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

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 δ13C 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 δ13C. 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.