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<< Diagnosis And Management Of Carbon Monoxide Poisoning In The Emergency Department

Diagnostic Studies

Co-oximetry

Standard pulse oximetry uses 2 wavelengths of light that are absorbed by oxyhemoglobin and deoxyhemoglobin, respectively. A calculation is then done based on the relative absorption of these 2 wavelengths as they pass through the fingertip, which yields the oxyhemoglobin percentage. Because the wavelength utilized in this instrument for oxyhemoglobin is also absorbed by COHb, this tool will be incapable of distinguishing between them.

A more recent technological innovation—noninvasive pulse co-oximetry—uses multiple wavelengths of light which are specifically absorbed by oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and in some instruments, methemoglobin. Two small studies prospectively evaluated this technology. One, with 20 CO-intoxicated patients and 5 healthy controls, compared results obtained via the noninvasive pulse co-oximetry against the gold standard blood gas co-oximetry analysis. Results showed a mean absolute error of 3.15% for the noninvasive method. It is worth noting that comparison of COHb levels between the various blood gas analyzers used in the department revealed a mean absolute variation of 2.4%.20 A second study, with 12 patients, compared pulse co-oximetry with blood gas co-oximetry. No difference was found between the 2 techniques (P > 0.05).21 A third study, published as a letter, found 5 false positive readings in 328 patients, and the study was halted after a single false negative reading was found.22 Although noninvasive pulse co-oximetry appears accurate on the basis of the small studies mentioned above, the equipment is relatively expensive and is not available in most EDs. It cannot be considered the standard of care.

Carboxyhemoglobin Testing

Any patient with known or suspected CO poisoning should undergo COHb-level testing. The relationship between COHb levels and poisoning severity is generally poor; accordingly, this should be obtained while initiating empiric treatment.23 Carboxyhemoglobin levels may be determined from venous or arterial blood, in a heparinized tube (typically a “green-top” or arterial blood gas syringe). Arterial COHb levels are ideal. In addition to being an accurate measurement technique, the additional information obtained from blood gas testing is helpful in assessing the acid-base status of the critically poisoned patient, but in most circumstances, venous samples are more rapidly and readily obtained by non-physician personnel. In addition, there is no clinically relevant difference between arterial and venous COHb levels, with 95% of the samples falling in the range of between 2.4% and -2.1% of each other.24 Therefore, venous COHb measurements are useful and accurate in the triage of multiple suspected poisonings, such as from a fire scene or industrial accident. If the clinical suspicion for acidosis is low, venous blood sampling causes less discomfort to patients than arterial blood sampling and may be preferable. Serial measurements of COHb are generally not helpful. An exception is in the case of CO poisoning via endogenous hepatic metabolism of methylene chloride to CO. In these patients, the apparent half-life of CO may be prolonged to 13 hours.25

A COHb level greater than 3% in non-smokers or greater than 10% in smokers suggests an abnormal CO exposure. Nonetheless, a patient may have toxicity with a normal or near-normal level, depending on the timing of exposure relative to sampling, length of exposure, degree of exposure, and any oxygen therapy initiated prior to sampling.

Electrocardiogram

All patients with a history suggestive of CO exposure should undergo ECG and cardiac monitoring because of the potential for myocardial ischemia or dysrhythmia. Ventricular dysrhythmias or ST-segment changes are sometimes seen.26,27

Cardiac Biomarkers

By consensus, most sources recommend cardiac biomarker testing be performed if patients have ECG changes or abnormalities suggestive of ischemia, symptoms consistent with myocardial ischemia, history of coronary artery disease, or age greater than 65.9

Pregnancy Testing

Qualitative urine or serum human chorionic gonadotropin (HCG) testing should be performed in female patients of childbearing age. Theoretically, elimination of CO from fetal hemoglobin takes over 3 times as long as it does for the mother; however, this is based on a sheep model.28

Pregnant women have been excluded from prospective trials of HBO therapy, so there is no evidence to suggest a difference in length of treatment or use of HBO therapy in the pregnant patient.

Chest Radiography

A chest x-ray is a useful diagnostic adjunct in the patient with abnormal lung findings, persistent hypoxia, or ventilatory failure. It may help distinguish between and guide treatment of pulmonary edema, aspiration, pneumonitis, pneumothorax, or an underlying preexisting condition such as chronic obstructive pulmonary disease (COPD).

Computed Tomography

Computed tomography (CT) scans of the head and cervical spine are useful in the patient with known or suspected trauma, such as those from the scene of a fire or explosion. Head CT is also helpful in the evaluation of the patient who presents with altered mental status and an unclear or unobtainable history in order to evaluate for traumatic intracranial injury, spontaneous intracranial hemorrhage, or structural abnormalities contributing to or causing the patient’s symptoms. For patients who will be undergoing HBO therapy (and will thus be difficult to examine) this is more important. Changes on the CT scan due to CO poisoning appear acutely within 12 hours of CO exposure but may not be apparent if the CT scan is obtained upon presentation to the ED.9 Therefore, neuroimaging is not useful and should not delay therapy in a patient with isolated CO poisoning and a well-established diagnosis. Hypodense lesions in the globus pallidus, caudate, and putamen are common findings.49

Toxicologic Testing

Toxicology screens, including assays for volatile alcohols, salicylates, acetaminophen, and drugs of abuse may be helpful in determining the cause of altered mental status. In patients who present after a suicide attempt or suicidal gesture, toxicology screens may identify poisons that are treatable if identified early, such as acetylsalicylic acid and acetaminophen. The test characteristics and potential inadequacies of toxicologic assays and drugs of abuse screens are well-discussed in the medical literature, and care must be taken not to ascribe a patient’s symptoms to toxic ingestion alone if other causes of altered mentation such as trauma, shock, and metabolic disturbance have not been thoroughly investigated.

Lactate

Protracted exposure to CO can cause an increase in the serum lactate due to increased anaerobic cellular metabolism in the setting of tissue hypoxia,31 but the quantitative lactate level does not correlate with the severity of CO poisoning. A very high lactate level in a patient who has suffered smoke inhalation might prompt empiric treatment for cyanide toxicity, but there is insufficient evidence to guide the threshold for such therapy.

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Last Modified: 11/23/2017
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