Wood smoke inhalation causes alveolar instability in a dose-dependent fashion

Wood smoke inhalation causes severe ventilation and oxygenation abnormalities. We hypothesized that smoke inhalation would cause lung injury by 2 mechanisms: (1) direct tissue injury by the toxic chemicals in the smoke and (2) a mechanical shear-stress injury caused by alveolar instability (ie, alveolar recruitment/derecruitment). We further postulated that alveolar instability would increase with the size of the cumulative smoke dose.

Anesthetized pigs were ventilated and instrumented for hemodynamic and blood-gas measurements. After baseline readings, the pigs were exposed to 5 separate doses of wood smoke, each dose lasting 1 min. Factors studied included hemodynamics, pulmonary variables, and in vivo photomicroscopy of alveolar mechanics (ie, the dynamic change in alveolar size with ventilation).

Smoke inhalation significantly increased alveolar instability with 4 min and 5 min of smoke exposure. Significant rises in carboxyhemoglobin levels and in pulmonary shunt were also observed at 4 min and 5 min of smoke exposure. Lung histology demonstrated severe damage characteristic of acute lung injury.

We demonstrated that wood smoke inhalation causes alveolar instability and that instability increases with each dose of smoke. These data suggest that smoke inhalation may cause a "2-hit" insult: the "first hit" being a direct toxic injury and the "second hit" being a shear-stress injury secondary to alveolar instability.