Neonatal physiology

9 min read Last updated April 8, 2026

Surgical approach (not applicable)

  • Neonatal physiology is not an operation; no surgical steps apply.

Anaesthetic management (contextual implications of neonatal physiology)

  • Type of anaesthesia
    • Most neonatal surgery: GA with controlled ventilation; regional techniques often adjuncts (caudal/epidural/spinal for selected cases).
    • Higher risk of postoperative apnoea in ex-preterm infants: consider regional-only (e.g., spinal) for brief lower abdominal surgery where appropriate.
  • Airway device
    • Usually cuffed or uncuffed ETT (microcuff increasingly used); SGA only for selected short, low-risk cases with experienced teams.
    • Neonates desaturate quickly: preoxygenate, minimise apnoea time, confirm tube position carefully (short trachea).
  • Duration
    • Procedure-dependent; physiology drives risk over time: heat loss, hypoglycaemia, fluid shifts, and ventilation/perfusion instability worsen with longer cases.
  • How painful
    • Neonates mount robust stress responses; provide multimodal analgesia (opioid-sparing where possible) while balancing risk of apnoea/respiratory depression.
  • Physiology-driven priorities
    • Prevent hypothermia (active warming, warmed fluids, humidified gases).
    • Maintain oxygenation and avoid large swings in PaCO2 (risk of IVH in preterm; pulmonary vascular reactivity).
    • Support circulation: limited ability to increase stroke volume; cardiac output is rate-dependent; avoid bradycardia and high afterload.
    • Glucose and fluids: avoid hypoglycaemia; careful isotonic fluids; monitor electrolytes (esp. Na+, Ca2+).

Definitions and key concepts

  • Neonate: birth to 28 days (term ~37–42 weeks gestation). Preterm: <37 weeks; extremely preterm <28 weeks.
  • Physiology is dominated by transition from fetal to neonatal life: lung aeration, fall in PVR, rise in SVR, closure of fetal shunts, thermoregulation, and metabolic adaptation.

Cardiovascular physiology

  • Fetal-to-neonatal transition
    • First breaths + oxygenation → ↓ PVR; cord clamping → ↑ SVR.
    • Functional closure: foramen ovale (minutes–hours), ductus arteriosus (hours–days; anatomical closure weeks).
    • Persistent pulmonary hypertension of the newborn (PPHN): failure of PVR to fall → right-to-left shunt (via PFO/PDA) → hypoxaemia.
  • Myocardial structure and function
    • Less compliant ventricle (fewer contractile elements, more connective tissue) → limited ability to increase stroke volume; CO mainly increased by heart rate.
    • Higher baseline HR; relatively fixed SV; sensitive to changes in preload/afterload.
    • Immature sympathetic innervation; relatively greater reliance on circulating catecholamines; vagal responses prominent → bradycardia with hypoxia, airway stimulation, opioids.
  • Blood pressure and organ perfusion
    • Lower MAP than older children; autoregulation may be limited (esp. preterm) → risk of IVH with hypotension/hypercapnia.
    • High PVR states (hypoxia, acidosis, hypothermia, high airway pressures) can reopen fetal shunts and worsen oxygenation.
  • Haematology relevant to circulation
    • Higher Hb at birth; HbF predominates (left-shifted O2 dissociation curve) aiding placental uptake but reducing tissue unloading.
    • Physiological anaemia of infancy: Hb falls over weeks (earlier and lower nadir in preterm).

Respiratory physiology

  • Airway anatomy
    • Large occiput, relatively large tongue, high/anterior larynx (C3–4), narrow nasal passages → airway obstruction risk.
    • Short trachea; main bronchi angles more similar than adults → endobronchial intubation risk with small movements.
  • Lung mechanics and FRC
    • Low FRC due to compliant chest wall and less elastic recoil; FRC maintained by laryngeal braking, rapid RR, and tonic diaphragmatic activity.
    • Anaesthesia abolishes these mechanisms → atelectasis and rapid desaturation; PEEP often required.
  • Oxygen consumption and ventilation
    • High metabolic rate: VO2 ~6–8 mL/kg/min (adult ~3) → limited apnoea tolerance.
    • Higher minute ventilation per kg; small reductions in ventilation quickly cause hypercapnia.
  • Control of breathing and apnoea
    • Immature respiratory control (esp. preterm): periodic breathing; blunted CO2 response; hypoxia may cause initial hyperventilation then depression.
    • Postoperative apnoea risk increased in ex-preterm infants (risk relates to post-conceptual age and anaemia).
  • Surfactant and compliance
    • Surfactant deficiency in preterm → RDS: low compliance, atelectasis, V/Q mismatch; sensitive to oxygen toxicity and barotrauma.

