Abbott apparently recognized ostium primum atrial septal defect (ASD) and common AV canal defect, but it was Rogers and Edwards who in 1948 recognized their morphologic similarity. ^, This concept was further elaborated by Wakai and Edwards in 1956 and 1958. ^, The terms partial and complete atrioventricular canal defects were introduced by these investigators, who realized nonetheless that not all cases fit their definitions. During this period, Lev was formulating his concepts of ostium primum ASD (or partial AV canal) and common AV orifice (or complete AV canal), and he described the position of the AV node and bundle of His in these malformations. Wakai and Edwards and later Bharati and Lev became dissatisfied with trying to compress all cases into two categories and added the terms intermediate and transitional . ^{, ,} During this period, Van Mierop’s scholarly studies added a great deal of knowledge about the overall anatomic features of AV septal defects.

By the early 1960s, surgical treatment of these defects provided a stimulus to further morphologic studies. In 1966, Rastelli and colleagues at the Mayo Clinic described in more detail the morphology of AV valve leaflets in cases with common AV orifice. The error made in this study was to compress into the common anterior leaflet designation a leaflet that was in fact divided in two by a commissure (i.e., the divided common anterior leaflet of type A). The description of AV valve leaflets by Rastelli and colleagues was accepted for some years, but in 1976 a publication by Ugarte and colleagues emphasized the idea of leaflets bridging the ventricular septum, a concept also held by Lev. Meanwhile, based on anatomic and cineangiographic studies and in accordance with the description of Baron and colleagues and Van Mierop and colleagues, it was recognized in the late 1960s that the basic defect in these malformations was absence of the AV septum . ^{, ,} These concepts were further expanded by Piccoli and colleagues under the direction of Anderson, who further emphasized that all the variations of the defect were part of a spectrum ( Fig. 32.1 ).

Diagrammatic representation of atrioventricular (AV) valves viewed from atrial side (surgical orientation). (A) Normal, with anterior and posterior mitral valve (MV) leaflets and septal, anterior, and posterior tricuspid valve (TV) leaflets. (B) Leaflets in partial AV septal defects. Left superior (LSL) , left inferior (LIL) , and left lateral (LLL) leaflets form left AV valve; right superior (RSL) , right inferior (RIL) , and right lateral (RLL) leaflets form right AV valve. (C) Leaflets in complete AV septal defects, or common AV orifice, are similar to those in B . However, LSL and LIL are not connected. LIL usually bridges a little (grade 1 or 2, based on 1 to 5) across crest of ventricular septum. LSL may bridge slightly or not at all (grade 0 or 1, Rastelli type A), moderately (grade 2 or 3, Rastelli type B), or markedly (grade 4 or 5, Rastelli type C). AL, Anterior leaflet; PL, posterior leaflet; SL, septal leaflet.

In 1951 at the University of Minnesota Hospital in Minneapolis, after a long period of laboratory investigation, Dennis and Varco attempted a cardiac operation in a human using a pump-oxygenator for the first time. The preoperative diagnosis was ASD, and at operation the defect was thought to be closed. The patient died, and autopsy showed the true diagnosis to be partial AV septal defect. The first successful repair of a complete AV septal defect was performed by Lillehei and colleagues in 1954, using cross-circulation and direct suture of the atrial rim of the septal defect to the crest of the ventricular septum. In 1954, Kirklin and colleagues successfully repaired a partial AV septal defect through the atrial well of Watkins and Gross, and in 1955 began repairing AV septal defects by open cardiotomy and use of the pump-oxygenator.

Early experiences with complete AV septal defects were all associated with a high hospital mortality, often related to complete heart block, postrepair left AV valve regurgitation, or creation of subaortic stenosis. Interestingly, in many of these early operations, a two-patch technique was used (see “ Two-Patch Repair Technique ” under Technique of Operation later in this chapter). In 1958, Lev’s description of the location of the bundle of His provided the basis for repair techniques that avoid heart block. In 1959, Dubost and Blondeau reported their early experience and emphasized that the “cleft” (zone of apposition; ZoA) in the “mitral leaflet” need not be sutured in repairing partial AV septal defects, a concept that is today largely refuted, although not uniformly. In 1962, Maloney and colleagues described two cases in which a single patch was used to close both defects and with the valve tissue suspended from the patch. This technique was again described by Gerbode in 1962 and was associated with decreased in-hospital mortality. McGoon recognized the importance of “taking from the tricuspid valve” to leave sufficient tissue from which to create an adequate left AV valve. These technical advances allowed repair of even the more complex variants of the defect. ^, Subsequently, good results were obtained in patients older than about 2 years of age, but results in infants remained relatively poor. Between 1968 and 1971, Barratt-Boyes successfully repaired this anomaly in four severely ill infants ; subsequently, improved results in infants were reported by many others. ^,

A fundamental concept regarding the left AV valve in repaired AVSD is its trifoliate structure. This understanding, combined with that regarding the completely different geometry of the mural leaflet, underlies all effective approaches to surgical repair. In 1978, Carpentier emphasized, as did Dubost and Blondeau, that generally the left AV valve functions best when repaired as a three-leaflet valve. ^, In contrast, increasing surgical experience in contemporary eras indicated that closure of the ZoA between the superior and inferior leaflet components of the left AV valve is associated with optimal left AV valve function. In contrast, recent evidence from Australia suggests that long-term outcomes are similar regardless of complete ZoA closure ^, and that preservation of the trifoliate left AV valve geometry is reasonable if prerepair valve function is good, although some question the study design and propensity-matching effectiveness of the Buratto study. As a result of these collective advances, risks of operation for nearly all types of AV septal defect are now low (see Results later).

Septation of the common AV canal into right and left components in the developing heart with corresponding formation of separate AV valves was historically attributed to, and substantial insight has been gained into, the complexity of the process underlying AV junction formation based on elucidation of molecular signalling, cellular mechanisms, and detailed embryologic anatomy of the developing heart. ^, It is now understood to be a highly complex process involving structures referred to as the AV mesenchymal complex that include the mesenchymal cap of the septum primum, the ventral (superior) and dorsal (inferior) major endocardial cushions, and the vestibular spine (dorsal mesenchymal protrusion). All of these structures contribute to septation of the AV junction, whereas the major endocardial cushions in concert with the smaller lateral endocardial cushions contribute to formation of the AV valve leaflets. ^, The vestibular spine is formed by ventral ingrowth of extracardiac mesenchyme derived from the secondary heart field and is a key contributor to formation of the normal AV junction and atrial septation. Evidence currently suggests that deficiency of the extracardiac mesenchyme contributing to the vestibular spine and reduced ventral protrusion of the vestibular spine contribute to the pathogenesis of AVSD, persistence of the ostium primum, and failure of fusion of the endocardial cushions.

