Approach to respiratory acidosis Quiz: Ace Your Exams

A 73-year-old man is brought to the emergency department after being found down outside in a nearby park next to a stone wall. EMS reports that a bystander called after the patient was unable to be aroused. He was given a one-time dose of naloxone intranasally in the field without significant improvement. The patient's past medical history is unknown, but bystanders say he usually sleeps in the park. Temperature is 37°C (98.6°F), heart rate 50/min, blood pressure 162/42 mmHg, respiratory rate 8/min and irregular, and O₂ saturation 93% on room air. The patient is moaning with slurred speech. He has a large hematoma on the right side of his head and significant ecchymoses on the right shoulder, elbow, and hip. He is moving all extremities spontaneously. Pupillary examination reveals normal-sized pupils bilaterally that are equal, reactive, and oval-shaped, Fundoscopic examination shows papilledema bilaterally. Cardiopulmonary examination reveals irregular respirations, but no murmurs are present. Lungs are clear to auscultation bilaterally. Abdominal exam is within normal limits. Point-of-care blood glucose is within normal limits. An arterial blood gas is drawn and shows a pH 7.27 and a pCO₂ 52. Which of the following tests will most likely confirm the underlying diagnosis?