Cyanosis

Pathogenesis

  • Blue blood flowing through to DERMAL CAPILLARIES< not through arteries and veins which lie too deep to contribute to skin colour.
  • Minimal amount of arterial deoxyHb to cause cyanosis is ~2.4 g/dL (or 4.2 g/dL in capillaries).
  • Absolute amount of arterial deoxyHb is required to produce cyanosis, therefore cyanosis may happen in a relatively high SaO2 in a polycythemic patient eg
    – Hb of a polycythemic patient = 23 g/dL
    – OxyHb/Total Hb = (23-2.4)/23 = 90% of oxygen saturation in artery. Cyanosis could happen in this relatively high saturation.

Where to find cyanosis

  • The best location is at places where the skin is thin and subepidermal vessels are abundant eg lips, mucous membrane of oral cavity, ears, cheeks, hands and feets.

Central cyanosis

  • Affects the areas mentioned above. mainly a supply problem (supply of O2-cardiopulmonary or Hb problems)
  • Blanching of the fingers will cause the blue colour to disappear as cyanosis depends on “blue blood” flowing through the vessels.
  • If oxygen administration does not diminish the degree of cyanosis, suspect methemoglobinemia (produces chocolate cyanosis).

Peripheral cyanosis

  • Only affects the hands and feets. Mainly a delivery problem (delivery of O2 to peripheries)
  • Rewarming the hands will cause vasodilation and the limbs will become less cyanotic. In central cyanosis (in which the problem is in supply of O2), rewarming the hands willl not cause the colour to go away.

Pseudocyanosis

  • Often due to drugs discolouration eg chloroquine, amiodarone and minocycline
  • Lips remain pink, and blanching of the fingers does not make the colour to disappear.

Cyanosis and oximetry:

  • Pulse oximetry detects the COLOUR of pulsatile waveform in digits –> peripheral perfusion
  • Co-oximetry is the measure of arterial blood O2 levels in lab
  • Therefore, co-oximetry can detect central cyanosis or abnormal Hb (methemoglobinemia).
  • Pulse oximetry can be falsely indicate central cyanosis in a patient with peripheral cyanosis (due to lack of good perfusion) or near normal oxygen saturation in abnormal Hb.

Does finding central cyanosis predict the arterial deoxyHb level?

  • Yes, finding central cyanosis detects arterial deoxyHb >2.4 g.dL with a +LR of 7.4.

Reference: Evidence-based Physical Diagnosis, Steven McGee – 2nd ed.

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