The Atrioventricular ConnectionAtrioventricular connections are concordant when the right atrium aligns with the right ventricle, and the left atrium aligns with the left ventricle, whereas they are discordant when the right atrium aligns with the left ventricle, and the left atrium aligns with the right ventricle. There are six types of atrioventricular connection. Three of these exist in hearts with two ventricles, termed biventricular atrioventricular connections, and three in hearts with a single functional ventricle, termed univentricular atrioventricular connections. Biventricular atrioventricular connections exists when each atrium connects to its own ventricle, and include concordant, discordant and ambiguous variants. The former connections exist when each one of the lateralized atria connects concordantly and discordantly with a ventricle, respectively, and are ambiguous when isomeric atria connect to the ventricles. The left atrioventricular (mitral) valve
Van Praagh and associates have advocated a shorthand method to designate the organizational pattern of the venoatrial connection, the pattern of ventricular looping, and the ventriculoarterial connection. By this method, the situs of each of these segments is recorded in the sequence {venoatrial connection, ventricular looping, ventriculoarterial connection}. Situs solitus indicates the usual or normal pattern, situs inversus the mirror-image of the normal pattern, and situs ambiguous denotes an ambiguous or indeterminate pattern. Univentricular atrioventricular connections are also of three types, and include double inlet ventricle, tricuspid atresia, and mitral atresia. Double inlet ventricle exists when both atria are connected to the same ventricle, and can be one of any of the four types of atrial arrangements, those being of normal, mirror-image, right or left isomeric arrangement. The ventricle to which the atria connect can be of either left, right or indeterminate morphology. An atrioventricular connection is absent when the involved atrium has a completely muscular floor and exhibits not even a rudiment of the atrioventricular junction. The atrial floor is then separated from the ventricular mass by the fibrofatty tissue of the atrioventricular groove. Tricuspid atresia is present when the left-sided atrium is connected to the ventricular mass, and mitral atresia is present where the right atrium is connected to the ventricular mass. The ventricular mass can again be of left, right, or indeterminate morphology. Despite being termed "single ventricles", most heart with a univentricular atrioventricular connection have two ventricles, albeit one is dominant and the other rudimentary. Rudimentary right ventricles are located anterosuperiorly relative to the dominant ventricle ('on the shoulders' of the ventricular mass) while rudimentary left ventricles are posteroinferior ('in the hip pocket'). Either type of rudimentary ventricle can be left- or right-sided. When both atria are connected to a single ventricle, then the two atrioventricular junctions can be guarded by either a common or two separate ventricular valves. In the presence of two ventricular valves, one of two valves may straddle and/or over-ride the ventricular septum. The term straddle is used when the chordae tendineae of the ventricular valve is attached to both ventricles, and under such circumstances the atrioventricular junction is usually also connected to both ventricles, although not necessarily so. Conversely, the term over-riding is used to refer to the biventricular connection (and not attachment) of a ventricular valve. The two features do not always co-exist, and can be found in isolation. The feature of over-riding is important in that the degree of over-ride also determines the type of atrioventricular connection present, hearts with over-riding atrioventricular junctions being intermediate between biventricular and univentricular connections. The valve is assigned to the ventricle connected to the greater part of its junction and the connection determined accordingly. A common ventricular valve is defined as one which drains both atrial chambers to the ventricular mass. Common ventricular valves usually straddle and over-ride the ventricular septum, except for hearts in which there is a univentricular connection, in which case they can be exclusively connected to the dominant ventricle. As with one of two valves, there is a spectrum of abnormalities in hearts having common valves with biventricular or univentricular atrioventricular connections. The modes of connection are strictly limited in hearts having absence of one atrioventricular connection, in that such hearts have a single ventricular valve. The ventricular valve most usually is exclusively connected to the dominant ventricle, although rarely it may straddle and/or over-ride. Ventricular spatial relationshipsThe relationships of the ventricles in hearts with biventricular atrioventricular connections is vital in diagnosis and description when they are not as anticipated for the given connection and ventricular topology. For example, in hearts with usual atrial arrangement and a concordant atrioventricular connection, it would be most unexpected for the morphologically right ventricle not to be right sided and relatively anteroinferior to the morphologically left ventricle. Yet, on rare occasions, the morphologically right ventricle in hearts with these connections