cynosis
central cyanosis can be detected reliably when the SaO2 has fallen to 85%; in others, particularly in dark-skinned persons, it may not be detected until it has declined to 75%
In general, cyanosis becomes apparent when the concentration of reduced hemoglobin in capillary blood exceeds 40 g/L (4 g/dL). 當毛細血管血中還原血紅蛋白的濃度超過 40 g/L (4 g/dL) 時，紫紺就會變得明顯。
Central Cyanosis
Decreased arterial oxygen saturation Decreased atmospheric pressure—high altitude Impaired pulmonary function Alveolar hypoventilation Inhomogeneity in pulmonary ventilation and perfusion (perfusion of hypoventilated alveoli) Impaired oxygen diffusion Anatomic shunts Certain types of congenital heart disease Pulmonary arteriovenous fistulas Multiple small intrapulmonary shunts Hemoglobin with low affinity for oxygen
Hemoglobin abnormalities
Methemoglobinemia—hereditary, acquired Sulfhemoglobinemia—acquired Carboxyhemoglobinemia (not true cyanosis)