The effect of bronchodilator drugs on breath-holding capability in asthma

A method of assessing the relationship between maximum voluntary breath-holding time (BHT) and the partial pressure of end-tidal carbon dioxide (PETCO2) during hyperoxic carbon dioxide rebreathing is described. The resulting index is referred to as the breath-holding capability (BHC). BHC and the maximum rate of early inspiratory pressure fall at the mouth while breathing oxygen at rest (RHdP/dtmax) were measured in normal and asthmatic subjects before and after treatment with nebulized solutions of salbutamol and ipratropium bromide. BHC was low in asthmatics when their airflow resistance was high and rose when the airflow resistance fell as the result of treatment; RHdp/dtmax was high in the asthmatics before treatment and fell after bronchodilator drugs were administered. The fall in respiratory drive which followed bronchodilatation in the asthmatic subjects cannot be solely explained in terms of the resulting fall in viscous and elastic work of breathing. Vagal afferent signals arising in the airways might play a part in modifying respiratory drive in such circumstances.