Anatomical and Physiological Differences: Child vs Adult
Children and neonates in particular are not simply small adults. Every organ system undergoes structural and functional maturation over the first years of life, and these differences directly shape anaesthetic risk, airway management, drug dosing, fluid therapy and perioperative monitoring.
1. Body Size, Proportions & Surface Area
Parameter | Neonate | Adult |
Body weight | ~1/20 of adult weight | Reference |
Body length | ~1/3 of adult length | Reference |
Body surface area (BSA) | ~1/9 of adult BSA | Reference |
Head length (% of body length) | ~25% | ~13% |
Surface area : volume ratio | ~70× greater than adult | Reference |
Body shape | Long torso, short limbs, large occiput | Proportional torso/limbs |
Anaesthetic Implications
● Increased BSA: weight ratio raises evaporative and radiant heat/fluid loss, warm the OR and use warmed fluids/actively warmed surfaces.
● The large head and short neck need a shoulder roll (not a pillow) for optimal airway alignment during laryngoscopy.
● All drug and fluid dosing must be weight-based; small absolute errors translate into large proportional errors.
● Topical/percutaneous drug absorption is proportionally greater; monitor for systemic toxicity with topical agents (e.g., local anaesthetic creams).
2. Airway & Respiratory System
Airway Anatomy
Feature | Neonate / Infant | Adult |
Head & neck | Large head, short neck, poor muscle control | Proportional, good control |
Breathing route | Obligate nose breather | Naso- or oro-breather |
Tongue | Relatively large for oral cavity | Proportional |
Jaw angle | More obtuse | More acute |
Epiglottis | Large, floppy, omega-shaped, cephalad | Flat, flexible, base of tongue |
Larynx level | C3 (newborn) → C4–5 (6 yr) | C5–6 |
Tracheal length | 2–5 cm (average 4 cm) | ~10–12 cm |
Narrowest airway point | Cricoid ring (complete, circular) | Vocal cords (irregular) |
Airway shape | Funnel-shaped | Cylindrical |
1 mm circumferential oedema | 60–70% ↓ cross-sectional area (4 mm cricoid) | ~19% ↓ (20 mm trachea) |
Protective reflexes | Immature — prone to laryngospasm/bradycardia | Mature cough reflex |
Respiratory Function
Variable | Neonate | Adult |
O₂ consumption (ml/kg/min) | 6.4 | 3.5 |
CO₂ production (ml/kg/min) | 6 | 3 |
Alveolar ventilation (ml/kg/min) | 130 | 60 |
Tidal volume (ml/kg) | 6 | 6 |
Respiratory rate (/min) | 35 | 15 |
Vital capacity (ml/kg) | 35 | 70 |
Functional residual capacity (ml/kg) | 30 | 35 |
Anaesthetic Implications
● Position with a shoulder roll (not a pillow) to align the airway axis for laryngoscopy.
● Because the cricoid, not the cords, is narrowest, accept a slight tube leak rather than a tight fit; a small margin of oedema costs far more cross-sectional area in a child than in an adult.
● Obligate nasal breathing makes nasal obstruction and nasogastric/nasal instrumentation higher-risk.
● Immature protective reflexes mean airway stimulation more readily triggers laryngospasm, breath-holding and bradycardia.
● High alveolar ventilation: FRC ratio gives rapid inhalational induction and emergence, but also rapid desaturation — pre-oxygenate thoroughly before any apnoeic period.
● Tidal volume cannot increase much; infants raise minute ventilation mainly by increasing rate, and weak intercostals plus low type I (fatigue-resistant) diaphragm fibres (25–30% vs 55% mature) mean the muscles fatigue quickly.
● Minimise apparatus dead space, resistance and bulk (even a 4 ml mask dead space can double a neonate’s total dead space); provide humidification and heating.
● Avoid gastric distension during mask ventilation; it splints the diaphragm and further compromises ventilation.
