Compartmental models of the chest wall and the origin of Hoover's sign

• Compartmental models express our understanding of chest wall mechanics.
• Recent data on diaphragm action provide the basis for a three-compartment model.
• Predicted model volume trajectories match those measured in COPD patients.
• Hoover's sign is a result of a smaller zone of apposition and increased resistance.

First, one- and two-compartment models of the chest wall are reviewed. Then, a three-compartment model is described, and the predictions of the model are compared with data on pressure and volume relationships for different breathing maneuvers in dogs. Finally, the three-compartment model is scaled to apply to humans, and dynamic solutions for periodic breathing are obtained. To model COPD, the area of the zone of apposition is decreased, and to model COPD patients who show Hoover's sign, pulmonary resistance and abdominal compliance are increased. The resulting traces match those reported by Binazzi et al. (2008. Respir. Physiol. Neurobiol. 160: 325–333) for controls and patients with COPD, both without and with Hoover's sign. We conclude that Hoover's sign is a dynamic phenomenon that results from the decrease in the area of the zone of apposition that occurs in COPD and a larger pulmonary resistance and abdominal compliance in those patients who show Hoover's sign.