In this article we will discuss about the meaning and factors causing cyanosis.
Meaning of Cyanosis:
Cyanosis is a clinical condition in which the skin and mucous membranes assume a bluish colour. It may be local or general. When general, it is best seen at the lips, nose, cheeks, ears, hands and feet. It is caused by an alteration in the character of blood circulating in the capillaries. Hence, when blood is pressed out of the part, cyanosis temporarily disappears.
It depends upon the absolute amount of reduced haemoglobin present in the blood. It may also be due to the presence of other haemoglobin derivatives having a darker colour (e.g., methaemoglobin and sulphaemoglobin).
At least 5 grams of reduced haemoglobin must be present (i.e., 5 × 1.34 ml = 6.7 ml of oxygen should be less per 100 ml of blood) before cyanosis can be produced. Consequently, in an anaemic subject containing less than 5 grams of haemoglobin (i.e., below 33% Hb) cyanosis is not possible.
Factors Causing Cyanosis:
i. Inadequate oxygenation of blood in the lungs due to:
a. Less O2 in the air as in high altitude.
b. Diseases of the lungs.
c. Collapse of the lungs.
d. Obstruction of the trachea and bronchus.
e. Heart failure.
f. CO poisoning, etc.
ii. Admixture of Venous and Arterial Blood:
This takes place in certain congenital heart diseases, where there is direct communication between right and left sides of the heart e.g. Patent interventricular septum, patent foramen ovale.
iii. Greater Reduction of Oxyhaemoglobin:
a. Local chilling.
b. Venous obstruction. This will retard the local circulation and allow more time for greater reduction of haemoglobin.
c. High metabolic rate and thereby higher degree of oxygen utilisation of a part occur.