Oxygen and hemoglobin values at which central cyanosis occurs

The threshold for central cyanosis is a capillary reduced hemoglobin content of 5 g/dL, which can occur at varying values of the two parameters that are measured most commonly, arterial oxygen saturation (SaO₂) and arterial hemoglobin content. The vertical axis shows values for venous, capillary, and arterial reduced hemoglobin (RHB, g/dL blood), and the horizontal axis shows a percent saturation of hemoglobin in arterial blood (SaO₂) along with corresponding PaO₂ (mmHg). Each diagonal line represents a different hemoglobin content (g/dL). For example, central cyanosis can manifest when SaO₂ is 79% in a patient with a hemoglobin of 15 g/dL.