1. The effect of adding low-level (2.7 cmH2O 1−1 s) external respiratory resistive loads on exercise-induced breathlessness has been examined in naive normal subjects; the intensity of this loading was chosen to simulate that confronting an asthmatic subject during exercise.

2. Each of 18 subjects performed two separate tests in which workload was oscillated while the respiratory loading was changed every minute between no loading, inspiratory loading only, and inspiratory plus expiratory loading. Each loading condition was given three times, and both these changes and those in workload were unpredictable as far as the subject was concerned.

3. The purpose was to ‘confuse’ subjects and obtain subjective estimates of their intensity of breathlessness independent of any expectation associated solely with the readily perceptible changes in external resistances to breathing. The study design was balanced for the group as a whole, both in terms of workload and respiratory loading condition.

4. The addition of these respiratory resistive loads during exercise did not result in a significant increase in the intensity of breathlessness.

5. Estimates of the rate of work of breathing revealed that this increased more with respiratory loading than it did as ventilation rose throughout the test; on the other hand, the intensity of breathlessness increased by a greater extent with continued exercise compared with the changes accompanying the addition of respiratory loads.

6. It is concluded that the intensity of the sensation of breathlessness experienced by normal subjects during exercise is not simply a reflection of an increased rate of work of breathing being performed by the respiratory muscles.

7. It is further suggested that similar studies in which internal resistances are increased experimentally are indicated in order to analyse the factors underlying the breathlessness of asthma.

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