Increased Carbon Monoxide Washout Rates in Newborn Infants

Endogenous carbon monoxide (CO) production is primarily due to heme degradation, which also results in the equimolar production of bilirubin. Thus, estimates of total body CO production can serve as indices of total body bilirubin formation. The elimination rate of CO from a person's body (CO washout rate) after exposure to an elevated ambient CO concentration is determined by a variety of factors, and is very different between babies and adults.

We determined CO washout rates for babies using a simplified technique to measure total body CO excretion rates (VeCO).

Using a simplified technique, we measured the times to reach an approximate steady state after a change in ambient CO concentration (decay time constant) and CO washout rates in normal newborn infants using non-linear least squares curve fitting.

We found a mean CO washout time of 18.7 ± 4.2 min and a CO equilibration (decay time) constant of 0.12 ± 0.04/min (0.08-0.21) for newborn infants.

We confirm that CO washout rates for babies are much faster than those for adults. Therefore, measurements of carboxyhemoglobin (COHb) or end-tidal CO (ETCO), corrected for ambient CO, (COHbc and ETCOc, respectively) can be used as surrogates for VeCO and can provide accurate estimates of endogenous CO (VCO) and bilirubin production rates under normal environmental conditions. Such measurements can be used to identify infants with severe hyperbilirubinemia due to hemolysis and thus at high risk for bilirubin neurotoxicity.