Acyanotic cardiac defects are a group of diverse cardiac malformations that share a common physiology of increased pulmonary blood flow in the setting of normal systemic perfusion and oxygen saturation. Most of these defects produce varying degrees of shunting of blood from the left side of the heart to the right side. Patients with shunt lesions often present initially to the general pediatrician with problems ranging from failure to thrive and poor weight gain to respiratory distress and exercise intolerance. Alternatively, the patient may be asymptomatic but have an abnormal cardiovascular physical examination. Some of these defects, when left untreated, can affect long-term cardiovascular health. Therefore, this chapter will address the pathophysiology of shunt lesions, modes of presentation, indications for referral, and general treatment algorithms of shunt lesions.
Pathophysiology of Shunt Lesions: General Principles
Although specific defects within the group of shunt lesions present at different ages depending on the location and degree of shunting, the common pathophysiology involves increased pulmonary blood flow.
In utero, the placenta provides oxygenated blood to the fetus. The pulmonary vasculature is constricted, and blood flow is largely shunted away from the lungs across the patent foramen ovale and the patent ductus arteriosus. The first breaths of life initiate a sharp decline in pulmonary vascular resistance that then continues for the first few months of life.1 The degree of shunting through some of the shunt lesions, such as patent ductus arteriosus and ventricular septal defect, depend on the relative resistances between the pulmonary and systemic vascular beds. Therefore, infants with these types of shunt lesions present beyond the neonatal period but within the first few months of life when pulmonary vascular resistance is low, promoting more pulmonary blood flow.
In contrast, the degree of shunting in other defects, such as atrial septal defects, depends on right ventricular compliance. The right ventricle is relatively stiff and noncompliant in utero. Unlike the relatively brisk drop in pulmonary vascular resistance after birth, the right ventricle can take months to remodel. Therefore, patients with these types of defects present in late childhood or early adulthood.
Clinical Manifestations: General Principles
Excessive pulmonary blood flow leads to respiratory symptoms of tachypnea and increased work of breathing. The exact mechanism of these symptoms is unknown, but it likely is explained by increased lymphatic flow. As pulmonary blood flow continues to increase, lymphatic channels in the lungs are overwhelmed and interstitial fluid accumulates, leading to peribronchial edema and inflammation.2 Tachypnea and retractions ensue. This increased work of breathing, in turn, makes it difficult to feed well and increases oxygen and caloric demands, leading to poor weight gain and failure to thrive. Unlike adults with low-output heart failure, infants and children with shunt lesions have high-output heart failure. Increased metabolic demand stimulates neurohormonal mechanisms and increases cardiac output. Hepatic veins dilate to accommodate the increased blood volume, and hepatomegaly ...