Which of the following does not contribute to hypoxemia?

Hyperventilation does not contribute to hypoxemia; rather, it can lead to a decrease in carbon dioxide levels (hypocapnia) and can sometimes improve oxygenation, depending on the circumstances. When a person hyperventilates, they are breathing at an increased rate or depth, which usually enhances the oxygen exchange in the alveoli and can transiently elevate blood oxygen levels.

In contrast, hypoventilation reduces the amount of fresh air (and therefore oxygen) reaching the alveoli, leading to a buildup of carbon dioxide and decreased oxygen levels in the blood, resulting in hypoxemia. Decreased atmospheric O2, occurring at high altitudes or in enclosed spaces, can also lead to hypoxemia due to the lower availability of oxygen for inhalation. Finally, a right-to-left shunt directs deoxygenated blood into the systemic circulation without passing through the lungs for oxygenation, which unequivocally causes hypoxemia.

Thus, hyperventilation is the only scenario listed that typically does not lead to hypoxemia but instead can improve oxygenation.