Thermoregulation

  • High heat loss risk: large surface area:mass, thin skin, little subcutaneous fat, limited shivering.
  • Heat production mainly via non-shivering thermogenesis (brown fat) driven by catecholamines and thyroid hormone; limited in preterm/sick neonates.
  • Cold stress consequences: ↑ VO2, metabolic acidosis, hypoglycaemia, ↑ PVR → worsened hypoxaemia/PPHN; impaired coagulation.

Renal physiology and fluids/electrolytes

  • Renal blood flow and GFR are low at birth; tubular function immature → limited ability to concentrate urine and handle sodium/water loads.
  • Total body water high (term ~75%, preterm up to ~85%); ECF proportion high → prone to rapid fluid shifts.
  • Sodium handling: limited reabsorption early; risk of hyponatraemia with hypotonic fluids; use isotonic maintenance strategies per local policy.
  • Potassium: levels may be higher early; monitor in sick/preterm neonates, haemolysis, transfusion, acidosis.

Hepatic physiology, glucose, and bilirubin

  • Glycogen stores limited (especially preterm/IUGR); high glucose utilisation → hypoglycaemia risk with fasting, sepsis, hypothermia.
  • Immature gluconeogenesis/ketogenesis early; stress response may cause hyperglycaemia with dextrose infusions—monitor and titrate.
  • Bilirubin: increased production + immature conjugation → physiological jaundice; risk of kernicterus with very high unconjugated bilirubin (esp. haemolysis, prematurity).
  • Drug metabolism: reduced phase I/II capacity at birth; protein binding reduced (low albumin, competing bilirubin) → higher free fraction of some drugs.

Central nervous system and neuromuscular physiology

  • Blood-brain barrier more permeable; immature autoregulation (esp. preterm) → vulnerable to fluctuations in BP, PaCO2, oxygenation.
  • Intraventricular haemorrhage risk in preterm: avoid hypoxia, hypercapnia, rapid volume expansion, large BP swings.
  • Neuromuscular junction immature: increased sensitivity to non-depolarising NMBAs; succinylcholine use requires caution (bradycardia, hyperkalaemia risk in undiagnosed myopathy).

Gastrointestinal physiology and aspiration risk

  • Lower oesophageal sphincter tone may be reduced; gastric emptying variable; higher aspiration risk in some neonates (e.g., obstruction, reflux).
  • Congenital GI obstruction often associated with fluid/electrolyte derangements and significant gastric losses; requires decompression and resuscitation.

Pharmacology and anaesthetic implications

  • MAC: volatile requirement is highest in early infancy then decreases; neonates may have slightly lower MAC than 1–6 months (agent-dependent). Dose to effect.
  • IV induction: larger volume of distribution for water-soluble drugs; reduced clearance; titrate carefully (propofol can cause significant hypotension).
  • Opioids: increased sensitivity and risk of apnoea; consider shorter-acting agents and regional/local techniques to reduce opioid dose.
  • Local anaesthetics: reduced protein binding and immature metabolism → higher toxicity risk; strict mg/kg dosing and incremental administration.

Haemostasis and transfusion considerations

  • Vitamin K-dependent factors low at birth; vitamin K prophylaxis routine; platelet function may differ but bleeding risk usually not increased in healthy term neonates.
  • Small blood volume (~80–90 mL/kg term; higher in preterm) → small absolute losses are significant; meticulous sampling and blood conservation.
  • Transfusion risks: hypocalcaemia (citrate), hyperkalaemia (stored blood), hypothermia, dilutional coagulopathy; use warmed blood and monitor electrolytes.
Describe the key physiological changes at birth and their relevance to anaesthesia.

Structure your answer: (1) circulation, (2) respiration, (3) thermometabolic changes, (4) implications.

  • Circulation: cord clamping → ↑ SVR; lung aeration/oxygenation → ↓ PVR; closure of PFO and PDA (functional then anatomical).
  • Respiration: first breaths clear lung fluid and establish FRC; surfactant reduces surface tension; anaesthesia reduces FRC and promotes atelectasis → consider PEEP.
  • Thermometabolic: high heat loss; reliance on brown fat non-shivering thermogenesis; cold stress increases VO2 and can worsen PPHN.
  • Anaesthetic relevance: avoid hypoxia/acidosis/hypothermia (increase PVR), avoid large PaCO2 swings, maintain HR (CO is rate-dependent), meticulous temperature and glucose control.
Why do neonates desaturate rapidly during apnoea? Give physiological reasons and practical implications.

Examiners want: oxygen store vs oxygen consumption, FRC, airway closure/atelectasis, and what you do about it.

