Assessment of responsiveness to carbon dioxide in patients with chronic airways obstruction by rate of isometric inspiratory pressure development.

1. Responsiveness to CO2 was measured in forty patients with chronic airways obstruction in terms of ventilation and rate of isometric inspiratory pressure change [(dP/dt)max]. 2. The ventilatory response was below the normal range in eighteen out of twenty-two patients with normal arterial CO2 tensions and in all of eighteen patients with CO2 retention. 2. The (dP/dt)max. response was distributed throughout the normal range in all but one of the patients with normal arterial CO2 tension. In all the patients with CO2 retention the (dP/dt)max. response was either at or below the lower limit of the normal range. 4. Although the ventilatory responses correlated significantly with FEV1 there was no such correlation for the (dP/dt)max. responses. 5. The (dP/dt)max. response showed a significant negative correlation with Pa,CO2. 6. It is believed that the (dP/dt)max. response to CO2 can be used to assess central CO2 responsiveness in subjects with airways obstruction independently of mechanical factors limiting their ventilation.