Evidence indicates that the brain is the organ most sensitive to changes in oxygenation. Accordingly, neurological symptoms such as headache, dizziness, lethargy and confusion are usually the first seen in patients exposed to low levels of CO. As the exposure continues or the concentration of gas is increased, patients will experience nausea, dyspnea, blurred vision, fatigue and agitation. The level of consciousness continues to decline, and patients eventually slip into a coma and then death. See Table I for a complete list of symptoms.
There are no physical assessment signs of CO poisoning, although a thorough primary and secondary are certainly still needed to rule out any trauma or medical sources of the symptoms. Especially when the patient has confusion or lethargy, he/she cannot be trusted to report injuries or other symptoms. Although widely reported in stories and training scenarios, the cherry red skin color of carbon monoxide poisoning is rarely seen and is more likely only noted in fatalities.4
Pulse oximetry in this setting should be used with caution. Standard pulse oximeters ignore hemoglobin containing CO, thus showing a normal to high SpO2 percentage. Without an understanding of CO poisoning, the EMT would misinterpret this as the patient being well-oxygenated and delay or forego important CO poisoning treatment. Even when CO exposure is suspected, SpO2 measurement should still be used to rule out conditions that might present with a low SpO2.
Specific devices are now available for measuring the level of carboxyhemoglobin in the blood. Hospital laboratories can measure HgCO by testing a sample of arterial or venous blood. Carboxyhemoglobin levels do not necessarily correlate with patients' conditions or their prognosis.9 Low levels of HgCO do not rule out a significant exposure, especially if the patient has already been receiving 100% oxygen by mask.
Although exhaled breath analyzers have been available for several years, they are not popular in the EMS setting, as they typically require both a cooperative patient to follow the testing procedure and regular calibration. A newer device called a pulse CO-oximeter allows immediate HgCO measurement in the field. The Masimo RAD-57 is the only prehospital pulse CO-oximeter approved by the FDA at this time. The unit functions similarly to a standard pulse oximeter and gives an accurate estimate of the patient's HgCO.10 The CO-oximeter only requires placement of a sensor on the patient's finger. The same unit provides SpO2 and CO, and can provide methemoglobin measurement if so configured.
Emergency departments are beginning to use pulse CO-oximeters to screen all patients for carbon monoxide poisoning. After implementing the devices at an emergency department in Rhode Island, triage nurses identified several patients who might otherwise not have had CO poisoning pinpointed as the cause of their complaints.11
Recognizing the increased risk of CO poisoning to citizens and rescuers, the Federal Emergency Management Agency (FEMA) has added the RAD-57 to the required equipment lists of each of its Urban Search and Rescue (USAR) task forces.12 USAR teams respond to a wide variety of natural and man-made disasters and often work in confined or poorly ventilated spaces. The hydrocarbon-powered tools they use to stabilize scenes and to locate and extricate victims create a risk of CO exposure. And, as described earlier, the patients rescue crews encounter may also be victims of CO. Early assessment of HgCO in these patients is very helpful in preventing ongoing poisoning, and in triage and management of limited medical resources.
In 2003, the National Fire Protection Association (NFPA) issued recommendations for firefighter rehabilitation. NFPA 1584 became a standard in 2008, setting the stage for required and recommended aspects of rehab on fire scenes and during training.13 Assessment of carbon monoxide exposure is considered a recommended vital sign in the medical monitoring portion of the standard. To help their fire districts comply with the standard, the state of Delaware recently purchased 150 Masimo RAD-57 pulse CO-oximeters and deployed them to each fire department in the state.14 The units will be used to assess citizen victims, as well as to monitor firefighters and other responders.