Historically, hearts with this anatomy have been referred to as having an AV canal defect, and this terminology is still used by some. While the term “AV canal” is consistent with the fundamental morphologic defect (see later), the more precise and literal term “atrioventricular septal defect” is now preferred.

AVSD accounts for 3% of all cardiac malformations, and 75% of AVSDs are complete. The International Pediatric and Congenital Cardiac Code classifies AVSD into four main groups: partial (or incomplete), intermediate (or transitional), complete, and unbalanced ( www.IPCCC.net ). ^, Thus, the spectrum of AV septal defects is categorized based on the presence and size of the ventricular septal defect (VSD) together with the configuration of the AV valves (a common orifice vs. two orifices). Partial AV septal defects, also known as primum ASDs, are characterized by absence of a VSD with two separate AV valve orifices created by fusion of the AV valve leaflets to the crest of the interventricular septum. Intermediate or transitional AV septal defects are defined by the presence of a shallow, restrictive VSD and a common AV valve that is divided into left and right orifices by what is usually a thin bridging ribbon of valvar tissue ( Fig. 32.2 ). Complete AV septal defects are characterized by a large unrestrictive interventricular communication beneath a common AV valve. There are several variations to these three basic subtypes of AVSD relating to abnormalities of the interatrial septum, variations of AV valvar and subvalvar anatomy, morphology of the septal defects, as well as commitment of the AV valve components to the respective ventricles, which will be reviewed in further detail in subsequent sections. All of these subtypes may be either balanced or unbalanced forms of AVSD. The following content is in specific reference to balanced forms of AVSD. Unbalanced atrioventricular septal defect (uAVSD) encompasses a wide range of morphologic abnormalities that are described and considered separately at the conclusion of this chapter.

Intermediate type of AV septal defect viewed from left ventricular aspect. Left superior (LS) leaflet is connected to left inferior (LI) leaflet by leaflet tissue, resulting in two AV valve orifices; yet there are interventricular communications between short, thick chordae connecting leaflets and their connection to scooped-out underlying ventricular septum. LS and LI leaflets, particularly the latter, are deficient.

(From Bharati et al. )

The main anatomic feature of the AVSD is a deficiency of the AV septum. This is a portion of the membranous septum that lies between the left ventricle and the right atrium, created by the attachment of the mitral valve at a slightly higher level (cephalad) than the tricuspid valve. In patients with AVSD, the mitral and tricuspid valves attach to the septum at the same level. AV septal defects have as defining characteristics a deficiency or absence of the AV septum, resulting in an ostium primum defect immediately above the AV valves and a deficiency (or scooped-out area) in the inlet (basal) portion of the ventricular septum immediately below the AV valves. ^{, ,}

Patients with partial AV septal defects usually have a normal septum secundum and fossa ovalis, and the ostium primum ASD is the result of absence of the relatively small AV septum plus some deficiency in the inlet portion of the ventricular septum. The deficiency in the inlet portion of the ventricular septum is variable but on average is greater in patients with complete AV septal defects than in those with partial defects. ^, These septal deficiencies may or may not result in interatrial or interventricular communications, depending on configuration and attachments of the AV valves.

Five or more AV valve leaflets of variable size are usually present (see Fig. 32.1 ), but there is a high degree of variability in the completeness of commissures and the prominence of crenations in the leaflets ( Fig. 32.3 ). For example, among the 43 hearts with all types of AV septal defects and 2 ventricles in the Green Lane Hospital (GLH; now Starship Children’s Hospital, Auckland, NZ) autopsy series in which the number of leaflets could be accurately assessed, 10 (23%) had 4 leaflets, 18 (42%) had 5 leaflets, 14 (33%) had 6 leaflets, and 1 (2%) had 7 leaflets. When a large interventricular communication was present (complete AV septal defect), the most common number of leaflets was 5 (16 of 28, or 57%).

AV valves in AV septal defects viewed from atrial aspect in a series of fixed specimens. (A) Specimen with partial AV septal defect in which left superior (LS) and left inferior (LI) leaflets are adherent to crest of ventricular septum and there is no interventricular communication. Note that as usual, LS leaflet does not bridge septum (there is no leaflet tissue in the position of the superior portion of the normal tricuspid septal leaflet). In this heart, as is not uncommon, there are two left lateral and two right lateral leaflets. Hearts with partial AVSD have two separate AV valve orifices. (B) Specimen with complete AV septal defect in which there are interventricular communications beneath LS and LI leaflets. LS leaflet does not bridge crest of septum. Right superior (RS) leaflet is characteristically large. LI leaflet is bridging (grade 2) and very distinct from right inferior (RI) leaflet. (C) Specimen of a complete AV septal defect in which LS leaflet markedly bridges crest of septum. Correspondingly, RS leaflet is small. LS leaflet is characteristically larger than LI leaflet. LA, Left atrium; LL, left lateral; RA, right atrium; RL, right lateral.

The left superior leaflet (LSL) and left inferior leaflet (LIL) are particularly variable in size, connections one to another, and degree of bridging across the crest of the ventricular septum (see Figs. 32.1 and 32.3 ). There may be one or two AV valve orifices within the common AV valve, depending upon the presence of AV valve leaflet attachments between the superior and inferior leaflets and to the septal crest. Partial and transitional (or intermediate) forms of AVSD have two separate AV valve orifices, and complete AVSD has a single or common orifice.

Hearts with AV septal defects are also characterized by absence of the usual wedged position of the aortic valve between the AV valves. Instead, it is “unwedged,” elevated and deviated anteriorly. ^{, ,} Details of the aortic-mitral fibrous continuity therefore differ from those in the normal heart. Continuity was abnormal in more than half of 21 specimens with normally related great arteries in the GLH autopsy series; continuity was to the base of the noncoronary cusp in only 5 (24%) and to both the noncoronary and right coronary cusps in 7 (33%). In addition, the left ventricular (LV) inflow tract is shortened in relationship to length of the outflow portion, and there is a related reduction in length of the diaphragmatic wall of the left ventricle ( Figs. 32.4 and 32.5 ). ^{, ,}

Pictorial depictions demonstrating the alteration in inlet and outlet portions of the left ventricle in normal hearts and hearts with atrioventricular septal defect.

(From Penkoske PA, et al. )

The collective effect of these abnormalities is that the LV outflow tract is not only long and serpiginous but also narrowed (so-called “gooseneck” deformity). It is rare for this narrowing to be of hemodynamic importance in the unrepaired heart.

