Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface having low oxygen saturation. Based on Lundsgaard and Van Slyke's work, it is classically described as occurring if 5.0 g/dL of deoxyhemoglobin or greater is present. This was based on an "estimate" of capillary saturation based on a mean of arterial versus peripheral venous blood gas measurements. Since estimation of hypoxia is usually now based either on arterial blood gas measurement or pulse oximetry, this is probably an overestimate, with evidence that levels of 2.0 g/dL of deoxyhemoglobin may reliably produce cyanosis. Since, however, the presence of cyanosis is dependent upon there being an absolute quantity of deoxyhemoglobin, the bluish color is more readily apparent in those with high hemoglobin counts than it is with those with anemia. Also the bluer color is more difficult to detect on deeply pigmented skin. When signs of cyanosis first appear, such as on the lips or fingers, intervention should be made within 3–5 minutes because a severe hypoxia or severe circulatory failure may have induced the cyanosis.
The name cyanosis literally means "the blue disease" or "the blue condition". It is derived from the color cyan, which comes fromkyanós, the Greek word for "blue".
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https://www.hindawi.com/journals/crira/2014/614647/ devendra v. Kulkarni, rahul g. Hegde, ankit balani, and anagha r. Joshi, “a rare case of pulmonary atresia with ventricular septal defect with a right sided aortic arch and a calcified pulmonary avm presenting in an adult without cyanosis,” case reports in radiology, vol. 2014, article id 614647, 4 pages, 2014. Doi:10.1155/2014/614647
https://www.hindawi.com/journals/crivam/2011/508171/ xue-yan ding, feng chen, xian-xian zhao, hong wu, shao-ping chen, and yong-wen qin, “a rare cause of cyanosis: hepatopulmonary syndrome caused by congenital extrahepatic portosystemic shunt,” case reports in vascular medicine, vol. 2011, article id 508171, 3 pages, 2011. Doi:10.1155/2011/508171