++
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.
++++
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,
measurement of arterial ...