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Neonatal and paediatric surgical physiology — fluids, kidneys, temperature and nutrition
A newborn is not a "small adult": it has more body water, kidneys that still neither concentrate nor filter well, rapid heat loss and a cardiac output that depends on heart rate — so fluids are calculated by weight, the infant is kept warm, and bradycardia in a newborn is a direct danger.
Why it matters in the exam
Body water: in a term newborn about 75%–80% of body weight, and more in a preterm infant. Blood volume in a term newborn is about 80–85 mL/kg. Maintenance fluids are calculated by Holliday-Segar: 100 mL/kg per day for the first ten kilograms, 50 mL/kg per day for the next ten and 20 mL/kg per day beyond that (per hour: 4, 2 and 1 mL/kg). Isotonic maintenance fluids are now preferred because of the danger of hyponatraemia. The urine output target in a child is 1–2 mL/kg/hour.
Kidneys: GFR is low at birth, rises quickly in the first weeks and reaches adult values only around two years of age; concentrating ability and the ability to excrete a sodium load are limited. Creatinine in the first days reflects the mother's creatinine. In children GFR is estimated by the bedside Schwartz formula: 0.413 times height in cm divided by creatinine in mg/dL. Heart: the newborn's stroke volume is relatively fixed, so cardiac output depends mainly on heart rate — bradycardia (in response to hypoxia or a vagal stimulus) immediately lowers cardiac output and needs treatment of the cause, above all oxygenation and ventilation.
Temperature: the newborn loses heat quickly because of a large surface area, thin skin and little insulating fat; it produces heat mainly by burning brown fat, without shivering. Hypothermia raises oxygen consumption, causes acidosis and hypoglycaemia and worsens coagulation, so the theatre is warmed and warming blankets are used. Nutrition: a newborn needs about 100–120 kcal/kg per day; when enteral feeding is not expected soon, parenteral nutrition is started, and over time it can cause liver disease (cholestasis). Fasting before anaesthesia in infants (ASA): clear fluids up to two hours, breast milk 4 hours, formula 6 hours.
Key points
- Maintenance fluids by 4-2-1.
- GFR reaches adult values around two years.
- Cardiac output in the newborn depends on heart rate.
- Hypothermia is dangerous — keep warm.
- Fasting: clear fluids two hours, breast milk 4 hours, formula 6 hours.
Summary table
| Topic | In the newborn and infant | Surgical meaning |
|---|---|---|
| Maintenance fluids | 4, 2 and 1 mL/kg/hour (Holliday-Segar) | Isotonic fluids; monitor sodium |
| Kidneys | Low GFR, adult values around two years | Care with drugs; Schwartz to estimate GFR |
| Heart | Output depends on heart rate | Bradycardia = falling output; treat oxygenation and ventilation |
| Temperature | Rapid heat loss, brown fat without shivering | Warm the room and fluids |
| Fasting before anaesthesia | Clear fluids two hours, breast milk 4 hours, formula 6 hours | Less unnecessary fasting — less hypoglycaemia |
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Sources
- Holcomb and Ashcraft's Pediatric Surgery, 7th edition (2019) — Fluid, Electrolyte and Nutritional Support of the Pediatric Surgical Patient
- KDIGO Clinical Practice Guideline for the Evaluation and Management of CKD, 2024 — Estimating GFR in children (bedside Schwartz)
- ASA Practice Guidelines for Preoperative Fasting, 2017 — Fasting intervals for infants
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.