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INNOVATIVE METHODOLOGY

INNOVATIVE METHODOLOGIES

Adaptation of the NDIR technology to ¹³CO₂ breath tests under increased inspiratory O₂ concentrations

¹Universitätsklinik für Anästhesiologie, Ulm; ²Münster University of Applied Science, Münster; ³ABB Automation, Frankfurt; and ⁴Fa. Fischer Analysen Instrumente, Leipzig, Germany

Submitted 15 July 2008 ; accepted in final form 11 May 2009

Nondispersive infrared spectroscopy (NDIR) allows the continuous analysis of respiratory gases. Due to its high selectivity, simple and robust setup, and small footprint, it is also used to support ¹³CO₂ breath tests to assess bacterial growth in the stomach, gut, or liver function. CO₂ NDIR signals, however, are biased by oxygen in the gas matrix. This complicates NDIR-based breath tests, if the inspired oxygen concentration has to be adjusted to the subject's requirements, or hyperoxia-induced effects were studied. To avoid the oxygen-induced bias, a "dilution" approach was developed: expired gas is mixed with N₂ to lower the oxygen content down to the usual range of 15–20%. Accuracy and precision were tested using synthetic gas mixtures with increasing ¹³CO₂-to-¹²CO₂ ratios (¹³CO₂/¹²CO₂), either based on synthetic air with 20% volume O₂ or on pure O₂. For samples with ¹³C values smaller than 300 (or ¹³CO₂/¹²CO₂ smaller than 0.003), the dilution does not significantly increase the bias in the ¹³CO₂/¹²CO₂ determination, and the within-run imprecision is smaller than 1 ¹³C. The practical use of this approach was validated in a pig study using a sepsis model reflecting a clinical situation that requires an increased oxygen concentration for respiration. The N₂ dilution eliminated the high bias in NDIR measurement, thus allowing the determination of the impact of oxygenation on glucose oxidation in patients ventilated with increased oxygen.

breath test; stable isotopes