Carbon Monoxide Poisoning in a Third-Trimester Pregnancy
A 29-year-old woman at 30 weeks' gestation is rescued from a house fire. She is drowsy but arousable, with headache and nausea. On 15 L/min oxygen by nonrebreather her SpO2 is 98%, carboxyhemoglobin is 18% measured 2 hours after exposure, and fetal heart rate is 170 beats/min. She has sickle cell trait. The nearest hyperbaric chamber is 45 minutes away and can accommodate her with an obstetric nurse; the local hospital has no maternity intensive care. The fire department reports heavy smoke and possible cyanide exposure. She is otherwise hemodynamically stable.
1. Does she meet criteria for hyperbaric oxygen, and what is the role of the 18% COHb?
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Pregnancy lowers the threshold for treating CO poisoning because fetal hemoglobin binds CO more avidly and the fetal circulation clears CO slowly. An 18% maternal COHb measured 2 hours after exposure, with drowsiness, headache, nausea, and fetal tachycardia, is concerning; the measured level underestimates peak exposure and fetal burden. I would treat with HBO, especially with neurologic symptoms or fetal distress, rather than rely on the number alone. If HBO is delayed, give 100% oxygen continuously. The judgment is that the decision is clinical and fetal, not a single threshold; pregnancy is a relative indication, and syncope or fetal compromise pushes toward treatment.
2. How should transport and fetal monitoring be arranged?
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Transport to the chamber with continuous 100% oxygen, maternal vital signs, and fetal monitoring if available; an obstetric nurse should accompany her. Because the local hospital lacks maternity intensive care, the chamber facility must have obstetric backup or a plan for transfer if she deteriorates. Avoid unnecessary delay for a COHb recheck; a repeat level can be falsely low after oxygen. Notify the chamber and obstetric team before arrival. The judgment is that the fetus is a second patient: maternal stabilization and fetal surveillance must be coordinated, not sequential.
3. How do possible cyanide exposure and sickle cell trait affect management?
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Smoke inhalation raises concern for cyanide toxicity, which can cause lactic acidosis, altered mental status, and cardiovascular collapse despite a normal SpO2. If there is unexplained acidosis or persistent neurologic depression, treat empirically with hydroxocobalamin in addition to oxygen; do not wait for a cyanide level. Sickle cell trait is not a contraindication to HBO, but dehydration, acidosis, and hypoxia should be corrected, and she should be monitored for vaso-occlusive complications. The judgment is to treat the clinically important coexposures and comorbidities, not to let the COHb value anchor the entire plan.