1. The primary cause of cyanosis is an increase in the reduced hemoglobin, or oxygen unsaturation, of the blood in the peripheral capillaries.

2. When the mean capillary oxygen unsaturation, which is calculated as the mean between venous and arterial unsaturation, and is normally about 2 to 3 volumes per cent, is increased to about 6 to 7 volumes per cent, cyanosis appears. For this reason 6 to 7 volumes per cent may be called the threshold value of mean capillary oxygen unsaturation for the incidence of cyanosis.

3. The increased mean capillary oxygen unsaturation is produced in two ways (secondary causes of cyanosis), either by an abnormally great reduction during passage through the capillaries (Text-fig. I, Diagram II) or by a state of partial reduction in the arterial blood entering the capillaries (Text-fig. I, Diagram III). The first condition (abnormally great reduction) occurs during exercise, or when the blood flow is retarded, as in decompensated heart condition. The second condition (partial arterial unsaturation) occurs in certain lung and heart diseases, and when the alveolar oxygen tension is greatly decreased, as at high altitudes.

4. If the blood is completely saturated with oxygen in the lungs, the oxygen unsaturation of the venous blood may increase to 13 to 14 volumes per cent before cyanosis appears.

5. If cyanosis appears at a venous oxygen unsaturation less than 13 to 14 volumes per cent, some arterial oxygen unsaturation may be assumed, and the more the lower the venous oxygen unsaturation is.

6. Even if neither râles nor dullness can be detected in the lungs, conditions may exist which prevent complete oxidation of the arterial hemoglobin. This is especially frequent in patients with mitral lesions.

7. Cyanosis cannot be produced in patients whose hemoglobin percentage is below 35 per cent on the Haldane scale (oxygen capacity of 6.5 volumes per cent).

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