Fig. 4

Left: baseline energy (red hatched triangle ABE), on which the inspiratory energy associated with the tidal volume (area BCDE) is added. Yellow hatched area to the right of line BC represents the inspiratory dissipated energy needed to move the gas, to overcome surface tension forces, to make the extracellular sheets slide across one another (tissue resistances), and possibly to reinflate collapsed pulmonary units. Light green hatched area on the left of line BC defines the elastic energy (trapezoid EBCD) cyclically added to the respiratory system during inspiration. Total area included in the triangle ACD is the total energy level present in the respiratory system at end inspiration. Right: energy changes during expiration. Of the total energy accumulated at end inspiration (triangle ACD), the area of the trapezoid EBCD is the energy released during expiration. The fraction of energy included in the hysteresis area (light blue hatched area) is dissipated into the respiratory system, while the remaining area (dark blue hatched area) is energy dissipated into the atmosphere through the connecting circuit. Note that whatever maneuver (as controlled expiration) reduces the hysteresis area will reduce the energy dissipated into the respiratory system (potentially dangerous?). PEEP positive end-expiratory pressure (Color figure online)