Anaes · Applied cardiovascular & respiratory physiology
Maternal physiology
Also known as Pregnancy physiology · Cardiovascular changes in pregnancy · Aortocaval compression · MAC in pregnancy · Aspiration risk in pregnancy · Obstetric physiology
Pregnancy transforms every organ system, and the anaesthetist must know the changes to manage the obstetric patient safely. The framework rests on five exam-critical ideas: a hyperdynamic vasodilated circulation with expanded volume and raised cardiac output and low vascular resistance (percentages are textbook furniture, not in any retrieved abstract; spinal haemodynamic responses differ by normotensive vs hypertensive status [3]); rapid desaturation from low oxygen store with high consumption plus difficult airway/aspiration physiology (FRC/VO2 percentages, gas values, and RSI week thresholds are textbook furniture; obstetric arrest needs adapted protocols and simulation [1]); the gravid uterus compresses the vena cava/aorta supine from mid-pregnancy, cutting venous return — left displacement/wedge defends preload (lumbar wedge cut hypotension in multiples [4]; 15-degree tilt did not alter phenylephrine ED50 in elective singletons [2]); and volatile/induction/neuraxial requirements fall with altered pharmacokinetics (MAC/local-anaesthetic percentages are textbook furniture; non-pregnant TCI models mispredict and PBPK needs validation [5]; Eleveld TCI feasible with EEG titration in a 35-week portal-hypertension case [6]). Built on the obstetric cardiac-arrest review (Astete 2026), the phenylephrine tilt-vs-supine ED50 study (Yu 2026), the caesarean-spinal-haemodynamics narrative review (Zagrodnik 2026), the multiples lumbar-wedge study (Kim 2025), the TCI-PBPK review for caesarean (Keresztes 2026), and the Eleveld-TCI portal-hypertension pregnancy case report (Lacassie 2026).
Checked against its sources on 14 Oct 2026
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Red flags
- The pregnant patient desaturates within seconds of apnoea: low store with high consumption shortens safe apnoea — preoxygenation and rapid-sequence readiness are mandatory [1] (exact FRC/VO2 percentages and halving maths are textbook furniture, not in any retrieved abstract).
- Aortocaval compression: the gravid uterus compresses the IVC/aorta supine from mid-pregnancy, reducing venous return and cardiac output — displace the uterus left or use a lumbar wedge [4] (exact week/degree cutoffs are textbook furniture; 15-degree tilt did not alter phenylephrine ED50 in elective singletons [2], while wedge cut hypotension in multiples [4]).
- Volatile/neuraxial requirements fall in pregnancy — less drug is needed for the same effect (exact MAC/local-anaesthetic percentages and progesterone mechanism are textbook furniture, not in any retrieved abstract; TCI models need pregnancy-specific validation [5]).
- Pregnancy is hypercoagulable with raised VTE risk persisting postpartum — VTE prophylaxis is a critical perioperative consideration (exact factor lists, fold-rises, and week durations are textbook furniture, not in any retrieved abstract).
- Plasma volume rises more than red cell mass, producing physiological dilutional anaemia at term (exact haemoglobin cutoffs are textbook furniture, not in any retrieved abstract).
Meet the patient
A term parturient with pre-eclampsia is rushed to theatre for an emergency caesarean under general anaesthesia. She is supine on the table, and the moment induction begins her saturation falls rapidly into the eighties within a minute of apnoea, and her blood pressure drops to severely hypotensive levels. The registrar is reaching for metaraminol — the real problem is that nobody tilted her left.[1]
This is applied maternal physiology failing on two clocks at once: the oxygen clock (low FRC, high consumption, rapid desaturation) and the venous-return clock (aortocaval compression from the gravid uterus on the inferior vena cava). Wind both clocks before you induce or block — tilt the patient left, preoxygenate meticulously, and have vasopressors drawn.[1]
References6ShowHide
- [1]Astete M, et al. Perioperative cardiac arrest in obstetric patients Rev Esp Anestesiol Reanim (Engl Ed), 2026.PMID 41850587
- [2]Yu XQ, et al. The Median Effective Dose of Phenylephrine Infusion for Prevention of Spinal-Induced Hypotension During Cesarean Delivery in the 15° Left-Tilt vs. Supine Positions: A Randomised Control Study Drug Des Devel Ther, 2026.PMID 42137121
- [3]Zagrodnik E, et al. Hemodynamic Changes During Cesarean Section Under Spinal Anesthesia in Normotensive and Hypertensive Pregnant Women-A Narrative Review J Clin Med, 2026.PMID 41899085
- [4]Kim JN, et al. Effectiveness of lumbar wedge in preventing intraoperative hypotension during cesarean delivery in multiple gestation: a retrospective analysis BMC Pregnancy Childbirth, 2025.PMID 41350653
- [5]Keresztes M, et al. Target-Controlled Infusion for Caesarean Delivery Under General Anesthesia: From Conventional Pharmacokinetic Models to Physiologically Based Pharmacokinetic Modeling Life (Basel), 2026.PMID 42195295
- [6]Lacassie HJ, et al. Propofol pharmacodynamics in a pregnant patient with portal hypertension: a case report Int J Obstet Anesth, 2026.PMID 42035514