Shunt Lesions



Shunt Lesions


Zachary L. Steinberg

Mathias Possner

Yonatan Buber



INTRODUCTION

Shunt lesions comprise a wide variety of anatomic abnormalities resulting in abnormal blood flow between cardiac chambers and great vessels. A shunt is labeled as “left-to-right” when oxygenated blood passes through a communication and mixes with deoxygenated blood. Left-to-right shunts do not result in hypoxemia and may therefore go unnoticed for an extended period of time. But an increase in pressure and volume through the cardiac chambers and pulmonary vasculature through which the shunted blood passes may result in significant and occasionally irreversible physiologic perturbations. Conversely, a shunt is labeled as “right-to-left” when deoxygenated blood passes through a communication and mixes with oxygenated blood. Right-to-left shunts may result in significant hypoxemia and are often diagnosed early in life owing to overt cyanosis. Most right-to-left shunts occur in response to a constellation of intracardiac abnormalities and are only rarely found as a result of a single isolated anatomic lesion.

This chapter reviews the anatomy, physiology, and management of some of the more common, isolated congenital and iatrogenic shunt lesions. As a rule, shunt physiology dictates treatment indications; however, shunt anatomy dictates treatment options. The remainder of this chapter is organized by the anatomic location of each shunt.


ATRIAL SEPTAL DEFECTS


EPIDEMIOLOGY AND PATHOGENESIS

Atrial septal defects (ASDs) include a group of abnormalities that result in a communication between the right and left atria. They are among the most common of all congenital heart defects with a prevalence of 1.6 per 1000 births worldwide.1 In the absence of additional pathology, ASDs result in a left-to-right shunt. Although almost uniformly these shunts occur at low pressure owing to relatively passive blood flow between two low-pressure chambers, the long-standing volume load on the right atrium, right ventricle, and pulmonary artery vasculature may lead to atrial arrhythmias, right-sided chamber enlargement and dysfunction, and, rarely pulmonary arteriolar hypertension.2

There are several types of ASDs, each resulting from a unique developmental abnormality (Figure 106.1). Most commonly encountered are secundum ASDs, which are located within the fossa ovalis, resulting from defects within the septum primum. Secundum defects vary greatly in size and shunt volume. Primum ASDs result from incomplete septation at the crux of the heart where the atrioventricular (AV) valves and atrial and ventricular septa meet and are part of a larger constellation of abnormalities known as endocardial cushion defects or AV septal abnormalities. These lesions, by definition, involve abnormalities of the developing AV valves and are often associated with ventricular septal defects (VSDs). As such, treatment indications and considerations for primum ASDs are described in a separate section (see section “Atrioventricular Septal Defects”). Sinus venosus ASDs are located superiorly at the junction of the superior vena cava (SVC) and the roof of the right atrium. These defects result from a deficiency in the development of the wall that normally separates the right pulmonary veins from the SVC and are accompanied by a posterosuperior ASD.3 Thus, the definition of a sinus venosus ASD includes abnormal pulmonary venous return and typically results in a significant left-to-right shunt. The least common of all ASDs, an unroofed coronary sinus, results from a deficiency of the septal tissue surrounding the ostium of the coronary sinus, creating a communication between the atria.








CLINICAL PRESENTATION AND DIAGNOSIS: ATRIAL SEPTAL DEFECTS

A substantial proportion of adult patients with an undiagnosed ASD have minimal to no recognizable symptoms. When present, the most common symptom is mild to modest exertional dyspnea. Frequently, it is only after an ASD is repaired that long-standing exertional intolerance is realized in retrospect. In the presence of large shunts or advanced age, symptoms of congestive heart failure may manifest. ASDs are also associated with early-onset atrial arrhythmias, and younger patients presenting with new-onset atrial fibrillation should prompt further evaluation for structural cardiac abnormalities.

