Status Asthmaticus

Excerpt

Asthma is a chronic inflammatory airway disease marked by recurrent wheezing, dyspnea, chest pain, and coughing (see Image. Asthma Pathology). The recommended treatments for acute attacks with mild-to-moderate symptoms include β₂-agonists and steroids. However, status asthmaticus arises when patients fail to respond to standard asthma treatments.

Status asthmaticus is a medical emergency, an extreme form of asthma exacerbation characterized by hypoxemia, hypercarbia, and secondary respiratory failure. All patients with bronchial asthma are at risk of developing this condition, which may progress and become poorly responsive to standard therapeutic measures. If not recognized and managed appropriately, status asthmaticus can lead to acute ventilatory failure and even mortality.

Status asthmaticus remains one of the most common causes of emergency department visits despite pharmacotherapeutic advances. No single clinical or diagnostic index has been known to predict the clinical outcomes of this condition. A multi-pronged approach, combining clinical evaluation, appropriate diagnostic tests, and rapid symptom relief, can improve outcomes for patients with this condition.

The Normal Lung

Embryologically, the respiratory system is a ventral foregut outgrowth. The trachea develops at the midline and gives rise to the lung buds. The right lung bud divides into 3 main bronchi, while the left divides into 2. The right main stem bronchus is more vertical than the left, thus more vulnerable to foreign body aspiration. Both right and left main bronchi branch into progressively smaller airways, forming the bronchioles, lobules, terminal bronchioles, acini, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Pulmonary and bronchial arteries follow airway and lung parenchymal branching. Several terminal bronchioles form a pulmonary lobule.

Pseudostratified columnar ciliated epithelium covers the laryngeal, tracheal, and bronchial surfaces. Cartilaginous tissue also supports these airways. Neuroendocrine cells abound in the bronchial mucosa, secreting serotonin, calcitonin, and bombesin. Submucosal mucus glands are also found in the tracheal and bronchial walls.

Cartilage and submucosal mucus glands disappear at the bronchiolar level. Terminal bronchioles have a diameter of less than 2 mm. Acini are spherical structures distal to the terminal bronchioles with a 7 mm diameter and comprised of respiratory bronchioles. The alveolar ducts arise from the respiratory bronchioles and branch into the alveolar sacs. Alveoli can arise from respiratory bronchioles and alveolar ducts and sacs.

Alveoli are the lungs' gas exchange units. The blood-air barrier comprises the capillary epithelium, basement membrane, interstitial tissue, alveolar epithelium, and alveolar macrophages. Type I pneumocytes are the flat cells constituting 95% of the alveolar epithelium. Type II pneumocytes are rounded cells occurring more sparsely in the alveolar space. Type II pneumocytes secrete pulmonary surfactant and repair the alveolar epithelium when type I pneumocytes are damaged. Pores of Kohn on the alveolar walls are potential sites for microbial and exudate spread between alveoli.