Children between birth and 17 years old with congenital or acquired heart disease who need to take medication to prevent blood clots are eligible for the study. Females cannot be pregnant and must not be actively breastfeeding. Patients can either be just starting anticoagulants for the first time or be on long-term medications like Vitamin K antagonists (VKA) [Warfarin, Coumadin] or low molecular weight heparin (LMWH) [Enoxaparin, Lovenox, Xaparin] for at least 6 months after a blood clot was found. Children with prosthetic and mechanical heart valves are not eligible to be in the study.

The study will be explained to you in detail by one of the study investigators. Once your questions have been answered, you will be asked to sign an informed consent form to enter the study.

The study has four major parts:

1. Screening: We will collect information about your child’s medical history and do a physical examination, which includes measuring heart rate, blood pressure, body temperature, height and weight; and draw blood for laboratory tests. Children 5 years and older will have a bone density scan. Females of child bearing potential will also be asked to provide a pregnancy test.
2. Randomization: Children will be randomly assigned to receive either apixaban (by mouth or feeding tube) or to receive one of the blood thinners commonly used to treat this condition: either VKA or LMWH. The study doctor will provide instructions about the particular study medication that the child is assigned to. Older participants will also be asked to answer survey questions about quality of life. A blood sample will be requested.
3. Treatment: Participants will take the study medication for 12 months, or until they no longer need a blood thinner, whichever is shorter. A medication diary will help families to keep track of the medication taken every day. Return visits to the study center will be at 2 weeks, 3 months, 6 months, 12 months and 14 months. At each visit, we will do a physical examination and draw blood for laboratory tests. These visits will also include questions about side effects, adverse events, and blood clots or bleeding. Some of these visits will also include a bone density scan and quality of life questionnaires. The study center will also call to check up on the child at 8 months and 10 months.
4. Follow-up: One final study visit, either in person or over the phone, will occur after the study medication is finished. This will be at about 14 months. We will ask again about illnesses, bleeding or clots, and hospital admissions or visits.

Participation can last for up to 14 months, or until anticoagulation is no longer needed – whichever is shorter.

• Apixaban may prevent formation of dangerous blood clots or prevent the growth of existing blood clots.
• Your child may not benefit from taking part in this research study. But the information we learn from this study may help improve the care of other children.

• Taking part in a research study involves risks or “side effects.” You should talk about these risks with the study doctor or your child’s regular doctor. There may be other side effects we do not know about yet.
• As with any anticoagulant medication, there is an increased risk of bleeding. This can be from a minor bleed (nosebleed, bleeding gums or bruising) or in rare occasions, more serous bleeding (in critical organs of the body, such as in the brain and stomach) or may be related to surgery or a medical procedure. Other possible side effects observed in people in previous studies of blood thinners include dizziness, rash, itchiness, and headache.
• Your child might have a black and blue mark or a small amount of bleeding where the needle is put in his or her body for the blood test. There is a very small chance that this area will get infected. Your child might feel lightheaded or dizzy or even faint after the blood test.
• A bone scan is a simple, quick and noninvasive procedure. The amount of radiation used during this scan is extremely small, less than 10% of the radiation received in a standard chest x-ray.
• You will be informed of any changes in the way the study will be done and any additional risks identified.

There will be no extra costs to you when you join this study. You must pay for all other costs related to your normal medical care such as hospital stays, surgery, drugs, lab tests, and doctor’s fees which are thought to be standard medical care for patients with your condition.