Oxygen dissociation (e.g., methemoglobinemia, carbon monoxide, or cyanide poisoning) Hypothermia, hyperthermia Sepsis Obstructive sleep apnea syndrome Acute chest syndrome in patients with sickle cell anemia Causes of acute respiratory failure can also be categorized with consideration of location in the respiratory system ( Table 99.1 ) In addition to primary pulmonary disease, many disorders outside the respiratory tract can lead to respiratory failure Primary pulmonary conditions must be considered Parenchymal lung disease can lead to acute respiratory failure, particularly in younger children, and those with underlying cardiopulmonary disease (e.g., bronchopulmonary dysplasia [BPD] or congenital heart disease) In such cases, the additional respiratory embarrassment from acute pulmonary infection can push these patients into respiratory failure Any condition that causes further narrowing or collapse of the intrinsically small pediatric airway can have a profound effect on air flow Edema from infectious, allergic, or caustic etiologies; foreign material in the airway; or obstruction by enlarged or compressing anatomic structures can restrict airflow These may occur in isolation or in combination Asthma is the most common etiology for lower airway disease, but infections such as bronchiolitis or viral pneumonia are also common Foreign-body aspiration can present acutely with airway obstruction, or may be a delayed diagnosis following the development of secondary postobstructive atelectasis, edema, or pneumonia Chest wall deformities and mechanical impairments prevent full expansion of the chest, leading to decreased vital capacity, decreased minute ventilation, and resultant hypercapnia Inefficient respiratory efforts can cause subsequent hypoxia Oftentimes, these patients maintain near-normal respiratory function until additional physiologic compromise occurs, often from illness as minor as an upper respiratory infection Disruption of nonpulmonary respiratory physiology often results from either reversible or irreversible causes of central nervous system (CNS) disease CNS disease may result in depressed respiratory drive, or inability to maintain