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).