Cyanosis is the visible appearance of hypoxia; it can result from an impaired local circulation but is more importantly a marker of a global lack of oxygen.

It is detected clinically when ≥ 5 g/dL deoxy-Hb can be seen in the skin or mucous membranes. With a [Hb] of 15 g/dL this is 33% of the total, with the remaining 67% being saturated.

In practice, patients appear cyanosed when the pulse oximetry reading is much higher, around 85%.

Cyanosis is seen in capillary blood, whilst a pulse oximeter reading is based on the arterial value, which will be substantially higher because:

• PO₂ is slightly higher in the artery than the capillary
• The saturation curve in the capillary has a small right shift (Bohr effect)