AV septal defects comprise a spectrum of malformations. At one end is the simplest type, in which there is an interatrial communication but no interventricular communication and a connection of variable width between the LSL and LIL; this is called a partial AV septal defect or ostium primum defect . At the other end of the spectrum is the most extreme form, with large deficiencies in atrial and ventricular septa, a common AV valve orifice, and large interatrial and interventricular communications; this is called a complete AV septal defect . Because a continuous spectrum of gradations lies between these extremes, some anomalies have been described as intermediate or transitional AV septal defects, although from a strictly morphologic perspective, there is no difference between any of the varieties of AVSD (partial, transitional, or complete). That is, they all lack an AV septum and therefore possess a common AV junction. Definitions of these intermediate types have varied but usually include presence of two AV valve orifices and a restrictive inlet VSD, with dense chordal attachments to the ventricular septum. Added complexity is provided by occurrence of a large variety of major and minor associated cardiac anomalies ( Tables 32.1 and 32.2 ). In addition, Down syndrome is present in 70% to 75% of patients ^, with a nonrestrictive interventricular communication.

Consequent to the deficient septation of the AV junction seen throughout the spectrum of AVSDs, AV valve morphology is in no way comparable to that in a normally septated heart with a normally developed AV septum. As briefly described earlier, a common AV valve that usually has five leaflet components forms over the common AV junction. Attachments of the AV valves to the crest of the ventricular septum in partial and intermediate AV septal defects, as well as their chordal attachments in complete AV septal defects, are displaced toward the apex of the heart because of deficiency of the inlet portion of the septum. This alters orientation of the AV orifices relative to the aortic orifice (i.e., the aortic valve is no longer wedged between the AV valves) and eliminates normal off-setting of the right and left AV valves. This lack of off-setting is an echocardiographic hallmark of AVSD. ^{, ,}

Two atrioventricular valve orifices.

Typically when two AV valve orifices are present, as in partial AV septal defects, the LSL and LIL are joined together to a variable extent anteriorly by leaflet tissue near the crest of the ventricular septum (see Figs. 32.1 and 32.5 ). Consequently, the left AV valve is trifoliate, being composed of the LSL, LIL, and left lateral (LLL) or mural leaflet, and is oriented differently than the normal mitral valve (see Figs. 32.1 and 32.2 ). ^, Specifically, the LSL and LIL components together occupy two-thirds or more of the circumference of the left AV valve orifice, whereas the LLL or mural leaflet accounts for the remaining one-third or less of the circumference, a ratio that is the proportional inverse to that of a normal mitral valve. ^, Commissures exist between the left superior and mural leaflets and between the left inferior and mural leaflets and are usually supported by respective papillary muscles. These commissures are positionally very different than a normal mitral valve, being posteriorly displaced and much closer together as they bound the margins of the base of the narrower LLL. Therefore, their position is defined by the width of the LLL, which is variable. The ZoA or “cleft” of the left AV valve resides between the LSL and LIL. The connection between the LSL and LIL may be only a thin strand of tissue (complete cleft), but more commonly it is 2 to 4 mm or more deep. This connection, too, is usually fused to the crest of the ventricular septum in partial AV septal defects. Occasionally, chordae pass from opposing edges of the LSL and LIL to the muscular ventricular septum beneath. The chordae that originate from the central edges of the LSL and LIL attach to different papillary muscles, which can cause a distracting force on the leaflets during closure at the ZoA. This contrasts with the normal commissure in which the chordae from adjacent leaflet edges attach to a single papillary muscle, encouraging coaptation. Rarely, separation into LSL and LIL is represented only by a notch in the center of the free edge of a nearly normal “anterior mitral leaflet.” The LLL is usually smaller than the other two leaflets and is triangular ( Fig 32.9 ). The morphology of the LLL including surface area and leaflet tip angle is variable and importantly contributes to left AV function in AVSDs. Rarely, the LLL can be absent, which may negate the ability to close the zone of opposition between the LSL and LIL. ^,

Illustrations of the variable but consistently triangular shape of the left lateral (mural) leaflet and the variable positioning of the underlying papillary muscles.

(From Macé L, Dervanian P, Houyel L, et al. Surgically created double-orifice left atrioventricular valve: a valve-sparing repair in selected atrioventricular septal defects. J Thorac Cardiovasc Surg . 2001;121(2):352-364.)

In aggregate, these left AV valve leaflet anomalies may make the valve regurgitant to a variable degree, sometimes severely. When LSL and LIL are nearly completely separated, an appreciable gap may occur during systole, producing regurgitation. When there is failure of valve coaptation at this site, leaflet tissue forming the margin usually becomes thickened and rolled. In other cases, regurgitation appears to be due to deficiency of leaflet tissue, particularly in the LIL, ^, which may contain an accessory cleft. The mechanism of severe left AV valve regurgitation is, however, not evident in some cases. Rarely, the left or right AV valve is stenotic, but this usually is associated with hypoplasia of the corresponding ventricle.

The right AV valve is also abnormal when there are two AV orifices, although less attention has been paid to it. It may consist of three leaflets—right superior leaflet (RSL), right lateral leaflet (RLL), and right inferior leaflet (RIL)—or of two or four leaflets (see Figs. 32.1 and 32.2 ). Leaflet tissue attached directly or by chordae to the crest or right side of the crest of the septum, and thus contributing to closure of the right AV valves, is considered to represent bridging of the LSL or LIL (see Fig. 32.1 ).

Usually in cases without an interventricular communication, the LSL does not bridge at all (previously, this finding was interpreted as absence or hypoplasia of the superior part of the tricuspid septal leaflet) and the LIL bridges moderately (see Fig. 32.2 A). Even with abnormalities of the right AV valve, it is unusual to see significant regurgitation.

Common atrioventricular orifice.

When the AV valve orifice is a common one and the interventricular communication is large (complete AV septal defect), the LSL and LIL are separate and a bare area is exposed on the crest of the ventricular septum ( Fig. 32.10 ; see Figs. 32.1 and 32.8 C). The LSL may be entirely on the LV side of the septum or may, to a variable degree, bridge the septum and extend onto the right ventricular side (see Fig. 32.2 B-C). This variability formed the basis for the classification by Rastelli and colleagues into types A, B, and C. Chordal attachments of the right ventricular extremity of the LSL vary according to degree of bridging ( Fig. 32.11 ). When there is no bridging, chordal attachments are to the ventricular crest (Rastelli A; Fig. 32.11 A). With mild bridging, they are to the medial papillary muscle in the right ventricle; with moderate bridging, to an accessory (often large) apical papillary muscle (Rastelli B; Fig. 32.11 B); and with marked bridging (Rastelli C), to the normally positioned (although often bifid) anterolateral papillary muscle of the right ventricle ( Fig. 32.11 C). When the LSL bridges the septum moderately or markedly and extends into the right ventricle, it is usually unattached to the underlying ventricular crest (free-floating), but it may occasionally be attached by chordae (tethered).

Complete atrioventricular septal defect viewed from right ventricular side in a specimen in which left inferior leaflet bridges over crest of septum onto right ventricular side. Left superior leaflet (poorly seen) does not bridge, resulting in a bare area of crest of ventricular septum on right ventricular side, where in a normal heart, superior aspect of septal leaflet touches septum. A small fossa ovalis atrial septal defect is also present. CoS, Coronary sinus; LI, left inferior leaflet.