Physical examination findings are often unremarkable; however, cardiac auscultation may reveal subtle signs of excessive pulmonary blood flow. A fixed split S2 may be heard throughout the respiratory cycle, and a pulmonary systolic flow murmur may be present over the second intercostal space. In rare cases, a loud and palpable P2, jugular venous distension, or dependent edema may be present as a sign of either significant pulmonary hypertension or right ventricular dysfunction.

An electrocardiogram (ECG) may reveal evidence of right atrial enlargement and a right bundle branch block with or without right axis deviation. Septum primum defects are the exception with a right bundle branch block associated with left axis deviation. Sinus venosus defects may show an ectopic atrial rhythm (ie, negative P waves in the inferior ECG leads). Transthoracic echocardiography (TTE) typically establishes the diagnosis demonstrating evidence of flow between the atria on color Doppler; however, transesophageal echocardiography (TEE) provides a more comprehensive assessment of defect anatomy. Sinus venosus defects and associated abnormal pulmonary venous return are frequently difficult to visualize in an adult by TTE imaging. A significantly dilated right ventricle without identifiable intracardiac pathology by TTE should raise concerns for this diagnosis and prompt further evaluation with either a TEE or cross-sectional imaging. Cross-sectional imaging with either cardiac magnetic resonance (CMR) imaging or a cardiac computed tomographic angiogram (CCTA) provides an excellent assessment of sinus venosus anatomy and the number and location of anomalous pulmonary venous drainage. Both CMR and CCTA are useful adjuncts in other types of ASDs and offer accurate assessment of chamber size, presence of additional abnormalities, and, in the case of CMR, an estimate of the shunt magnitude.






Cardiac catheterization can aid in the diagnosis by confirming the presence of a shunt. Often, direct passage of a catheter through the defect is possible, helping to identify shunt location. Additionally, cardiac catheterization provides important hemodynamic information regarding intracardiac filling pressures, shunt magnitude, and pulmonary vascular resistance, all of which may influence treatment decisions.


MANAGEMENT: ATRIAL SEPTAL DEFECTS

ASD closure is recommended in the presence of impaired functional capacity, right atrial or ventricular enlargement, right ventricular dysfunction, or paradoxical embolism (Algorithm 106.1). Closure is not recommended and possibly harmful in the presence
of significant pulmonary vascular resistance at greater than one-half to two-thirds systemic levels.4 These patients should be comanaged with a pulmonary hypertension specialist.

Both surgical and transcatheter techniques exist for ASD closure; however, not all ASDs allow for transcatheter closure at present. Transcatheter ASD closure is the treatment of choice for secundum ASDs when anatomically feasible. In the presence of adequate circumferential tissue, device closure of secundum ASDs has a high rate of success at low risk.5 Device embolization is infrequent and often successfully managed in the cardiac catheterization laboratory.6 Device erosion is a life-threatening complication often requiring urgent surgical intervention. This rare complication remains poorly understood and, while associated with ASD rim tissue deficiency, no single identifiable subpopulation is recognized to be at substantially elevated risk to warrant contraindication of device closure.7

Surgery remains the mainstay of therapy for individuals with primum ASDs, sinus venosus defects, and unroofed coronary sinuses; however, new transcatheter techniques are emerging for the treatment of a subpopulation of sinus venosus lesions.8 Surgical ASD closure remains an effective option for those with contraindications to transcatheter closure with very low mortality and an excellent long-term prognosis.5,9 In general, patients with an indication for ASD closure who are not eligible for transcatheter occlusion should be referred for surgery.


FOLLOW-UP PATIENT CARE: ATRIAL SEPTAL DEFECTS

Lifelong follow-up is typically recommended regardless of whether the ASD has been treated. Post ASD device closure, patients should receive intermittent imaging follow-up within the first year to evaluate for device malposition, residual shunting, and erosion. Long-term follow-up is recommended following device and surgical closure every 3 to 5 years with an ECG and TTE imaging to assess for new-onset atrial arrhythmias, residual shunting, right ventricular size and function, pulmonary artery pressures, and, in the case of device closure, late erosion.4 Patients in whom ASD closure has not been pursued, long-term follow-up every 3 to 5 years with either TTE or cross-sectional imaging is recommended in the absence of chamber enlargement, ventricular dysfunction, or elevated pulmonary artery pressures.


