Anaes · Obstetric anaesthesia
Maternal physiology of pregnancy for the anaesthetist
Also known as Pregnancy physiology anaesthesia · Aortocaval compression · FRC pregnancy desaturation · MAC reduction pregnancy
Exam-exhaustive maternal physiological changes by system with anaesthetic implications: aortocaval compression, reduced FRC and rapid desaturation, airway oedema, aspiration risk, hypercoagulability, reduced MAC and local anaesthetic requirements, and positioning/preoxygenation practice.
Practise this topic
On this page
Study tools
Your progress
Saved on this device.
Target exams
Red flags
- From about 20 weeks, avoid flat supine without left lateral tilt or uterine displacement — aortocaval compression reduces venous return.
- Pregnant patients desaturate rapidly — FRC falls and oxygen consumption rises; preoxygenate meticulously before GA.
- Airway is more oedematous and friable; failed intubation risk is higher — plan per OAA/DAS.
- Assume full stomach from second trimester for GA planning.
- Hypercoagulability raises VTE risk — prophylaxis is part of perioperative care.
Meet the patient
A 34-year-old at 39 weeks, BMI 33, is wheeled to theatre for a category-one caesarean under spinal. The block goes in, she is laid flat, and within ninety seconds her pressure craters and the fetal heart decelerates — not because the spinal was wrong, but because nobody tilted the table and the gravid uterus just closed her vena cava.[1]
The two questions that decide every obstetric anaesthetic are two physiology answers in disguise: can she tolerate lying flat? (aortocaval compression) and how long can she hold her breath? (the FRC and oxygen-use desaturation clock). Hold those two and every system change slots into place.[1]
References4ShowHide
- [1]Mushambi MC et al. Obstetric Anaesthetists' Association and Difficult Airway Society guidelines for the management of difficult and failed tracheal intubation in obstetrics Anaesthesia, 2015.PMID 26449292
- [2]Ngan Kee WD et al. Prophylactic phenylephrine infusion for preventing hypotension during spinal anesthesia for cesarean delivery Anesth Analg, 2004.PMID 14980943
- [3]WOMAN Trial Collaborators. Effect of early tranexamic acid administration on mortality, hysterectomy, and other morbidities in women with post-partum haemorrhage (WOMAN): an international, randomised, double-blind, placebo-controlled trial Lancet, 2017.PMID 28456509
- [4]Delgado C, Ring L, Mushambi MC General anaesthesia in obstetrics BJA Educ, 2020.PMID 33456951