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1. PEEP 10-5 r = 0.187 p=NS PEEP 10-5 r = 0.232 p=NS PEEP 15-5 r = 0.37 p=NS PEEP 15-5 r = 0.57 p<0.05 900 1200 800 1000 700 600 800 ∆LV20-abs(ml) ∆LV20-rel (ml) 500 600 400 300 400 200 200 100 0 0 0 50 100 150 200 250 0 50 100 150 200 250 ∆EELV0 (ml) ∆EELV0 (ml) Figure E3. Linear correlations between the changes in EELV0(∆EELV0) and the recruitments measured at 20 cmH2O airway pressure. The relative volume method (left panel) did not include EELV0 changes (∆LV20-rel), and the absolute volume method (right panel) did include changes in EELV0 (∆LV20-abs). Open circles represent recruitment at PEEP 10 with respect to PEEP 5; solid circles represent recruitment at PEEP 15 with respect to PEEP 5. Correlation between recruitment and gas exchange variables There were no significant correlations between recruitment variables and gas exchange variables when recruitment variables were expressed in absolute values, i.e., in ml of change. There was a better correlation when recruitment variables were expressed as the percent variations in absolute lung volume, both at ZEEP (percent changes in EELV0), and at 20 cmH2O of airway pressure (percent changes in LV20-abs). The correlations between recruitment variables and absolute changes in QVA/Q, percent changes in alveolar pressure of oxygen (PaO2), and percent changes in alveolar pressure of carbon dioxide (PaCO2)are shown in Figures E4 (see Supplemental Digital Content 6, http://links.lww.com/CCM/A134), E5 (see Supplemental Digital Content 7, http://links.lww.com/CCM/A137), and E6 (see Supplemental Digital Content 8, http://links.lww.com/CCM/A138), respectively. It is worth noting that changes in EELV0 showed a better correlation than changes in LV20-abs. As we suggest in the discussion section, the alveoli that contribute to changes in EELV0, i.e., recruited alveoli that remained recruited at ZEEP, may better represent normal function and mechanics than those that collapse at zero end expiratory pressure.