VENTRICULAR SEPTAL DEFECTS


EPIDEMIOLOGY AND PATHOGENESIS

VSDs are open communications in the interventricular septum, which result in a shunt between the left and right ventricles. They are the most common congenital heart defects worldwide with a prevalence of 2.6 per 1000 births,1 although many are small and spontaneously close during somatic growth, such that the prevalence of isolated VSDs in adulthood is much lower.10 VSDs are often isolated lesions but commonly occur as part of a constellation of defects such as in the case of atrioventricular septal defects (AVSDs), tetralogy of Fallot, and transposition of the great arteries.

In the absence of additional pathology, VSDs result in left-to-right shunting. The magnitude of shunting and clinical presentation widely vary and are predominantly dictated by the size of the defect. Large VSDs expose the pulmonary vascular bed to substantial volume and pressure. If left untreated, they almost always result in irreversible pulmonary arteriolar hypertension and, ultimately, reversal of the shunt (so-called Eisenmenger syndrome). Moderate-sized VSDs are often pressure restrictive, protecting the pulmonary vasculature from excessive pressure overload, but permit substantial shunted blow flow through the pulmonary vasculature and into the left heart, resulting in left-sided chamber enlargement and dysfunction and, occasionally, slowly progressive pulmonary arteriolar hypertension. Small-sized VSDs are both pressure and volume restrictive and are typically tolerated for many years without hemodynamic perturbations. However, the presence of a long-standing, high-velocity jet may result in adverse sequelae including right ventricular outflow tract hypertrophy and obstruction (ie, double outlet right ventricle), progressive aortic insufficiency, and endocarditis.

VSDs are classified based on their location within the interventricular septum although a well-defined nomenclature remains a topic of ongoing debate. In general, there are four major VSD classifications: perimembranous, muscular, conoventricular (also referred to as supracristal, juxtaarterial, subarterial, subpulmonic, or infundibular), and inlet VSDs (Figure 106.2).

Perimembranous VSDs are located in the membranous ventricular septum and are the most common of all VSDs. Perimembranous VSDs are often congenital in etiology but may also result iatrogenically (eg, postaortic valve replacement). Muscular VSDs may be present anywhere within the muscular portion of the interventricular septum and can be found as a single defect or in multiples. Like perimembranous defects, muscular VSDs are most frequently congenital in etiology, although acquired muscular VSDs are a well-recognized
complication of myocardial infarction (ie, postinfarct VSD) or surgical intervention (eg, postseptal myectomy). Conoventricular defects, located within the subpulmonary infundibulum, do not exist in isolation but rather as one of a constellation of defects in more complex congenital heart disease.11 Inlet VSDs result from developmental abnormalities within the endocardial cushion and are associated with atrial and AV valve abnormalities (see section “Atrioventricular Septal Defects”).







CLINICAL PRESENTATION AND DIAGNOSIS: VENTRICULAR SEPTAL DEFECT

The clinical presentation of a VSD depends on the size of the defect, the duration of the shunt, and the presence or absence of associated cardiac lesions. Individuals with small volume and pressure-restrictive VSDs (pulmonary to systemic blood flow ratio [Qp:Qs] < 1.5:1) often remain asymptomatic. Those with moderately sized VSDs (Qp:Qs 1.5-2:1) may develop symptoms of exertional intolerance and congestive heart failure. A long-standing volume load within the pulmonary circulation may lead to irreversible pulmonary arteriolar hypertension in select cases. Individuals with large VSDs are typically quite symptomatic with significantly reduced functional capacity and symptoms of congestive heart failure owing to excessive pulmonary circulation. If large VSDs are left untreated, rising pulmonary arteriolar hypertension often results in significant hypoxia caused by shunt reversal (right-to-left) known as Eisenmenger syndrome.







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May 8, 2022 | Posted by in CARDIOLOGY | Comments Off on Shunt Lesions
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