Final answer:
The best indication of the effectiveness of furosemide in treating a preterm infant's patent ductus arteriosus is an increase in urine output that exceeds fluid intake. This result shows the drug's efficacy in reducing fluid overload and pulmonary pressure.
Step-by-step explanation:
The nursing assessment that provides the best indication of the effectiveness of furosemide (Lasix) in a preterm infant being treated for persistent patent ductus arteriosus is urine output exceeds fluid intake. Furosemide is a diuretic, which means it helps to remove excess fluid from the body by increasing urine output. In cases of patent ductus arteriosus, this can help reduce the additional fluid that is moving toward the lungs, thereby decreasing pulmonary pressure and aiding respiration. Monitoring urine output ensures that the medication is effectively reducing fluid overload, which is important as the neonatal kidneys are immature and inefficient at concentrating urine. An increase in urine output relative to fluid intake can indicate that the furosemide is effectively reducing fluid accumulation that may be contributing to symptoms like dyspnea, tachycardia, and an enlarged heart.