Aortocaval compression (supine hypotensive syndrome) was first reported in 1931. The inferior vena cava and aorta become compressed by the pregnant uterus (the vena cava may be totally occluded), causing reduction in venous return and cardiac output, thus compromising the mother, the fetus or both. Vasovagal syncope may follow aortocaval compression. Maternal symptoms and signs vary from asymptomatic mild hypotension to total cardiovascular collapse, partly dependent on the efficacy of the collateral circulation bypassing the inferior vena cava. Onset of symptoms and signs is associated with lying in the supine or semi-supine position, and in most cases is relieved by turning to the full lateral position.
Problems and special considerations
Aortocaval compression is not confined to the woman at term. The condition has been reported in the fifth month of pregnancy. Women with multiple pregnancy or polyhydramnios are at increased risk because of the increased size of the gravid uterus.
It is important to appreciate that a normal blood pressure and lack of maternal symptoms do not exclude a significant fall in cardiac output and placental perfusion.
Onset of symptoms may occur within 30 seconds, but may be delayed by 30 minutes. Severity of symptoms is not a reliable guide to the severity of hypotension.
Slight changes in maternal position may cause significant change in symptoms. A 15- degree lateral tilt does not always relieve aortocaval compression, and even a 45-degree tilt does not guarantee abolition of hypotension.
Catastrophic hypotension, and even cardiac arrest, may occur if general anaesthesia is induced in a woman who is experiencing severe aortocaval compression (e.g. in the supine position). Even mild degrees of aortocaval compression may lead to severe hypotension after spinal or epidural anaesthesia.
It is impossible to perform effective cardiopulmonary resuscitation on a pregnant woman in the supine position, and the uterus must be displaced off the vena cava and aorta by tilting the pelvis or using manual displacement.
Management options
Women in late pregnancy will not voluntarily adopt positions in which aortocaval compression occurs, and therefore the condition is largely iatrogenic, occurring after a woman has been placed in the supine position by her midwifery or medical attendants. A history suggestive of aortocaval compression in late pregnancy may indicate an increased risk of developing the condition during labour and delivery. All healthcare staff must be aware of aortocaval compression and of the need to avoid the supine position. This is particularly important if the woman is unable to change her own position because of administration of analgesia or anaesthesia.
Uterine displacement (usually to the left, although occasionally improved symptomatic relief will be obtained by displacement to the right) must be used during all vaginal examinations, for fetal blood sampling, and for both vaginal and operative delivery, and is especially important if regional analgesia or anaesthesia is used. This can be achieved manually or by use of table tilt or a wedge under the hip. Use of uterine displacement rather than the full lateral position is a compromise between maternal safety and obstetricians’ convenience. Use of the full lateral position for caesarean section has been reported.
Extreme vigilance is necessary during general anaesthesia, as the mother will not be able to report symptoms. During regional anaesthesia for operative delivery, complaints of faintness, dizziness, restlessness and nausea should alert the anaesthetist to the onset of hypotension. Pallor, particularly of the lips, yawning and non-specific feelings of anxiety are also warning signs of aortocaval compression. Continuous fetal monitoring may indicate signs of fetal distress when the mother adopts the supine or semi-supine position, and occasionally this may be the only indicator of the condition. Turning the mother into the full left lateral position should be the first step in the treatment of hypotension or cardio-tocographic abnormalities.
Key points
• No pregnant woman should lie supine beyond 16-18 weeks.
• The uterus must be displaced off the aorta and vena cava during vaginal examinations and during caesarean section. This can be done manually, with a wedge under the hip, or by using lateral tilt of the operating table.
• Manual displacement of the uterus is essential for effective cardiopulmonary resuscitation in advanced pregnancy.
Further reading
Cluver C, Novikova N, Hofmeyr GJ, Hall DR. Maternal position during caesarean section for preventing maternal and neonatal complications. Cochrane Database Syst Rev 2013; (3): CD007623.
Lee A, Landau R. Aortocaval compression syndrome: time to revisit certain dogmas. Anesth Analg 2017; 125: 1975-85.