Ventricular Septal Defect and Double-Outlet Right Ventricle

CHAPTER 117 Ventricular Septal Defect and Double-Outlet Right Ventricle





VENTRICULAR SEPTAL DEFECT


A ventricular septal defect (VSD) is a hole between the left and right ventricles. A VSD may occur as an isolated anomaly or with a wide variety of intracardiac anomalies, such as tetralogy of Fallot or transposition of the great arteries. This chapter deals with isolated VSD.


Banding of the pulmonary artery as a palliative maneuver was first described in 1952.1 This decreased left-to-right shunting and as a consequence prevented the development of pulmonary vascular obstructive disease and left-sided volume overload. Until the mid 1960s when primary VSD closure became safer, pulmonary artery banding was the procedure of choice in managing VSDs. The first VSD closure was performed in 1954 by Lillehei and associates2 at the University of Minnesota, using controlled cross-circulation between the child and parent. Nineteen of the 27 patients who underwent this procedure survived. In 1955, Kirklin and associates3 at the Mayo Clinic closed a VSD using a heart–lung machine. In 1958, transatrial VSD closure was performed,4 followed in 1969 by the popularization of primary repair in symptomatic infants by Barratt-Boyes and associates5 using cardiopulmonary bypass, deep hypothermia, and circulatory arrest.



Anatomy



Anatomy of the Tricuspid Valve, Right Ventricular Septum, and Conduction System


Surgeons planning VSD surgery must have intimate knowledge of the tricuspid valve, the right ventricular septal anatomy, and the conduction system.


The tricuspid valve has three leaflets: anterior, septal, and posterior. The anterior leaflet is connected via the chordae tendineae cordis to the anterior papillary muscle (located on the anterior right ventricular free wall) and to the septal papillary muscle (sometimes called muscle of Lancisi). The septal papillary muscle is itself part of the septal band of the septomarginal trabecula. The posterior leaflet is attached to the anterior and posterior papillary muscles, and the septal leaflet attaches to the posterior and septal papillary muscles.


The right ventricular septum has five components (Fig. 117-1):








Unlike the left ventricular septum, which is free of any papillary muscle attachment (the mitral valve can be called “septophobic,” whereas the tricuspid valve can be called “septophilic”), the right ventricular septum is where the septal (sometimes called medial) papillary muscle and part of the posterior papillary muscle originate. The septal papillary muscle is a portion of the septal band, which runs along the septum (hence its name). The septal band itself is a portion of the septomarginal trabecula, which also comprises the moderator band. The moderator band itself links the septum to the anterior papillary muscle (called moderator band because it was erroneously thought to “moderate” the right ventricular free wall—that is, keep in sync with the rest of the ventricle). The membranous septum is the only fibrous component of the septum. It is wedged between the aortic valve, the tricuspid valve, and the mitral valve. Because the tricuspid valve is normally apically displaced vis-à-vis the mitral valve, a portion of membranous septum ends up between the right atrium and the left ventricle, called the atrioventricular part of the membranous septum. The portion of membranous septum located between both ventricles is called the interventricular part.


Knowledge of the conduction system of the heart also is critical when approaching VSDs so as to avoid damaging it (Fig. 117-2). The various atrial conduction tracts all converge toward the AV node of Aschoff-Tawara. The AV node is located in the inferior-posterior portion of the membranous septum, just inferior to the anteroseptal commissure of the tricuspid valve. A different description of its location is that it occupies the apex of the triangle of Koch, which is limited by the ligament of Todaro posteriorly, the orifice of the coronary sinus inferiorly, and the tricuspid valve annulus superiorly (see Fig. 117-2). From the AV node, the common AV bundle of His descends within the interventricular part of the membranous septum (or, in the case of a membranous VSD, the posteroinferior rim of the VSD), traverses the septum, and then courses along the left ventricular aspect of the septum. It then separates into a right bundle branch, which travels back to the right ventricular surface, as well as a left bundle branch. At the anteroinferior border at the level of the muscle of Lancisi, the right bundle branch descends toward the right ventricular apex.




Anatomic Classification of Ventricular Septal Defects


A useful surgical classification of VSDs was initially developed in 1980 by Soto and associates6 (Fig. 117-3) and then further modified by Van Praagh and associates (Fig. 117-4).7 Variations of this classification are used in most pediatric cardiac centers. VSDs can be classified as follows:









