When a relaxed subject is passively inflated by a ventilator, the mechanical work necessary to inflate the total respiratory system can easily be determined by measuring inhaled volume and the airway pressure (Paw). During passive ventilation, Paw represents the pressure difference across the respiratory system (Prs). Work performed on the elastic properties of the respiratory system can be shown graphically as a plot of Prs against volume, as represented by the triangular area ABCA in
Fig 1. Airway pressure obtained at point C is determined under static or quasistatic conditions. If compliance is assumed to be constant, the elastic work of inspiration can be calculated as (VT*0.5)x pressure change; alternatively, it can be calculated as mean pressure * volume change.
Fig. 1 Work performed in inflating the total respiratory system against elastic forces in (a) a normal subject and (b) a patient with decreased respiratory compliance. Work, represented by the triangular shaded area ABCA, is greater in the patient with decreased compliance. The slope of AC represents compliance of the respiratory system. (Reproduced with permission from Tobin ^
Work performed in overcoming air flow resistance, and to some degree resistance due to movement of the chest wall and lung tissue, during passive inflation can be shown on a plot of volume versus Paw; the inspiratory curve is bowed to the right of the compliance line, and the area ACDA represents the work of inspiration against air flow resistance (Fig 2).
Fig. 2 Work performed in overcoming flow resistance during passive inflation is represented by the shaded area ACDA in (a) a normal subject and (b) a patient with increased resistance. Work performed against elastic forces is represented by the area ABCA. (Reproduced with permission from lobinandV^^^
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