  • Low oxygen stores: low FRC (compliant chest wall, reduced elastic recoil) and anaesthesia further reduces FRC.
  • High oxygen consumption: VO2 ~6–8 mL/kg/min (about double adult per kg).
  • Atelectasis and airway closure occur easily → V/Q mismatch and shunt.
  • Practical: effective preoxygenation, gentle mask ventilation if needed, minimise intubation time, use appropriate PEEP, avoid excessive airway pressures that raise PVR.
Explain why cardiac output in neonates is described as 'rate-dependent'. What are the anaesthetic consequences?

Link myocardial compliance, stroke volume limitation, and vagal bradycardia triggers.

  • Neonatal ventricles are less compliant with limited ability to augment stroke volume; SV is relatively fixed → CO increases mainly via HR.
  • Bradycardia (often vagal, commonly due to hypoxia) can cause rapid fall in CO and hypotension.
  • Anaesthetic consequences: prevent/treat hypoxia promptly; avoid deep vagal stimulation; consider atropine in high-risk situations per local practice; avoid drugs causing profound myocardial depression or afterload increase.
What factors increase pulmonary vascular resistance (PVR) in neonates and why does this matter perioperatively?

This is a common FRCA viva theme: triggers of PVR and shunt physiology.

  • PVR increases with hypoxia, hypercapnia, acidosis, hypothermia, pain/stress, and high mean airway pressure/overdistension.
  • Raised PVR can promote right-to-left shunting via PFO/PDA → refractory hypoxaemia (PPHN physiology).
  • Management principles: optimise oxygenation/ventilation, avoid acidosis, maintain normothermia, use gentle ventilation strategies, adequate analgesia/sedation, consider pulmonary vasodilators (e.g., iNO) in specialist settings.
Discuss thermoregulation in neonates and the consequences of perioperative hypothermia.

Aim for mechanisms of heat loss, heat production, and systemic consequences.

  • Heat loss: convection, radiation, conduction, evaporation; neonates have large surface area:mass and thin skin.
  • Heat production: mainly non-shivering thermogenesis (brown fat); limited in preterm/sick neonates.
  • Consequences: ↑ VO2, hypoglycaemia, metabolic acidosis, ↑ PVR/PPHN risk, coagulopathy, delayed drug metabolism, delayed recovery.
  • Prevention: warm theatre, forced-air warming, warmed fluids, humidified warmed gases, plastic wrap/hats (esp. preterm), minimise exposure.
Outline renal differences in neonates and how these affect perioperative fluid prescribing.

Focus on GFR/tubular immaturity, water balance, sodium handling, and monitoring.

  • Low renal blood flow and low GFR at birth; immature tubules → limited concentrating ability and sodium handling.
  • High total body water and ECF fraction → sensitive to fluid overload and dehydration.
  • Avoid hypotonic fluids (hyponatraemia risk); use isotonic strategies and add glucose as required; monitor UO, Na+, glucose, weight, and acid-base.
Explain the oxygen dissociation curve in neonates (HbF) and the implications for oxygen delivery.

Examiners like: left shift, loading vs unloading, and what changes it.

  • HbF predominance causes a left shift (higher O2 affinity) aiding placental loading but reducing tissue unloading at a given PaO2.
  • As HbA replaces HbF over months, curve shifts right; anaemia and low CO can still limit delivery.
  • Remember modifiers: acidosis, hypercapnia, temperature, 2,3-DPG (lower in HbF) affect unloading; perioperative hypothermia and alkalosis can further impair tissue delivery.
Postoperative apnoea in ex-preterm infants: what are the risk factors and how would you manage perioperatively?

This is a recurring FRCA viva topic; give risk factors, mitigation, and monitoring plan.

  • Risk factors: lower post-conceptual age, history of apnoea, anaemia, ongoing respiratory disease, sepsis, hypothermia, opioids/sedatives.
  • Mitigation: minimise opioids (regional/local), maintain normothermia, correct anaemia if appropriate, consider caffeine in specialist protocols, avoid residual anaesthetic/sedative effects.
  • Management: plan postoperative monitoring (cardiorespiratory and oximetry) for an appropriate duration; consider HDU/NICU depending on risk and comorbidity.
How does neonatal pharmacology differ from adults? Discuss distribution, metabolism, and protein binding with examples relevant to anaesthesia.

Aim for a structured answer: Vd, clearance, protein binding, and clinical consequences (dose/interval/toxicity).

  • Distribution: higher total body water and lower fat (term) → larger Vd for water-soluble drugs; dosing may require different loading doses.
  • Metabolism: immature hepatic enzymes (phase I/II) and reduced hepatic blood flow in illness → reduced clearance and prolonged half-life for many drugs.
  • Protein binding: lower albumin and competition with bilirubin → increased free fraction (e.g., local anaesthetics) → toxicity risk; dose strictly mg/kg.
  • Renal excretion: low GFR/tubular immaturity → prolonged elimination of renally cleared drugs.

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