Effect of hematocrit on exhaled carbon monoxide in healthy newborn infants.

Objective: We tested the hypothesis that, the red blood cell (RBC) mass, estimated by hematocrit (HCT) or hemoglobin (Hb) level, influences the carbon monoxide (CO) production rate.
Study Design: The relationship between end tidal CO corrected for ambient carbon monoxide level (ETCOc) and the RBC mass have been studied in 58 full-term infants at the mean age 4.9 hours.
Results: Mean ETCOc was 1.88 ppm (1.3 to 3.4 ppm). ETCOc correlated significantly with HCT (R 2=10.1%, p=0.015) and with Hb (R 2=11%, p=0.011). Infants with a capillary HCT >65% had significantly higher ETCOc (mean 1.99+/-0.49 ppm) than infants with a capillary HCT <65% (1.74+/-0.39 ppm), p=0.035. When CO production was corrected for HCT (ETCOc/HCT), this difference did not longer exist.
Conclusions: In newborn infants ETCOc significantly correlates with RBC mass. Comparing different infant's CO generation rate one should take into consideration their initial RBC level. In order to adjust for the existing differences in RBC, we suggest the use of the ETCOc/HCT index.