Group averages and SD for (A) end-inspiratory lung volume (EILV) versus positive end-expiratory pressure (PEEP); (B) EILV versus peak inspiratory pressure (PIP). Values obtained during both ascending and descending PEEP ramps are displayed with solid and dashed lines, respectively. Two-way ANOVA for changing PEEP: *P < 0.001; for type of ramp (ascending vs. descending): ¶P < 0.001. Paired t test for PEEP 0 versus 9 cm H₂O: ‡P < 0.001. The area under the curve subtended by EILV was larger in the descending than in the ascending PEEP ramp (P < 0.01).