The most appropriate diagnostic test is measurement of the carboxyhemoglobin level. The history of exposure to an idling car in an enclosed space is a risk factor for carbon monoxide poisoning. Symptoms of carbon monoxide poisoning vary and include headache, confusion, nausea, vomiting, and, in severe cases, loss of consciousness. Routine pulse oximetry measures and compares the light absorption of oxygenated and deoxygenated hemoglobin to calculate the percentage of hemoglobin saturated with oxygen. However, it is not able to detect the presence of other abnormal hemoglobin moieties, such as methemoglobin or carboxyhemoglobin. Because of this, standard pulse oximetry may not indicate the presence of either of these abnormal hemoglobins, and a normal hemoglobin saturation by pulse oximetry is inadequate to exclude their presence. Therefore, co-oximetry of an arterial blood gas sample, which measures both types of hemoglobin, is indicated if either of these conditions is suspected, as in this patient.

This patient's troponin level and electrocardiogram findings suggest the presence of myocardial ischemia. Myocardial ischemia occurs in roughly one third of patients with carbon monoxide poisoning, often independent of any known history of, or risk factors for, coronary artery disease. Vascular occlusive coronary artery disease is also less likely in this patient without clear cardiovascular risk factors. Therefore, cardiac catheterization would not be an appropriate next diagnostic step in this patient with possible myocardial ischemia due to carbon monoxide poisoning.

Electroencephalography is also not appropriate for this patient. Although he may have developed a seizure or acute coronary syndrome after pulling his car into the garage, neither possibility fully accounts for the multiple signs of end-organ damage seen with carbon monoxide poisoning. This patient has a history of seizures and could have postictal encephalopathy with an accompanying lactic acidosis, but this would not explain his myocardial ischemia. There are no corroborating signs of seizure such as a tongue laceration. Given the absence of hemodynamic instability, acute coronary syndrome is unlikely to present with encephalopathy.

This patient's clinical scenario is highly consistent with carbon monoxide poisoning, and the combination of obtundation, preserved respiratory drive, low blood pressure, and normal pupil findings are not typical of sedating or sympathomimetic drug ingestions.