So, I found a few interesting things about cyanosis.
Cyanosis is bluish discolouration of skin and mucous membranes.
It occurs when amount of reduced Hemoglobin is more than 4g/dl. (Harrison's, 19E)
Even small amounts of methemoglobin (1.5g/dl) and sulphmethemoglobin (0.5g/dl) can evoke cyanosis!
(Just remember, bigger the name lesser the amount required :P)
Central and Peripheral cyanosis
What happens in both of them?
Central cyanosis has either i) low arterial oxygen saturation or ii) abnormal Hemoglobin derivatives so skin as well as mucous membranes are involved!
Peripheral cyanosis is due to either i)increased oxygen extraction or ii)reduced amount of blood reaching periphery (simply, vasoconstriction [so you may find cold skin] or reduced cardiac output =D) so only skin is involved.
Where to check for cyanosis?
Lips, nail beds, ears and malar eminences.
1. Cyanosis in heart failure can be mixed i.e. Central plus peripheral
2. "Differential cyanosis" is when lower limbs are cyanosed and NOT the upper limbs! It happened when there's a Patent Ductus Arteriosus (PDA) with Reversal of shunt!
3. So can cyanosis of only upper limbs happen? Yeah, if there's a PDA with Reversal of shunt with Transposition of great vessels!
Most interesting one:
4. In PDA with reversal of shunt with Pre-ductal coarctation of aorta all limbs except for Right Upper Limb can be cyanosed! If you're wondering why, then the answer is that if the coarctation is before the origin of left Subclavian Artery you may find its blood supply being hampered and so the cyanosis of left upper limb. Both the lower limbs get its blood supply from arteries after the level of coarctation, so it's invariably going to be cyanosed!
Quite a lot, right?