Paediatrics: Age-Specific Normals and Compensatory Limits
Paediatrics tests growth and development, neonatal adaptation, fluid and electrolyte management, common infections, congenital heart disease and immunisation. Almost every item turns on an age-specific normal value or on the child's limited reserve: infants compensate well until they decompensate abruptly, and a normal blood pressure in a child may already represent hypotension.
Core rules
- Development. Gross motor, fine motor, language and social milestones are assessed together; loss of a previously acquired skill is always abnormal and prompts investigation.
- Neonatal jaundice. Jaundice in the first 24 hours is pathological. Plot bilirubin against age in hours and gestational age; treat with phototherapy and use exchange transfusion for severe or refractory cases.
- Fluids. Maintenance uses the 4-2-1 rule (4 mL/kg/h for the first 10 kg, 2 for the next 10, 1 thereafter). Assess dehydration by weight loss, cap refill, mucous membranes and urine output; oral rehydration solution is first-line for mild to moderate dehydration.
- Congenital heart disease. Ductal-dependent lesions present with collapse as the duct closes; start prostaglandin E1 and avoid high inspired oxygen in suspected ductal-dependent pulmonary flow lesions.
- Immunisation. Live vaccines are avoided in significant immunodeficiency and during pregnancy; catch-up schedules follow age-specific rules.
Worked example
A 5-week-old boy has projectile non-bilious vomiting after feeds, weight loss, visible gastric peristalsis and an olive-shaped mass. Electrolytes show pH 7.52, bicarbonate 34 mmol/L, potassium 3.1 mmol/L, chloride 88 mmol/L, with paradoxically acidic urine. This is hypertrophic pyloric stenosis: vomiting loses hydrochloric acid, producing hypochloraemic hypokalaemic metabolic alkalosis, and the kidney conserves sodium by excreting hydrogen ions, which acidifies the urine. Confirm with ultrasound, resuscitate with isotonic saline plus potassium chloride, then perform pyloromyotomy. The reasoning: correct the electrolyte and volume deficit before anaesthesia because alkalosis increases the risk of postoperative apnoea.
Traps that cost marks
- Giving bicarbonate, which worsens the alkalosis of pyloric stenosis.
- Operating before correcting hypokalaemia, hypochloraemia and dehydration.
- Calling jaundice at 36 hours "physiological" without checking the hour-specific bilirubin level.
- Missing ductal-dependent heart disease in a collapsing neonate.