Question

pathophysiology discussion about congenital heart disease

150-200 words long

Answer 1

Congenital heart disease is common, occurring in ≈8 of 1000 live births.1 With the successes in cardiothoracic surgery over the past 3 decades and the ongoing improvements in the diagnostic, interventional, and critical care skills of pediatric cardiologists, ≈90% of children born with heart defects now survive to adulthood.2 In addition, using improved noninvasive techniques, adult cardiologists are increasingly identifying adults with septal defects that were undiagnosed in childhood. The adult congenital heart disease patients carry a spectrum of disease, from small septal defects and minor valvar obstructions to complex single-ventricle lesions that have been palliated with staged surgical repairs. It is estimated that >1 million adults in the United States now have congenital heart disease, outnumbering their pediatric counterparts for the first time.

While the adult cardiology community struggles with a population that once was the exclusive domain of pediatricians, governmental agencies, national physician associations, and cardiology advisory boards are trying to define the scope of this national healthcare issue and to figure out how to train current and future generations of doctors.5 This specialized cardiac care will require the diagnosis of adult congenital heart disease in patients presenting de novo with new or chronic symptoms, the long-term maintenance of those previously diagnosed, and the ability to recognize when primary or additional interventions are required. As these patients increasingly present to cardiologists’ offices for care, healthcare professionals will need to develop a better level of comfort with adult congenital heart disease.

This 3-part series focuses on the pathophysiology of congenital heart lesions, which are seen commonly in adult patients. In this first portion, simple shunt lesions are reviewed. For each, the natural history and common clinical presentations resulting from the shunt are discussed. A discussion of therapeutic options and the literature supporting these options is beyond the scope of this series. Patient management is limited to a discussion of which patient requires intervention. The second article in the series examines the pathophysiology of simple congenital obstructive lesions; the third looks at the fascinating physiologies of some of the more complex congenital heart malformations.

Shunting Lesions

Perhaps no aspect of cardiology is as uniquely identified with congenital heart disease as intracardiac shunting lesions. Most adult congenital heart disease patients who require therapy present with a shunt.

With normal cardiac anatomy, there is complete septation of oxygenated and deoxygenated blood. The 2 circulations run in parallel, each feeding the other, and maintain a 1-to-1 volume relationship on the systemic and pulmonary sides of the circulation. The deoxygenated, systemic venous return to the right atrium (RA) is pumped to the lungs as the pulmonary blood flow (abbreviated Qp). Once oxygenated, the blood returns via the pulmonary veins to the left atrium (LA) and is pumped to the aorta as the systemic blood flow or cardiac output (Qs). The term “shunt” refers to an abnormal connection allowing blood to flow directly from one side of the cardiac circulation to the other. A left-to-right shunt allows the oxygenated, pulmonary venous blood to return directly to the lungs rather than being pumped to the body. A right-to-left shunt allows the deoxygenated, systemic venous return to bypass the lungs and return to the body without becoming oxygenated. In each case, the circulation is less efficient and creates increased demand on the ventricles. In most patients, the volume of shunted blood determines the severity of symptoms.

Left-to-Right Shunting

The metabolic needs of the body’s tissues are highly variable, depending on the patient’s level of activity. To maintain normal aerobic respiration at the cellular level, oxygen must be delivered in quantities sufficient to meet those needs. One measure of how well the cellular needs are being supplied is tissue oxygen delivery, the mathematical product of systemic arterial oxygen content and cardiac output.6 By definition, a left-to-right shunt allows a portion of the pulmonary venous return to escape back to the lungs, thereby reducing the cardiac output by the amount of the shunted volume. Tissue oxygen delivery is thereby reduced. The pathophysiology associated with each congenital shunt is reviewed in more detail below.

Right-to-Left Shunting

With normal cardiac anatomy, lung function, and hemoglobin levels, arterial blood oxygen contents vary only to the extent that pulmonary alveolar oxygenation changes. Under most physiological conditions, the blood oxygen content changes little and is more than adequate to supply the needs of the tissues. With a right-to-left shunt, however, deoxygenated systemic venous blood returns directly to the systemic arterial circulation. The oxygen content of the systemic arterial blood falls in proportion to the volume of systemic venous blood mixing with the normal pulmonary venous return. With reduced oxygen content, even with normal cardiac output, tissue oxygen delivery falls and the work capacity of the muscles is limited.6

Quantifying Shunt Volumes

The ratio of total pulmonary blood flow to total systemic blood flow, the Qp/Qs ratio, is a useful tool for quantifying the net shunt. A Qp/Qs ratio of 1:1 is normal and usually indicates that there is no shunting. A Qp/Qs ratio of >1:1 indicates that pulmonary flow exceeds systemic flow and defines a net left-to-right shunt. Similarly, a Qp/Qs ratio of <1:1 indicates a net right-to-left shunt. Both left-to-right and right-to-left (bidirectional) shunting may be present in the same patient. If the left-to-right shunt equals the right-to-left shunt in magnitude, it is possible to have a Qp/Qs of exactly 1:1.

Atrial Septal Defect

The formation of the atrial septum is a complex process, consisting of the growth and partial reabsorption of 2 tissue membranes, septum primum and septum secundum; the fusion of these membranes to the forming endocardial cushions; and the reabsorption of the fetal sinus venosus into the structure that will ultimately become the RA. In ≈4 of 100 000 newborns,7 an error in this developmental process will result in a defect in the wall separating the 2 atria, an atrial septal defect (ASD). There are a number of types of ASD (Figure 1), including the ostium primum defect (a result of the deficiency of endocardial cushion tissue), the ostium secundum defect (a result of excessive reabsorption of septum primum), and the sinus venosus defect (resulting from an error in the incorporation of the sinus venosus chamber into the RA).8 Although the following discussion of ASD pathophysiology is true for all types of ASD, the sinus venosus ASD also may be associated with anomalous pulmonary venous return and the ostium primum ASD with significant atrioventricular (AV) valve abnormalities. These additional features may complicate the physiology further and are beyond the scope of this review.

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