Responsiveness of patient-reported breathlessness during exercise in persistent asthma

The purpose of the study was to examine the responsiveness of a computerized system whereby the patient reports spontaneously any change in the intensity of breathlessness during exercise. The hypotheses were that hypercapnia would increase and hyperoxia would decrease the slope of power production-breathlessness ratings compared with a control condition during cycle ergometry.

Thirty adult subjects (15 women and 15 men) with persistent asthma (mean [+/- SD] FEV(1)/FVC ratio, 57 +/- 10%) completed an initial familiarization visit and three study visits. All subjects inhaled two puffs of albuterol (180 microg) in order to standardize lung function prior to exercise. At visits 2 to 4, subjects breathed one of the three gas mixtures administered in a random order while performing a ramp exercise test. The experimental conditions were as follows: hypercapnia (5% carbon dioxide); hyperoxia (40% oxygen); and control (room air).

Lung function was the same before and after exercise with the three experimental conditions. With hypercapnia, peak ventilation was increased, peak oxygen consumption, and power production were reduced, the slope of power-breathlessness was increased, and 25 patients (83%) reported breathlessness as the limiting symptom. With hyperoxia, peak ventilation was decreased, peak power production and the slope of power-breathlessness were unchanged, and 16 patients (53%) reported leg discomfort as the limiting symptom.

Breathing 5% carbon dioxide altered physiologic responses and the slope of power production-breathlessness during exercise. The responses to hyperoxia were inconsistent. The continuous method for patient-reported breathlessness was responsive to hypercapnia, but not to hyperoxia, during incremental exercise.