The direct effect on pulmonary stretch receptor discharge produced by changing lung carbon dioxide concentration in dogs on cardiopulmonary bypass and its action on breathing.

1. Single fibre pulmonary stretch receptor discharge was recorded in dogs on cardiopulmonary bypass. 2. Inhalation of CO2 depressed pulmonary stretch receptor discharge despite the absence of changes in arterial PCO2. This effect was particularly marked with airway CO2 levels below 5%. 3. Changing arterial PCO2, without changing airway CO2, had only small and insignificant effects on pulmonary stretch receptor discharge. 4. The effect of changes in airway CO2 on pulmonary stretch receptor discharge was rapid and correlated well in time with the reflex tachypnoea produced when CO2 was inhaled in conditions of cardiopulmonary bypass. 5. Stimulation of the central end of the cut vagus nerve was triggered from simultaneously recorded action potentials from a single pulmonary stretch receptor. 6. In these conditions, the reflex response to CO2 could be simulated provided that the pulmonary stretch receptor had an end-expiratory discharge. 7. It is suggested that the vagally mediated tachypnoeic response to changes in airway CO2 seen in conditions of cardiopulmonary bypass is due to the effect of CO2 on the end-expiratory discharge of pulmonary stretch receptors.