The Act of Breathing and How It Fails

¹ Meakins-Christie Laboratories, Montreal Chest Hospital, McGill University, Montreal, Canada.

1. Ventilatory failure is due to failure of the respiratory pump. A framework in which it can be examined is described in terms of the four coupling levels within the respiratory system: (1) stimulation excitation, (2) excitation-contraction, (3) contraction-ventilation, (4) ventilation-stimulation.

2. The inspiratory muscles can be conveniently divided into 2 groups, the diaphragm and its muscles and the inspiratory muscles of the rib cage. Neither group is vital, but both groups taken together are. The diaphragmatic muscles are more important than the inspiratory rib cage muscles because the latter expand only the rib cage, while deflating the abdomen whereas the former expand both rib cage and abdomen.

3. Of the 2 compartments of the chest wall, preservation of rib cage motion is more important than abdominal motion. Abdomen restriction, while making the chest wall less compliant, has the compensating feature that the diaphragm operates at a more efficient length, producing the same Pdi for a lesser degree of activation. The rib cage muscles can act synergistically with the diaphragm. With rib cage restriction, the diaphragm must shorten more and faster, requiring a greater activation for the same pressure, while the synergistic action of the rib cage muscles is completely neutralized. The rib cage muscles, if they can do anything at all, can only antagonize the inspiratory action of the diaphragm on the abdomen.

4. Rib cage distortions cannot be ignored and can be quantified by modeling the rib cage as 2 compartments. By so doing, the pressures resulting from distortions as well as those arising from rib cage muscle contraction can be estimated. During quiet breathing, distortions are an important source of pressure expanding the costal surface of the lung. They relieve the rib cage muscles of work because they supply approximately 50% of the pressures required to expand the pulmonary rib cage and lung.