Anaes · Applied cardiovascular & respiratory physiology
Neonatal physiology
Also known as Neonatal circulation · Fetal circulation · Circulatory transition at birth · Ductus arteriosus · Foramen ovale · Neonatal pharmacokinetics
The neonate undergoes the most dramatic physiological transition of any human — from placental dependence to independent air-breathing — and the anaesthetist managing the newborn must understand the fetal circulation, the transition at birth, and the pharmacokinetic and thermoregulatory differences that make neonatal anaesthesia a distinct discipline. The framework rests on five exam-critical ideas: fetal circulation shunts blood past the lungs via the foramen ovale (right-to-left atrial) and the ductus arteriosus (pulmonary artery to aorta) because pulmonary vascular resistance is high; at birth the first breaths lower pulmonary vascular resistance, pulmonary blood flow rises, left atrial pressure rises (closing the foramen ovale), and the ductus arteriosus closes (oxygen and falling prostaglandins) — converting to the adult pattern; neonatal thermoregulation is precarious (large surface area to mass ratio, brown fat non-shivering thermogenesis, limited glycogen); neonatal pharmacokinetics differ (higher total body water, lower protein binding, immature hepatic metabolism, more permeable blood-brain barrier); and the neonatal airway and cardiovascular response to hypoxia (bradycardia, not tachycardia) make rapid desaturation and bradycardia during induction the defining hazards. Built on the paediatric sedation behaviour study (Nikula 2026), the dexmedetomidine pharmacokinetics study (Tsai 2026), the paediatric TIVA review (Quintao 2026), the maternal-neonatal circulation study (Piani 2026), the neonatal resuscitation review (Krishnaprasadh 2026), and the neonatal left-ventricle study (Sehgal 2023).
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Red flags
- Neonates desaturate and bradycardise with hypoxia (unlike adults who tachycardise) — preoxygenation and rapid intubation are essential; the first sign of hypoxia is bradycardia, which must be treated with 100 percent oxygen, not adrenaline.
- The ductus arteriosus closes functionally within 72 hours of birth (oxygen and falling prostaglandins) but may reopen in hypoxia, acidosis or sepsis — reverting to fetal circulation (right-to-left shunting through the reopened ductus), causing profound hypoxaemia.
- Neonatal thermoregulation: large surface area to mass ratio (4x adult), limited subcutaneous fat, brown fat non-shivering thermogenesis (the only thermogenic mechanism in the neonate), and limited glycogen — hypothermia causes apnoea, metabolic acidosis and pulmonary vasoconstriction (worsening hypoxaemia).
- Neonatal drug pharmacokinetics: higher total body water (larger Vd for water-soluble drugs), lower plasma protein binding (more free drug), immature hepatic Phase II conjugation (slower clearance, longer drug action), and a more permeable blood-brain barrier (greater CNS sensitivity).
- The neonatal airway differs: large occiput (flexes the neck), relatively large tongue, narrow nasal passages (obligate nose breather), and a larynx positioned higher (C3 to C4, descending to C6 with age) — requiring different laryngoscopy technique and tube sizing.
Meet the patient
A 3-week-old ex-preterm infant born at 32 weeks presents for inguinal hernia repair. The parents are anxious. You explain that this baby has four physiological clocks the adult does not: the heart rate is the cardiac output, the oxygen store is tiny, the cold is dangerous, and the liver clears drugs slowly.[5]
Every neonatal anaesthetic is built on anticipating those four clocks. Preoxygenation is limited because the FRC is small and closing capacity encroaches. Bradycardia from brief hypoxia crashes cardiac output. Hypothermia raises pulmonary vascular resistance and oxygen consumption. And drug effects last longer because hepatic conjugation is immature.[5]
References6ShowHide
- [1]Nikula A, et al. Behavioral Changes in Children Are Uncommon 4 Weeks After Procedural Sedation and Analgesia With Intranasal Dexmedetomidine and Nitrous Oxide Paediatr Neonatal Pain, 2026.PMID 42311919
- [2]Tsai YF, et al. Dexmedetomidine Dosing Strategies in Sedation and Anesthesia: Pharmacokinetics, Safety, and Clinical Applications - A Narrative Review Drug Des Devel Ther, 2026.PMID 42232093
- [3]Quintão VC, Carlos RV, von Ungern-Sternberg BS Update on total intravenous anesthesia in children Curr Opin Anaesthesiol, 2026.PMID 41817234
- [4]Piani F, et al. Linking maternal and neonatal circulation in preeclampsia Am J Physiol Heart Circ Physiol, 2026.PMID 41525138
- [5]Krishnaprasadh D. Pediatric and Neonatal Resuscitation 2026.PMID 34283435
- [6]Sehgal A, et al. The left ventricle in well newborns versus those with perinatal asphyxia, haemodynamically significant ductus arteriosus or fetal growth restriction Transl Pediatr, 2023.PMID 37814715