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Changes in the estimated VD/VT ratio during exercise at similar percentages of VO 2peak in the healthy adolescents and the adolescents with CF. Dashed lines correspond to the ventilatory threshold in both groups. Abbreviations: CF = cystic fibrosis; VD/VT ratio = physiological dead space ventilation; VO2peak = peak oxygen uptake. **P < 0.01; ***P < 0.001.
