Effect of repetitive exercise testing on breathlessness in humans

There are conflicting reports on the reproducibility of the visual analogue scale (VAS) and the modified Borg scale for the estimation of breathlessness during exercise. In an attempt to clarify the situation, two groups of healthy subjects undertook a progressive exercise test either daily (Group A) or weekly (Group B) on 10 separate occasions. Breathlessness was estimated every 1 min using the VAS. After 10 occasions, both Group A (P<0.05) and Group B (P<0.01) showed a significant increase in the mean intercept of the breathlessness/ventilation (VAS/V^I) relationship. The increase was not progressive; using change point regression, reproducible values were found to occur after approximately the fifth occasion in both subject groups. As the slope of the VAS/V^I relationship was highly reproducible and did not change with repeat testing, it would appear that at least two mechanisms are involved in the generation of the sensation of breathlessness. A decrease in the exercise heart rate over the same time period was significantly correlated with changes in the VAS/V^I intercept in both groups (P<0.01 and P<0.005 respectively). The relationship is unlikely to be causal, but may be indicative of a common underlying mechanism. It is suggested that breathlessness scores are likely to decrease as a direct result of repetitive testing over, on average, the first five periods of assessment. On the basis of this study, it may be inferred that a physiological mechanism contributes to the modulation of breathlessness during repetitive exercise testing.