Cyanosis is a clinical observation of bluish discoloration of the the patient. It is best seen in capillary rich tissues, such as lips, nail beds and mucous membranes. It occurs because unoxygenated hemoglobin is bluish-purple in color, whereas, oxygenated hemoglobin is bright red. If enough unoxygenated hemoglobin enters the systemic arterial circulation, one can observe this discoloration. The amount of unoxygenated hemoglobin necessary to produce minimal clinical cyanosis is approximately three grams per one hundred milliliters of blood. This is an absolute amount, not a proportion or percentage.
Effect of hemoglobin level of
Notice the important effect of a change in hemoglobin. One gram of hemoglobin carries 1.34 milliliters of oxygen at 100% saturation.
An individual with fifteen grams of hemoglobin per 100 ml of blood carries approximately 200 ml of oxygen per liter of blood. If this person developed anemia and hemoglobin was low at 10 grams per 100 ml of blood, oxygen carrying capacity would be 134 ml per liter of blood. In order to produce clinical cyanosis, oxygen saturation would have to fall to seventy percent, providing three grams of unoxygenated hemoglobin. Oxygen content would only be 94 ml per liter of blood.
Conversely, If hemoglobin were high at 18 grams per one hundred milliliters of blood, a compensation that normally takes place in individuals with cyanotic congenital heart disease, this individual's oxygen carrying capacity would be 241 milliliters per 100 milliliters of blood. Three grams of unoxygenated hemoglobin would be approximately 200 milliliters per liter of blood. The same amount as present with hemoglobin of fifteen grams per one hundred milliliters at 100% saturation.