Intracardiac Shunts

Etiology

The etiology of Intracardiac shunt formation is multifactorial and combines genetic and external factors. Genetic factors include chromosomal deletions, trisomies, and single-gene mutations. For example, 40 to 50% of those born with trisomy 21 or Down syndrome have ASD, VSD, TOF, or TGA.[2] Maternal factors that increase the risk for intracardiac shunt formation are pregestational diabetes, marijuana use, rubella, ibuprofen, influenza, febrile illness, and vitamin A exposure.[3] 

Epidemiology

Intracardiac shunts are the most common congenital heart defects. One in 8 newly born infants worldwide has some congenital heart anomaly. The prevalence of congenital heart defects varies with geography. Asia has the highest prevalence, with 9.3 cases reported for every 1,000 live births, and the least prevalence was noticed in Africa, with 1.3 cases reported for every 1,000 live births. North America has 6.9 cases per 1,000 live births. The difference in prevalence could also be attributed to the better availability of diagnostic modalities in certain parts of the world than in others. In the US, the prevalence of congenital heart defects is slightly higher in whites and Hispanics than in the black population. Overall, female predominance was seen in the US, with 8.03 females affected compared to 7.67 males for every 1,000 live births.[4] Due to the invention of better medical and surgical therapies, the survival of patients with congenital heart diseases is increasing, which also increases their prevalence in the adult population.[5]

Pathophysiology

Intracardiac shunts are anatomic defects that occur from derangements in the embryologic development of the heart, mainly in the first trimester of pregnancy. Fetal heart development occurs from the mesoderm. It is a complex process where a single heart tube is initially formed, which later develops into 4 chambers with separate inflow and outflow tracts by cell migration and looping of the structures.[6] Incomplete separation of right and left-sided structures leads to defects like ASD, VSD, and PDA. Since left-sided cardiac pressures are higher, defects in atrial or ventricular septums or patency of ductus arteriosus lead to blood shunting into the right side where pressures are lower. Since all deoxygenated blood from systemic circulation makes its way to the lungs, oxygenation is not a problem. Shunting only sends additional blood into the lungs; this blood is already oxygenated. Therefore, these lesions are called acyanotic shunts.

Small defects can close by themselves with time; however, larger defects can sometimes require surgical intervention. Left to right shunts are often asymptomatic in childhood; however, symptoms can appear in adolescence and adulthood. Presentation often includes shortness of breath, exercise intolerance, and easy fatigability. In these defects, the right side of the heart receives more blood, which causes pulmonary hypertension over time in some patients. A rare complication of these shunts is Eisenmenger syndrome, in which shunt reversal occurs due to severe pulmonary hypertension to the point that right-sided pressures exceed left-sided pressures. This syndrome causes cyanosis and carries a high mortality.[7] 

The other group of intracardiac shunts is called cyanotic shunts. In these disorders, anatomic defects impede the flow of deoxygenated blood to the lungs. Oxygenation is impaired in these disorders, and partial pressure of oxygen in arterial circulation is low. Tissues do not get enough oxygen supply, and characteristic bluish skin discoloration, called cyanosis, is seen. Cyanotic lesions are associated with significant morbidity and mortality and typically present early after birth; however, small lesions could be minimally cyanotic or even completely asymptomatic at first. Symptoms include cyanosis, clubbing of fingers and toenails, dyspnea, poor weight gain, hemoptysis, and recurrent infections.[8] A classic phenomenon historically seen in patients with cyanotic spells was squatting, which increases systemic vascular resistance, thereby increasing pulmonary flow. This improves oxygen and relieves symptoms. Squatting is not seen as common now due to early diagnosis and improved therapeutic interventions. 

History and Physical

Congenital disorders are present at the time of birth, and history is often not practicable. However, history is important as a child grows up and can talk. Patients should be asked for clinical symptoms, including dyspnea, exercise capacity, recurrent infections, skin discoloration, hemoptysis, presyncope or syncope, and pedal edema when possible. Physical examination is critical and often is the only indication of the disease. It should include thorough inspection, palpation, and auscultation. Inspection should be conducted for cyanosis, nail clubbing, growth stunting, jugular venous distension, pedal edema, and breathing difficulty. Palpation should be done to notice the apex beat's thrill, heave, or displacement. Characteristic murmurs are present in different shunts at different locations, aiding diagnosis. Typical murmurs heard are fixed widely split S2 with ejection systolic murmur at the pulmonary area in ASD, holosystolic murmur at the left lower sternal border in VSD, and continuous machinery murmur in PDA. The murmur in TOF would be due to the pulmonic stenosis and not due to the concomitantly present VSD, and a systolic ejection sound is heard at the left upper sternal border.[9] Murmurs can often be unclear, and a formal echocardiogram should be obtained to evaluate a pathological murmur.

