A patient who is taking digoxin is admitted to the hospital for treatment of congestive heart failure. The prescriber has ordered furosemide [Lasix]. The nurse notes an irregular heart rate of 86 beats/min, a respiratory rate of 22 breaths per minute, and a blood pressure of 130/82 mm Hg. The nurse auscultates crackles in both lungs. Which laboratory value causes the nurse the most concern?

Choice A reason: A sodium level of 140 mEq/L is within the normal range (135-145 mEq/L). While furosemide can cause hyponatremia, this value is not concerning. Hypokalemia is a greater risk with furosemide and digoxin, as it potentiates digoxin toxicity, making this choice less critical than potassium. Choice B reason: Oxygen saturation of 95% is normal (95-100%). Crackles suggest pulmonary edema, but this saturation doesn’t indicate severe hypoxia requiring immediate action. Hypokalemia poses a greater risk with digoxin and furosemide, as it increases toxicity potential, making this choice less concerning. Choice C reason: A potassium level of 3.0 mEq/L (normal: 3.5-5.0 mEq/L) indicates hypokalemia, exacerbated by furosemide’s diuretic effect. In digoxin use, low potassium increases cardiac toxicity risk, causing arrhythmias, especially with an irregular heart rate noted, making this the most concerning value requiring immediate attention. Choice D reason: A blood glucose level of 100 mg/dL is normal (70-110 mg/dL fasting). It’s unrelated to digoxin or furosemide’s primary risks (e.g., electrolyte imbalances, arrhythmias). Hypokalemia is a more immediate concern due to its synergistic toxicity with digoxin, making this choice less critical.