Use of End-Tidal Carbon Monoxide to Correct End-Tidal Hydrogen in Neonates

Abstract

We evaluated the usefulness of end-tidal CO (ET_CO) as an internal standard for reducing the error in end-tidal H₂ (ET_H2) measurements due to contamination of repeated breath samples with nonalveolar gas. Triplicate end-tidal samples were drawn from 12 healthy premature infants in small (< 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 ET_CO to correct ET_H2 from samples deliberately contaminated with ambient air can significantly reduce the variability of ET_H, 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.