フレイルチェスト I: 概要

Flail chest occurs when three or more adjacent ribs fracture in multiple places, creating an unstable chest wall section. Common causes include blunt trauma from motor vehicle accidents or steering wheel injuries, falls in elderly individuals with osteoporosis, and assaults with sharp objects. These traumatic events fracture ribs, costal cartilage, or sternum, destabilizing the thoracic cage.

During inspiration, the flail segment moves inward, limiting air intake and reducing ventilation efficiency. During expiration, it moves outward, hindering exhalation. This paradoxical movement increases dead space—ventilated areas not participating in gas exchange—leading to compromised gas exchange and hypoxemia as oxygen delivery becomes inefficient.

Patients experience chest pain from fractures and wall movement, dyspnea from compromised lung expansion, hypoxemia with low blood oxygen, and tachypnea as the body compensates for reduced lung function. Cyanosis may develop due to poor oxygenation. These symptoms reflect the respiratory impairment caused by the unstable chest wall.

Severe cases can cause pneumothorax when air accumulates in the pleural space due to chest wall instability. Crepitus occurs as fractured bone ends rub together. Poor tissue perfusion and metabolic acidosis result from reduced cardiac output, while atelectasis develops as lung tissue collapses, further compromising respiratory efforts and oxygenation.

During inspiration, the diaphragm moves downward, creating negative pressure that exacerbates the inward movement of the flail segment. This intensifies the paradoxical motion, reducing ventilation efficiency and limiting air intake. The negative pressure pulls the unstable segment inward rather than allowing normal chest wall expansion.

Normal breathing involves coordinated chest wall movement for efficient gas exchange. In flail chest, the unstable segment moves independently and paradoxically, disrupting this coordination. This causes dyspnea, tachypnea, increased dead space, and compromised gas exchange, forcing the body to work harder to maintain adequate oxygenation and ventilation.

Fractures of ribs, costal cartilage, or sternum create an unstable chest wall that cannot maintain normal structural integrity. This instability allows the detached flail segment to move independently during breathing, causing paradoxical movement that impairs respiratory function, reduces ventilation efficiency, and triggers compensatory mechanisms like tachypnea and increased metabolic demand.