The effects of carbon dioxide on exercise-induced asthma: an unlikely explanation for the effects of Buteyko breathing training

OBJECTIVES: To examine the effect of breathing 3% CO2 on exercise-induced asthma (EIA), as a raised airway CO2 level is suggested to mediate the effects of Buteyko breathing training (BBT). DESIGN: Double-blind crossover study, using a standard laboratory-based exercise challenge, with EIA defined as a fall of 15% or greater in the forced expiratory volume in one second (FEV1) within 30 minutes of completing a standard exercise protocol. SUBJECTS: 10 adults with confirmed EIA. INTERVENTION: Air enriched with 3% CO2 during and for 10 minutes after exercise. OUTCOME MEASURES: Maximum percentage fall in FEV1 after exercise. Area under curve (AUC) of the decrease in FEV1 with time. RESULTS: Mean maximum fall in FEV1 was similar: 19.9% with air, and 26.9% with 3% CO2 (P = 0.12). The mean AUC for the total 30-minute post-exercise period was 355 for air and 520 for 3% CO2 (P = 0.07). After discontinuing the 3% CO2 at 10 minutes after exercise, there was a further and sustained fall in FEV1. Mean AUC for the period 10-30 minutes post-exercise was significantly greater for CO2 than air (275 and 137, respectively [P = 0.02]). Mean minute ventilation was increased when subjects exercised breathing 3% CO2: 77.5 L/min for 3% CO2, compared with 68.7 L/min for air (P = 0.02). CONCLUSION: Breathing 3% CO2 during exercise does not prevent EIA. The shape of the FEV1 response curve after 3% CO2 suggests that a greater degree of EIA (because of increased minute ventilation during exercise) was opposed by a direct relaxant effect of CO2 on the airway. Increased airway CO2 alone is an unlikely mechanism for the reported benefits of BBT; nevertheless, further study of the effects of voluntary hypoventilation in asthma is warranted.