Carboxyhaemoglobin levels in infants with hypoxic ischaemic encephalopathy

Hypoxic ischaemic encephalopathy (HIE) is associated with oxidative stress. A potential marker of oxidative damage is carboxyhaemoglobin (COHb) which is the product of the reaction between carbon monoxide and haemoglobin and is routinely assessed on blood gas analysis. Our objective was to test the hypothesis that higher COHb levels would be associated with worse outcomes in infants treated for HIE.

A retrospective, observational study was performed of all infants who received whole body hypothermia for HIE at a tertiary neonatal intensive care unit between January 2018 and August 2021. For each participating infant, the highest COHb level per day was recorded for days one, three and five after birth.

During the study period, 67 infants with a median (IQR) gestational age of 40 (38-41) weeks underwent therapeutic hypothermia for HIE. The median (IQR) COHb level on day three was higher in infants without electroencephalographic seizures (1.4 [1.1-1.4] %) compared with infants with seizures (1.1 [0.9-1.3] %, p=0.024). The median (IQR) COHb on day five was higher in infants without MRI brain abnormalities (1.4 [1.2-1.7] %) compared with infants with MRI abnormalities (1.2 [1.0-1.4] %, p=0.032). The COHb level was not significantly different between the nine infants who died compared to the infants who survived.

COHb levels were higher in infants with HIE without seizures and in those with normal MRI brain examinations. We suggest that carbon monoxide has a potential protective role in HIE.