Carbon monoxide toxicity
The case
A 22-year-old G1P0 woman at 18 weeks estimated gestational age, who is otherwise healthy and has had an uncomplicated pregnancy, presents with 1 day of constant, diffuse headache which was gradual in onset. She has no history of migraines or other primary headache disorders. She has never smoked. She denies fever, neck stiffness, vomiting, vision changes, focal weakness, syncope, or other associated symptoms. She notes that her mother and boyfriend, who live in the same house, had also complained of headaches. She also adds that, due to recent cold weather, they have been running an old space heater in the house for the past 2 days.
Her vital signs are within normal limits. Her complete neurologic examination is normal. Transabdominal ultrasound demonstrates normal fetal activity and a fetal heart rate of 140 bpm.
Her arterial blood gas is as follows:
pH 7.4
pCO2 38
PO2 100
HCO3 26
COHb 5%
Remaining labs including complete blood count, comprehensive metabolic panel, and urinalysis are within normal limits. ECG shows no ischemic changes.
The brush up
Simply put, carbon monoxide (CO) binds to hemoglobin, displacing oxygen. Think about toxicity in terms of organs with high oxygen consumption: brain and heart. Consider CO poisoning in your evaluation of the undifferentiated headache, especially during winter months, and certainly if multiple people from the same household are affected. In patients with chest pain, have a high index of suspicion for myocardial ischemia/infarction, especially if there are risk factors for coronary artery disease. The diagnosis is commonly confirmed with carboxyhemoglobin testing from arterial blood (level may not correlate well with symptoms but COHb > 25% is significant). Consider CO poisoning in any patient presenting after a fire exposure/smoke inhalation as well.
Standard pulse oximetry can't distinguish between oxyhemoglobin and COHb. If you suspect CO poisoning clinically, put the patient on high flow supplemental oxygen (ideally 100% concentration) as soon as possible. Recall that CO causes a leftward shift in the oxyhemoglobin dissociation curve, and that oxygen will reverse this through competitive binding to hemoglobin:
Consequently, the elimination half life of CO decreases from about 6 -> 1 hour when the patient goes from breathing room air to 100% oxygen, respectively. Hyperbaric oxygen (HBO2) therapy will further reduce the half-life to 20-30 minutes. Generally speaking, any patient with an abnormal neurologic exam, evidence of end-organ damage, women with viable pregnancies, and symptoms refractory to normobaric oxygen therapy should be considered for HBO2 therapy, especially if the COHb level is > 25%.
In 2016, ACEP revised the 2008 clinical policy on the management of CO poisoning in adults and additionally addresses the evaluation component. The full text is worth a read, but here is a summary of the recommendations. All of these are Level B recommendations:
1. Do not use noninvasive COHb measurement (plus CO oximetry) to diagnose CO toxicity in patients with suspected acute CO poisoning
2. Emergency physicians should use HBO2 therapy or high-flow normobaric therapy for acute CO-poisoned patients. It remains unclear whether HBO2 therapy is superior to normobaric oxygen therapy for improving long-term neurocognitive outcomes
3. In ED patients with moderate to severe CO poisoning, obtain an ECG and cardiac biomarker levels to identify acute myocardial injury, which can predict poor outcome
The complete policy can be accessed here (requires login).
The conclusion
After the initial evaluation, the patient was placed on 100% oxygen by face mask. Her family members were contacted to come in for evaluation, and additionally they were instructed to turn off and discard the space heater. The patient's symptoms were completely resolved after 2 hours. Repeat HbCO was less than 2%. She was discharged to a hotel room for the night with plans to follow up with her primary care physician the next day.
References
Olson KR & California Poison Control System. (2012). Poisoning & drug overdose. New York: Lange Medical Books/McGraw-Hill.
Clardy PF, Manaker S, Perry H. Carbon monoxide poisoning. In: UpToDate, Grayzel J (Ed), UpToDate, Waltham, MA, 2017.
Wolf SJ, Maloney GE, Shih RD, Shy BD, Brown MD. Clinical policy: critical issues in the evaluation and management of adult patients presenting to the emergency department with acute carbon monoxide poisoning. Ann Emerg Med 2017;69:98-107.
Henry CR, Satran D, Lindgren B, Adkinson C, Nicholson CI, Henry TD. Myocardial injury and long-term mortality following moderate to severe carbon monoxide poisoning. JAMA 2006;295(4):398-402.
Weaver LK, Hopkins RO, Chan KJ, Churchill S, Elliott CJ, Clemmer TP, Orme JF Jr, Thomas FO, Morris AH. Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med 2002;347(14):1057-1067.