Cyanosis is a blue discolouration of the skin and mucous membranes. It
is due to the presence of desaturated haemoglobin and becomes visible when
levels rise above 5 g/dL. Cyanosis is not present in very anaemic patients due
to the low haemoglobin levels. Cyanosis is divided into two categories:
Peripheral cyanosis, which is seen in the fingertips and peripheries.
When occurring without central cyanosis it is due to poor perfusion, as the
sluggish circulation leads to increased desaturation of haemoglobin. This may
be as a result of normal vasoconstriction in the cold, poor peripheral
circulation or a poor cardiac output.
Central cyanosis also affects the warm mucous membranes such as the
tongue. It is a result of failure of oxygenation. This may be a result of blood
bypassing the lungs (right to left shunting) or due to severe lung disease.