Complete atrioventricular septal defects with varying degrees of bridging of left superior (LS) leaflet. (A) Nonbridging (bridging grade 0) LS leaflet (Rastelli type A). This surgical specimen (the patch having been removed) is viewed from right atrium. Arrow marks mildly bridging left inferior (LI) leaflet. (B) Moderate (grade 2 or 3) bridging of LS leaflet. Chordae from its right ventricular extremity go to a papillary muscle in right ventricle. Arrow indicates bridging portion of LI leaflet. (Rastelli and colleagues termed this type B , but it is just part of the spectrum of bridging.) (C) Marked (grade 5) bridging of LS leaflet (Rastelli type C). Arrow marks bridging part of LI leaflet. RS, Right superior leaflet; S, ventricular septal crest.

(From Rastelli and colleagues. )

The LIL typically bridges moderately, but it, too, varies in this respect. It is not uncommon for a bridging LIL to be attached to the underlying ventricular crest either completely or by short, thick chordae with interchordal spaces.

Chordal attachments of the leftward components of the common AV valve in the left ventricle may be relatively normal, but variation in chordal number and distribution is a consistent feature of hearts with AVSD. In addition, chordal attachments to the interventricular septum are frequently present. Leaflet and subvalvar abnormalities can exist that result in commissural deficiencies (particularly diminished or absent LIL-mural commissure) and/or a single dominant papillary muscle or more closely spaced papillary muscles than usual. The leftward components of the LSL or LIL may have secondary or tertiary chordal attachments to the ventricular septum that may contribute to later development of left ventricular outflow tract obstruction after AVSD repair. There may be only one papillary muscle, producing a parachute-type valve that is difficult to repair. This was true in 7 of 53 (13%) cases in the GLH autopsy series, in 14% of the specimens described by David and colleagues, and in 4% of 155 surgical cases reported by Ilbawi and colleagues. ^, Abnormalities of the subvalvar apparatus and hypoplasia or deficiency of LLL can contribute to left AV valve dysfunction following AVSD repair. ^, The right ventricular portion of the common AV valve has superior, lateral, and inferior leaflets, but as in partial AV septal defects, they vary considerably in number and size (see Fig. 32.2 ). When bridging of the LSL is absent or mild, the RSL is large, whereas with more extensive bridging, it is smaller.

When leaflets of the common AV valve close appropriately during ventricular systole, AV valve regurgitation is absent or mild. However, important left AV valve regurgitation may be present ( Table 32.3 ) that is usually directed through the ZoA between the LSL and LIL or centrally. The mechanism of the regurgitation is likely to be multifactorial and may include valvar dysplasia and left heart dilation due to volume-loading from the VSD shunt.

TABLE 32.3

Preoperative Atrioventricular Valve Regurgitation in Patients with Atrioventricular Septal Defect without Major Associated Cardiac Anomalies

Data from Studer M, Blackstone EH, Kirklin JW, et al. Determinants of early and late results of repair of atrioventricular septal (canal) defects. J Thorac Cardiovasc Surg . 1982;84:523.

	TOTAL		WITHOUT INTERVENTRICULAR COMMUNICATION			WITH INTERVENTRICULAR COMMUNICATION
Magnitude of AV Valve Regurgitation	No.	% of 305 a	No.		% of 154	No.		% of 151
0	29	10	15		10	14		9
1	39	13	26		17	13		9
2	85	28	48		31	37		25
3	98	32	41		27	57		38
4	43	14	16	65/154(42%) b	10	27	87/151 (58%) b	18
5	11	4	8		5	3		2
TOTAL	305		154		151

Anatomic studies by Kanani and colleagues have emphasized the marked valvar abnormalities in this malformation, not only of the anular component but also of the subvalvar apparatus (with deficiency of chordal arrangement) and leaflet tissue (which is often deficient in coaptation surface and pliability following repair).

The defect in the AV septum often displaces the coronary sinus ostium inferiorly, which may appear to lie in the left atrium, especially when the ostium primum atrial defect is particularly large. The AV node is also displaced inferiorly (caudally) and lies in the posterior right atrial wall between the orifice of the coronary sinus and ventricular crest ( Fig. 32.12 ) in what has been termed the nodal triangle . The bundle of His passes forward and superiorly from the node to the ventricular crest, reaching it where the crest fuses posteriorly with the AV valve anulus. It then courses along the top of the ventricular septum beneath the bridging portion of the LIL, giving off the left bundle branches. As it reaches the midpoint of the crest of the ventricular septum, it becomes the right bundle branch, which continues along the crest a little farther before it descends toward the muscle of Lancisi and trabecula septomarginalis. These anatomic findings have been supported by electrophysiologic studies at operation. ^, Current clinical investigations to achieve intraoperative imaging of conduction tissue may allow surgeons to visualize conduction tissue in all patients, theoretically eliminating the threat of acquired postoperative complete heart block and mitigation strategies predicated on anatomic boundaries that approximate its location. The morphology of the conduction system in AVSD is a determinant of its characteristic electrocardiographic pattern: left axis deviation, superior QRS frontal plane axis, and counterclockwise depolarization pattern (see “ Electrocardiogram ” later). ^,

Location of the conduction system in atrioventricular septal defect. The Triangle of Koch and the supplementary nodal triangle are denoted as well as the pathway of the Bundle of His and the location of the AV node. See text for further details.

In AVSD, the inflow portion of the left ventricle is foreshortened due to the downward displacement of the LAVV (see “ General Morphologic Characteristics ” under Morphology, earlier in this chapter). The left ventricle will often appear smaller than the right ventricle due to left-to-right shunting and resultant right ventricular enlargement; however, absolute volumetrics should generally be within normal limits in balanced AVSD. LV hypoplasia is usually associated with an unevenly distributed AV valve complex. This is referred to as right dominant unbalanced AVSD and is addressed later in this chapter (see Special Situations and Controversies ).

The right ventricle has no specific anomalies but is usually enlarged secondary to the left-to-right shunt. It, too, may be frankly hypoplastic with associated maldistribution of the AV valve, in which case the arrangement is referred to as left dominant unbalanced AVSD.

The LV outflow tract is characteristically elongated and narrowed ( Fig. 32.13 ) in all types of AV septal defect (see “ General Morphologic Characteristics ” under Morphology earlier in this chapter).

The left ventricular outflow tract as viewed from the ventricular apex in (A) a normal heart, and (B) in complete atrioventricular septal defect. The narrowness of the LVOT is apparent.

Septal malalignment may refer to either the atrial or ventricular septum. True atrial septal malalignment is unusual in balanced AVSD but quite common in unbalanced AVSD.