Evaluation

After the initial history and physical examination, further workup should be done as warranted. The extent of evaluation for intracardiac shunts depends on a case-by-case basis. It could include blood testing, electrocardiogram (EKG), echocardiogram, chest x-ray, CT scan, and cardiac catheterization (see Image. Transesophageal Echocardiogram). EKG could indicate signs of pulmonary hypertension from right ventricular hypertrophy, right axis deviation, and right atrial enlargement. Right ventricular hypertrophy shows tall R wave in V1, dominant S wave in leads V5 and V6 and can also produce p pulmonale and right ventricular strain pattern on EKG that could be evidenced by ST depressions and T wave inversions in the right precordial (V1-V4) and inferior (II, III, aVF) leads.[10] 

A chest X-ray could show characteristic heart shapes in different defects. TOF could make the heart boot-shaped on CXR, TAPVR could show a snowman-shaped heart, and TGA could give the heart an "egg-on-string" appearance.[11] Blood work could show reactive polycythemia in cyanotic defects, indicating the severity of hypoxemia. Cardiac catheterization is not typically needed for diagnostic purposes but should be done once the surgical intervention is planned. It gives accurate pressures in heart chambers. Induction of oxygen or nitric oxide could be done in the pulmonary system to see vascular reactivity, which could help decide if the patient is a candidate for surgical intervention or pulmonary vasodilator therapy.

Treatment / Management

Management of a shunt depends on the disease type and ranges from simple clinical observation, medical therapy, and lifestyle modification to surgical intervention. Small noncyanotic shunts can often be clinically observed. Large left-to-right shunts do need closure to avoid the development of complications. Cyanotic shunts are associated with higher morbidity and mortality and often require surgical intervention for correction. For example, TOF needs surgical intervention, often done in stages. In the first phase, a connection is created between the subclavian artery and the pulmonary artery called the Blalock-Tussing shunt.

Some surgeons choose not to perform this procedure and directly proceed to the next steps. The superior vena cava is connected to the right pulmonary artery via the Glenn shunt in the second phase. In the third phase, the inferior vena cava is connected to the pulmonary artery in a Fontan procedure. In cyanotic conditions, vessel thrombosis could occur due to increased hematocrit from reactive polycythemia. Phlebotomy is done on a case-by-case basis to correct the hematocrit. For female patients desiring to get pregnant, a discussion should be held with an obstetrician before conception to discuss the plans. Some disorders are associated with high obstetric mortality, and pregnancy is not advised.[12]

Differential Diagnosis

Careful evaluation should be done as clinical features overlap between different types of intracardiac shunts and can also mimic extracardiac disease processes. Cyanotic intracardiac shunts could mimic pulmonary, neurologic, and infectious diseases. Transient tachypnea of newborns, bronchopulmonary dysplasia, pulmonary hypoplasia, laryngotracheomalacia, vocal cord paralysis, neonatal myasthenia gravis, spinal muscular atrophy, infantile botulism, and pneumonia should be considered in the differential diagnosis. 

Prognosis

The prognosis depends on the particular type of shunt. Noncyanotic shunts carry remarkably better prognosis than cyanotic shunts. Small lesions can often close spontaneously or could remain asymptomatic for life. Large lesions need closure to avoid complications. Once severe pulmonary hypertension has developed, the prognosis worsens, and heart-lung transplantation is often warranted, which is not widely available. Severe cyanotic lesions present soon after the birth, and often, there is no chance of survival without timely surgical intervention. 

Complications

Complications of intracardiac shunts include failure to thrive, recurrent infections, pulmonary hypertension, right heart failure, Eisenmenger syndrome, infective endocarditis, dysrhythmias, and death. However, not all shunts result in complications, and most are benign.