CO in pregnancy
1. What every clinician should know
Carbon monoxide (CO) is the leading cause of poisoning morbidity and mortality in the United States. There is a high incidence of fetal CNS damage and stillbirth after severe maternal CO poisoning. The fetal brain is considered to be at greater risk for CO toxicity than the adult brain. Persistent or delayed cognitive sequelae may occur in up to 50% of patients with symptomatic poisoning. Clinical manifestations of CO toxicity are relatively nonspecific and may include headache, nausea, dizziness, weakness, chest pain, vomiting, confusion, syncope, cardiac arrhythmias and myocardial ischemia.
CO is lipid soluble with adipose tissue accumulation and storage resulting in prolonged excretion in cases of severe and/or prolonged CO exposure. Some animal data suggest that elimination of CO from the fetus takes up to 3.5 times longer than maternal CO elimination. The fetus is considered to be at greater risk than adults.
2. Diagnosis and differential diagnosis
History is very important in considering this diagnosis. Obtain a blood carboxyhemoglobin (COHb). Normal blood COHb is 0–5%. Neonates and patients with hemolytic anemia may have COHb up to 5%. One-pack-per-day cigarette smokers typically have COHb of about 6% but may be up to 10%. Elevated COHb confirms exposure, but specific COHb levels do not predict symptoms or outcome. COHb is misinterpreted as oxyhemoglobin by pulse oximeters, so pulse oximeters overestimate oxyhemoglobin by the approximate amount of COHb present. A COHb greater than 25% may be considered an indication for hyperbaric therapy.
Depending on the patient’s presentation, the differential may be quite wide. History is important to raise suspicion that CO exposure may have occurred.
Begin oxygen administration. In more severe exposures use non-rebreather mask (delivers only 70–90% oxygen) or endotracheal intubation to deliver oxygen. COHb serum half-life is 5 hours on room air (21% oxygen); it is 1 hour (range approximately 0.5–1.5 hours) on 100% oxygen at one atmosphere pressure. Hyperbaric oxygen therapy indications may include fetal distress, syncope, coma, seizure, confusion, altered mental status, COHb greater than 25%, and abnormal cerebellar examination. Referral and transport to a reasonably near hyperbaric facility (if available) may be needed. Endpoint of treatment is resolution of symptoms and COHb less than 5%. Consultation with a regional poison center 800-222-1222 and a physician medical toxicologist is recommended.
Seriously symptomatic pregnant patients should receive 100% oxygen by rebreather face mask at least until symptoms resolve or longer. Pregnant patients with loss of consciousness or COHb greater than 25% may be considered for hyperbaric chamber therapy.
Persistent or delayed adverse effects may include cognitive deficits, incontinence, peripheral neuropathy, amnesia, apraxia, agnosias, chorea, parkinsonism, psychosis, paralysis, dementia, and cortical blindness. Most patients with delayed neurologic sequelae had loss of consciousness in the acute phase of toxicity. Children appear to have greater recovery compared with adults.
Hyperbaric chamber therapy requires myringotomy. Greater than 2 to 3 atmospheres of 100% oxygen for greater than 2 to 3 hours increases the risk for seizures.
5. Prognosis and outcome
Prognosis depends in part on the severity of poisoning. Pregnant women who present with normal mental status and no loss of consciousness have excellent outcomes, and their infants have no subsequent delay in obtaining developmental milestones. There is no high-quality evidence that hyperbaric therapy poses a risk to the fetus.
Long-term health prognosis relates in part to the severity of presentation. Infants and children appear to recover more readily than adults from the adverse effects of CO poisoning, but severe persistent fetal/neonatal adverse effects have occurred.
6. What is the evidence for specific management and treatment recommendations
Koren, G, Sharav, T, Pastuszak, A, Garrettson, LK, Hill, K, Samson, I. “A multicenter prospective study of fetal outcome following accidental carbon monoxide poisoning in pregnancy”. Reprod Toxicol. vol. 5. 1991. pp. 397–403. (All asymptomatic pregnant patients had excellent neonatal outcomes. In three severely symptomatic pregnant patients who did not receive hyperbaric therapy, there were two stillbirths and one case of cerebral palsy.)
Van Hoesen, KB, Camporesi, EM, Moon, RE, Hage, ML, Piantadosi, CA. “Should hyperbaric oxygen be used to treat the pregnant patient for acute carbon monoxide poisoning? A case report and literature review”. JAMA. vol. 261. 1989. pp. 1039-43.
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