Which condition may cause cyanosis despite a normal PaO2?

• A. Anemia
• B. Polycythemia
• C. Hypoxemia Due to RDS
• D. Heart Failure

Answer: (B)

Cyanosis reflects how much deoxygenated hemoglobin is in the blood, not the amount of dissolved oxygen in the plasma. PaO2 measures oxygen dissolved in the blood, while how saturated the hemoglobin is with oxygen determines how much deoxygenated Hb is present. In polycythemia, there is a higher total hemoglobin content. If not all of that hemoglobin is fully oxygenated, the absolute amount of deoxygenated hemoglobin can become large enough to cause a bluish discoloration, even though PaO2 is normal. In contrast, with anemia, there’s less hemoglobin overall, so the total deoxygenated Hb available to produce cyanosis is reduced, making cyanosis less likely at a normal PaO2. Hypoxemia from conditions like RDS would typically lower PaO2 and lead to cyanosis via reduced dissolved oxygen, and heart failure can vary, but the classic scenario for cyanosis with a normal PaO2 is polycythemia.