A 20-year-old woman is brought to the emergency department by her boyfriend after being found unresponsive in her apartment. She has a medical history of major depression and generalized anxiety disorder. There is no evidence of trauma. On examination, the patient is obtunded and responds only to painful stimuli. There are no indications of recent vomiting. Lungs are clear to auscultation. Toxicology screen is positive for benzodiazepines. Which of the following sets of arterial blood gas findings (in mm Hg) are most likely present in this patient?

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11ec160b_3ce6_2611_9249_9d5a864402cc_MD0001_00 This patient has an acute benzodiazepine overdose. Benzodiazepines (eg, alprazolam, lorazepam) cause sedation and central respiratory depression with hypoventilation and CO₂ retention; therefore, affected patients have acute respiratory acidosis (low pH and PaCO₂ >40 mm Hg). Because full metabolic compensation by the kidneys (ie, HCO₃⁻ retention) requires approximately 72 hours, there is minimal compensation in the acute setting, and the expected serum HCO₃⁻ is near normal (24 mEq/L). The alveolar-arterial (A-a) O₂gradient is the difference between the partial pressure of alveolar oxygen (PAO₂) and the partial pressure of arterial oxygen (PaO₂); it represents the efficiency of gas transfer between the lungs and the circulation. A small gradient (eg, 4-15 mm Hg) is normal in part because there is physiologic shunting of unoxygenated bronchial blood, which decreases PaO₂. In addition to CO₂ retention, hypoventilation also causes low PAO₂that leads directly to hypoxemia (PaO₂ <75 mm Hg). The efficiency of gas transfer is intact and a normal A-a O₂ gradient is expected. (Choice A) PaCO₂ <40 mm Hg indicates respiratory alkalosis. Respiratory alkalosis (ie, hyperventilation) with hypoxemia and a normal A-a O₂ gradient is expected at high altitude. The efficiency of gas transfer is intact, but low partial pressure of inspired oxygen (PiO₂) causes low PAO₂. (Choice B) Respiratory alkalosis with hypoxemia and an elevated A-a O₂ gradient is expected with lung pathology that causes acute V/Q mismatch (eg, pulmonary embolism and pneumonia). (Choices C and E) Respiratory acidosis with hypoxemia and an elevated A-a O₂ gradient is expected with chronic obstructive pulmonary disease. These patients have chronic CO₂ retention, and O₂ exchange is impaired by emphysematous disruption of gas diffusion across the alveolar-capillary membrane. Educational objective: Acute benzodiazepine (eg, alprazolam, lorazepam) overdose causes central respiratory depression with hypoventilation. The hypoventilation leads to CO₂ retention and acute respiratory acidosis. It also decreases the PAO₂, leading directly to hypoxemia (PaO₂ <75 mm Hg). The efficiency of gas transfer between the lungs and the circulation is intact; therefore, a normal alveolar-arterial O₂ gradient (eg, <15 mm Hg) is expected.

A 20-Year-Old Woman Is Brought to the Emergency Department by Her