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 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 (I) 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 I (6 mL/kg = 9726 +/- 848 microm; 15 mL/kg = 9,637 +/- 884 microm ), E (6 mL/kg = 9747 +/- 800 microm; 15 mL/kg = 9742 +/- 853 microm ), or I-Edelta (6 mL/kg = -21 +/- 240 microm; 15 mL/kg = -105 +/- 229 microm ). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 +/- 1032 microm; 15 mL/kg = 13,917 +/- 1214 microm ), at E (6 mL/kg = 10,462 +/- 906 microm; 15 mL/kg = 12,000 +/- 1066 microm ), and I-Edelta (6 mL/kg = 825 +/- 276 microm; 15 mL/kg = 1917 +/- 363 microm ). Moreover, alveolar instability (increased I-Edelta) 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.