End-tidal carbon monoxide as an indicator of the hemolytic rate.

In the first days of life, low grade jaundice is essentially universal. The source of the elevated bilirubin level giving rise to "physiological jaundice of the newborn" is only partly known. We hypothesized that it is, at least in part, the result of active and specific hemolysis involving a physiological mechanism to lower the high fetal hematocrit, appropriate for the relatively low oxygen environment in utero, to a lower level appropriate for the state of oxygen abundance after birth. We tested this by quantifying end tidal carbon monoxide (ETCO) as a marker of the rate of heme metabolism to bilirubin. We found that ETCO values of 20 neonates and children with known hemolytic disorders were higher than 20 age-matched healthy controls (p<0.0001), indicating that this instrumentation recognizes hemolysis in neonates and children. We also found that ETCO reference intervals were indeed higher in healthy neonates during the first three days after birth (5th to 95th percentile reference range, 1.4 to 1.7ppm) than after 1month of age (all ≤1.0ppm, p<0.0001). These results suggest to us that hemolysis is physiological during the first days after birth. The cellular and molecular mechanisms responsible for transient hemolysis after birth are topics of current investigation.