Comparative mammalian respiratory mechanics.

Respiratory mechanics can be studied in mammals of all sizes by the same methods used in man, although some definitions need to be modified and special attention given to small size and fast events. Mechanical properties vary with body size in regular ways. Operating pressures are invariant with size, volumes and compliances varying with body mass (M) approximately to the first power, and ventilation and conductance with M3/4. Thus, time constants vary with M1/4 and frequencies with M -1/4. Tracheal cross-section varies with M3/4 and appears to be constrained by expiratory flow-limitation mechanics. Smaller mammals have more compliant chest walls and lower relaxation volumes, so FRC is more likely to be influenced by reflex and dynamic mechanisms. Methods and procedures are not standardized; variability of observations is great and our ability to discriminate specialized characteristics is undeveloped. Recent fundamental challenges to conceptual foundations of scaling are unresolved. Thus, uncertainty and skepticism about existing views are appropriate.