Obstetric Anesthesia Core Knowledge Modules-Labor and Delivery-specific Physiology Quick Reference Guide

Labor and Delivery-specific Physiology Quick Reference Guide

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<h3>Labor and Delivery Physiology—Summary</h3>

With each contraction, cardiac output rises by about 10–25% and 300–500 mL of blood is shifted into circulation. Labor pain increases catecholamines and systemic vascular resistance, which can reduce uterine perfusion. An epidural can reduce catecholamines and vascular resistance, improving uteroplacental blood flow.

After delivery, relief of inferior vena cava compression and blood returning from the uterus can increase cardiac output by up to 80%. This is a high-risk period for pulmonary edema and decompensated cardiomyopathy.

Uteroplacental circulation has no autoregulation: it depends directly on maternal mean arterial pressure, cardiac output, and vascular resistance. Maternal hypotension can therefore lead to fetal acidosis.

Typical pregnancy changes include high cardiac output, low vascular resistance, mild tachycardia, reduced functional residual capacity and PaCO₂, low creatinine, hypercoagulability, and reduced anesthetic requirements.

<h3>Red Flags</h3>

The guide identifies these as concerning: PaCO₂ around 40 mmHg (possible hypoventilation); creatinine above 1.0 mg/dL; hemoglobin below 10 g/dL early in pregnancy; platelets below 100,000; fibrinogen below 200 mg/dL (severe postpartum hemorrhage risk); persistent blood pressure at or above 140/90; oxygen saturation below 95%; severe dyspnea at rest; and resting heart rate above 120.

Keywords

labor physiology

cardiac output

uteroplacental circulation

epidural anesthesia

maternal hypotension

fetal acidosis

pregnancy changes

postpartum hemorrhage

pulmonary edema

red flags