In clinical studies on breathlessness, the relationship between breathlessness and ventilation is a convenient way of summarizing the responses of single subjects or groups of subjects. This relationship is usually based on measurements during progressive exercise. The objective of this study was to determine whether the relationship was maintained in different conditions or whether breathlessness could alter independently of ventilation. Six healthy subjects undertook progressive exercise so that the relationship of breathlessness to ventilation could be described; the reproducibility was established. The same subjects also undertook exercise at two constant workloads. After 4 min, stable values of ventilation were achieved but the breathlessness scores continued to increase (p less than 0.05). These findings show that the relationship between breathlessness and ventilation may be disturbed without external intervention, and this provides further evidence that breathlessness is not simply a sensing of the ventilation achieved.

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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.

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