A 24-year-old woman with a large secundum atrial septal defect and established Eisenmenger syndrome is placed on 100% inspired oxygen. After 20 minutes her PaO2 is 58 mmHg. Which mechanism explains the failure of PaO2 to rise substantially?

• A Elevated hemoglobin affinity prevents oxygen release into end-pulmonary blood
• B Diffusion limitation across thickened alveolar membranes persists even at high FiO2
• C Hypoventilation keeps alveolar PO2 below the level needed for full saturation
• D Right-to-left shunted venous blood admixes with oxygenated end-pulmonary-capillary blood ✓

Explanation

In a fixed right-to-left shunt, a portion of venous blood bypasses ventilated alveoli entirely and mixes with fully oxygenated pulmonary capillary blood downstream. Because dissolved oxygen added at FiO2 1.0 is tiny compared with hemoglobin-bound oxygen in the admixed desaturated blood, PaO2 remains low. Diffusion impairment improves markedly with 100% oxygen, and hypoventilation is readily overcome by the high inspired PO2.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.