In a single center retrospective analysis of 31 patients with AVSD including 22 patients undergoing biventricular repair, Ahmad and colleagues described the angle formed by the axis of the atrial and ventricular septa (termed the AV septal angle). In the 22 patients undergoing biventricular repair, the mean AV angle was 7.4 +/- 11.1 degrees, whereas in patients requiring univentricular repair it was 28.6 +/- 3.04 degrees ( P <.0002) using multiplanar three dimensional datasets. Consequently, the AV septal angle provides useful information to guide assessment for ventricular hypoplasia and imbalance when assessing complete AVSD anatomy.

Important LV outflow tract obstruction (LVOTO) occurs rarely in unoperated AVSD, unless severely unbalanced right dominant AVSD is present. ^{, ,} It most often becomes apparent in the postrepair setting, occurring in about 5% of AVSD patients at a mean interval of 5 to 7 years following primary repair. It is surprising that it is not more frequent, in view of the elongation and narrowing of this area in affected hearts. ^, The incidence of LVOTO after AVSD repair is similar in patients with partial and complete AVSD.

As reviewed previously, the elongation and narrowing of the LVOT results from multiple morphologic inputs including “unwedging” of the aortic root, discrepancy between the inflow and outflow septal lengths which is a consequence of the downward displacement of the AV valve and a more extensive area of direct fibrous continuity between the aortic valve and the LSL than is present normally between the aortic and mitral valves. Short, thick chordae may anchor the LSL to the crest of the ventricular septum thereby crowding the distal LVOT. The anterolateral muscle bundle of the left ventricle (AML), described by Moulaert and Oppenheimer-Decker, is a horizontal muscle bundle located between the left coronary cusp and the aortic leaflet of the mitral valve. It is present in 40% of normal hearts and 100% of hearts with AVSD, and its encroachment on the LVOT is additive to LVOTO in AVSD. Resection of the AML should be a considered element of any reoperation for LVOTO after AVSD repair. ^{, ,} In addition to these basic arrangements tending to narrow the LV outflow tract, LV obstruction may be contributed to by morphologically discrete subaortic stenosis or by excrescences of AV valve tissue heaped up in the LV outflow tract. ^{, ,} It may also result from abnormally positioned papillary muscles, which are, again, rotated counterclockwise and may be more closely spaced in AVSD. Specifically, the anterior papillary muscle may have an abnormally high insertion, creating proximal LVOT obstruction. Occasionally, the substrate to create LVOTO may be overlooked preoperatively, and obstruction becomes apparent or develops only after operation.

Important LV inflow obstruction may occur rarely. ^, This may be from simple narrowing of the AV valve entrance into the underlying ventricle or its inflow angle relative to diastolic loading of the ventricle. This is an important marker of unbalance and may occur with either the left or right-sided AV valve (see Special Situations and Controversies later). As previously noted, it may occur in relation to the presence of an accessory AV valve orifice on the left side. It may also result from cor triatriatum (see Chapter 31 ) or a supravalvar fibrous ring.

Table 32.1 presents the prevalence of the major cardiac anomalies associated with AV septal defects.

Table 32.2 lists minor cardiac anomalies associated with AV septal defects.

In partial AV septal defects, as in other types of ASDs, pulmonary vascular disease is uncommon, whereas in complete AV septal defects, as with large VSDs, the risk of pulmonary vascular disease begins to rise by 6 months of age and progresses. ^, Morphologically, pulmonary vascular disease associated with complete AV septal defects is similar to that associated with large VSDs (see “ Pulmonary Vascular Disease ” under Morphology in Section I of Chapter 33 ). However, it tends to progress more rapidly in patients with complete AV septal defects. Correlation between histologic findings and pulmonary vascular resistance is similar in the two conditions. The pulmonary vascular changes are more frequent and can occur as early as 6 months of age in patients with Down syndrome with complete AV septal defects compared with patients without Down syndrome.

Down syndrome is rare in patients with partial AV septal defects but common (50%-75%) in those with complete AV septal defects. While advanced pulmonary vascular disease is more frequent in patients with Trisomy 21, left-sided obstructive lesions are 10 times less common in such patients, and other associated anomalies are probably also less common.

It is important to note that an isolated inlet (AV septal) type of VSD (see “ Morphology ” in Section I of Chapter 33 ) occurs without any of the features of an AV septal defect as defined in this chapter, except that it involves the inflow portion of the ventricular septum beneath the septal tricuspid valve leaflet and usually also the area of the membranous ventricular septum. The AV septum, however, is intact, and the mitral and tricuspid annuli and aortic orifice lie in normal positions with normal off-setting of the AV valves. This feature allows these VSDs to be readily differentiated echocardiographically and angiographically from AV septal defects. Interestingly, in isolated inlet VSD, the anterior mitral leaflet is often cleft.

In the contemporary era with access to standard pregnancy care, most congenital heart defects are detected prenatally by screening morphologic ultrasound and confirmed by dedicated fetal echocardiography. Diagnosis can be made between 18 and 24 weeks of gestation but is also possible as early as 11 to 14 weeks and includes all forms of AVSD with complete AVSD being the most straightforward to detect. The detection rate for AVSDs with obstetric ultrasound is approximately 43%. The four-chamber obstetric ultrasound view is optimal for detection with the ability to screen for absence of the cardiac crux, absent differential insertion of the AV valves, and defects in the atrial and/or ventricular septum at the crux cordis. At least 50% of fetuses with complete AVSD will have associated cardiac and noncardiac anomalies and 60% will have associated Trisomy 21. Multiple other chromosomal abnormalities including trisomy 18 and 13 are also associated with complete AVSD. In cases of unbalanced complete AVSD, 25% to 50% will have associated heterotaxy syndrome. For cases of complete AVSD, around 30% of parents will elect to terminate the pregnancy and there is a significant rate of fetal demise, still birth and neonatal death, particularly for complex AVSD associated with other cardiac and/or extracardiac anomalies.

Patients without an interventricular communication (partial AV septal defect) and with absent or mild left AV valve regurgitation often have minimal symptoms and their clinical presentation is virtually identical to that of patients with secundum ASD (see “ Fossa Ovalis Defect ” under Morphology in Chapter 29 ). If there is audible left AV valve regurgitation, then the patient may have an apical systolic/holosystolic murmur which radiates toward the axilla. The electrocardiogram (ECG) is quite distinct, characterized by the presence of a superior axis directed more toward the left and commonly with signs of RV volume overload such as rsR′ or rR′. ^, Patients with moderate or severe left AV valve regurgitation with partial AV septal defects may be symptomatic earlier, and such patients may develop congestive heart failure and require surgical repair in infancy. In these patients with heart failure, in addition to the usual signs of ASD with a wide and fixed split S2, the heart is more active in association with a loud apical pansystolic murmur, mid-diastolic murmur (relative AV valve stenosis), apical displacement of apical impulse, and possible S3 gallop rhythm. Tachypnea, tachycardia, sweating while feeding, failure to thrive, and hepatomegaly can also be present.

