Maternal physiology of pregnancy for the anaesthetist

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.[2]

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]

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1. [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. [2]Ngan Kee WD et al. Prophylactic phenylephrine infusion for preventing hypotension during spinal anesthesia for cesarean delivery Anesth Analg, 2004.PMID 14980943
3. [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