Calculation of work per liter of ventilation (Pavg) in a passive patient on constant-flow mechanical ventilation. Pavg can be calculated by three methods. (a) Dividing the integral of the airway pressure (Paw) by the inspiratory time (Ti). (b) Recording the airway pressure at the mid-inspiratory time (Ti/2). (c) Calculating Pd - (Ps - Pex)/2, where Pd = peak inspiratory pressure, Ps = estimate of end-inspiratory pressure, and Pex = estimate of end-expiratory pressure. Reprinted from  with permission from Elsevier.