3. Cardiovascular System
Transitional Circulation & Hypoxic Response
Feature | Neonate | Adult |
Foetal shunts | Foramen ovale, ductus arteriosus closing | Closed |
Ductus arteriosus | Functional closure 10–15 h; fibrous by 2–3 wk | Closed (ligamentum arteriosum) |
Foramen ovale | Anatomical closure ~6 weeks | Closed |
Response to hypoxia | Pulmonary + systemic vasoconstriction, bradycardia, ↓CO | Vasodilation, tachycardia |
Haemoglobin | 70–90% HbF at birth (P₅₀ 20 mmHg) | HbA (P₅₀ 27 mmHg) |
Myocardial contractile tissue | ~30% less; more connective tissue | Mature contractile mass |
Cardiac output driver | Heart-rate dependent (fixed stroke volume) | Rate + stroke volume dependent |
Haemodynamic Reference Values
Parameter | Neonate | Adult |
Blood volume | 90–100 ml/kg | 60–70 ml/kg |
Heart rate (normal range) | 100–170 /min | 60–100 /min |
Cardiac arrest threshold (HR) | < 85 /min | — |
Systolic BP at birth | 50–65 mmHg | ~120 mmHg |
Cardiac output | 150–200 ml/kg/min (1 wk) | ~70 ml/kg/min |
Anaesthetic Implications
● Keep the heart rate toward the higher end of normal; cardiac output is largely rate-dependent, not stroke-volume-dependent.
● Avoid hypoxia, acidosis, hypothermia, hypoglycaemia, hypocalcaemia and overhydration; each can trigger reversion to a foetal shunt pattern.
● Rigorously exclude air bubbles from all IV lines given the potential for right-to-left shunting even in a structurally normal heart.
● Use systolic BP trends (not absolute values) as a guide to volume status and the need for blood replacement; up to 9 months, BP is normally higher in the legs than the arms.
● Anticipate bradycardia with airway manipulation and treat promptly with oxygen and ventilation.
● Poor baroreceptor/capacitance-vessel control means fluid deficits and blood loss must be identified and corrected early.
4. Central Nervous System
Feature | Neonate | Adult |
Brain weight (% body weight) | ~10% at birth (×2 by 6 mo, ×3 by 1 yr) | ~2% |
Blood–brain barrier | Poorly developed, more permeable | Mature, selective |
Neuromuscular junction | Immature; ACh sensitivity along whole nerve until ~12 wk | Mature; confined to end-plate |
Cerebral capillary architecture | Fragile, thin-walled, right-angle branching | Mature, well-supported vasculature |
Pain pathways | Thalamocortical connections functional from 3rd trimester | Fully mature |
Anaesthetic neurotoxicity risk | Possible window: late pregnancy to ~3 yr (animal data) | Not demonstrated |
Anaesthetic Implications
● Avoid abrupt swings in arterial or venous pressure (rapid osmotic boluses, awake intubation without adequate anaesthesia); the fragile periventricular capillaries predispose to intraventricular haemorrhage, especially in preterm or asphyxiated neonates.
● Provide adequate, titrated multimodal analgesia; neonates and preterm infants do feel pain; under-treatment is not protective.
● A more permeable blood–brain barrier can increase CNS penetration of sedatives/opioids; titrate doses carefully.
● Where clinically feasible, limit unnecessary duration or repetition of anaesthetic exposure given ongoing uncertainty about developmental neurotoxicity.
5. Thermoregulation
Heat Balance
Feature | Neonate | Adult |
Thermal mass/insulation | Decreased | Normal |
Heat production mechanism | Non-shivering thermogenesis (brown fat) | Shivering thermogenesis |
Brown fat reserve | 2–6% body weight (term); minimal in VLBW | None functional |
Effect of general anaesthesia | Blunts the infant’s main heat-generating mechanism | Suppresses shivering response |
Neutral & Critical Temperature
Age group | Neutral temp. (°C) | Critical temp. (°C) |
Preterm neonate | 34 | 28 |
Term neonate | 32 | 23 |
Adult | 28 | 1 |
Anaesthetic Implications
● Maintain OR ambient temperature 25–28°C and use active warming (warmed fluids, forced-air warming); infants cannot compensate for cold stress the way adults do.
● Treat hypothermia as a serious perioperative complication, not a benign finding: it worsens acidosis, coagulopathy, drug clearance, dysrhythmia risk and emergence, and increases infection/transfusion needs.
● Remember GA blunts the infant’s only real heat-generating mechanism (non-shivering thermogenesis); unlike in adults, where shivering is merely suppressed, but the option remains post-op.
● Monitor both core and peripheral temperature; a falling peripheral temperature with a near-normal core suggests poor perfusion, not just cooling; give more fluid.
