10.1 Definition

Cyanosis is a bluish discoloration of the skin and mucous membranes. It occurs when the hemoglobin is not adequately saturated with oxygen. Cyanosis becomes clinically visible when the amount of unsaturated hemoglobin in the veins of the skin exceeds 4 to 5 g per 100 mL of blood.

The extent of cyanosis correlates with the absolute amount of unsaturated (reduced) hemoglobin. Cyanosis may therefore not be visible in anemia (low hemoglobin content in blood), although oxygen saturation is low. However, cyanosis quickly becomes apparent even in patients with polyglobulia (high hemoglobin content in blood) who have relatively low undersaturation with oxygen.

10.2 Classification

A differentiation is made between central and peripheral cyanosis.

Central cyanosis

Central cyanosis stems from low arterial oxygen saturation. It is caused by intracardiac right-to-left shunts, by insufficient oxygenation of the blood in the lungs, or rarely by reduced oxygen-binding capacity of hemoglobin (e.g., associated with methemoglobinemia).

Peripheral cyanosis

Peripheral cyanosis stems from increased oxygen extraction from the blood when arterial oxygen saturation is normal. In most cases, it is caused by a large reduction of cardiac output due to heart failure. The skin is noticeably cool in this case.

One cause of localized peripheral cyanosis is reduced local perfusion or obstructed venous blood flow, for example, due to a thrombosis, venous congestion, or impaired peripheral blood flow. Increased vasoconstriction due to cold also leads to increased oxygen extraction (blue lips from the cold).

10.3 Diagnostic Measures

Symptoms

The leading symptom is the bluish discoloration of the skin and mucosa, which is best seen at the fingernails, lips, ear lobes, and oral mucous membranes. Clinically, a distinction is made between central and peripheral cyanosis:

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  In central cyanosis, the tongue is cyanotic and the mucosae are a deep red. The skin is warm.

  
  
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  In peripheral cyanosis, the tongue is not cyanotic. In peripheral cyanosis resulting from reduced cardiac output, the skin is cool.

  
  
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  Impaired venous outflow (thrombosis or venous congestion) leads to swelling proximal to the congestion.

Oxygen saturation

In central cyanosis, arterial oxygen saturation is reduced. In peripheral cyanosis, arterial oxygen saturation is normal, but because of increased oxygen extraction in the periphery, mixed venous oxygen saturation is reduced.

Hyperoxia test

The hyperoxia test is used to distinguish between cardiac or pulmonary cyanosis. The cyanotic patient is given 100% oxygen to breathe for a few minutes. If there is a pulmonary cause, the cyanosis disappears or is considerably reduced and there is a relevant increase in oxygen saturation. If there is a cardiac cause, oxygen saturation remains unchanged because the cardiac right-to-left shunt is not overcome by applying oxygen.

Echocardiography

Cyanotic heart defects can generally be reliably diagnosed by echocardiography. Echocardiography is therefore indispensable in patients with cyanosis.

Differential diagnoses

The most common cyanotic heart defects are:

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  Transposition of the great arteries

  
  
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  Fallot group:

  
  
  
  
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    Tetralogy of Fallot

    
    
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    Pulmonary atresia with ventricular septal defect

    
    
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    Pulmonary atresia with intact ventricular septum

    
    
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    “Double-outlet right ventricle”

  
  
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  Univentricular heart:

  
  
  
  
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    Tricuspid atresia

    
    
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    Single ventricle

    
    
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    Hypoplastic left heart syndrome

  
  
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  Ebstein anomaly

  
  
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  Total anomalous pulmonary venous connection

  
  
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  Truncus arteriosus communis

The most common differential diagnoses for cyanosis are summarized in Table 10.1.

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