Cyanosis occurs when the concentration of deoxyhaemoglobin is high (greater than 5g/dL), because deoxyhaemoglobin absorbs differing wavelengths of light than oxyhaemoglobin and looks bluer.
Central cyanosis occurs when there is high deoxyhaemoglobin levels in the arterial blood, caused by:
- Shunt of deoxygenated blood into the systemic arteries
- Impaired oxygen uptake through the alveolar membrane
- High levels of overall haemoglobin overwhelming oxygen diffusing capacity (eg polycythaemia rubra vera)
- Low delivery of blood to the alveolar capillaries
This is because these central areas have high capillary blood flow which reduces the effect of tissue oxygen extraction.
Peripheral cyanosis occurs when there is high tissue oxygen extraction, caused by:
- Localised low blood supply (eg peripheral vasoconstriction)
- High tissue metabolic demand
So you'd always see peripheral cyanosis if there is central cyanosis, but not necessarily central cyanosis when there is peripheral cyanosis.