ObjectiveWe investigated the evolution of lung injury in an animal model with multiple rib fractures, both with and without acute lung injury, and the influence of spontaneous breathing with continuous positive airway pressure (CPAP) therapy on the relative distributions of alveolar ventilation (&OV0312;a) and perfusion (&OV0422;).DesignProspective, randomized laboratory investigation using an established porcine model with instrumentation for measurement of ventilation/perfusion distribution, pulmonary mechanics and gas exchange, and cardiovascular variables.SettingUniversity experimental research laboratory.SubjectsTwenty-nine domestic swine.InterventionsAnesthetized pigs were assigned randomly to undergo chest wall dissection alone or chest wall dissection and bilateral fractures of ribs with or without oleic acid-induced acute lung injury.Measurements and Main ResultsGas exchange was evaluated by blood gas analysis and multiple inert gas elimination technique. After baseline data were collected, subsequent data were collected at 60 and 120 mins after experimental injuries, and at 180 mins, which was 60 mins after titration of CPAP therapy. The range of CPAP was 4–22 cm H2O. Shunt (&OV0312;a/&OV0422; < 0.005), venous admixture &OV0312;a/&OV0422; < 0.1), and functional deadspace (&OV0312;a/&OV0422; > 10) before injury were similar among all animals and ranged from 3.4% to 4.5%, 4.2% to 5.0%, and 54.4% to 56.5%, respectively. There were no changes, throughout the study, in lung regions with low &OV0312;a/&OV0422; (0.005 < &OV0312;a/&OV0422; ≤ 0.1) in any group of animals. Shunt of control animals increased to 10.5 ± 8.8% (p< .05) at 60 mins but demonstrated no further changes in &OV0312;a/&OV0422; at subsequent measurements. Shunt also increased after animals underwent bilateral rib fractures without (12.7%,p< .05) and with (19.9%,p< .05) acute lung injury; however, it decreased in both groups after the application of CPAP (4.6% and 6.6%, respectively,p< .05). All changes in venous admixture directly reflected the change in shunt at all intervals. Functional deadspace was unaffected by chest wall dissection, rib fractures, or subsequent lung injury but decreased after CPAP therapy in all animals.ConclusionsAcute lung injury exacerbated the right-to-left intrapulmonary shunt seen within the first hour after disruption of the chest wall. Application of CPAP decreased shunt, improved matching of &OV0312;a/&OV0422;, and reduced the requirement for supplemental oxygen, without any significant impairment in cardiovascular function.