A mathematical model for breath gas analysis of volatile organic compounds with special emphasis on acetone |
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Authors: | Julian King Karl Unterkofler Gerald Teschl Susanne Teschl Helin Koc Hartmann Hinterhuber Anton Amann |
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Institution: | (1) Department of Anaesthesia and Intensive Care Medicine, University of Rostock, Schillingallee 70, 18057 Rostock, Germany;(2) Joint Mass Spectrometry Centre, Chair of Analytical Chemistry, University of Rostock, Dr. Lorenz Weg 1, 18059 Rostock, Germany;(3) Cooperation Group Analysis of Complex Molecular Systems, Helmholtz Zentrum Muenchen, German Research Center for Environmental Health, Ingolstaedter Landstr. 1, 85764 Neuherberg, Germany; |
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Abstract: | Recommended standardized procedures for determining exhaled lower respiratory nitric oxide and nasal nitric oxide (NO) have
been developed by task forces of the European Respiratory Society and the American Thoracic Society. These recommendations
have paved the way for the measurement of nitric oxide to become a diagnostic tool for specific clinical applications. It
would be desirable to develop similar guidelines for the sampling of other trace gases in exhaled breath, especially volatile
organic compounds (VOCs) which may reflect ongoing metabolism. The concentrations of water-soluble, blood-borne substances
in exhaled breath are influenced by: (i) breathing patterns affecting gas exchange in the conducting airways, (ii) the concentrations
in the tracheo-bronchial lining fluid, (iii) the alveolar and systemic concentrations of the compound. The classical Farhi
equation takes only the alveolar concentrations into account. Real-time measurements of acetone in end-tidal breath under
an ergometer challenge show characteristics which cannot be explained within the Farhi setting. Here we develop a compartment
model that reliably captures these profiles and is capable of relating breath to the systemic concentrations of acetone. By
comparison with experimental data it is inferred that the major part of variability in breath acetone concentrations (e.g.,
in response to moderate exercise or altered breathing patterns) can be attributed to airway gas exchange, with minimal changes
of the underlying blood and tissue concentrations. Moreover, the model illuminates the discrepancies between observed and
theoretically predicted blood-breath ratios of acetone during resting conditions, i.e., in steady state. Particularly, the
current formulation includes the classical Farhi and the Scheid series inhomogeneity model as special limiting cases and thus
is expected to have general relevance for a wider range of blood-borne inert gases. The chief intention of the present modeling
study is to provide mechanistic relationships for further investigating the exhalation kinetics of acetone and other water-soluble
species. This quantitative approach is a first step towards new guidelines for breath gas analyses of volatile organic compounds,
similar to those for nitric oxide. |
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