The causes of cyanosis may be quickly recalled by applying the basic science of physiology. Cyanosis is due to decreased oxygenation of the blood. The decrease, however, cannot be mild; there must be at least 5 g of reduced hemoglobin per 100 mL of blood if cyanosis is to appear. It should be understood from the above that cyanosis will appear with less severe anoxia in polycythemia than it will in anemia. For example, a patient with 20 g of hemoglobin needs only one fourth of his or her blood unsaturated to show cyanosis, whereas a patient with 10 g of hemoglobin needs one half of his or her blood unsaturated to do the same.
Decreased oxygenation of the blood may result from obstruction to the intake of oxygen (e.g., acute laryngotracheitis, chronic bronchial asthma, chronic bronchitis, and emphysema or foreign body); from the decreased absorption of oxygen, as in conditions with alveolar–capillary block (sarcoidosis, pulmonary fibrosis, pneumonia, pulmonary edema, and alveolar proteinosis); or from a ventilation–perfusion defect (e.g., emphysema, pneumoconiosis, or sarcoidosis). Decreased oxygenation of the blood may also result from decreased perfusion of the lung with blood in shock, pulmonary embolism, pulmonary vascular shunts or bypasses such as occur in pulmonary hemangiomas and congenital heart disease. Another cause of reduced intake of oxygen is an atmosphere with reduced concentration of oxygen. The hemoglobin may be unable to latch onto the oxygen in carbon monoxide poisoning and methemoglobinemia, but the cyanosis is associated with a cherry-red color to the lips and tongue in the former and a brownish hue in the latter; polycythemia vera may be associated with a cyanotic hue to the face in cold weather, but the arterial oxygen saturation is not necessarily decreased (Table 21).
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Cyanosis |
Another approach to developing a differential diagnosis of cyanosis is to apply the mnemonic VINDICATE to the heart and lungs. This is suggested as an exercise for the reader.
Approach to the Diagnosis
The workup of cyanosis includes pulmonary function studies before and after bronchodilators, arterial blood gases, routine and before-and-after breathing 100% oxygen, venous pressure and circulation times, chest x-rays, ECGs, and ventilation–perfusion scans. It is unusual not to be able to pinpoint the cause.
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TABLE 21. Cyanosis |
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Other Useful Tests
1. CBC (pneumonia)
2. Tuberculin test (tuberculosis)
3. Lung scan (pulmonary embolism)
4. Echocardiogram (CHF, valvular heart disease)
5. Test for methemoglobinemia
6. Serial cardiac enzymes and ECGs (myocardial infarction)
7. Cardiac catheterization and angiocardiography (arteriovenous [A-V] shunts, valvular heart disease)
8. Pulmonary angiography (pulmonary embolism)
