The persistence of hypoxia after 5 to 10 minutes of 100% oxygen exposure is indicative of which mechanism?

• A. Right to Left Shunt
• B. Atelectasis
• C. Increased Work of Breathing
• D. Pulmonary Edema

Answer: (A)

When hypoxemia does not improve after a patient breathes 100% oxygen, it points to blood bypassing ventilated alveoli. That’s shunt physiology. In newborns, a right-to-left shunt—such as persistent fetal circulation with blood moving through a patent foramen ovale or ductus arteriosus, or severe pulmonary hypertension with intracardiac/extrapulmonary shunting—causes deoxygenated blood to reach the systemic circulation regardless of the oxygen in the lungs, so increasing FiO2 won’t fully correct the low oxygen level. Atelectasis and other causes like increased work of breathing or pulmonary edema can often improve somewhat with high oxygen or recruitable alveoli, but a true shunt remains refractory to 100% oxygen, making it the best explanation in this scenario.