MECHANICAL WORK OF BREATHING DERIVED FROM RIB CAGE AND ABDOMINAL V-P PARTITIONING

Estimates of the mechanical work of breathing [in humans] derived from measurements of separate rib cage and abdominal volume displacements, each plotted against transthoracic pressure, include the elastic cost of chest wall distortion which may occur during breathing. Inspiratory work was partitioned between the diaphragm and the rib cage musculature by adding measurements of transabdominal pressure. The mechanical work of breathing derived from separate rib cage and abdominal volume-pressure (V-P) tracings (the sum of work done by the diaphragm, rib cage and abdominal musculature) was compared with ventilatory work estimated from the Campbell diagram (which did not include any distortional work). During resting breathing, the 2 estimates were closely comparable, consistent with little or no distortion of the chest wall during quiet breathing. As ventilation increased the estimate developed from rib cage and abdominal tracings revealed systematically greater mechanical work than was estimated from the Campbell diagram, consistent with distortion of the chest wall from the relaxed thoracoabdominal configuration at higher levels of ventilation. At ventilations achieved during exercise, the Campbell diagram may underestimate the work of breathing by up to 25%.