From the Guidelines
Carboxyhemoglobin (COHb) levels above 25% are concerning for carbon monoxide poisoning, as they warrant hyperbaric oxygen (HBO) therapy, especially in patients with loss of consciousness, ischemic cardiac changes, neurological deficits, or significant metabolic acidosis. According to the practice recommendations in the diagnosis, management, and prevention of carbon monoxide poisoning 1, the role of carboxyhemoglobin level is to confirm the clinical diagnosis, although it correlates poorly with symptoms or prognosis.
- Key points to consider in carbon monoxide poisoning include:
- Symptoms are nonspecific and can range from headache, dizziness, nausea/vomiting, confusion, fatigue, chest pain, shortness of breath, and loss of consciousness
- The management of carbon monoxide poisoning involves administering 100% oxygen while waiting for COHb level, with normobaric oxygen therapy as the initial treatment
- Hyperbaric oxygen therapy is considered for patients with severe poisoning, including those with COHb levels above 25%
- The goals of HBO therapy are to prevent neurocognitive sequelae, with the optimal protocol currently unknown
- Patient follow-up is crucial, with accidental poisoning requiring follow-up in 4–6 weeks to screen for cognitive sequelae, and intentional poisoning necessitating psychiatric follow-up due to the high rate of subsequent completed suicide. The concern for carbon monoxide poisoning stems from its high affinity for hemoglobin, which displaces oxygen and causes tissue hypoxia, emphasizing the need for prompt treatment based on both clinical symptoms and measured COHb levels 1.
From the Research
Carboxyhemoglobin Levels in Carbon Monoxide Poisoning
- Carboxyhemoglobin (COHb) levels are used to diagnose carbon monoxide poisoning, but they do not always correlate with the clinical severity of the poisoning 2, 3.
- A study of 1603 patients with CO poisoning found that statistically higher COHb measurements were associated with certain factors, such as male sex, adult age range, and loss of consciousness, but the clinical significance of these differences was minimal 3.
- The presence of clinical signs and symptoms after known exposure to carbon monoxide should not be ignored, even if COHb levels are not elevated 4.
- There is no clear consensus on what level of COHb is concerning for carbon monoxide poisoning, but levels above 10-15% are generally considered elevated 5, 4.
- A case report of a 22-year-old woman with CO poisoning found that she had significant neurological damage despite an initial COHb level of only 2.5%, highlighting the importance of considering clinical symptoms and not just relying on COHb levels 6.
Factors Affecting Carboxyhemoglobin Levels
- Delay and interval oxygen administration before obtaining COHb measurements can affect the accuracy of the results 3.
- COHb measurements can be influenced by factors such as sex, age, and source of CO exposure 3.
- The use of hyperbaric oxygen therapy can increase CO elimination and improve outcomes in patients with CO poisoning, but its benefit in limiting or reversing cardiac injury is unknown 2, 6.