A 45-year-old woman is brought to the emergency department by EMS after being found confused in her apartment. She complains of a severe, throbbing diffuse headache and nausea that began approximately 4 hours ago. She reports feeling dizzy and unsteady on her feet. Her boyfriend, who lives with her, is complaining of similar symptoms. It is currently winter, and she mentions they recently started using an older gas heater in the living room. She has a history of migraines and smokes half a pack of cigarettes daily. She denies chest pain, palpitations, or loss of consciousness.

Initial laboratory studies:

WBC: 9,000/mm³(4500-10,000)

Hemoglobin: 13.8 g/dL (12.0-16.0)

Platelets: 240,000/mm³(150,000-400,000)

Na⁺: 138 mEq/L (136-146)

K⁺: 4.0 mEq/L (3.5-5.0)

Creatinine: 0.8 mg/dL (0.6-1.1)

Glucose: 105 mg/dL