Status Asthmaticus

Excerpt

Asthma is a chronic inflammatory airway disease marked by recurrent wheezing, dyspnea, chest pain, and coughing (see Image. Pathophysiology of Asthma). Recommended treatments for mild-to-moderate asthma exacerbations include β2-agonists and corticosteroids. Patients with status asthmaticus are typically refractory to these standard therapies and may require intravenous medications, continuous nebulizer treatments, and mechanical ventilation.

Status asthmaticus is a medical emergency characterized by hypoxemia, hypercarbia, and secondary respiratory failure. The condition may progress to acute respiratory failure, end-organ dysfunction, and death if not promptly recognized and treated aggressively.

No single clinical or diagnostic index reliably predicts the outcomes of severe asthma exacerbations. A multipronged approach combining thorough clinical evaluation, judicious diagnostic testing, and rapid symptom management optimizes outcomes in these patients.

Anatomy of the Normal Lung

Embryologically, the respiratory system arises as an outgrowth of the ventral foregut. 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 lung bud divides into 2. Both main bronchi branch into progressively smaller airways, forming bronchioles, lobules, terminal bronchioles, acini, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Several terminal bronchioles constitute a pulmonary lobule. Pulmonary and bronchial arteries accompany the airway and lung parenchymal branching.

The proximal airways conduct air to the distal lung regions. Cartilaginous tissue supports these airways to maintain patency. Pseudostratified columnar ciliated epithelium lines the larynx, trachea, and bronchi. Neuroendocrine cells in the bronchial mucosa secrete serotonin, calcitonin, and bombesin. Tracheal and bronchial walls also contain submucosal mucous glands.

Distal airways have progressively smaller diameters. Cartilage and submucosal mucus glands disappear at the level of the bronchioles. Acini, composed of respiratory bronchioles, are spherical structures distal to the terminal bronchioles and measure approximately 7 mm in diameter. Alveolar ducts arise from the respiratory bronchioles and branch into alveolar sacs. Alveoli develop from respiratory bronchioles, alveolar ducts, or alveolar sacs.

The alveolar anatomy is specialized for gas exchange. The blood-air barrier consists of the capillary epithelium, basement membrane, interstitial tissue, alveolar epithelium, and alveolar macrophages. Type I pneumocytes are flat epithelial cells constituting 95% of the alveolar surface. The remaining epithelium comprises Type II pneumocytes, rounded cells that secrete pulmonary surfactant and repair the alveolar epithelium following Type I pneumocyte injury. Pores of Kohn in the alveolar walls connect adjacent alveoli.