Average PaO2 in the Uterine Artery
The average partial pressure of oxygen (PaO2) in the uterine artery is approximately 100 mmHg (13.3 kPa) in normal pregnancy.
Physiological Basis of Uterine Artery Oxygenation
Uterine arteries undergo significant physiological adaptations during pregnancy to ensure adequate oxygen delivery to the developing fetus. These adaptations include:
- Increased uterine blood flow from 50 mL/min to nearly 1000 mL/min during pregnancy 1
- Reduced uterine vascular resistance
- Enhanced vasodilation and blunted vasoconstriction
- Receipt of up to 20% of maternal cardiac output at term 1
Evidence for Uterine Artery PaO2 Values
Serial measurements in normal pregnant women have demonstrated that:
- Mean maternal arterial PaO2 remains consistently above 100 mmHg throughout pregnancy 2
- PaO2 values show a slight decrease from 106.4 mmHg at 12 weeks gestation to 101.8 mmHg at 38 weeks 2
- Despite this small decrease, there is no significant change in alveolar-arterial oxygen gradient with increasing gestation 2
Maternal Physiological Changes Affecting Oxygen Levels
Several pregnancy-related changes affect oxygen levels and delivery:
- Increased oxygen consumption (20-33% above baseline by third trimester) due to fetal demands 1
- Decreased functional residual capacity (10-25%) as the enlarging uterus elevates the diaphragm 1
- Increased ventilation (20-40% above baseline) mediated by elevated progesterone 1
- Mild respiratory alkalosis with arterial CO2 pressure of 28-32 mmHg 1
- Right-shifted oxyhemoglobin dissociation curve in the mother (P50 increases from 27 to 30 mmHg) 1
Clinical Implications
Understanding normal uterine artery PaO2 values is important because:
- Hypoxemia can rapidly develop in pregnant women due to reduced oxygen reserves and increased consumption 1
- In high-altitude pregnancy (3,820m), uterine artery PaO2 decreases to approximately 60 mmHg, which affects vascular reactivity 3
- Maintaining adequate oxygenation is crucial for fetal development and maternal health
Monitoring Considerations
When monitoring oxygen levels in pregnancy:
- Pulse oximetry (SpO2) correlates with PaO2 but with increasing variability at higher saturations 4
- At SpO2 of 90%, PaO2 varies by approximately ±10 mmHg
- At SpO2 of 98%, PaO2 varies by approximately ±28 mmHg 4
- Direct measurement via arterial blood gas remains the gold standard for accurate PaO2 determination 5
In summary, while uterine artery PaO2 values may vary slightly throughout pregnancy, they typically remain above 100 mmHg in normal pregnancies at sea level, with a slight decrease toward term.