In obstetric anaesthesia, a parturient at 28 weeks gestation has aortocaval compression in the supine position. Which physiological mechanism explains why this causes worse hypotension than in non-pregnant patients?

• A Increased blood viscosity in pregnancy
• B Uterine arterial vasospasm reducing placental flow
• C Progesterone-induced myocardial depression
• D Inferior vena caval compression reduces venous return and cardiac output; absence of adequate compensation through the paraspinal venous plexus ✓

Explanation

From 20 weeks gestation, the gravid uterus compresses the IVC and aorta when supine. IVC compression reduces venous return to the right heart, decreasing cardiac output by up to 25–30%. The azygos and paraspinal venous plexuses provide collateral return, but this is often insufficient. The enlarged uterus also partially compresses the aorta (Poseiro effect), reducing uteroplacental blood flow. This can cause supine hypotensive syndrome in 10–15% of parturients. The 15-degree left lateral tilt or a Cardiff wedge displaces the uterus leftward, restoring IVC patency. During spinal anaesthesia for LSCS, this displacement is critical.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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