Perioperative fluids in children and neonatal physiology relevant to surgery

Overview

Children are not small adults. Their fluid needs, body proportions, airway and ability to keep warm all change with age. Most serious fluid errors in children come from giving too much hypotonic fluid, which causes hyponatraemia and brain swelling.

This chapter covers the age terms used in paediatric surgery, how to prescribe intravenous fluids around surgery, and the parts of neonatal anatomy and physiology that change how we operate.

Age terms

Basic numbers to know

Normal vital signs

Why children are given IV fluids: the four reasons

Maintenance fluid calculation

The standard (Holliday–Segar) scheme:

The usual ceiling for an adult-sized child is about 2400 mL/day (100 mL/hour).

Electrolytes in maintenance

A simple rule from paediatric texts: about 3 mmol sodium and 2 mmol potassium per 100 mL of maintenance fluid. Glucose in maintenance gives only a small part (about a fifth) of daily calories, so it is not nutrition.

Tonicity and the danger of hyponatraemia

• Fluids are now judged by their electrolyte content. Glucose is ignored because it is quickly metabolised. So 0.18% saline with 4% glucose and 0.45% saline with 5% glucose count as hypotonic.
• UK guidance is that maintenance fluids should contain sodium 131–154 mmol/L (that is, isotonic).
• Stress (surgery, pain, nausea, trauma, head injury, chest infection) causes ADH release and non-osmotic water retention.
• Brain swelling from low sodium (hyponatraemic encephalopathy) starts at a less severe sodium level in children than in adults, because the brain is large relative to the skull and has little room to swell.

Deaths and brain damage have followed: (1) hypotonic fluid used to resuscitate or replace losses; (2) maintenance given at 3–5 times the real requirement.

Postoperative practice

• Restricting maintenance to about 70% for the first 24 hours after major surgery can be appropriate.
• Check electrolytes daily in any child on IV fluids, and weigh them.
• Hyponatraemia in surgical children is usually from too much water, not too little sodium.

Assessing dehydration

Normal daily water losses are mainly urine, then insensible losses through skin and lungs, with a small amount in stool. Insensible loss rises with fever, fast breathing, radiant warmers and phototherapy.

Fluids in the newborn

Term neonate:

• Days 1–2: 10% glucose at 60 mL/kg/day (newborns are naturally fluid-overloaded and have little need for sodium in the first day).
• Day 2: add sodium (as 0.18% saline) and potassium (0.15% KCl).
• From day 3: about 100–120 mL/kg/day (4–5 mL/kg/hour).
• Preterm or under 2 kg: may need up to 180 mL/kg/day (high skin losses).
• Keep blood glucose above 2.6 mmol/L.

Neonatal anatomy and physiology that affect surgery

Temperature control

Babies lose heat quickly because they have:

• a large surface area to weight ratio;
• a large head (about 20% of body surface, compared with about 9% in adults);
• little subcutaneous fat and immature control of skin blood vessels;
• no effective shivering.

They make heat by non-shivering thermogenesis in brown fat (around the scapulae, neck, kidneys). Cold stress releases noradrenaline, which breaks down fat in brown adipose tissue to produce heat. Hypoxia and beta-blockade reduce this response.

Neonatal hypothermia is a core temperature below 36°C. It leads to hypoglycaemia, metabolic acidosis, apnoea, bradycardia, clotting problems and higher mortality. Preterm and growth-restricted babies are at highest risk.

In theatre: warm room, cover the head, warm all fluids, warm and humidify gases, monitor core temperature, use active warming for long cases. Nurse sick babies in a thermoneutral environment (the temperature at which oxygen use is lowest while body temperature stays normal).

Airway

Body shape and incisions

• Infants have a wide abdomen and shallow pelvis; the liver edge sits lower than the rib margin and much of the bladder lies in the abdomen rather than the pelvis.
• So transverse supraumbilical incisions give better access than vertical midline ones, and the liver and bladder are easily injured.
• The umbilicus sits low, which matters when siting a stoma.
• Flexible ribs rarely break; a rib fracture in an infant should raise concern about abuse.

Glucose, kidneys, liver and blood

• Glucose: small glycogen stores and immature gluconeogenesis cause hypoglycaemia with fasting, sepsis and stress. Monitor glucose around surgery.
• Kidneys: the newborn kidney cannot concentrate or dilute urine well, and handles sodium poorly. Babies dehydrate easily but are also easily overloaded.
• Liver: immature drug metabolism and conjugation (physiological jaundice); low vitamin K stores, so vitamin K is given at birth.
• Blood volume is small: a 3.5 kg baby has under 300 mL of blood, so small losses matter.

Preoperative fasting and perioperative care

Fasting before anaesthesia:

• 2 hours for clear fluids
• 4 hours for breast milk
• 6 hours for solids (and formula)

Avoid long fasting (risk of hypoglycaemia and dehydration). Consent comes from a person with parental responsibility. Use gentle tissue handling and fine sutures; abdominal wounds and skin are usually closed with absorbable sutures. Watch for respiratory depression with opioids.

Quick revision

• Maintenance: 100/50/20 mL/kg/day = 4/2/1 mL/kg/hour.
• Bolus 10–20 mL/kg 0.9% saline; reassess; senior help after about 40 mL/kg.
• Maintenance must be isotonic (sodium 131–154 mmol/L); no 0.18% saline outside neonates.
• Low urine output after surgery alone is not a reason for more fluid.
• Symptomatic hyponatraemia under 125 mmol/L: 3% saline and PICU.
• Neonate day 1: 10% glucose 60 mL/kg/day; day 3 onward 100–120 mL/kg/day.
• Weight 2 × (age + 4); blood volume 80 mL/kg (90 in infants).
• Brown fat makes heat; core under 36°C is hypothermia.
• Cricoid narrowest; straight blade under 1 year.

Written by The House Job from standard references (Bailey & Love; Basis of Paediatrics).

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