The main finding of this study was that substituting helium for nitrogen in the hyperoxic ambient air did not improve the maximal performance of exercise of trained cyclists during an incremental exercise test to exhaustion. This finding is contrary to results from most previous studies that have evaluated the effects of breathing a lighter gas on performance of exercise.9,12,25
Furthermore, the perceived ventilatory effort was not significantly attenuated when subjects breathed heliox. Thus, although the work of the respiratory muscles was potentially reduced by breathing a gas with a density of one fifth and a viscosity 1.12 times greater than the nitrox air,26
the sensation of the effort of breathing was not reduced.
previously reported that the work of breathing is not altered when the density of the inspired air is reduced, as ventilatory volume was increased at submaximum workloads when heliox was breathed; however, in this study, minute ventilation was depressed at submaximum workloads (fig 2). A likely explanation for this discrepancy is the enormous difference in subject samples between the studies. A prerequisite for inclusion into Babb's study was pathological air flow limitation, whereas our subjects were extremely well‐trained, healthy people. Therefore, breathing a lighter gas probably exerts a separate effect in populations that experience restricted breathing conditions. It seems logical that for people who have air flow limitations, and who therefore experience an attenuated ventilatory volume, breathing a lighter gas will improve their ventilation towards normal—that is, the ventilatory volume will increase. Certainly, Puente‐Maestu et al27
showed that a reduction in tidal volume is the limiter to exercise tolerance in patients with chronic obstructive pulmonary disease (COPD). Eves et al22
previously showed that in patients with COPD, the submaximal tidal volume is increased when patients breathe a heliox gas mixture, but does not change when the patients breathe a hyperoxic gas even though both gas mixtures improve exercise tolerance to the same extent. This suggests that the mechanisms through which heliox and hyperoxia improve performance are different, a postulate that is supported by their observation that a hyperoxic heliox mixture exhibits a performance improvement effect greater than either hyperoxia or normoxic heliox individually.
In healthy people whose ventilation is compromised through hypobaric exposure, the supplementation of helium for nitrogen in the ambient air in hypobaric conditions has a similar effect to the COPD studies of increasing submaximal ventilation towards normobaric values through an increase in tidal volume.28
Furthermore Esposito and Ferretti12
reported that VO2
max and peak power were improved in hypoxic conditions when a heliox gas was inspired; however, they did not find any difference in either VO2
max or peak power when heliox was substituted in normoxic conditions. Interestingly, however, maximal expired and maximal alveolar ventilation were increased in both hypoxia and normoxia when heliox was substituted for nitrox. In people who have no pathological limitations to their ventilation, an effect of inspiring a less dense gas on respiratory work or ventilatory dynamics may be to reduce tidal volume at submaximal workloads. A lower ventilation and oxygen uptake at submaximum workloads, such as that observed in our study, implies superior gas exchange and unchanged airway resistance—that is, a lower ventilation is required to deliver oxygen, thus oxygen uptake is lower. Interestingly, the reduction in mean oxygen consumption at submaximum workloads observed during the heliox trial (about 8%) is similar to the oxygen cost that has been determined for breathing normal air during exercise (4.6–10%).29
Although there was a reduction in submaximal VE
, the perceived ventilatory effort remained similar between trials. This can probably be explained by the fact that the reduction in VE
was attained through a reduced tidal volume and not a change in the breathing frequency. A change in the rate of breathing is the respiratory variable that has been associated with the perception of dyspnoea.27
Our study differed from other studies that have looked at maximal exercise capacity in healthy subjects breathing a heliox gas9,12
in two important ways: (1) our subjects were highly trained cyclists and (2) our subjects inspired a hyperoxic gas mixture. Esposito and Ferretti12
and Powers et al9
reported an increase in maximal minute ventilation while breathing a heliox mixture, but Powers et al
only reported an increase in VO2
max and workload under normoxic conditions. We have previously alluded to the fact that the effects of breathing a heliox gas may be twofold: an improved ventilatory capacity and improved ventilatory dynamics. With regard to the improved ventilatory capacity, the subjects in our study are accustomed to working close to their maximal capacity and therefore their respiratory system would be trained to cope with the volume of air that is moved in and out of the lungs at peak workloads. However, in less well‐trained people, the respiratory system would be unaccustomed to the ventilatory volumes, especially at the higher workloads (which might explain why Powers et al
and Esposito and Ferretti only noted differences in submaximal VE
at higher workloads) and therefore were not able to attain their functional maximal ventilation while breathing nitrox gas. However, as in the case of subjects with restricted breathing, heliox allowed them to ventilate closer to their maximal volume.
What is already known on this topic
- Breathing a heliox mixture improves exercise tolerance in hypoxic conditions and in patients with COPD.
- The performance benefits derived from breathing a lighter gas have been associated with both a decrease in the sensation of ventilatory effort and an enhancement of arterial blood saturation.
Additionally, we argue that the effects of the improved pulmonary gas exchange while breathing heliox, evidenced in this study by the lower submaximal ventilation, would have been even more pronounced had the exercise not been conducted in hyperoxic conditions. This argument is indirectly supported by Esposito and Ferretti,12
who observed significantly improved maximal alveolar ventilation when heliox was inspired under hypoxic conditions as compared with normoxic conditions. Although alveolar ventilation did improve in normoxic conditions, it was to a lesser extent, and not statistically significant. Therefore, seemingly, breathing helium may be beneficial to improve work capacity in subjects who have respiratory pathologies or are not habituated to high ventilatory volumes, as well as in conditions of low inspired oxygen concentrations.
It is well documented that exercise‐induced arterial hypoxaemia occurs at higher exercise intensities in some highly trained athletes.30
Therefore, it could be argued that a compromised oxygen delivery to the working muscles limited the exercise capacity of these subjects before they reached the ventilatory volumes that would terminate exercise. However, it has been shown that the arterial pO2
is better maintained during severe exercise when a heliox gas is inhaled compared with normal air.9,19
Furthermore, Dempsey et al19
have shown that the arterial desaturation associated with maximal work is completely counteracted when subjects breathe a hyperoxic gas mixture (24% and 30%, respectively; table 1).
Table 1Measured mean arterial oxygen pressure and mean arterial oxygen saturation during maximal exercise in a study performed in the same chamber as those in this study31
Therefore, it seems unlikely that in this study maximal exercise capacity was limited by arterial desaturation in either condition.
The Bland–Altman plots for peak power, VO2
max and maximal VE
show the close limits of agreement between the trials for the peak power (−4.0% to 2.9%) compared with both VO2
max (−16.3% to 19.7%) and maximal VE
(−25.6% to 17.5%). These observations are similar to those of Laplaud et al
who reported an interclass correlation of 1 for peak power using a similar protocol, and Kuipers et al
who showed a coefficient of variation in peak power and VO2
max of 2.95–6.83% and 4.20–11.35%, respectively. Owing to the greater variability associated with the VO2
max and maximal VE
coupled with the variability previously reported for biological variables,33
it seems doubtful that the termination of the exercise was due to a single physiological correlate but rather to a multivariable evaluation of integrated afferent feedback that probably includes mechanoreceptors, metaboreceptors and chemoreceptors.