ObjectivesTo compare data derived from a computer model of the pulmonary circulation with data from a case series of patients with chronic obstructive pulmonary disease (COPD). To evaluate the specific factors contributing to CO2retention due to oxygen therapy in patients with acute exacerbations of COPD.DesignData from a computer model of the pulmonary circulation were compared with a previous case series.PatientsPatient data were derived from previous case series.InterventionsSimulated application of oxygen therapy.Measurements and Main ResultsThe computer model of the pulmonary circulation generates data comparable with those data from a series of patients with COPD treated with supplemental oxygen and permits identification of the causes for hypercarbia. Therapy with supplemental oxygen alters hypoxic pulmonary vasoconstriction and modulates the Haldane effect, resulting in changes in physiologic deadspace.ConclusionChanges in physiologic deadspace are sufficient to account for the hypercarbia developed by patients with acute exacerbations of COPD when treated with supplemental oxygen.(Crit Care Med 1996; 24:23-28)