A 20-year-old fireman comes to the emergency room complaining of headache and dizziness after putting out a garage fire. He does not complain of shortness of breath, and the arterial blood gas shows a normal PO2. There is no cyanosis. Which of the following is the best first step in the management of this patient?

With symptoms of headache and dizziness in a fireman, the diagnosis of carbon monoxide poisoning must be addressed quickly. A venous or arterial measure of carboxyhemoglobin must first be obtained, if possible, before oxygen therapy is begun. The use of supplementary oxygen prior to obtaining the test may be a confounding factor in interpreting blood levels. Oxygen or even hyperbaric oxygen is given after blood for carboxyhemoglobin is drawn. Methemoglobinemia causes cyanosis, which is not present in this patient. ECG is unlikely to be abnormal in this young healthy patient without chest pain. Central nervous system imaging would not be indicated, and there are no diagnostic patterns that are specific to carbon monoxide poisoning. Anemia might cause dizziness, but the symptom would not occur as acutely as in this case.