In patients with a large interventricular communication (complete AV septal defect), symptoms of pulmonary overcirculation develop usually before the age of 6 months. Typically, left-to-right shunting is minimal in the first few weeks of life, becoming more pronounced at 4 to 8 weeks as pulmonary vascular resistance falls. Tachypnea and failure to thrive emerge as congestive failure physiology supervenes. Clinical signs and symptoms are similar to those of patients with isolated VSD. The larger the VSD, the softer the systolic murmur, but a systolic murmur can be heard at the left sternal border with a mid-diastolic rumble often heard in the apex. In patients with important AV valve regurgitation, a systolic murmur with radiation toward the axillae is present. Tachypnea, tachycardia, sweating with feeds, failure to thrive, and hepatomegaly can be present. Occasionally, heart failure symptomatology is minimal or absent and the patient has good weight gain. Chest radiograph will characteristically show clear lung fields, a relatively normal heart size, and prominent pulmonary vasculature at the hila. This clinical picture is a marker of elevated pulmonary vascular resistance and should prompt preoperative evaluation for pulmonary vascular disease, including either noninvasive or catheterization-based assessment of pulmonary vascular resistance and reactivity. It is extremely rare in the developed world, but when presentation is delayed until some years later, there is frequently severe pulmonary vascular disease and Eisenmenger complex (see “ Clinical Findings ” under Clinical Features and Diagnostic Criteria in Section I of Chapter 33 ).

In those patients with morphology intermediate between the partial and complete AV septal defects, clinical features depend on the size of the interventricular communication and severity of left AV valve regurgitation. In particular, the auscultatory examination will feature a higher-pitched and louder murmur as the VSD becomes more restrictive.

In the current era, infants with Trisomy 21 routinely undergo screening echocardiography. If diagnosed in utero with Trisomy 21, then a fetal echocardiogram is indicated and can rule out AVSD. However, for subtle and smaller defects, postnatal echocardiography is still indicated. Given this current approach, the vast majority of infants with Trisomy 21 and AVSD are diagnosed prior to becoming symptomatic. It should be noted that patients with Trisomy 21 are at higher risk of developing pulmonary vascular disease with unrepaired intracardiac shunts.

In patients with partial AVSD without important left AV valve regurgitation, the chest radiograph is the same as in other large ASDs with right heart enlargement and increased pulmonary vascular markings. When moderate or severe left AV valve regurgitation is present, the radiograph usually shows marked cardiomegaly with evidence of cardiac chamber enlargement (can be LA/LV and/or RA/RV depending on the degree of MR and shunt size at the atrial level) and pulmonary plethora.

In complete AVSD, cardiomegaly and pulmonary plethora are evident in infants and young children presenting with heart failure. In patients who have elevated pulmonary vascular resistance, the heart is less enlarged, central pulmonary arteries are large, and lung fields can be relatively clear.

ECG findings are rather specific. The most characteristic feature of the ECG in AVSD is a superior QRS axis directed more toward the left, although this has been noted by several investigators to be variable and affected by the underlying anatomic substrate. ^, The PR interval is frequently prolonged. Partial right bundle branch block with rsR′ and RV enlargement is also typical. A superior axis of the P wave is seen in left atrial isomerism. ^,

Echocardiography is, today, the gold standard for characterization of essentially all aspects of AVSD anatomy. In the majority of cases, it is the only diagnostic modality (with the exception of standard chest radiograph (CXR) and ECG) required prior to definitive repair. Many sophisticated and nuanced echocardiographic measures have been introduced and refined over the last decade and are beyond the scope of this chapter. However, there are well-established fundamental echocardiographic features of AVSD. ^, These include the following:

• •

  Loss of normal off-setting of AV valves (absence of the AV septum)

• •

  Abnormal configuration of the AV valves

• •

  Trifoliate appearance of the left AV valve (so-called “cleft”)

• •

  Unwedged or “sprung” aortic valve

• •

  Left ventricular inlet-outlet disproportion (so-called “gooseneck deformity”)

• •

  Abnormal position of the left ventricular papillary muscles (counterclockwise rotation)

The four-chamber view clearly demonstrates the loss of normal off-setting of the AV valves. That is, the valvar complex lies in the same plane rightward and leftward of the interventricular septum, as opposed to the slight cephalad displacement of the mitral valve in normal hearts. This finding is true regardless of whether a VSD is present or not and is essentially diagnostic of AVSD. Also in this view, chordal attachments and degree of leaflet bridging can be assessed. The elongated left ventricular outflow tract and unwedged position of the aortic valve are best characterized in the precordial long-axis view, although unwedging is also appreciated on the four-chamber view. Valvar abnormalities such as leaflet dysplasia, accessory “clefts,” location of regurgitant jets, and papillary muscle abnormalities require skill and experience to precisely identify. By using a thorough combination of parasternal and subcostal short-axis and long-axis sweeps, in addition to four- and five-chamber views, many, if not all, of these important features can be identified. The degree of AV valve regurgitation can be evaluated with the use of color flow Doppler. Double orifice left AV valve may be challenging to image, particularly in hearts with a large ventricular septal defect (VSD), but is best seen in short-axis views. Using the subcostal 30-degree left anterior oblique (LAO) view, the degree of commitment of the common AV orifice to the right and left ventricles can be ascertained. Other measures of unbalance are also obtained by echocardiography (see “ Unbalanced Atrioventricular Septal Defect ” under Special Situations and Controversies). Likewise, echocardiography can be used to diagnose associated malformations such as tetralogy of Fallot, double outlet right ventricle, transposition of the great arteries, and additional VSDs.

A patent ductus arteriosus can be identified by echocardiography unless pulmonary vascular resistance is elevated and left-to-right shunting thereby minimized. For this reason, empiric ligation of the PDA or ligamentum arteriosum is a reasonable practice at the time of AVSD repair.

While patients suspected of having pulmonary vascular disease and elevated pulmonary vascular resistance should undergo preoperative catheterization for proper assessment (see “ Cardiac Catheterization ” later), echocardiography can also be employed to assess this possibility. In the setting of unrepaired VSD, the direction and magnitude of ventricular level shunting (right to left, bidirectional, left to right) with and without oxygen and/or inhaled nitric oxide can be ascertained. Evaluation of the pulmonary regurgitation velocity and presence of notching in the pulmonary arteries can also be clues to elevated pulmonary artery pressures, as are interventricular septal position, PR velocity, and presence of RV hypertrophy. Using continuous wave Doppler across the tricuspid valve to assess peak tricuspid regurgitation (TR) velocity, right ventricular pressure is easily measured in the septated heart (postrepair or prerepair in hearts with absent or restrictive interventricular communication). This can be particularly useful in the postrepair setting wherein pulmonary hypertension may occur suddenly and unexpectedly.

