Reproducibility of flow rates measured with low density gas mixtures in exercise-induced bronchospasm

We have studied the reproducibility of the change in maximum expiratory flow rates after breathing helium/oxygen (He/O2) mixtures in 12 asthmatics at rest and after exercise. Each subject performed four identical exercise tests which caused a similar degree of exercise-induced bronchospasm (EIB) on each occasion. We compared flow rates at 50% of the vital capacity (V50) breathing He/O2 to those breathing air at rest, and with the lowest V50 on air after exercise. Those subjects showing an increase of greater than 20% in V50 with He/O2 compared to the corresponding air value were termed "responders". At rest the responder status after He/O2 was more consistent than during EIB. Six subjects were non-responders consistently on up to 12 separate measurements at rest while the other five subjects were non-responders on all but one occasion and the remaining subject a responder on seven of eight measurements. During EIB all but one subject showed a He/O2 response. A response was seen consistently in six subjects but the actual percentage change in V50 with helium varied greatly. One subject remained a non-responder after exercise and the other five were He/O2 responders after only two or three of the four test runs, and non-responders on the remainder. The lack of consistency of our data, particularly during EIB makes the interpretation of the He/O2 breathing test less useful than originally claimed.