Fig. 24.3.

Semi-static pressure/volume (P/V) relationship (i.e., compliance) of the respiratory system in normal and ARDS individuals (b), obtained with serial insufflation of equal volumes of gas a few seconds apart (semi-static) through a calibrated super syringe (a). The normal P/V relationship is nearly linear, with a compliance of approximately 80 mL/cmH₂O. The ARDS relationship is moved to the right (lower compliance) and is sigmoid, indicating that compliance varies with lung volume. Lower and upper “inflection points” indicate segments of the curve at which the compliance changes; these two points suggest values of airway pressure to set the PEEP (lower) and the inspiratory plateau pressure (higher). The segment of the curve between these set PEEP and plateau pressure should have the most favorable compliance to ventilate the patient at (Reproduced with permission from Wolters Kluwer Health: Crimi E, Hess DR: Respiratory Monitoring. In: Bigatello LM Senior Ed.: Critical Care Handbook of the Massachusetts General Hospital, Fifth Edition. Wolters Kluwer Health, 2009.) [93]