The catabolism of heme produces equimolar amounts of bilirubin and carbon monoxide (CO) and the measurement of end tidal CO concentration, corrected for ambient CO (ETCOc) is therefore auseful measurement of bilirubin production. We studied 50 term and near term infants (gestation 38.3 ± 1.9 wk) who were readmitted 2-16 days after birth because of hyperbilirubinemia (mean bilirubin level 19.9 + 2.7 mg/dL). 46 were breast fed and 4 had positive Coombs tests (all OA incompatibility). We compared ETCOc measurements in the jaundiced babies with ETCOc in 102 Coombs negative infants ≥ 36 weeks gestation, 48-96 hours old. Mean ETCOc in the 50 jaundiced infants was 1.66± 0.74 ppm vs 1.25 ± 0.64 ppm in the control infants (p=0.0006). There was a significant correlation between the serum bilirubin level on admission and ETCOc (r=.342, p=.0149) but no correlation between ETCOc and reticulocyte counts or hemoglobin levels. The 95th percentile for our control population is an ETCOc of 2.4 ppm. Only 6 (12%) of the hyperbilirubinemic babies had an ETCOc > 2.4 ppm but 25 (50%) had ETCOc levels at or above the 75th percentile (1.6 ppm). In the 4 Coombs positive infants, ETCOc was 1.1, 1.3, 1.5 and 1.6 ppm.

These data are consistent with previous clinical observations which suggest that only a few of these babies are actually hemolyzing while most (or all) have difficulty clearing their bilirubin. Nevertheless, as a group, their distribution of ETCOc is shifted to the right and they have a mean CO (and therefore bilirubin) production that is about 30% higher than normal. In our normal nursery population there is a decrease of ETCOc from a mean of 1.86± 0.95 ppm in the first 24 hours to 1.25 ± 0.64 ppm after 48 hrs, but it appears that certain infants maintain a higher rate of heme turnover for several days. When combined with sluggish bilirubin clearance, this can lead to significant elevation of serum bilirubin levels.