An 18-year-old, previously healthy man comes to the emergency department after being stabbed in the chest. The patient reports shortness of breath. Blood pressure is 136/84 mm Hg, pulse is 96/min, and respirations are 20/min. Oxygen saturation is 96%. Physical examination reveals a 2.5-cm stab wound on the right lateral chest wall. Breath sounds are decreased on the right side. Chest x-ray is shown in the exhibit. Which of the following changes have most likely occurred in this patient's right hemithorax and right lung compared to his preinjury state?
Show Explanatory Sources
This patient's chest x-ray shows no peripheral lung markings on the right side and a tissue mass representing the collapsed right lung, findings consistent with pneumothorax; the stab wound likely breached the chest wall and pleura, allowing air to enter the intrapleural space.
The lungs exert collapsing force at all lung volumes, and the chest wall exerts expanding force at all but very high lung volumes. In the normal respiratory system, the lung and chest wall are in equilibrium (represented by the blue dot) at the functional residual capacity (FRC), and they move in concert throughout the respiratory cycle with the opposing forces working together to maintain negative intrapleural pressure.
When an abnormal communication pathway is created between the lung (or exterior chest) and the intrapleural space, intrapleural pressure equalizes with alveolar (ie, atmospheric) pressure, resulting in loss of intrapleural negative pressure. With the lung and chest wall no longer pulling each other in opposite directions, the chest wall springs outward to its equilibrium position (red dot), increasing hemithorax volume.
Loss of negative intrapleural pressure also causes the lung to collapse toward its equilibrium position (black dot). Compliance is defined as change in volume per change in pressure and is represented by the slope of the pressure-volume curve (eg, high compliance is illustrated by a steep-sloping curve). Lung compliance demonstrates hysteresis (ie, compliance differs during inspiration and expiration) due to the effects of alveolar surface tension. The flatness of the inspiratory lung compliance curve at very low lung volumes reflects the difficulty in inflating a collapsed lung and represents decreased compliance compared to that at the FRC.
Educational objective:
The lungs exert collapsing force, and the resting chest wall exerts expanding force. These opposing forces are in equilibrium at the functional residual capacity and generate sustained intrapleural negative pressure that allows the lung and chest wall to move together as a combined respiratory unit. Loss of intrapleural negative pressure (eg, pneumothorax) disrupts the combined system, causing the chest wall to spring outward to its equilibrium position, enlarging the hemithorax. Similarly, the lung collapses to its equilibrium position where inspiratory compliance is decreased.