Fig. 2. Schematic representation of the Campbell diagram, used to calculate the work of breathing (WOB). For one entire breath, the esophageal pressure change is plotted against tidal volume to generate a loop. The relaxation line of the chest wall is traced for the corresponding tidal volume. The negative lung static pressure volume curve passes through the zero flow points. The increase in lung volume relative to functional residual capacity (FRC) due to dynamic hyperinflation indicates that the beginning of the inspiratory effort is shifted to the right of the zero flow point (or to the right of the beginning of inspiratory flow) by an amount equal to the intrinsic positive end-expiratory pressure (PEEPi). This value is further corrected for expiratory muscle activity (not shown). The inspiratory WOB is the area enclosed between the chest wall relaxation line and the inspiratory pressure volume loop. The area right of the chest wall relaxation line during expiration represents active expiratory work (ExpWOB).