Carbon Monoxide Poisoning - Diagnosis

Diagnosis

As many symptoms of carbon monoxide poisoning also occur with many other types of poisonings and infections (such as the flu), the diagnosis is often difficult. A history of potential carbon monoxide exposure, such as being exposed to a residential fire, may suggest poisoning, but the diagnosis is confirmed by measuring the levels of carbon monoxide in the blood. This can be determined by measuring the amount of carboxyhemoglobin compared to the amount of hemoglobin in the blood. Carbon monoxide is produced naturally by the body as a byproduct of converting protoporphyrin into bilirubin. This carbon monoxide also combines with hemoglobin to make carbooxyhemoglobin, but not at toxic levels. The ratio of carboxyhemoglobin to hemoglobin molecules in an average person may be up to 5%, although cigarette smokers who smoke two packs/day may have levels up to 9%.

As people may continue to experience significant symptoms of CO poisoning long after their blood carboxyhemoglobin concentration has returned to normal, presenting to examination with a normal carboxyhemoglobin level (which may happen in late states of poisoning) does not rule out poisoning.

A CO-oximeter is used to determine carboxyhemoglobin levels. Pulse CO-oximeters estimate carboxyhemoglobin with a non-invasive finger clip similar to a pulse oximeter. These devices function by passing various wavelengths of light through the fingertip and measuring the light absorption of the different types of hemoglobin in the capillaries.

The use of a regular pulse oximeter is not effective in the diagnosis of carbon monoxide poisoning as people suffering from carbon monoxide poisoning may have a normal oxygen saturation level on a pulse oximeter. This is due to the carboxyhemoglobin being misrepresented as oxyhemoglobin.

Breath CO monitoring offers a viable alternative to pulse CO-oximetry. Carboxyhemoglobin levels have been shown to have a strong correlation with breath CO concentration. However, many of these devices require the user to inhale deeply and hold their breath to allow the CO in the blood to escape into the lung before the measurement can be made. As this is not possible in a nonresponsive patient, these devices are not appropriate for use in on-scene emergency care detection of CO poisoning.