Milrinone for Prevention of Post-ligation Cardiac Syndrome Trial

Researchers will compare milrinone to a placebo saline solution to see if it helps the heart work better by supplying oxygen to the lungs and tissues. After randomization the following will happen in both the control and treatment groups: * Infant will start receiving either a low dose (0.33 μg/kg/min) of milrinone in the treatment group, OR saline solution in the control group through an intravenous (IV) line. The study drug will be started within 2 hours of the PDA closure procedure. * For the first 2 to 4 hours, the study team will monitor very closely to see if there are any major side effects from the study drug. * If the infant continues to have high blood pressure, which places an added stress on the heart, the dose will go up to 0.66 μg/kg/min. One more increase will be allowed to 0.75 μg/kg/min. The study drug will then stay at this dose. No more changes will happen to the dose. If there are side effects, then the study drug will be stopped and will not be started again. * The study drug will be stopped after 24 hours if the infant only required the lowest drug and remains well. If the required higher doses of milrinone, it will take longer to wean them off the medication. Some infants may receive milrinone for 3-5 days. If the infant's heart starts to work better by using less oxygen from the breathing machine (ventilator), the study drug may be stopped before 3 days. * Information will be collected from the infant's medical record including demographic information, gestational age, blood pressures, heart rates, respiratory rates, information about his or her breathing, medications, details of medical treatment for PDA, medical procedures including echocardiograms (ultrasound of the baby's heart), details of PDA closure (by surgery or transcatheter closure), head ultrasounds, and diagnoses. The investigators will also be collecting information from the maternal medical record including ultrasounds that may be relevant to the infant's course. * Right after the study drug is stopped (which could be 5 days or less) the doctor may decide to give the infant the drug milrinone. This is a decision that will be made by the infant's doctor. * After the infant goes home from the hospital, they will be scheduled for follow-up exams in the clinic. The follow-up visit will be done when the infant is about 2 years old. The visit will take about two hours to do. During the follow-up visit the investigators will test the infant's movement, behavior, and development. The investigators will also check the infant to make sure they do not have high blood pressure or any evidence of kidney disease.