A 66-year-old patient with heart disease requires ampicillin (Principen) IV for an infection. Which overnight change in the patient's assessment findings is the nurse's priority?

The nurse's priority finding is the weight gain of 0.68 kg (approximately 1.5 pounds) overnight. Ampicillin is a penicillin-class antibiotic that is formulated as a sodium salt; the intravenous preparation delivers a significant sodium load with each dose. In a 66-year-old patient with underlying heart disease, excess sodium intake increases intravascular fluid volume, which can precipitate or worsen heart failure, leading to fluid retention and weight gain. An overnight weight gain of even 0.68 kg is a clinically significant early indicator of fluid accumulation and demands prompt assessment for signs of worsening heart failure such as dyspnea, edema, and crackles. A potassium level of 4.0 mEq/L is within normal range (3.5–5.0 mEq/L) and requires no intervention. A heart rate of 94 beats per minute is mildly elevated but not immediately alarming on its own and does not represent an acute change requiring priority action. A hemoglobin of 10.5 g/dL indicates mild anemia, which should be monitored, but it does not represent an acute overnight change that would supersede concern for fluid overload in a cardiac patient. Identifying fluid retention early is essential because deterioration in cardiac function can occur rapidly in vulnerable populations. Weight gain is the most sensitive early indicator of fluid retention.