Atrial pressure, right and left ventricular systolic and diastolic pressure, and the presence or absence of pressure gradients across AV valves and/or outflow tracts, arterial and venous saturations, direction and magnitude of shunting, pulmonary and systemic pressures, resistances, and relative flows can all be measured and calculated from data obtained at cardiac catheterization. These studies are now required only when major cardiac anomalies coexist or when operability is questioned because of clinical evidence of pulmonary vascular disease, particularly with late presenting cases.

Anatomic angiocardiographic features of AV septal defects were well described by Baron and colleagues in 1964 and further refined by the work of Brandt and colleagues, Bargeron and colleagues, and Macartney and colleagues ^{, , , ,} in the era prior to development of echocardiography. These are well portrayed by the line drawings of Baron and colleagues and representative cineangiograms ( Figs. 32.14 through 32.16 ). Detailed cineangiography is now largely of historic interest, being mostly supplanted by enormous advances in echocardiocardiography and, more recently, advanced cardiac imaging.

Diagrams of altered attachment of left AV valve leaflets in AV septal defect compared with normal heart. Heart is shown in its in vivo position as seen in a frontal angiogram. Right ventricle (RV) and most of the ventricular septum and right atrium have been removed. Dashed line indicates portion of line of attachment of left AV valve hidden by other structures. (A) Normal heart. Attachment of anterior mitral leaflet (A) begins at anterolateral commissure (AL) and runs anteriorly for a short distance along free wall of left ventricle (LV) . It is then continuous with root of aorta in relation to adjacent portion of left coronary (L) and noncoronary (N) aortic cusps. Line of attachment proceeds downward along AV septum to posteromedial commissure (PM) . Attachment of mitral valve to AV septum is profiled in right anterior oblique view. (B-C) Partial AV septal defect shown in diastole (B) and systole (C). Scooped-out crest of basal portion of ventricular septum is shown considerably thinner than it actually is. Right AV valve leaflets are shown only at their sites of attachment. Diastolic figure (B) depicts left superior and left inferior leaflets as open; their line of attachment to aortic root is nearly normal, but it then passes along the superior rim of the scooped-out ventricular septal crest. Left superior leaflet is displaced upward into the left ventricular outflow tract, narrowing it. Left inferior leaflet is folded back against left ventricular aspect of sinus septum. In systolic figure (C), left superior and inferior leaflets are closed. Increased left ventricular pressure balloons them toward the atria. Arrow marks their point of coaptation. AP, Anterior papillary muscle; AV, atrioventricular septum; I, left inferior leaflet of atrioventricular valve; LA, left atrium; MS, muscular ventricular septum; P, posterior mitral leaflet; PP, posterior papillary muscle; R, right aortic cusp; S, left superior leaflet of atrioventricular valve; T, tricuspid valve.

(From Baron and colleagues. )

Diagrammatic representations of cineangiograms of a normal heart and hearts with AV septal defects, in oblique and axial views. (A) Mitral valve orifice and leaflet attachments (interrupted line) in right anterior oblique (RAO) and left anterior oblique (LAO) projections. (1) Normal mitral orifice is approximately profiled in 40-degree RAO projection but is overlapped by left ventricular (LV) outflow tract. Rightward posterior border of normal LV outflow tract is formed by AV septal tissue, not mitral valve. (2) In 50-degree LAO projections, the rightward anterior margin of the normal outflow tract consists of the basal ventricular sinus (inlet) septum: membranous above, muscular below. Mitral valve attachments do not reach the septal margin. (3, 4) In AV septal defects, absence of AV septum and adjacent deficiency of basal (inlet) ventricular septum modify left AV valve attachments and position and shape of left AV orifice and LV outflow tract. (B) LV cineangiograms in 40-degree RAO projections. (1, 2) Normal features can be compared with those of typical partial (3, 4) and complete (5, 6) AV septal defect in systole and diastole. In the normal heart, mitral (left AV valve) leaflets contribute only to the lowest portion of rightward posterior LV outflow margin in systole, the relatively immobile AV septum forming the remainder of this margin throughout cardiac cycle in diastole. Line of attachment (m) of mural (posterior) leaflet of mitral valve can be identified, because contrast is trapped between leaflet and adjacent LV wall. In AV septal defects, rightward posterior margin of LV outflow tract consists of mobile leaflet tissue: left superior leaflet (s) above and left inferior leaflet (i) below. AV septum is absent. Mural leaflet attachment lies in relatively normal position. When there is complete attachment of left superior and inferior leaflets to septal crest (dashed line) , LV outflow tract deformity is well marked in systole, and position of septal crest can be identified in diastole, with contrast being trapped between open leaflets and septum. When there is a large interventricular communication with superior and inferior leaflets free-floating or attached to septal crest by thin chordae only (5, 6) , systolic deformity may be less marked and septal crest may be invisible, because contrast is washed away by non-radiopaque inflow. RAO view separates an AV regurgitant stream (AVR) from an interventricular shunt (VS) . (C) Left ventricular (LV) cineangiograms in 50-degree left anterior oblique (LAO) projection. (1, 2) In normal heart, septal margin of LV outflow tract is uninterrupted in systole and diastole. In AV septal defect (3-8) , septal margin is interrupted by defect in basal (inlet) ventricular septum, the defect being continuous with left AV orifice. (3, 4) When left superior (s) and inferior (i) leaflets are completely attached to septal crest, as in partial AV septal defect, leaflet tissue bulges into defect in systole, and position of septal crest (c) can be identified in diastole (7, 8) . When there is a large interventricular communication, systolic flow into right ventricle can be observed passing beneath left superior (upper arrow) or left inferior (lower arrow) leaflets. In diastole, a common AV orifice is identified (5, 6) . In some cases, attachments to septal crest are present but leave smaller interventricular communications. AV valve regurgitation tends to obscure valve detail as overlying atria opacify. (D) LV or left atrial cineangiograms of AV septal defect in 50-degree LAO with cranial angulation (axial, hepatoclavicular, or four-chamber view). (1) Arrows in 40-degree RAO view illustrate why conventional LAO (part C) shows full height of the AV orifice, providing the best separation of left superior from left inferior leaflets. Cranially tilted version of LAO (CR LAO) foreshortens AV orifice and tends to superimpose these leaflets (2, 3) . However, the characteristic deformity of septal and AV orifice anatomy seen in conventional LAO view can be appreciated in axial LAO views, which are shown in both systole and diastole. In addition, midmuscular and apical parts of the sinus septum are better separated from basal defect so that additional muscular defects in cross-hatched area (e.g., at x ) may be identified. AV valve regurgitation obscures detail, but partial separation of atria from ventricles improves differentiation of regurgitation from interventricular shunting. With a left atrial or right upper pulmonary venous injection in 50-degree LAO with cranial angulation (4) , contrast flow (arrows) early in the sequence shows position of atrial septal defect adjacent to AV valves. Contrast enters right atrium, right ventricle, and left ventricle. Cranial angulation rarely achieves a perfect profile of AV anulus, and ventricular opacification obscures detail later in the sequence. AV, Atrioventricular septum; AVR, atrioventricular regurgitant stream; bs, basal (inlet) portion of ventricular septum; c, septal crest; CR LAO, cranial left anterior oblique; i, left inferior leaflet; L, left coronary sinus of aortic root, LA, left atrium; LAO, left anterior oblique; LV, left ventricle; m, line of attachment of posterior mitral leaflet; N, noncoronary sinus of aortic root; R, right coronary sinus of aortic root; RA, right atrium; RUPV, right upper pulmonary venous injection; RV, right ventricle; s, left superior leaflet; VS, ventricular shunt.

