Total Respiratory Inertance and Its Gas and Tissue Components in Normal and Obese Men *

Total respiratory inertance may be defined as that component of the total impedance to breathing that relates to acceleration of respiratory gas and of all tissue elements (lungs, rib cage, diaphragm, and abdominal contents) that move during respiration. It is expressed in units of pressure required to overcome this inertial component of the total impedance and produce air flow acceleration of 1 L per second per second. Thus its units are centimeters H2O per liter per second2. Intuitively, one would suspect that the inertance of the chest and the abdomen should be increased in severely obese subjects. In investigations of total respiratory mechanics in excessively obese persons, we made measurements of total respiratory inertance. In addition, we attempted to separate the total respiratory inertance into its gas and tissue (including lung tissue, chest wall, and abdominal) components in three normal and three excessively obese subjects. These data form the substance of this report. Inertance data on the normal human respiratory system were first published by DuBois, Brody, Lewis, and Burgess (1) in 1956. These authors, assuming that the respiratory system behaves mechanically as a simple or lumped physical system, determined the total compliance and the natural frequency of the respiratory system. They then calculated the total inertance of the respiratory system by using the simple physical relationship by which the natural frequency of a system may be predicted if its elastic and inertial characteristics are known: fn = 1/274 1/jI *C, where fn