CYANOSIS

DAVID A. LOWE, MD AND ANNE M. STACK, MD

Cyanosis, a bluish-purple discoloration of the tissues, is a disturbing condition commonly confronted by the pediatric emergency physician. It is most easily appreciated in the lips, nail beds, earlobes, mucous membranes, and locations where the skin is thin and may be enhanced or obscured by lighting conditions and skin pigmentation.

PATHOPHYSIOLOGY

The three factors that ultimately determine the occurrence of cyanosis are the total amount of hemoglobin (Hb) in the blood, the degree of Hb oxygen saturation or qualitative changes in the Hb, and the state of the circulation.

Oxygenated Hb is bright red, and deoxygenated Hb is purple. Cyanosis is evident when the total reduced or deoxygenated Hb in the blood exceeds 5 g per dL or when oxygen saturation approaches 85%. When the total amount of Hb is decreased, as in anemia, the patient may not appear cyanotic even in the presence of unsaturated Hb. Conversely, polycythemic patients may appear ruddy because of the increased red cell mass, and the relative increase in unsaturated Hb will add a blue hue to the skin.

The degree of Hb saturation is determined by several factors, including the partial pressure of oxygen (Po₂) in the alveoli, the ability of oxygen (O₂) to diffuse across the alveolar wall into the red cell and the Hb molecule itself. First, if the level of alveolar ventilation falls, so does the alveolar Po₂, resulting in a fall in arterial Po₂, resulting in desaturation and subsequent cyanosis. Second, the ability of O₂ to diffuse across the alveolar wall into the red cell, or blood–gas barrier, is greatly affected by the circumstances of the barrier itself. According to Fick’s law, any condition that diminishes alveolar surface area and/or increases the thickness will decrease gas diffusion and hence arterial Po₂. Third, the Hb molecule itself has unique properties that affect the amount of oxygen it can carry. The color of whole blood is determined in part by the state of the Hb molecule. Under normal circumstances, oxygen binds reversibly to the iron molecule of the Hb subunit, changing its conformation from a purple deoxygenated form to a bright red oxygenated Hb. Consequently, factors that affect O₂ binding to Hb will affect the color of the blood. For example, carbon monoxide competitively binds to hemoglobin at an affinity 200 times more than that of oxygen to form carboxyhemoglobin. This abnormal form of Hb has a cherry red color, despite the fact that little oxygen is bound to the Hb molecule. Another important conformational change in Hb occurs when heme iron is oxidized from its normal ferrous to a ferric state, to form methemoglobin. Hemoglobin in this state is brownish-purple in color, is incapable of binding O₂, and results in cyanosis if the level exceeds 10% to 15% of total Hb.

The state of the circulation plays an important role in the presence and degree of cyanosis. Cyanosis can result from shunting. A shunt is defined as a mechanism by which deoxygenated blood that has not traveled through the ventilated alveolar capillary bed mixes with oxygenated arterial blood, hence reducing the arterial Po₂. If the shunt is large, the reduction in arterial Po₂ can be severe, leading to marked cyanosis. Oxygen is unloaded to the tissues as blood travels through a capillary, with the relative concentration of unsaturated Hb increasing from one end of the capillary bed to the other. Poor perfusion states and cold temperature favor the unloading of oxygen and thus increase the amount of unsaturated Hb in the tissue capillaries. In an upright lung, the apex is ventilated more than the base, and the base is perfused more than the apex resulting in ventilation/perfusion (V/Q) mismatch. In healthy subjects, this is not clinically relevant; however, in patients with diseased lungs, the contribution of V/Q inequality to lowering of blood Po₂ can be significant.

DIFFERENTIAL DIAGNOSIS

The most common causes of cyanosis are cardiac and respiratory diseases but many other conditions can also cause a patient to appear blue (Tables 16.1 and 16.2). Consideration of the pathophysiologic framework outlined previously allows an orderly approach to the differential diagnosis of cyanosis. Life-threatening causes of cyanosis are summarized in Table 16.3.

Polycythemia, as in newborns with twin–twin transfusion, infants of diabetic mothers or children with high erythropoietin states, may give the appearance of cyanosis. This can occur even with a normal saturation because absolute the amount of unsaturated Hb is above 5 g per dL, however that makes up only a small percentage of the total Hb.

The degree of Hb saturation is affected by many factors, which can be grouped conveniently by systems. First is the significant contribution from respiratory conditions. Any circumstance leading to a decrease in the concentration of inspired oxygen, such as a house fire, confinement to a small unventilated space, or high altitude, can lead to diminished arterial Po₂

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