Chapter 49

Cyanosis, or bluish discoloration of the skin, is derived from the Greek word kuaneos, meaning dark blue. Cyanosis is caused by the presence of deoxygenated hemoglobin in the blood vessels that is most visible on the surface of the skin and mucosa. In general, cyanosis occurs because (1) the binding of oxygen to hemoglobin is abnormal so that blood does not carry much oxygen despite having a normal partial pressure of oxygen, or PO2 (eg, methemoglobin or carboxyhemoglobin); (2) the perfusion of the skin is poor, such that the venous and capillary blood are very deoxygenated even though the arterial blood may be well oxygenated (eg, cold environment or circulatory shock); or (3) the arterial, and therefore the capillary and venous blood, is poorly oxygenated (eg, a right-to-left shunt with congenital cardiac disease, parenchymal pulmonary disease, or hypoventilation). Cyanosis tends to become apparent when there is about 3 to 5 g/dl of deoxygenated hemoglobin, but detection varies widely depending on lighting, observer differences, and pigmentation of the skin, among other factors. The oxygen binding capacity of the fetal hemoglobin in the newborn also alters the degree of desaturation at a given PaO2. For example, at a PaO2 45, the saturation of adult hemoglobin would fall below 80%, typically creating a cyanotic appearance but fetal hemoglobin saturation would remain in the mid 80s, which may not be associated with overt cyanosis (see eFig. 49.1). There is urgency to determine the cause of the cyanosis because of the high risk of tissue injury or death posed by poor oxygenation and in order to guide important interventions to improve tissue oxygenation. Although a specific diagnosis may not necessarily be determined at the bedside without special studies, the underlying nature of the disturbance usually can be derived with common clinical tools and the physical examination.

###### eFigure 49.1.

Representation of the characteristics of oxygen binding in fetal versus adult hemoglobin. For a hypothetical PaO2 of 45, the saturation of adult hemoglobin would fall below 80%, typically creating a cyanotic appearance. However, the binding characteristics of fetal hemoglobin would allow for the saturation to remain in the mid 80s, which may not be associated with overt cyanosis.

Arterial O2 saturation using pulse oximetry should immediately be measured in any cyanotic newborn infant. It is particularly important to measure blood saturation of tissue that is likely perfused from the aorta proximal to the ductus arteriosus—generally, the right hand or, if possible, an ear lobe—and from a lower extremity. Although there is some imprecision in oximeters, especially if perfusion is poor (see Chapters 103 and 106), this approach will help establish if the hypoxemia is a valid finding. Furthermore, right-to-left shunting across the ductus arteriosus may be detected when the upper and lower body saturations differ consistently by more than 3% to 5%. If there is indeed hypoxemia, ...

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