Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation


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Abstract

ObjectiveWe utilized in vivo microscopy to measure the impact of increasing tidal volume on individual alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) in the living porcine lung.DesignIn three anesthetized, mechanically ventilated pigs, we observed normal alveoli (n = 27) and alveoli after surfactant deactivation by Tween 20 lavage (n = 26) at three different tidal volumes (6, 12, and 15 mL/kg). Alveolar area was measured at peak inspiration (I) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I−EΔ).Measurements and Main ResultsIn normal alveoli, increasing tidal volume did not change alveolar area at I (6 mL/kg = 9726 ± 848 μm2; 15 mL/kg = 9,637 ± 884 μm2), E (6 mL/kg = 9747 ± 800 μm2; 15 mL/kg = 9742 ± 853 μm2), or I−EΔ (6 mL/kg = −21 ± 240 μm2; 15 mL/kg = −105 ± 229 μm2). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 ± 1032 μm2; 15 mL/kg = 13,917 ± 1214 μm2), at E (6 mL/kg = 10,462 ± 906 μm2; 15 mL/kg = 12,000 ± 1066 μm2), and I−EΔ (6 mL/kg = 825 ± 276 μm2; 15 mL/kg = 1917 ± 363 μm2). Moreover, alveolar instability (increased I−EΔ) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli.ConclusionsWe conclude that high tidal volume ventilation does not alter alveolar mechanics in the normal lung; however, in the surfactant-deactivated lung, it causes alveolar overdistension and exacerbates alveolar instability.

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