tools and approach. Appropriate patient monitoring and therapeutic strategies are necessary for early recognition, intervention and treatment. Causes Airway obstruction: Obstruction may occur in the upper and lower airway. Upper airway: Obstruction of the upper airway is common in infants less than 3 months old because they are nose breathers. Nasal blockage may easily lead to upper airway obstruction in infants. For other ages, upper airway obstruction may occur from a foreign body or edema of the pharynx, larynx, or trachea. In cases of decreased or total loss of consciousness, the tongue can lose muscle tone and obstruct the upper airway. Other potential causes of obstruction include tumors of the upper respiratory tract (oral cavity, pharynx, larynx), bodily fluids (blood, mucus, vomit), and trauma to the upper airway. The most common type of tumor of upper respiratory tract is squamous cell carcinoma, with the greatest risk factors for this condition being alcohol and tobacco use, with HPV (genotype 16) being another important risk factor. An epidemiological study of over 5 million cases of head and neck trauma in the United States resulting in visits to the emergency department found that the majority occur due to falls or blunt force, with foreign body injuries being more common in the pediatric population. Lower airway: may occur from bronchospasm, drowning, or airspace filling disorders (e.g. pneumonia, pulmonary edema, pulmonary hemorrhage). Obstructive conditions of the lower airway, including severe asthma or COPD episodes, can also lead to respiratory arrest. During these episodes, known as exacerbations, airway resistance is increased due to inflammatory changes in the lungs. This leads to increased work of breathing and decreased oxygen delivery to tissue. In asthma, this involves bronchiolar constriction whereas in COPD this involves small airway collapse during expiration and subsequent air-trapping. One of the ways the body attempts to compensate for these increased respiratory demands is by increasing respiratory rate, which in turn worsens respiratory muscle fatigue of the diaphragm and can eventually lead to respiratory arrest and death without timely medical intervention. Decreased respiratory effort: Central nervous system impairment leads to decreased respiratory effort. The respiratory center of the brain is located in the pons and medulla and is primarily driven by elevated carbon dioxide levels in the blood (hypercapnia) with decreased oxygen levels (hypoxemia) serving as a less potent stimulus. Central nervous system disorders, such as stroke and tumors, may cause hypoventilation. Drugs may decrease