
|---|---|---|---|
| Premature neonate | 80–90% | 50–60% | 30% |
| Term neonate | 75–80% | 40–45% | 35% |
| Infant (1–12 months) | 65–75% | 30–35% | 40% |
| Child (1–12 years) | 60–65% | 25–30% | 35–40% |
| Adult | 55–60% | 20% | 40% |
The Hyponatraemia Risk — The 2007 NICE Guidance Change
Prior to 2007, hypotonic fluids (0.18% NaCl + 4% dextrose, often called “one-fifth normal saline”) were routinely used in paediatric maintenance. This caused iatrogenic hyponatraemia and deaths. NICE 2015 now recommends isotonic saline (0.9% NaCl or Hartmann’s) as the maintenance fluid of choice for all children >28 days old.
The mechanism: ADH secretion (from pain, surgery, nausea, illness) causes free water retention. If hypotonic fluid is given simultaneously, serum sodium plummets. The brain swells. Children die or sustain permanent neurological injury.
The Key Fluid Types
| Fluid | Na⁺ (mmol/L) | K⁺ (mmol/L) | Cl⁻ (mmol/L) | Glucose (g/L) | Tonicity |
|---|---|---|---|---|---|
| 0.9% NaCl (“Normal Saline”) | 154 | 0 | 154 | 0 | Isotonic |
| Hartmann’s (Lactated Ringer’s) | 131 | 5 | 111 | 0 | Isotonic |
| 0.45% NaCl + 5% Dextrose | 77 | 0 | 77 | 50 | Hypotonic |
| 0.18% NaCl + 4% Dextrose (“1/5 saline”) | 31 | 0 | 31 | 40 | Hypotonic — avoid routine use |
| 5% Dextrose | 0 | 0 | 0 | 50 | Hypotonic (effectively) |
| 10% Dextrose | 0 | 0 | 0 | 100 | Hypertonic |