• Exhaled airway markers
• respiratory measurement

Analysis of volatile trace gases within exhaled breath, for the purpose of non-invasive disease detection and monitoring, is a rapidly emerging field of research.1 2 Recent technological developments such as proton transfer reaction-mass spectrometry (PTR-MS) have allowed on-line and real-time detection of multiple trace gases in breath, leading to novel discoveries in cancer, infectious disease and metabolism.2 3

One of the greatest lessons on clinical applicability of breath analysis has been the recognition that multiple physiological variables can influence the quantification of exhaled nitric oxide (NO), necessitating international consensus guidelines for its standardised measurement.4 There remains however limited experimental evidence defining the impact of confounding factors which may influence the quantification of other exhaled volatile trace gases.5 Herein we present the finding of a study investigating the influence of respiratory variables on the on-line detection and quantification of a judiciously selected and potentially clinically relevant panel of expiratory trace gases.

We examined the variation in the concentrations of selected trace gases (methanol, acetaldehyde, ethanol, acetone, isoprene, acetonitrile, propanol, dimethyl sulphide and butyric acid) over a range of expiratory flows (50, 100, 250 ml/s) and after the 30-s periods of breath hold and paced hyperventilation. These volatiles were compared to exhaled NO and carbon dioxide. On-line measurement of breath samples from healthy volunteers (n=10) was performed by combining PTR-MS (Ionimed Analytik GmbH, Innsbruck, Austria) with the LR2500 multiple-gas analyser (Logan Research Ltd, Rochester, UK). Quantification of trace gases by PTR-MS was achieved by calibration experiments using accurately known gas standards and a purpose built gas calibration unit (Ionimed). (Further details of methodology are provided as supplementary digital content).

In contrast to NO, exhibiting an inverse relationship with expiratory flow rate, exhaled acetone increased with higher flows (805 vs 838, 898 ppb, p=0.02) (figure 1). After a 30-s breath hold, levels of acetone (805 vs 869 ppb, p<0.01), methanol (206 vs 217 ppb, p=0.02), isoprene (348 vs 390 ppb, p=0.02) and dimethyl sulphide (113 vs 136 ncps, p=0.02) increased significantly. Levels of methanol (206 vs 179 ppb, p<0.01), dimethyl sulphide (113 vs 103 ncps, p<0.01), acetaldehyde (26 vs 22 ppb, p<0.01) and ethanol (410 vs 208 ppb, p=0.01) fell significantly following the 30-s hyperventilation (figure 1). Variation in respiratory parameters did not significantly alter the levels of acetonitrile, propanol and butyric acid (table 2 in online supplement).

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Figure 1

Influence of respiratory physiological variables on the concentrations of selected trace gases measured within the exhaled breath of healthy volunteers. Trace gas level are presented as the ratio of the difference in breath manoeuvres versus their respective control breath measures at a flow rate of 50 ml/s.

This work constitutes the first concerted attempt to discern the effect of ventilatory variables on breath analysis by an on-line MS-based analytical technique. The principal findings of this study are (i) PTR-MS evidence for the flow dependency of exhaled acetone; (ii) changing minute ventilation can both increase and decrease the concentrations of selected exhaled trace gases; and (iii) concentrations of certain volatiles were not significantly altered by respiratory manoeuvres in healthy volunteers.

These preliminary observations may have important implications regarding the standardisation requirement for measuring and reporting the concentrations of exhaled trace gases in the future. Further larger studies both in healthy and diseased subjects are necessary to expand on these observations and to provide mechanistic insights into exchange kinetics of affected volatiles. Such studies may help to further define the exact role of on-line MS technologies in non-invasive diagnosis and monitoring pulmonary and systemic diseases.