ABSTRACT
The performance of the inspiratory pump can be characterized by the pressure, flow, and volume achieved, and these in turn depend on the force, velocity, and length of the inspiratory muscles (Rahn et al., 1946; Agostoni and Rahn, 1960; Agostoni and Penn, 1960; Loring et al., 1985). The description of performance in terms of pressure, flow, and volume is analogous to that which has been applied so successfully over the past 30 years to the cardiac pump, leading to a better understanding of ventricular performance (Fry et al., 1964; Gault et at., 1968; Brutsaert and Sonnenblick, 1969; Strobeck and Sonnenblick, 1986) and to a number of routine invasive and noninvasive tests of cardiac performance. The situation in the respiratory system is different from the heart for several reasons. The inspiratory pump is comprised of a number of muscles that not only operate in concert with one another to sustain ventilation but are also used to perform nonrespiratory trunk tasks. These muscles vary in their operational lengths and the lung volume at which optimal tension is developed (Braun et al., 1982; Decramer and DeTroyer, 1984). The configuration of the respiratory system and the manner in which the muscles are coupled to the system profoundly impact on performance. Finally, the inspiratory pump depends on neural activation rather than pacemaker cells, and the degree of neural activation may vary among muscles and from breath to breath. Nonetheless,
This chapter discusses how the pressure-flow-volume and force-velocitylength frameworks may be applied to the performance of the inspiratory pump, its energetics, its ability to sustain ventilation against a load, and how this approach may clarify some clinical issues.
