From the Guidelines
In carbon monoxide poisoning, arterial blood gas (ABG) results typically show a normal partial pressure of oxygen (PaO2) despite the patient being hypoxic, which is a key diagnostic clue. The oxygen saturation measured by pulse oximetry (SpO2) will appear falsely normal because standard pulse oximeters cannot distinguish between oxyhemoglobin and carboxyhemoglobin 1. The ABG will reveal a metabolic acidosis due to tissue hypoxia, with decreased pH and decreased bicarbonate levels. The PaCO2 may be normal initially but can decrease as the patient hyperventilates to compensate for the acidosis.
- The most important diagnostic finding is an elevated carboxyhemoglobin (COHb) level, typically above 10% in symptomatic patients, which directly measures carbon monoxide bound to hemoglobin 1.
- Severe poisoning may show COHb levels exceeding 25% 1.
- The calculated oxygen saturation from the blood gas analyzer will be decreased, reflecting the true oxygen-carrying capacity, unlike pulse oximetry.
- Lactate levels are often elevated due to anaerobic metabolism from tissue hypoxia. These ABG findings, particularly the elevated COHb level with normal PaO2, are essential for diagnosing carbon monoxide poisoning and guiding treatment decisions regarding oxygen therapy. The goal of treatment is to prevent long-term and permanent neurocognitive dysfunction, and hyperbaric oxygen therapy should be considered in all cases of serious acute CO poisoning 1.
From the Research
Arterial Blood Gas Results in Carbon Monoxide Poisoning
- The symptoms of carbon monoxide (CO) poisoning are nonspecific, ranging from dizziness and headache to unconsciousness and death 2.
- In patients with severe carbon monoxide poisoning, an ECG should be obtained and biomarkers for cardiac ischemia should be measured 2.
- The reduced oxygen-carrying capacity of the blood, impairment of the cellular respiratory chain, and immune-modulating processes can lead to tissue injury in the myocardium and brain even after lowering of the carboxyhemoglobin (COHb) concentration 2.
- Hyperbaric oxygen therapy (HBOT) should be critically considered and initiated within six hours in patients with neurologic deficits, unconsciousness, cardiac ischemia, pregnancy, and/or a very high COHb concentration 2.
Carboxyhemoglobin Levels
- The mean baseline COHb level of patients with acute carbon monoxide poisoning was 22.5% (SD 8%) 3.
- The mean half-life of carboxyhemoglobin was determined as 36.8 minutes (SD 9.26 min) with high flow nasal cannula oxygen 3.
- COHb levels were halved during the first 40 minutes in 67% of the study patients 3.
- Diagnosis of carbon monoxide poisoning is made by prompt measurement of carboxyhemoglobin levels 4.
Treatment and Outcomes
- The initiation of 100% oxygen breathing as early as possible is the most important treatment for carbon monoxide poisoning 2.
- Hyperbaric oxygen benefits the brain more than normobaric oxygen by improving energy metabolism, preventing lipid peroxidation, and decreasing neutrophil adherence 5.
- Randomized controlled trials have definitively shown hyperbaric oxygen as the only efficacious therapy for acute CO poisoning if delayed neurological sequelae are to be minimized 5.
- A case study reported a patient with significant neurological damage related to CO exposure who improved after hyperbaric oxygen therapy, despite an initial carboxyhaemoglobin level of 2.5% 6.