Neonatal physiology

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: &lt,37 weeks, extremely preterm &lt,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.

Test yourself…

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 &#039,rate-dependent&#039,. 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.