Voluntary decrease in breathing frequency in exercising asthmatic subjects.

Exercise rehabilitation programmes are increasingly recommended in young asthmatics, but it is unclear whether or not training should incorporate instructions on breathing pattern. In this study, we examined the effects of voluntarily decreasing breathing frequency on their ventilatory equivalents for oxygen and carbon dioxide (minute ventilation (V'E)/oxygen consumption (V'O2) and V'E/CO2 production (V'CO2), respectively), noninvasively determined physiological dead space/tidal volume (VD/VT) and dyspnoea. Fifteen young asthmatic subjects were assigned to two groups: low frequency breathing (LFB) and controls. They first underwent an exercise test at a cardiac frequency of 150 beats x min(-1). They were trained at this level for nine sessions. LFB subjects were instructed to decrease respiratory frequency by 40% during exercise. Control subjects received no instructions. A second test was then performed in the same conditions. LFB subjects decreased V'E/V'O2, V'E/V'CO2 and VD/VT by 22, 19 and 12%, respectively. Arterial oxygen saturation (Sa,O2) fell to 89+/-4% and end-tidal carbon dioxide tension (PET,CO2) rose to 6.5+/-0.7 kPa (49+/-5 mmHg). In controls, these variables were identical in the two tests. Dyspnoea was lower in the second test in all subjects. In conclusion, breathing pattern may be profoundly altered during exercise without concomitant increase in dyspnoea. However, directing breathing patterns for exercise rehabilitation requires an individual assessment of the desired pattern in order to prevent hypercapnia or hypoxia.