Polycythemia, as in newborns with twin–twin transfusion, infants of diabetic mothers, or children with high erythropoietin states may cause cyanosis This can occur even with normal pulse oximetry because the absolute amount of deoxygenated Hb is above g/dL, but with a normal proportion of oxygenated to deoxygenated 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 PaO2 and cyanosis Upper airway obstruction, as with a foreign body, croup, epiglottitis, bacterial tracheitis, tracheal/bronchial disruption, or congenital airway abnormalities, if severe, can cause hypoxemia and consequent cyanosis Age, events leading to presentation, and examination features, such as barking cough, can help distinguish among these diagnoses Cyanosis ensues rapidly when chest wall movement or lung inflation is impeded This condition is often a result of trauma and includes external chest compression, flail chest, or hemothorax Tension pneumothorax, whether traumatic or as a result of pre-existing lung disease such as asthma or cystic fibrosis, is diagnosed by dyspnea, deviated trachea, and possibly distended neck veins with diminished breath sounds on the affected side Empyema or pleural effusion caused by infection, malignancy, or large chylothorax may be associated with fever, respiratory distress, dullness to percussion, and asymmetric breath sounds on auscultation Importantly, any lung dysfunction that directly affects pulmonary gas exchange can lead to cyanosis The most common conditions in children are asthma, bronchiolitis, pneumonia, cystic fibrosis, foreign-body aspiration, and pulmonary edema TABLE 21.1 CAUSES OF CYANOSIS I Respiratory A Decrease in inspired O2 concentration B Severe upper airway obstruction Foreign body Croup Epiglottitis Bacterial tracheitis Traumatic disruption Congenital anomalies (e.g., vascular malformation, hypoplastic mandible, laryngotracheomalacia) C Chest wall External compression Flail chest D Pleura Pneumothorax Hemothorax Empyema/effusion Diaphragmatic hernia E Lower airway Asthma Bronchiolitis Cystic fibrosis Pneumonia Acute respiratory distress syndrome Foreign body/aspiration Congenital hypoplasia II Vascular A Cardiac Cyanotic congenital defects a Tetralogy of Fallot b Transposition of the great vessels c Truncus arteriosus d Pulmonary atresia e Severe pulmonary stenosis with patent foramen f Tricuspid atresia g Ebstein anomaly h Total anomalous pulmonary venous drainage i Atrioventricular canal defect Congestive cardiac failure Cardiogenic shock B Pulmonary Pulmonary edema Primary pulmonary hypertension of the newborn Pulmonary hypertension Pulmonary embolism Pulmonary hemorrhage C Peripheral Moderate cold exposure Shock: septic/cardiogenic Acrocyanosis of the newborn Complex regional pain syndrome III Neurologic A Drug or toxin-induced respiratory depression (e.g., morphine, barbiturates) B Central nervous system lesions (e.g., intracranial hemorrhage, contusion) C Seizure D Breath holding E Brief resolved unexplained event (BRUE) F Neuromuscular disease (e.g., Guillain–Barré, spinal muscular atrophy) IV Hematologic A Polycythemia B Methemoglobinemia V Dermatologic A Blue dye B Pigmentary lesions C Tattoos D Amiodarone therapy TABLE 21.2 COMMON CAUSES OF CYANOSIS I Peripheral cyanosis A Acrocyanosis of the newborn B Moderate cold exposure II Central cyanosis A Respiratory dysfunction B Congenital heart disease Circulatory or vascular conditions leading to diminished PaO2 are also associated with cyanosis One of the most common causes of cyanosis in children