The Fontan procedure is a specialized heart surgery performed to help children born with complex heart defects where only one pumping chamber works properly. Understanding how doctors diagnose these conditions and determine whether a child needs this surgery is essential for families navigating this challenging journey.

Introduction: Who Should Undergo Diagnostics

Diagnostic testing for conditions requiring the Fontan procedure begins at birth or even before. Babies born with single ventricle defects, which means they have only one working pumping chamber in their heart instead of two, need comprehensive evaluation from the moment they are diagnosed. In a normal heart, the right ventricle pumps blood to the lungs to pick up oxygen, while the left ventricle pumps oxygen-rich blood to the body. When a baby has only one functional ventricle, that single chamber must do double duty, pumping blood both to the lungs and to the body, which creates a mixture of oxygen-rich and oxygen-poor blood.^[1]

Parents should seek diagnostic evaluation if their newborn shows signs of cyanosis, which is a bluish discoloration of the skin, lips, or nails caused by low oxygen levels in the blood. Other warning signs include difficulty feeding, poor weight gain, rapid breathing, or signs of congestive heart failure. These symptoms suggest that the heart is not effectively delivering oxygen to the body, which requires immediate medical attention.^[3]

Children who have already undergone earlier heart procedures, such as the Norwood procedure or Glenn procedure, need ongoing diagnostic monitoring to determine the right timing for the Fontan surgery. The Fontan procedure is typically performed when children are between 18 and 36 months old, though some centers may perform it earlier or later depending on the individual child’s condition. The decision about timing depends on careful assessment of how well the child’s heart and lungs are functioning.^[1][2]

Classic Diagnostic Methods

Several heart conditions can lead to the need for a Fontan procedure, including hypoplastic left heart syndrome, which is the most common reason for this surgery, as well as tricuspid atresia, double outlet right ventricle, pulmonary atresia, and other complex congenital heart defects. Each of these conditions involves either a ventricle that is too small to function properly or a heart valve that did not form correctly before birth.^[2][4]

Echocardiography, commonly called an echo, is one of the primary diagnostic tools used to evaluate children who may need the Fontan procedure. This ultrasound test of the heart creates detailed images using sound waves, allowing doctors to see the structure of the heart chambers, valves, and blood vessels. Echocardiograms help cardiologists assess how well the single ventricle is pumping, check for any leaky valves, and evaluate blood flow patterns through the heart. This test is painless and does not use radiation, making it safe to repeat as often as needed to monitor a child’s condition.^[1]

Electrocardiograms, known as EKGs or ECGs, measure the electrical activity of the heart. These tests involve placing small sticky patches on the chest that connect to a machine that records the heart’s rhythm. EKGs help doctors identify abnormal heart rhythms, which are common complications in children with single ventricle defects. Heart rhythm problems can develop at any time, even years after the Fontan procedure, so regular EKG monitoring is essential for detecting issues early.^[1]

Cardiac catheterization is a more invasive procedure that provides detailed information about how the heart is working. During this test, a thin tube called a catheter is inserted into a blood vessel, usually in the leg, and carefully guided to the heart. This allows doctors to measure pressures inside the heart chambers and blood vessels, check oxygen levels in different parts of the heart, and take detailed images of the heart’s structure. Cardiac catheterization is particularly important before deciding whether a child is ready for the Fontan procedure because it measures pulmonary pressures, which are the pressures in the blood vessels of the lungs. Low pulmonary pressures are essential for the Fontan circulation to work properly.^[1][2]

Laboratory blood tests provide important information about how well the body’s organs are functioning. Tests that measure oxygen levels in the blood, kidney function, and liver function help doctors understand whether the single ventricle is providing adequate blood flow to the body. When the body does not receive enough oxygen-rich blood, organs can become stressed or damaged over time. Regular blood tests help doctors catch these problems early before they become serious.^[1]

Chest X-rays create images of the heart and lungs using small amounts of radiation. These images help doctors see the size and shape of the heart and check for fluid buildup in the lungs, which can indicate that the heart is not pumping effectively. X-rays are quick and relatively inexpensive, making them useful for routine monitoring, though they provide less detailed information than other imaging methods.

Diagnostics for Clinical Trial Qualification

Children enrolled in clinical trials studying the Fontan procedure or related treatments undergo additional standardized testing to ensure they meet specific criteria for participation. These diagnostic requirements help researchers collect consistent data across different patients and medical centers, which is essential for understanding which treatments work best.

Age is one of the most basic qualifying criteria. Most clinical trials specify that children should be between 2 and 15 years old for the Fontan procedure, with the typical range being 3 to 5 years. However, some trials may focus on younger children or examine outcomes in adolescents and adults who had the procedure years earlier. The age requirement ensures that researchers are studying patients at similar stages of their treatment journey.^[2]

Pulmonary artery pressure measurements obtained through cardiac catheterization are critical qualifying factors for clinical trials. The Fontan circulation depends on blood flowing passively from the body’s veins directly to the pulmonary arteries and into the lungs without a ventricle actively pumping it forward. This passive flow only works when the pressures in the lung blood vessels are low. Clinical trials typically require pulmonary pressures to be below a certain threshold, demonstrating that the lungs can accept this passive blood flow without resistance.^[2][3]

Assessment of ventricular function is another key requirement. The working ventricle must be strong enough to pump all the oxygen-rich blood returning from the lungs out to the body without assistance from a second pumping chamber. Echocardiography and sometimes advanced imaging such as magnetic resonance imaging (MRI) measure how efficiently the ventricle contracts and empties with each heartbeat. Weak ventricular function may disqualify a patient from certain clinical trials or indicate that alternative treatments should be considered.^[2]

Documentation of previous surgical procedures is essential for trial enrollment. The Fontan procedure is typically the third surgery in a staged approach to treating single ventricle defects. Most children first undergo either the Norwood procedure or placement of a Blalock-Taussig-Thomas shunt shortly after birth, followed by the Glenn procedure or hemi-Fontan procedure around 6 months of age. Clinical trials need detailed records of these earlier surgeries, including dates, techniques used, and any complications that occurred. This information helps researchers understand how the timing and sequence of surgeries affect long-term outcomes.^[2][5]

Some research studies investigating new approaches to the Fontan procedure require additional diagnostic testing beyond standard clinical care. These may include exercise testing to measure how much physical activity a child can tolerate, specialized blood tests to check for markers of heart stress or organ function, or advanced imaging techniques that are not yet routine clinical practice. Families considering clinical trial participation should discuss with their medical team what additional testing their child would need and how these tests differ from standard care.