Cyanosis is bluish discoloration of skin and mucosa due to reduced Hb of 5gm/dl. This may be central when arterial saturation is low or peripheral when arterial saturation is normal and cyanosis is due to increased oxygen extraction from blood due to decreased blood flow as in shock and vasoconstriction.

The best place to look for central cyanosis is the tip of the tongue as this place is never pigmented and always has a good blood flow.

When in doubt, determine arterial oxygen. Note that due to left shift of Hb dissociation curve in a newborn, 90 % SaO2 correlates with only 45 mm Hg PO2 while in an older person it correlates with 65 mm Hg PO2.

 Normally, venules contain 2 gm/dl of reduced Hb, so an additional 3 gm/dl is needed to cause cyanosis. This is accomplished by a reduction of SaO2 to 80% in a person with Hb level of 15 gm/dl ( 15 x 20%= 3).

Causes of central cyanosis are cardiac, respiratory and nervous. Cardiac cause is right to left shunt. Pulmonary causes include lung diseases and pulmonary AV fistula. Nervous system causes are those which affect ventilation like CNS depression, Pickwickian syndrome and respiratory muscle paralysis.

Causes of peripheral cyanosis are heart failure, shock, acrocyanosis of newborn, and methemoglobinemia.

The hyperoxitest helps to differentiate cardiac from pulmonary disease as the cause of cyanosis in newborn. With inhalation of 100% oxygen, in right to left shunt the PO2 does not rise to more than 100 mm Hg while with pulmonary disease it does. In right to left shunt the PO2 does not increase because even though the PO2 of the blood passing through the lung increases to 600 mm Hg, this gets mixed with the low PO2 blood passing to the left side through the right to left shunt and the resulting mixture has still a low PO2 because of the sigmoid shape of the Hb dissociation curve. (SaO2 of 50% is achieved at a PO2 of 27 mm Hg. This occurs in the sharp rising part. The plateau starts at an SaO2 of around 90%.)

Polycythemia occurs in cyanotic children and is beneficial as long as hematocrit is below 65%. Higher values cause hyperviscosity syndromes. 

Clubbing occurs due to megakaryocytes bypassing the pulmonary circulation and getting trapped in the nailbed where they deliver growth factors. It occurs at 6 months of age and occurs first in the thumb.

Paradoxical embolism can occur.

Cortical venous thrombosis can occur, especially in age less than 2 yrs, due to hyperviscosity.

Brain abscess can occur due to bypassing of phagocytic filtering in the lungs.

Platelets are decreased in number and function. PT and APTT are prolonged.

IQ is lower than expected.

Scoliosis is often present.

Hyperuricemia occurs in adults.