Predominantly Right-to-Left (Complex) Shunts

Predominantly Right-to-Left (Complex) Shunts

Lesions within this group (some also called mixinglesions) often produce both ventricular outflowobstruction and shunting. The obstruction favors shunt flow toward the unobstructed side. When the obstruction is relatively mild, the amount of shunting is affected by the ratio of SVR to PVR, but increasing degrees of obstruction fix the direc-tion and magnitude of the shunt. Atresia of any one of the cardiac valves represents the extreme form of obstruction. Shunting occurs proximal to the atretic valve and is completely fixed; survival depends on another distal shunt (usually a patent ductus arteriosus [PDA], patent foramen ovale, ASD, or VSD), where blood flows in the opposite direction. This group of defects may also be divided according to whether they increase or decrease pulmonary blood flow.

Tetralogy of Fallot

This anomaly classically includes right ventricu-lar outflow obstruction, right ventricular hyper-trophy, and a VSD with an overriding aorta. Right ventricular obstruction in most patients is due to infundibular stenosis, which is due to hypertrophy of the subpulmonic muscle (crista ventricularis).

At least 20% to 25% of patients also have pulmonic stenosis, and a small percentage of patients has some element of supravalvular obstruction. The pulmonic valve is often bicuspid, or, less commonly, atretic. Infundibular obstruction may be increased by sym-pathetic tone and is therefore dynamic; this obstruc-tion is likely responsible for the hypercyanotic spells observed in very young patients. The combinationof right ventricular outflow obstruction and a VSD results in ejection of unoxygenated right ven-tricular blood, as well as oxygenated left ventricu-lar blood into the aorta. The right-to-left shuntingacross the VSD has both fixed and variable compo-nents. The fixed component is determined by the severity of the right ventricular obstruction, whereas the variable component depends on SVR and PVR.

Neonates with severe right ventricular obstruc-tion may deteriorate quickly, as pulmonary blood flow decreases when a PDA starts to close. Intravenous prostaglandin E₁ (0.05–0.2 mcg/kg/ min) is used to prevent ductal closure in such instances. Surgical palliation with a left-to-right sys-temic shunt or complete correction is then usually undertaken. For the former, a modified Blalock– Taussig (systemic–pulmonary artery) shunt is most often used to increase pulmonary blood flow. In this procedure, a synthetic graft is anastomosed between a subclavian artery and an ipsilateral pulmonary artery. Complete correction involves closure of the VSD, removal of obstructing infundibular muscle, and pulmonic valvulotomy or valvuloplasty, when necessary.

The goals of anesthetic management in patients with tetralogy of Fallot should be to maintainintravascular volume and SVR. Increases in PVR, such as might occur from acidosis or excessive air-way pressures, should be avoided. Ketamine (intra-muscular or intravenous) is a commonly used induction agent because it maintains or increases SVR and therefore does not aggravate the right-to-left shunting. Patients with milder degrees of shunting generally tolerate inhalation induction. The right-to-left shunting tends to slow the uptake of inhalation anesthetics; in contrast, it may accel-erate the onset of intravenous agents. Oxygenation often improves following induction of anesthesia. Muscle relaxants that release histamine should be avoided. Hypercyanotic spells may be treated with intravenous fluid and phenylephrine (5 mcg/kg). Beta blockers (eg, propranolol) may also be effective in relieving infundibular spasm. Sodium bicarbon-ate to correct the resulting metabolic acidosis, may also be helpful when the hypoxemia is severe and prolonged.

Tricuspid Atresia

With tricuspid atresia, blood can flow out of the right atrium only via a patent foramen ovale (or an ASD). Moreover, a PDA (or VSD) is necessary for blood to flow from the left ventricle into the pulmonary circulation. Cyanosis is usually evident at birth, and its severity depends on the amount of pulmonary blood flow that is achieved. Early survival is depen-dent on prostaglandin E₁ infusion, with or without a percutaneous Rashkind balloon atrial septostomy. Severe cyanosis requires a modified Blalock–Taussig shunt early in life. The preferred surgical manage-ment is a modified Fontan procedure, in which the venous drainage is directed to the pulmonary circu-lation. In some centers, a superior vena cava to the main pulmonary artery (bidirectional Glenn) shunt may be employed before or instead of a Fontan procedure. With both procedures, blood from the systemic veins flows to the left atrium without the assistance of the right ventricle. Success of the pro-cedure depends on a high systemic venous pressure and maintaining both low PVR and a low left atrial pressure. Heart transplantation may be necessary for a failed Fontan procedure.

Transposition of the Great Arteries

In patients with transposition of the great arteries, pulmonary and systemic venous return flows nor-mally back to the right and left atrium, respectively, but the aorta arises from the right ventricle, and the pulmonary artery arises from the left ventricle. Thus, deoxygenated blood returns back into the systemic circulation, and oxygenated blood returns back to the lungs. Survival is possible only through mixing of oxygenated and deoxygenated blood across the foramen ovale and a PDA. The presence of a VSD increases mixing and reduces the level of hypoxemia. Prostaglandin E₁ infusion is usually necessary. Rashkind septostomy may be necessary if surgical correction is delayed. Corrective surgical treatment involves an arterial switch procedure in which the aorta is divided and reanastomosed to the left ventricle, and the pulmonary artery is divided and reanastomosed to the right ventricle. The coro-nary arteries must also be reimplanted into the old pulmonary artery root. A VSD, if present, is closed. Less commonly, an atrial switch (Senning) proce-dure may be carried out if an arterial switch is not possible. In this latter procedure, an intraatrial baffle is created from the atrial wall, and blood from the pulmonary veins flows across an ASD to the right ventricle, from which it is ejected into the systemic circulation.

Transposition of the great vessels may occur with a VSD and pulmonic stenosis. This combina-tion of defects mimics tetralogy of Fallot; however, the obstruction affects the left ventricle, not the right ventricle. Corrective surgery involves patch closure of the VSD, directing left ventricular outflow into the aorta, ligation of the proximal pulmonary artery, and connecting the right ventricular outflow to the pulmonary artery with a valved conduit (Rastelli procedure).

Truncus Arteriosus

With a truncus arteriosus defect, a single arte-rial trunk supplies the pulmonary and systemic circulation. The truncus always overrides a VSD, allowing both ventricles to eject into it. As PVR gradually decreases after birth, pulmonary blood flow increases greatly, resulting in heart failure. If left untreated, PVR increases, and cyanosis develops again, along with Eisenmenger physiology. Surgical correction closes the VSD, separates the pulmonary artery from the truncus, and connects the right ventricle to the pulmonary artery with a conduit (Rastelli repair).

Hypoplastic Left Heart Syndrome

Th is syndrome describes a group of defects charac-terized by aortic valve atresia and marked under-development of the left ventricle. The right ventricle is the main pumping chamber for both systemic and pulmonary circulations. It ejects normally into the pulmonary artery, and all (or nearly all) blood flow entering the aorta is usually derived from a PDA. Surgical treatment includes both the Norwood repair and a hybrid approach to pallia-tion. In the Norwood repair, a new aorta is created from the hypoplastic aorta and the main pulmo-nary artery. Pulmonary blood flow is delivered via a Blalock–Taussig shunt. The right ventricle becomes the heart’s systemic pumping ventricle. A hybrid approach has also been advocated for the treatment of hypoplastic left heart syndrome. In this approach, the pulmonary arteries are banded to reduce pul-monary blood flow, and the PDA is stented to pro-vide for systemic blood flow.