The role of carbon dioxide in the development of pulmonary insufficiency.

The effects of carbon dioxide (PCO2 = 40 versus PCO2 = 20) on pulmonary function changes during 2 hours of hemorrhagic hypotension followed by resuscitation are evaluated in 21 dogs. Pulmonary hemodynamics, mechanics, gas exchange, functional residual capacity, and morphology are studied. In the preshock period, hypocapnia is associated with a decreased cardiac output, increased dead space, and increased alveolar-arterial (A-a) gradiant (room air). During the period of hypotension, all parameters in both groups changed similarly. After resuscitation, the A-a gradients in the two groups further widened. Following the return to control levels of normocapnia in all animals, the group which had been hypocapnic during the hypotensive episode continued to show increased shunting (20 versus 13 percent, p less than 0.05). These results correlated well with cinemicroscopic findings, which showed the normocapnic group to have less interstitial edema and better capillary flow.