Other Xenobiotics

• Aniline dye derivatives
• Chlorobenzene
• Isobutyl nitrite
• Naphthalene
• Nitrates (e.g., well water)
• Nitrites
• Nitrous gases

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Respiratory Disorders

Respiratory disorders (including both upper and lower airway disorders) should be considered in the evaluation of any infant presenting with cyanosis. Disorders that occlude the upper airway such as choanal atresia, Pierre Robin Sequence, airway hemangioma, vascular rings/slings, cystic hygroma, and micrognathia may cause respiratory distress, stridor, and cyanosis. For example, choanal atresia, although usually diagnosed at birth, may include respiratory distress that improves with crying. Newborns as obligate nasal breathers normally breathe through the nose at rest but through the mouth when crying. Because these infants have an obstructed nasal passage, they will have respiratory distress when attempting to breathe through the nose. It is often diagnosed when the physician is unable to pass a catheter through the nares.

The most common lower airway disorders that cause cyanosis are pneumonia and atelectasis, which result in a ventilation-perfusion mismatch. Neonatal pneumonia typically presents with diffuse infiltrates on the chest X-ray. Most common pathogens include Escherichia coli and group B streptococci. Consider Chlamydia in neonates with a history of apnea and cough.

Less common pulmonary disorders include persistent pulmonary hypertension of the newborn (often associated with a diaphragmatic hernia), interstitial lung disease (diffusion abnormality), arteriovenous malformations (extracardiac shunt), congenital cystic adenomatoid malformation, pulmonary sequestration, and congenital lobar emphysema.

Hemoglobinopathies

Any hemoglobin disorder that interferes with transport of oxygen will result in cyanosis. Polycythemia and anemia can both result in cyanosis. Polycythemia may cause pulmonary hypertension secondary to increased viscosity of the blood. Severe anemia, in contrast, may result in cyanosis due to lack of oxygen delivery to vital tissues. A structurally abnormal hemoglobin molecule can have an impaired ability to carry oxygen.

Methemoglobinemia is a disorder in which there is a structurally abnormal hemoglobin molecule and most commonly results from exposures to an oxidizing chemical (see Table 2) but may also arise from genetic or dietary etiologies or may be idiopathic.

In this disorder, the ferrous (Fe2+) irons of heme are oxidized to ferric (Fe3+) state. Hemoglobin can transport oxygen only when iron is in the ferrous form, and so this oxidation shifts the oxygen dissociation curve to the left. Infants younger than 3 months are particularly susceptible to methemoglobinemia for several reasons: 1) they have lower amounts and activity of NADH-cytochrome b5 reductase; 2) they have lower gastric pH resulting in the proliferation of intestinal flora that reduces ingested nitrates to nitrites; and 3) fetal hemoglobin is more easily oxidized to methemoglobin than adult hemoglobin is. Infants with methemoglobinemia usually present with central cyanosis and no respiratory distress.