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While in central cyanosis we see cyanotic mucosae, we have in peripheral cyanosis a cyanotic peripheral.

In central cyanosis we have a oxygenation problem and in peripheral cyanosis a low perfusion.

Why doesn't lead a peripheral cyanosis not to cyanotic mucosae and a central cyanosis not to cyanotic peripheral? Because if there is an oxygenation problem I would think that there's also too less O2 in the peripheral and vice versa.

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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.

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