Respiratory and Haemodynamic Response to Carbon Dioxide Breathing in Patients with and without Bronchial Obstruction

1. The effect of breathing a mixture of 5–6% carbon dioxide in air was examined in seven patients with obstructive bronchitis and in eight patients with lung disease but without bronchial obstruction.

2. Hyperventilation during CO₂ breathing was less marked in patients with bronchial obstruction than in the others. Cardiac output did not change in patients with airways obstruction but rose in the control group. The increase of arterial carbon dioxide tension, hydrogen ion concentration, pulmonary arterial and pulmonary wedge pressures (P̄_pa and P̄_paw was comparable in both groups. The increase of P̄_pa and P̄_paw was proportional to the Pa,co₂ but the change of cardiac output was not. The ventilatory response to CO₂ breathing was correlated with the spirometric measurements, but the haemodynamic changes were not.

3. No significant changes of pulmonary vascular resistance or of the P̄_pa–P̄_paw pressure gradient were observed. The increase of pulmonary arterial pressure during CO₂ breathing appears to be due to increased pulmonary wedge pressure.

4. The contribution of depressed myocardial function during CO₂ breathing to the response of cardiac output is discussed on the basis of a negative correlation between Pa,co₂ and stroke volume during CO₂ breathing.