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

Objective: We 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. Design: In 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 (1) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I-EDelta). Measurements and Main Results. In normal alveoli, increasing tidal volume did not change alveolar area at 1 (6 mL/kg = 9726 +/- 848 mum(2); 15 mL/kg 9,637 +/- 884 mum(2)), E (6 mL/kg = 9747 +/- 800 mum(2); 15 mL/kg 9742 853 mum(2)), or I-EDelta (6 mL/kg = -21 +/- 240 mum(2); 15 mL/kg = -105 +/- 229 mum(2)). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at 1 (6 mL/kg = 11,413 +/- 1032 mum(2); 15 mL/kg = 13,917 +/- 1214 mum(2)), at E (6 mLkg = 10,462 +/- 906 mL/Kg 2; 15 mLkg = 12,000 +/- 1066 mum(2)), and I-EDelta(6 mL/kg = 825 +/- 276 mum(2); 15 mL/kg = 1917 +/- 363 mum(2)). Moreover, alveolar instability (increased I-EA) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli. Conclusions: We 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.