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Figure 3 (A) Extended studies: sub-group 2. Qp increased by 21% during a standard study (NPV₁), and then increased by a further 38% during when the period of negative pressure ventilation was extended by 20 to 30 min (NPV₂). The total increase in Qp was 67%. (B) Sub-group 2: restrictive versus nonrestrictive patients. The initial increase in Qp was lower during a standard study (1) for restrictive patients (19%), than nonrestrictive patients (26%). When the period of negative pressure ventilation was further extended (2), the increase was higher for restrictive patients (54%) than nonrestrictives (20%), and the total increase at the end of the study period (3) was 67%.