Conoventricular (or Membranous) Defects


Conoventricular defects are located between the conal septum and the ventricular septum. They are centered around the membranous septum and comprise 80% of all VSDs. They may be located exclusively in the membranous septum, or they can extend beyond the boundaries of the membranous septum in the inferior, posterior, or anterior direction and are then sometimes called “perimembranous” or “paramembranous” VSDs. The prefix peri-, appearing in loan words from the Greek, means “surrounding” (e.g., perimeter). Thus, a truly perimembranous VSD would surround the membranous septum. In contrast, the prefix para-, also from the Greek, means “adjacent to” or “beside” and more accurately reflects the notion of a defect adjacent to the membranous septum. Neither perimembranous nor paramembranous correctly describes the typical defect involving the membranous septum and extending into the adjacent septum. The current recommendation is to call these defects either membranous VSDs or conoventricular defects. Malalignment of the conal septal plane vis-à-vis the ventricular septal plane results in the typical conoventricular defect. The malalignment can be anterior, as seen in tetralogy of Fallot, for example, or posterior, as seen in interrupted aortic arch. In addition to resulting in a VSD, anterior conal septal malalignment also results in right ventricular outflow tract obstruction, whereas posterior malalignment of the conal septum results in left ventricular outflow tract obstruction. Important landmarks in conoventricular septal defects are the anteroseptal commissure of the tricuspid valve inferiorly and the noncoronary cusp of the aortic valve. When the ventricular portion of the membranous septum is entirely absent, the VSD extends to the base of the aortic valve (sometimes called “subaortic” VSD). The medial papillary muscle (muscle of Lancisi) located at the inferoposterior border of the defect is also an important landmark. Both the septal and anterior tricuspid valve leaflets are attached to it.






Commonly Associated Defects


VSDs are an intrinsic portion of many, if not most, complex cardiac malformations. These VSDs are discussed separately with their respective entities (see other chapters). Almost half of patients who undergo surgical treatment of primary VSD have an associated lesion.


A large patent ductus arteriosus (PDA) is present in about 25% of symptomatic neonates or infants with VSDs.5 This is important to know because preoperative echocardiography may fail to show a PDA in the presence of a large amount of left-to-right shunting. Furthermore, intraoperative transesophageal echocardiography (TEE) is notoriously unreliable in excluding PDAs. Therefore the possibility of a PDA should be kept in mind when approaching a VSD, and if there is any doubt, or if there is a large amount of backflow through the pulmonary arteries on cardiopulmonary bypass, the PDA should be ligated or clipped.


A hemodynamically significant aortic coarctation is present in approximately 10% of cases. Because of the unique pathophysiology here (more left-to-right shunting across the VSD because of increased afterload caused by the coarctation), these patients usually have presenting symptoms before 3 months of age.8


Congenital valvar or subvalvar aortic stenosis, resulting in left ventricular outflow tract obstruction, is seen in approximately 4% of patients requiring an operation for VSD.9 The most common type of subaortic stenosis associated with VSDs involves the discrete fibromuscular membrane of the VSD that is located inferior or upstream to it. Congenital mitral valve stenosis is rare and occurs in about 2% of patients.


Other significant anomalies include large atrial septal defects (ASDs), right ventricular outflow tract obstruction, vascular ring, and persistent left superior vena cava.



Pathophysiology





Pulmonary Vascular Disease


The classic description of the pathology of hypertensive pulmonary vascular disease is that of Heath and Edwards.10 They correlated the PVR of patients with large VSDs with the histologic severity of pulmonary vascular changes. Grade 1 changes were defined as medial hypertrophy without intimal proliferation; grade 2 as medial hypertrophy with cellular intimal reaction; grade 3 as intimal fibrosis and medial hypertrophy; grade 4 as generalized vascular dilation, an area of vascular occlusion by intimal fibrosis, and plexiform lesions; grade 5 as other “dilatation lesions” such as cavernous and angiomatoid lesions; and grade 6 as necrotizing arteritis. It is assumed that Heath-Edwards grade 3 or greater is not reversible. The importance of lung biopsies has decreased over the years, with catheterization-based data increasing in importance in terms of suitability for repair.



Natural History and Indications for Surgery


Approximately 30% of infants with severe symptoms such as intractable congestive heart failure or failure to thrive require surgery within the first year of life.11 The remainder can usually be managed medically, because the natural history of VSDs is well known.12 Aggressive medical management is indicated because a majority of membranous and muscular VSDs tend to close spontaneously.12 Malalignment conoventricular VSDs or inlet-type VSDs are unlikely to close spontaneously, and therefore closure at the time of diagnosis is recommended, regardless of age or weight. Asymptomatic children with isolated small restrictive VSDs can be followed safely with serial echocardiograms.


The development of pulmonary vascular disease is a tragedy that can be prevented with virtually no mortality by VSD closure. If in doubt, cardiac catheterization and measurement of PVR-to-SVR ratio should help with decision making. In addition, pulmonary artery (PA) pressures greater than one half the systemic pressure in a child older than 1 year indicate the need for surgery. If PA pressures are greater than one half the systemic, the response of the pulmonary vasculature to inhaled nitric oxide and 100% inspired oxygen should be studied during catheterization. Even children who have significant pulmonary hypertension with a reversible component to it can become operative candidates.


During the first decade of life, a small proportion (5%) of patients with membranous or outlet VSDs develop prolapse of an aortic cusp into the VSD. This usually results in a gradual decrease of the effective orifice and shunt flow, and also in increasing aortic regurgitation. Increasing aortic cusp prolapse and regurgitation is an indication to operate.

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Jul 30, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Ventricular Septal Defect and Double-Outlet Right Ventricle
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