Passive total respiratory system compliance (C_RS) and gas exchange measurements were performed in nine newborns during the course of hyaline membrane disease. None of the subjects presented bronchopulmonary dysplasia at follow‐up investigations. Gestational age ranged from 29 to 37 weeks. C_RS was measured by the multiple occlusion technique. Gas exchange parameters were the fraction of inspired oxygen concentration (F_IO₂) and the arterial/alveolar ratio for oxygen (a/Ao₂ ratio). In each subject four tests were performed: test 1 during the first day of life; test 2 during the second day of life; test 3 between the fourth and the seventh days of life; test 4 after extubation. C_RS/BW (C_RS normalized for body weight) was not statistically different at tests 1–3, but it significantly increased (P < 0.001) between tests 3 and 4. F_IO₂ and a/Ao₂ ratio presented no statistical difference at tests 1–2 but several significant differences were noted thereafter; F₁₀₂ decreased significantly (P < 0.001) when results from tests 2 and 3 were tabulated. The a/Ao₂ ratio increased significantly between tests 2 and 3 (P < 0.001), and a further significant increase (P < 0.01) was also noted when results obtained during tests 3 and 4 were compared. A significant relationship existed during the evolution of the disease between C_RS/BW and gas exchange parameters (F_IO₂ and a/Ao₂ ratio) (P < 0.01), but gas exchange improved earlier than lung mechanics.

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