Use of end-tidal carbon monoxide to correct end-tidal hydrogen in neonates

We evaluated the usefulness of end-tidal CO (ETCO) as an internal standard for reducing the error in end-tidal H2 (ETH2) measurements due to contamination of repeated breath samples with nonalveolar gas. Triplicate end-tidal samples were drawn from 12 healthy premature infants in small (less than 1 cc) increments through a posterior nasopharyngeal catheter at end-expiration, determined from the infant's chest wall movement. CO and H2 determinations were made on each sample by a reduction gas detector capable of determining CO and H2 concentrations to +/- 0.001 and 0.010 ppm, respectively. Respiratory breath samples were corrected for ambient CO and H2 concentrations. Since the alveolar gas fraction has the highest CO concentration of all tidal gases, the end-tidal sample with the highest CO peak was assumed to be most representative of uncontaminated alveolar gas. The other samples were "corrected" using a factor that was the ratio of the patient's highest CO peak to the given sample's CO value. The use of ETCO to correct ETH2 from samples deliberately contaminated with ambient air can significantly reduce the variability of ETH2 values. However, such correction is probably not necessary when comparing groups of infants using a standard collection technique. For individual infants, correction may reveal more marked short-term fluctuations in true alveolar H2 concentration.