6. Renal Function, Fluid, Electrolyte & Glucose Homeostasis
Renal & Metabolic Features
Feature | Neonate | Adult |
Renal maturity | Immature until ~18 months | Mature |
GFR / urine concentrating ability | Low / poor | Normal |
Sodium handling | Obligatory salt losers | Conserves sodium well |
Maintenance sodium requirement | 2–3 mEq/kg/day (up to 5 if <30 wk preterm) | ~1–2 mEq/kg/day |
Maintenance potassium requirement | 2–3 mEq/kg/day | ~1 mEq/kg/day |
Glucose requirement to maintain normoglycaemia | 3–5 mg/kg/min (term); 5–6 (preterm) | ~2 mg/kg/min (basal) |
Hypoglycaemia threshold | < 40 mg/dl | < 70 mg/dl (clinical) |
Body Water Distribution (% of Total Body Weight)
Compartment | Premature | Neonate | Infant | Adult |
Extracellular fluid | 50% | 35% | 30% | 20% |
Intracellular fluid | 30% | 40% | 40% | 40% |
Plasma | 5% | 5% | 5% | 5% |
Total body water | 85% | 80% | 75% | 65% |
Anaesthetic Implications
● Calculate maintenance fluids with the Holliday–Segar rule, not adult weight-based estimates: 4 ml/kg/hr (0–10 kg), + 2 ml/kg/hr for each kg 10–20 kg, + 1 ml/kg/hr for each kg over 20 kg.
● Monitor glucose regularly perioperatively, especially after prolonged fasting or interrupted TPN, and in preterm/low-birth-weight infants.
● Expect reduced clearance of renally excreted drugs; adjust dosing intervals accordingly.
● Correct hypocalcaemia and hypokalaemia promptly, particularly after massive transfusion or in sick/preterm neonates.
● Avoid excessive free water or hypotonic fluids; immature kidneys handle water and sodium loads poorly, and overhydration risks pulmonary oedema, PDA reopening and hyponatraemia.
7. Pharmacologic Response to Anaesthetic Drugs
Parameter | Neonate / Infant | Adult |
Inhalational induction/emergence | Rapid (high alveolar ventilation: FRC, high CO) | Slower |
MAC (volatile agents) | ~25% lower <1 mo; peaks ~1–6 mo, then declines | Reference adult MAC |
NMJ sensitivity to non-depolarising relaxants | ~3× more sensitive | Reference |
Volume of distribution (ECF-dependent drugs) | Larger (expanded ECF) | Smaller / reference |
Succinylcholine dose | Higher, up to 2 mg/kg IV | 1–1.5 mg/kg IV |
Cholinesterase activity | Reduced in preterm/term newborn | Normal |
Anticholinesterase reversal | Faster, effective at smaller doses | Standard dosing |
Local anaesthetic therapeutic index | Narrower (larger Vd + immature hepatic clearance) | Wider |
Anaesthetic Implications
● Titrate volatile agents to age-adjusted MAC values, not a single paediatric “standard” dose.
● Expect rapid inhalational induction and emergence — monitor closely for hypotension and bradycardia at high MAC.
● Non-depolarising relaxant doses may need less frequent (not necessarily lower) dosing, since sensitivity is offset by a larger volume of distribution but elimination is prolonged.
● Use higher weight-based succinylcholine dosing when indicated, aware of reduced cholinesterase activity in newborns.
● Reduce anticholinesterase reversal doses accordingly.
● Apply cautious, weight-scaled local anaesthetic infusion limits and monitor for early signs of toxicity.
Key Clinical Pearls
● A 1 mm rim of subglottic oedema removes 60–70% of the cross-sectional airway area in an infant, versus ~19% in an adult; accept a small leak around the tracheal tube rather than a tight fit.
● The infant cannot meaningfully increase tidal volume to raise minute ventilation; watch respiratory rate and fatigue closely, and pre-oxygenate thoroughly before any apnoeic period.
● Cardiac output in the neonate is rate-dependent, not stroke-volume-dependent; bradycardia is a haemodynamic emergency, not just an ECG finding.
● Hypoxia, acidosis, hypothermia, hypoglycaemia and hypocalcaemia can each trigger reversion to a foetal circulation pattern; correct all five aggressively in a deteriorating neonate.
● Hypothermia is not a benign finding — it worsens acidosis, coagulopathy, drug clearance and emergence, and should be actively prevented, not just treated once it occurs.
● Neonates and preterm infants do feel pain and require titrated multimodal analgesia; undertreatment is not protective.
References
Understanding Paediatric Anaesthesia, 4th edition: chapters 1, 3, 4.
Smith’s Anaesthesia for infants and children, 8th edition: chapter 1.