and topologies can be left-sided. Similarly, in hearts with usual atrial arrangement, discordant atrioventricular connection and left-hand topology, the morphologically right ventricle can be found in right-sided rather than its expected left-sided position. These unexpected ventricular relationships, which are the result of rotation of the ventricular mass along its long axis, produce the so-called criss-cross heart. The essence of the criss-cross heart is the finding of unexpected ventricular relationships for the given connection. A criss-cross heart also present in hearts in which ventricular topology not as expected, so that both the topology as well as the relationship are disharmonious with the connection. The 'upstairs-downstairs' ventricular relationship is a less extreme form of unexpected chamber locations, in which case the ventricular mass is tilted along its long axis in one or other direction so that the ventricles are arranged in layered rather than upright fashion. Ventricular situsThe situs of the ventricular mass is designated according to the embryological looping pattern. Ventricular D-looping is the normal arrangement (situs solitus) in which the morphologically right ventricle is right-sided and the morphologically left ventricle is left-sided. As viewed from the right ventricular free wall, as one would in the operating room, the normal, D-looped right ventricle is organized from right to left, such that
Ventricular L-looping results in the inverse arrangement in which the right ventricle is left-sided, and the left ventricle is right-sided. Here, the right ventricle is organized from left to right, such that
Ventricular situs ambiguous (designated as X-looping) is exceptionally rare, and would designate anatomic uncertainty or indeterminate ventricular looping. Notably, there are no defined syndromes of ventricular situs ambiguous as there are for venoatrial situs ambiguous. The situs of the atrioventricular connection is based on the relationships of the atria to the ventricles, and is derived from the ventricular looping pattern. This is because the ventricular valves are virtually always associated with their appropriate ventricles, so that the left ventricular valve (mitral valve) is virtually always connected to the left ventricle, while the right ventricular valve is virtually always connected to the right ventricle. The reason for this very strong association is that the ventricular valves are embryologically derived in large part from their corresponding ventricle, the left ventricle forming the great majority of the left ventricular valve, while the right ventricle forms the great part of the right ventricular valve. (This is also the reason while the size of a ventricular valve corresponds closely to the size of the ventricle to which it is attached.) Only that portion of the ventricular valves arising from the atrioventricular septum is derived from a source other than the ventricles, and while that relationship is important for the separation of the two ventricular valves, it does not play prominently in the association of the ventricular valve to the ventricular mass. There are two reasons why this relationship is important: First, the location of either the right or left ventricular valve can be derived from the ventricular looping pattern. For the normal, D-looped ventricle, the right ventricular valve is on the right side, while the left ventricular valve is on the left side, whereas for the L-looped ventricle, the right ventricular valve is on the left side, while the left ventricular valve is on the right side. Second, atrioventricular concordance can be derived from noting the relationship of the venoatrial situs to that of the ventricular looping pattern. For example, in venoatrial situs solitus and a D-looped ventricle, {S,D,-}, the morphologically right venoatrial connection is normal and right-sided, and the morphologically right ventricular valve is also right-sided, hence atrioventricular concordance is present. For venoatrial situs inversus and a D-looped ventricle, {I,D,-}, the morphologically right venoatrial connection is left-sided, while the morphologically right ventricular valve is right-sided, hence atrioventricular discordance is present. In general, if both the venoatrial and ventricular situs are either solitus or inversus, then atrioventricular concordance is present, whereas if one is situs and the other inversus, then atrioventricular discordance is present. For venoatrial situs ambiguous, or the exceptionally rare X-looped heart, atrioventricular connection remains incompletely defined. |
The left ventricular, or mitral, valve has two leaflets of markedly
dissimilar shape and circumferential length. The anterior mitral valve leaflet (AMVL) is
square in shape and takes up only one-third of the annular circumference. Its most
characteristic feature is its fibrous continuity with the leaflets of the aortic valve.
The posterior mitral valve leaflet (PMVL) is long and thin, taking up two-thirds of the
annular circumference. It is attached throughout its length to the diaphragmatic wall of
the ventricle. The commissure between the leaflets is oriented in posteromedial and
anterolateral position, which are supported by the posteromedial and anterolateral
papillary muscles, respectively. A characteristic feature of the left ventricle is that
the mitral valve never possesses chordal attachments to the septum.