Predominantly Left-to-Right (Simple) Shunts

Predominantly Left-to-Right (Simple) Shunts

Simple shunts are isolated abnormal communica-tions between the right and left sides of the heart. Because pressures are normally higher on the left side of the heart, blood usually flows across from left to right, and blood flow through the right heart and the lungs increases. Depending on the size and location of the communication, the right ventricle may also be subjected to the higher left-sided pres-sures, resulting in both pressure and volume over-load. Right ventricular afterload is normally 5% that of the left ventricle, so even small left-to-right pressure gradients can produce large increases in pulmonary blood flow. The ratio of pulmonary (Qp) to systemic (Qs) blood flow is useful to determine the directionality of the shunt.

A ratio greater than 1 usually indicates a left-to-right shunt, whereas a ratio less than 1 indicates a right-to-left shunt. A ratio of 1 indicates either no shunting or a bidirectional shunt of opposing magnitudes.

Large increases in pulmonary blood flow pro-duce pulmonary vascular congestion and increase extravascular lung water. The latter interferes with gas exchange, decreases lung compliance, and increases the work of breathing. Left atrial distention also compresses the left bronchus, whereas distention of pulmonary vessels compresses smaller bronchi.

Over the course of several years, chronic increases in pulmonary blood flow produce vascu-lar changes that irreversibly increase PVR. Elevation of right ventricular afterload produces hypertro-phy and progressively raises right-sided cardiac pressures. With advanced disease, the pressures within the right heart can exceed those within the left heart. Under these conditions, the intracardiac shunt reverses and becomes a right-to-left shunt (Eisenmenger syndrome).

When a communication is small, shunt flow depends primarily on the size of the communication (restrictive shunt). When the communication is large (nonrestrictive shunt), shunt flow depends on the relative balance between PVR and SVR.

An increase in SVR relative to PVR favors left-to-right shunting, whereas an increase in PVRrelative to SVR favors right-to-left shunting. Common chamber lesions (eg, single atrium, single ventricle, truncus arteriosus) represent the extreme form of nonrestrictive shunts; shunt flow with these lesions is bidirectional and totally dependent on relative changes in the ventricular afterload.

The presence of shunt flow between the right and left hearts, regardless of the direction ofblood flow, mandates the meticulous exclusion of air bubbles and particulate material from intravenous fluids to prevent paradoxical embolism into the cerebral or coronary circulations.

Atrial Septal Defects

Ostium secundum atrial septal defects (ASDs) are the most common type and usually occur as isolated lesions in the area of the fossa ovalis. The defect is sometimes associated with partial anomalous pul-monary venous return, most commonly of the right upper pulmonary vein. A secundum ASD may result in single or multiple (fenestrated) openings between the atria. The less common sinus venosus and ostium primum ASDs are typically associated with other car-diac abnormalities. Sinus venosus defects are located in the upper interatrial septum close to the superior vena cava; one or more of the right pulmonary veins often abnormally drains into the superior vena cava. In contrast, ostium primum ASDs are located in the lower interatrial septum and overlie the mitral and tricuspid valves; most patients also have a cleft in the anterior leaflet of the mitral, valve and some have an abnormal septal leaflet in the tricuspid valve.

Most children with ASDs are minimally symp-tomatic; some have recurrent pulmonary infections. Congestive heart failure and pulmonary hyperten-sion are more commonly encountered in adults with ASDs. Patients with ostium primum defects often have large shunts and may also develop significant mitral regurgitation. In the absence of heart failure, anesthetic responses to inhalation and intravenous agents are generally not significantly altered in patients with ASDs. Large increases in SVR shouldbe avoided because they may worsen the left-to-right shunting.

Ventricular Septal Defects

Ventricular septal defect (VSD) is a common con-genital heart defect, accounting for up to 25% to 35% of congenital heart disease. The defect is most frequently found in the membranous part of the interventricular septum (membranous or infracris-tal VSD) in a posterior position and anterior to the septal leaflet of the tricuspid valve. Muscular VSDs are the next most frequent type and are located in the mid or apical portion of the interventricular sep-tum, where there may be a single defect or multiple openings (resembling Swiss cheese). Defects in the subpulmonary (supracristal) septum are often asso-ciated with aortic regurgitation because the right coronary cusp can prolapse into the VSD. Septal defects at the ventricular inlet are usually similar in development and location to AV septal defects (see the following section).

The resulting functional abnormality of a VSD is dependent on the size of the defect, PVR, and the presence or absence of other abnormalities. Small VSDs, particularly of the muscular type, often close during childhood. Restrictive defects are associated with only small left-to-right shunts (pulmonary– systemic blood flow ratios less than 1.75:1). Large defects produce large left-to-right shunts (shunts larger than 2:1) that vary directly with SVR and indirectly with PVR. Recurrent pulmonary infec-tions and congestive heart failure are common with pulmonary–systemic flow ratios of 3–5:1. Patients with small VSDs are treated medically and fol-lowed by electrocardiography (for signs of right ventricular hypertrophy) and echocardiography. Surgical closure is usually undertaken in patients with large VSDs before pulmonary vascular disease and Eisenmenger physiology develop. As with atrial defects, in the absence of heart failure, anesthetic responses to inhalation and intravenous agents are generally not significantly altered. Similarly, increases in SVR worsen the left-to-right shunting.When right-to-left shunting is present, abrupt increases in PVR or decreases in SVR are poorly tolerated.

Atrioventricular Septal Defects

Endocardial cushion (AV canal) defects produce contiguous atrial and ventricular septal defects, often with very abnormal AV valves. This is a com-mon lesion in patients with Down syndrome. The defect can produce large shunts both at the atrial and ventricular levels. Mitral and tricuspid regurgi-tation exacerbate the volume overload on the ven-tricles. Initially, shunting is predominately left to right; however, with increasing pulmonary hyper-tension, Eisenmenger syndrome with obvious cya-nosis develops.

Patent Ductus Arteriosus

Persistence of the communication between the main pulmonary artery and the aorta can produce restrictive or nonrestrictive left-to-right shunts. This abnormality is commonly responsible for the car-diopulmonary deterioration of premature infants and occasionally presents later in life when it can be corrected thoracoscopically. Anesthetic goals should be similar to atrial and ventricular septal defects.

Partial Anomalous Venous Return

This defect is present when one or more pulmo-nary veins drains into the right side of the heart; the anomalous veins are usually from the right lung. Possible anomalous entry sites include the right atrium, the superior or inferior vena cava, and the coronary sinus. The resulting abnormality produces a variable amount of left-to-right shunting. The clinical course and prognosis are usually excellent and similar to that of a secundum ASD. A very large coronary sinus on TEE suggests anomalous drainage into the coronary sinus, which may complicate the management of cardioplegia during cardiac surgery. Total anomalous venous return is corrected imme-diately after birth.