Cineangiograms of AV septal defects. (A) Partial AV septal defect shown by left ventriculogram in four-chamber position: diastolic frame (left) and systolic frame (right) . Loss of straight line contour from aortic valve to crux cordis indicates absence of major part of AV septum. (B) AV septal defect with two AV valve orifices and an interventricular communication. (C) Complete AV septal defect shown by left ventriculogram in four-chamber view. Anulus of valve is seen as a negative shadow formed by accumulation of contrast medium between the leaflets and ventricular free wall (arrowheads) ; it includes right (RV) and left ventricles (LV) in almost equal proportions. On right, contrast medium outlines left lateral leaflet (LLL) , which is related to aorta (Ao) and separated from left superior leaflet by a commissure (arrowhead) marking position of papillary muscle of conus. Outline of left superior leaflet is separated from left lateral leaflet by a commissure marking position of anterior papillary muscle of left ventricle. This represents an example of Rastelli type A defect. (D) Partial AV septal defects with moderate (left) and severe (right) regurgitation of left AV valve. Contrast medium has passed from left ventricle into right atrium (RA) through regurgitant valve. LAVV, Left atrioventricular valve; PT, pulmonary trunk; RAVV, right atrioventricular valve.

Presence of common atrium generally can be identified preoperatively by echocardiography and shows nearly complete absence of both atrial and AV septum ( Fig. 32.17 ). Finding mild arterial desaturation in a patient with clinical findings of an AV septal defect, but without an interventricular communication or pulmonary artery hypertension, suggests presence of common atrium. ^{, , ,} Presence of a left superior vena cava in such a setting suggests both unroofed coronary sinus syndrome and common atrium, because they frequently coexist. ^, In patients with common atrium and atrial isomerism, even more complex associations occur including DORV, partial or total anomalous pulmonary venous connection, and azygos extension of the inferior vena cava (see “ Morphology ” in Chapter 53 ). ^{, , ,}

Echocardiograms of AV septal defects. (A) Apical four-chamber view of a heart with an unbalanced AV septal defect. In this figure, the common AV valve (arrow) is positioned such that there is malalignment between interatrial septum and interventricular septum and a disproportionate size of the ventricles. Smaller arrow shows primum atrial septal defect. (B) Apical four-chamber view showing common AV valve (arrows) with virtual absence of interatrial septum. (C) Parasternal long axis view showing a “gooseneck” appearance of left ventricular outflow tract caused by displacement of left-sided portion of a common AV valve (arrow). (D) Coronal image of heart as viewed from apex (apical four-chamber view). This heart has a complete AV septal defect. There are both primum and secundum atrial septal defects.(E) Coronal image of a heart with a complete AV septal defect as viewed from the apex (apical four-chamber view). Arrows point to the common AV valve. There are both primum and secundum atrial septal defects. (F) Coronal image of a heart with a partial AV septal defect as viewed from the apex (apical four-chamber view). Thin arrows point to common AV valve. Thick arrow points to tissue occluding the inlet ventricular septal defect right AV valve pouch formation. Ao, Aorta; IVS, interventricular septum; LA, left atrium; LV, left ventricle; RA, right atrium, RV, right ventricle; V, inlet ventricular septal defect; 1°, primum; 2°, secundum.

It is essential to recognize functionally important LVOTO . The distinctive geometry results from the disproportionate lengths of the inflow and outflow axis of the left ventricle and anterior displacement of the left AV valve complex (see “ Morphology ” previously) ( Fig. 32.18 ). It rarely results in preoperative LVOTO in balanced AVSD. Instead, and in the vast majority of cases, important obstruction only becomes evident postoperatively, early or late. It is typically multifactorial, secondary to the elongated LVOT, abnormal or accessory valvar tissue or insertions, as well as discrete fibromuscular obstruction (see “ Left Ventricular Outflow Tract Obstruction ” later). Infrequently, it occurs as a result of the ventricular patch component pulling the left AV valve apically and anteriorly such that the left AV valve apparatus narrows the outflow tract. Thus, the initial repair must place the left AV valve at the appropriate level: caudad toward the apex may result in LV outflow tract obstruction; cephalad toward the atria may result in left AV valve regurgitation. When the former situation occurs, either in the setting of no or a very small interventricular communication or as a result of repair, resuspension of the LSL after patch closure of a created or enlarged VSD has been described as a technical approach to relief of LVOTO. ^{, ,}

Anatomic specimens (above) and drawings of them (below) contrasting outflow angle in a normal heart compared with that in AV septal defect. (A) Wide angle between plane of outlet septum and plane of septal crest (outflow angle) in a normal heart. (B) Narrow angle in a heart with AV septal defect. Septal crest is scooped out, shortened, and anteriorly displaced; outflow axis is elongated.

(From Van Arsdell and colleagues. )

Predictors of postoperative LVOTO have been sought by many investigators. Evidence is mixed with regard to the impact of subtype of AVSD (partial, transitional, or complete) on the incidence of LVOTO after repair. Most recent series report similar reoperation rates among AVSD subtypes. ^{, ,} Similarly, Rastelli type A configuration of the common AV valve has been widely understood to predispose to LVOTO after AVSD repair compared with types B and C, but data from the recent literature do not support this view. ^{, ,} However, there are data to suggest that a number of specific echocardiographic metrics are associated with the development of LVOTO after AVSD repair. In a single institution analysis of 415 patients with repaired AVSD, indexed LVOT diameter, decreased LV inflow/outflow length ratio, the presence of chords crossing the LVOT, decreased modified atrioventricular valve index (mAVVI), and coarctation were all found to be associated with LVOTO compared to controls. In multiple logistic regression analysis, the presence of chordae in the LVOT and indexed LVOT diameter ≤ 2.5 cm/m ² were independently associated with the development of LVOTO. Aortoseptal angle and high insertion of the anterior papillary muscle have also been associated with the development of LVOTO. ^,