A 65-year-old man is scheduled to undergo staged bilateral cataract extractions under conscious sedation. Medical history is notable for hypertension and asthma. Medications are amlodipine, an albuterol inhaler as needed, and a fluticasone inhaler.

On physical examination, blood pressure is 122/74 mm Hg. The cardiac examination is normal. The lungs are clear.

Laboratory studies from 1 year ago show normal serum electrolyte and serum creatinine levels.

Which of the following is the most appropriate diagnostic test to perform next?