Diagnosis of Carbon Monoxide Poisoning in the ER

Diagnosing carbon monoxide poisoning in the emergency room starts with measuring carboxyhemoglobin (COHb) levels. If the patient arrived by ambulance, oxygen treatment likely began at the scene and continued during transport. This early oxygen therapy is important, but it also lowers COHb levels, which must be considered when interpreting results.

How Carbon Monoxide Exposure Is Measured

There are two common ways to measure carbon monoxide exposure. The first and quickest method is the use of what is called the “pulse CO oximeter.” This device is placed on a finger, much like the standard fingertip pulse oximeter used in hospitals and emergency rooms, but it is designed specifically to detect carbon monoxide. But the CO pulse oximeter is a different device from the standard pulse oximeter. The CO pulse oximeter can be carried by EMTs and those readings when taken at the scene or in the ambulance, are the most helpful in making a diagnosis.  

Although a CO pulse oximeter isn’t as precise as a blood test performed later, it is usually given much closer to the time of exposure, making it the best indicator of poisoning for those unfamiliar with how COHb levels decrease over time. Unfortunately, there’s a common mistake of disregarding a higher CO pulse oximeter reading if a later blood test shows a lower COHb level. This should be avoided for two reasons. First, CO pulse oximeters are actually more likely to underestimate, rather than overestimate, the true COHb level. So, readings like 11 or 16 percent are quite serious. Second, the CO pulse oximeter gives the most accurate picture of what the COHb levels were at the time the patient was evacuated. 

The challenge with using a blood gas test performed later is that there’s often a significant delay and the patient’s blood has already been ventilated before the sample is taken. In most cases, one to two hours pass between when the patient leaves the toxic environment and when the blood is drawn. For an adult male who has not received oxygen, this delay might result in only a 20–30% decrease from the peak COHb level. However, for a child who has received 30–60 minutes of oxygen, the COHb level could drop by as much as two-thirds. The younger the child, the greater the drop is likely to be. That’s why it’s crucial for the diagnosing physician to know not just the COHb level at the time of testing, but also to consider what the level was when the patient first left the exposure. 

Understanding COHb Metabolism Rates

To estimate the peak COHb level during a carbon monoxide poisoning event based on later emergency room testing, it’s important to understand the concept of “half-life.” The half-life is the amount of time it takes for the COHb level in the blood to drop by half—for example, from 20% to 10%. Traditionally, the half-life of CO in an adult male without oxygen treatment is about five hours. So, if an adult male has a COHb level of 15% a few hours after exposure, his peak level was probably around 20%. However, if the patient received oxygen in the ambulance (which is almost always the case), and continues to receive it in the ER, the half-life will be much shorter. In that case, a 15% COHb level might actually mean the peak was over 30%. 

Why does this matter? In theory, it shouldn’t, since any COHb level above 10% carries a significant risk of long-term brain damage. But in practice, it does matter, because treatment decisions are often based on these numbers. Most emergency rooms will refer a patient with COHb levels in the 30s for hyperbaric oxygen therapy, while patients with levels between 10% and 20% are rarely referred for this treatment. This means the recorded COHb level can impact both immediate care and long-term health outcomes. 

The risk of underestimating COHb levels is especially serious for children. Kids metabolize COHb nearly twice as fast as adults, and the younger or smaller the child, the quicker this happens. When a small child is given oxygen, the COHb half-life can drop to just 20 minutes. So, a COHb reading of 5% after an hour of oxygen could actually mean the child’s peak level was between 30% and 40%.   

Permanent brain damage from carbon monoxide poisoning can be diagnosed even if COHb levels are not elevated at the time of testing. If a person’s blood wasn’t tested while their COHb was still high, that doesn’t rule out CO poisoning. Doctors can estimate peak COHb levels based on other information, such as the carbon monoxide concentration in the air and how long the person was exposed. Environmental and epidemiological details can help determine whether CO poisoning occurred, even without abnormal blood test results. 

If other people involved in the same poisoning event have high COHb levels, you can estimate the COHb level of someone who wasn’t tested or didn’t go to the hospital. For example, if adults exposed to the same air have peak COHb levels of 15–20%, then a reading of 3% in a child should be taken seriously, as it may indicate a much higher peak exposure. 

CO-oximetry is a quick and easy way to check carbon monoxide levels in the blood, which helps understand the extent of exposure in emergencies. But it’s not always perfect, so it should be supported by other testing methods when available. In all cases, half-life and oxygen treatment should be accounted for when estimating peak COHb levels in patients.