A method to classify airborne chemicals which alter the normal ventilatory response induced by CO2.

Guinea pigs inhaling 10% CO2, 19% O2, and 71 N2 increase their tidal volume (VT) by a factor of 2 to 3 and their respiratory frequency (f) by a factor of 1.2 to 1.5 above their normal values while breathing room air. While exposing guinea pigs to 10% CO2, a variety of aerosols can be added to determine how they modify the normal ventilatory response to this agent. Aerosols of carbamylcholine, serotonin, or propranolol all decreased VT of guinea pigs when added to 10% CO2. We propose here that chemicals evoking reductions in tidal volume can be classified into two groups based on the set of concurrent respiratory responses. Those chemicals evoking "obstruction," not only reduced VT but also decreased (f), increased resistance to airflow in conducting airways, and interrupted airflow. The prototype of this family would be carbamylcholine. The second group of chemicals evoked what is described as "reflex restriction." These agents reduced VT but increased f and lowered resistance to airflow. Propranolol would be the prototype for this family. Serotonin, however, evoked both types of response patterns which were time dependent. At the beginning of exposure, obstruction was prevalent while at the end of a 30-min exposure, reflex restriction was prevalent.