Chest wall strapping. An old physiology experiment with new relevance to small airways diseases.

Chest wall strapping (CWS) induces breathing at low lung volumes. Mild to moderate obesity can lead to similar changes in lung volumes, due to chest wall and abdominal restriction. Chest wall strapping is also conceptually similar to a mismatch between significantly oversized donor lungs transplanted into a recipient with a smaller chest cavity. Chest wall strapping increases lung elastic recoil, reduces pulmonary compliance, and substantially increases maximal expiratory flows. The interactions between elastic properties of the lung parenchyma and small airways are critical for pulmonary function. Chest wall strapping lowers residual volume and closing volume, likely from the interdependence between increased elastic recoil and airways, leading to greater radial distending forces on small airways and small airway dilation. Chronic obstructive pulmonary disease (COPD) and chronic rejection of the transplanted lung, bronchiolitis obliterans syndrome (BOS), are primarily diseases of the small airways, and are characterized by progressive obstruction and subsequent loss of small airways. In COPD, higher body mass index (BMI) (conceptually like being more tightly strapped) is associated with lower lung volumes, increased airway conductance, and lower risk of progression to emphysema or death. Likewise, in lung transplantation, oversized donor lungs have been linked to higher expiratory airflows, lower risk of bronchiolitis obliterans syndrome, and improved survival. This article reviews the physiology of chest wall strapping and explores how it could enhance the understanding or even the treatment of small airway diseases, such as COPD and bronchiolitis obliterans syndrome.