Peripheral cyanosis improves with oxygen therapy. Conversely, central cyanosis does not respond to oxygen therapy because of the underlying intrapulmonary or intracardiac shunt which is responsible for mixing the nonoxygenated venous blood and oxygenated arterial blood. All causes of central cyanosis may cause peripheral cyanosis.

• The diagnosis of cyanosis is made when the following diagnostic criteria are met:

• Transient bluish discoloration of hands and feet in the neonate or acrocyanosis
• Response to oxygen therapy in peripheral cyanosis
• NO response to oxygen therapy in central cyanosis
• Discoloration of hands and feet in peripheral cyanosis
• Discoloration of mucous membrane and lips in central cyanosis

• The diagnosis of central cyanosis in the neonate is made when at least one of the following diagnostic criteria are met:

• Insufficient pulmonary oxygen intake such as respiratory failure, upper airway obstruction, hypoventilation
• Insufficient pulmonary blood flow such as increased pulmonary vascular resistance, congenital heart disease
• Unusual pattern of pulmonary blood flow such as transposition of great arteries (TGA), total anomalous pulmonary venous connection (TAPVC), pulmonary arteriovenous malformation
• Normal PO2 level such as methemoglobinemia, polycytemia

• Echocardiography is the gold standard test for the diagnosis of cyanotic congenital heart diseases.^[1]