EXPOSURE TO CARBON MONOXIDE IN PREGNANCY

Carbon monoxide is a colourless, odourless and tasteless gas produced by incomplete combustion of carbon-containing products. Common sources of exposure include faulty heating appliances (gas and solid fuel boilers), BBQs being used inside homes, caravans and tents, car exhausts, open fires, kerosene stoves and cigarette smoke.

Following maternal exposure carbon monoxide crosses the placenta and may reach higher concentrations in the fetus than in the mother. The elimination half-life in the fetus may be up to 4-5 times longer than in the mother. High maternal carboxyhaemoglobin (COHb) concentrations may confer a greater risk to both the mother and fetus, but the correlation between maternal COHb blood concentrations and the clinical severity of maternal poisoning is weak. The possible need for treatment of asymptomatic pregnant women therefore needs to be considered on a case-by-case basis.

Adverse outcomes have been reported after acute carbon monoxide poisoning in pregnancy. Fetal and neonatal death, congenital malformations and neurological problems have all occurred in association with reported moderate to severe (loss of consciousness/coma) maternal toxicity. However, the available data suggest that an increased risk of adverse outcomes cannot be excluded in the absence of maternal toxicity, for example following low level chronic exposure. Some studies investigating chronic environmental in utero exposure to carbon monoxide have reported associations with preterm delivery, low birth weight, congenital malformations, sudden infant death and neurodevelopmental problems. Increased risks for these adverse outcomes have also been reported with maternal smoking in pregnancy – please refer to the UKTIS monograph on ‘Use of tobacco in pregnancy’ for further information.

Maternal toxicity is likely to be a major determinant of risk to the fetus. UK guidance on antenatal testing for carbon monoxide exposure during pregnancy is available online from Public Health England (click here).

There are no published guidelines on the management of carbon monoxide poisoning during pregnancy. Initial management of the poisoned pregnant patient should be the same as for the non-pregnant patient, although treatment may need to be continued for longer given the reduced capacity of the fetus to clear CO. For current guidelines on the management of carbon monoxide poisoning the reader should consult TOXBASE or contact UKTIS. Where exposure to carbon monoxide has occurred, even in the absence of maternal toxicity, enhanced fetal monitoring may be warranted. Discussion with UKTIS is recommended for all cases of carbon monoxide poisoning in pregnancy.