The action of carbon dioxide on constricted airways.

1. In artificially ventilated open-chest cats and dogs ventilation with 5-15% CO(2) reversed the bronchoconstriction caused by drugs or by pulmonary artery occlusion. Total lung resistance, ;static' lung compliance, and intratracheal or intrabronchial pressure at constant pump stroke were measured.2. CO(2) reduced resistance and increased compliance of the lung during infusions of 5-hydroxytryptamine (5-HT), histamine and acetylcholine in cats. In dogs CO(2) reduced resistance during 5-HT infusions; it caused small reductions in intratracheal pressure but no significant change in resistance during infusions of histamine and acetylcholine. Even in cats CO(2) had a larger effect during 5-HT than during histamine and acetylcholine infusions.3. Occlusion of a pulmonary artery caused increases in resistance and decreases in compliance in the affected lobes of both cats and dogs. These changes were partly reversed by ventilation with high CO(2) mixtures.4. The bronchodilator action of CO(2) took place over a wide range of P(a, CO2) values (20-100 torr).