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.