Obstetric Anesthesia Core Knowledge Modules-Pregnancy Physiology Quick Reference Guide

Pregnancy Physiology Quick Reference Guide

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Pregnancy causes chronic compensated respiratory alkalosis, with PaCO₂ typically 28–32 mmHg. Functional residual capacity falls by 15–20%, the diaphragm rises about 4 cm, and oxygen consumption increases by about 20%. These changes shorten safe apnea time and can cause rapid desaturation, making preoxygenation especially important. A PaCO₂ of 40 mmHg may indicate hypoventilation; end-tidal CO₂ targets are generally lower, around 30 mmHg.<br /><br />Plasma volume increases by 40–50%, compared with a 20–30% rise in red-cell mass, often producing physiologic anemia (hemoglobin around 10–11 g/dL). Pregnancy is also hypercoagulable, with increased fibrinogen and factors VII, VIII, and X, and reduced protein S; venous thromboembolism risk rises roughly four- to fivefold.<br /><br />For neuraxial anesthesia, engorged epidural veins reduce epidural space volume and local anesthetic dose requirements by about 25–30%. Spinal anesthetics may spread faster, increasing the risk of a high spinal. Anesthetic requirements also change: MAC decreases by about 30%, sensitivity to volatile agents and opioids increases, and propofol requirements may be reduced.<br /><br />Airway mucosal edema and tissue friability can make airway management difficult; a smaller endotracheal tube may be needed. The stomach is considered full after the second trimester.

Keywords

pregnancy

respiratory alkalosis

PaCO2

functional residual capacity

preoxygenation

physiologic anemia

hypercoagulable

venous thromboembolism

neuraxial anesthesia

airway edema