Conus - Infundibulum

The conus and infundibulum in congenital heart disease

The conus, also called the infundibulum, is a central component of ventricular outflow anatomy. Its presence, extent and relationship to the atrioventricular (AV) valves help explain the position of the arterial roots, the relationship of a ventricular septal defect (VSD) to the great arteries, and the pathway that can be constructed during surgical repair. A diagnosis such as transposition of the great arteries (TGA) or double outlet right ventricle (DORV) does not, by itself, describe this anatomy. Each outflow must be examined individually. The clinical importance extends beyond naming the malformation. When a surgical repair directs blood through a pathway bordered by conal muscle, that pathway may become obstructed as its geometry changes or the muscle hypertrophies. Understanding the conus therefore helps connect anatomical description with operative planning and long-term surveillance.

Terminology and normal anatomy

In the descriptive framework used here, conus and infundibulum refer to the muscular subarterial outlet supporting a semilunar valve and separating it from the adjacent atrioventricular valve. The terms are often used interchangeably, although their boundaries and developmental implications vary among anatomical traditions. Describing the actual muscular and fibrous relationships is more informative than relying on the terminology alone. The normal heart has a subpulmonary infundibulum. This smooth muscular outlet leads from the right ventricle (RV) to the pulmonary valve and separates the pulmonary valve from the tricuspid valve. The pulmonary root is therefore supported above the ventricular cavity by an infundibular sleeve. It normally lies anterior, leftward and superior to the aortic root. The normal aortic outlet has a different arrangement. The aortic valve is in fibrous continuity with the anterior leaflet of the mitral valve through the aortomitral curtain. There is no intervening muscular subaortic conus separating these valves. This does not mean that the left ventricular (LV) outflow tract has no muscle: the ventricular septum forms a substantial muscular boundary. Absence of a subaortic conus means absence of the muscular separation between the aortic and mitral valves, not absence of all subaortic myocardium. This distinction is fundamental. A short outlet with atrioventricular to arterial fibrous continuity differs from an elongated outlet in which the semilunar valve is separated from the atrioventricular valve by muscle. Both can permit unobstructed flow, but their anatomy and responses to surgical rerouting differ.

Developmental context

Early in development, the common outflow tract connects with the developing right ventricle. Its separation, alignment and incorporation into the ventricles establish the definitive aortic and pulmonary outlets. This developmental arrangement is the basis of the statement that the embryonic heart initially has a double outlet right ventricular configuration; it is a normal developmental stage, not a diagnosis of congenital DORV. The traditional conal model describes persistence of subpulmonary muscle and regression or remodelling of subaortic muscle, allowing aortomitral continuity. Modern developmental anatomy adds an important qualification: the outflow cushions fuse, partly muscularise and remodel, while the arterial channels acquire separate walls. Much of the embryonic outlet septal tissue becomes incorporated into the right ventricular outlet and free-standing pulmonary infundibulum. Adult anatomy cannot therefore be explained simply by persistence or disappearance of two fixed muscular cones. For clinical imaging, the practical task remains to identify the resulting muscular support, fibrous continuity and alignment of each outflow, irrespective of the developmental model used to explain it.

The four principal conal patterns

When two separate semilunar valves are present, four broad patterns provide a useful descriptive framework. They are anatomical patterns, not diagnoses.

These associations should guide inspection without determining the diagnosis in advance. TGA, DORV and double outlet left ventricle can exhibit different conal configurations. In particular, bilateral conal tissue is not a requirement for DORV, and finding a subaortic conus does not by itself establish transposition. Conal tissue is also not simply present or absent. Its length, thickness, completeness and relationship to the outlet septum vary. A description of bilateral conal tissue should not automatically be interpreted as two complete circumferential muscular rings: the arterial valves can retain fibrous continuity with one another where the muscular outlet septum is deficient. A substantial conus tends to elevate its arterial valve and increase its separation from the atrioventricular valves and the interventricular communication. In many hearts with usual ventricular topology, it is associated with a more anterior arterial root and greater alignment with the right ventricle. These are spatial associations rather than universal rules. Root position also depends on ventricular topology, cardiac orientation and the alignment of the entire outlet complex.

The conus and the conal septum are different structures

The conal septum, also called the infundibular or outlet septum, is the partition between the two subarterial pathways when such a partition is identifiable. It is one component of the outlet architecture. It is not synonymous with the complete conus surrounding an arterial outlet. In the normal heart, the different levels of the aortic and pulmonary valves complicate the concept of a shared outlet septum. Much of the structure seen along the posterior or medial aspect of the pulmonary infundibulum is a free-standing right ventricular wall. It does not constitute a substantial muscular partition directly separating the two ventricular cavities. Developmental remodelling explains why the embryonic outlet septum largely loses its identity as a true septum in the normal postnatal heart. In congenital malformations, a distinct outlet septum may remain readily recognisable. It can be displaced relative to the muscular ventricular septum and may lie predominantly within the right ventricle. Its position determines which outflow is narrowed and how the interventricular communication relates to the arterial roots. The consequences of displacement must be described in relation to the actual channels involved: an anterior or posterior direction has different implications when the great arteries are abnormally related. The assessment should establish whether the outlet septum is muscular, fibrous or markedly deficient; its thickness and orientation; its attachment to the septal band and inner heart curvature; and whether atrioventricular valve chordae attach to it or cross a proposed surgical pathway. These features can vary independently of the muscular separation between an arterial valve and an atrioventricular valve.

Ventriculoarterial connections and competing classification conventions

Ventriculoarterial connection describes which morphological ventricle supports each arterial root. Great artery relationship describes where the arteries lie relative to each other. Conal anatomy describes their subvalvar support. These are related observations, but each requires a separate description. A widely used convention assigns an overriding arterial root to the ventricle supporting more than half of its valvar junction. DORV is then a ventriculoarterial connection in which both arteries arise entirely or predominantly from the morphological right ventricle. In principle, assessment concerns the three-dimensional valvar junction, rather than a diameter measured in a single image. Ventriculoarterial assignment does not, by itself, describe the conal or infundibular anatomy. Two strategies are used for this designation. The Anderson/European approach applies the 50% rule to both great arteries under all conditions. The Van Praagh/American approach considers mitral–aortic continuity to always assign the aorta to the left ventricle. When there is discontinuity between the mitral and aortic valves, the 50% rule is applied to assign the aorta. The 50% rule is also used to assign the pulmonary artery. These conventions can produce different labels for the same borderline anatomy. A markedly overriding aorta with aortomitral continuity may be classified within tetralogy of Fallot by one approach and within DORV with Fallot-type morphology by another. The convention should be made explicit when it affects interpretation, and the underlying morphology should always be reported. Determining the degree of override is particularly difficult when the ventricular septum is malaligned, the ventricles are arranged superiorly and inferiorly, or cardiac motion changes the apparent relationship across an imaging plane. Even the normal aortic root has a complex relationship to the crest of the septum, so apparent overlap in one projection does not establish pathological commitment to both ventricles (see figure below inspired by Bharucha et al.). Multiple planes and a clear understanding of the septal boundaries are essential. The percentage of override is useful, but it cannot replace description of valve continuity, subarterial muscle, VSD position or the route available for ventricular egress.

Transposition of the great arteries

In complete TGA, atrioventricular connections are concordant and ventriculoarterial connections are discordant: the right ventricle connects to the aorta and the left ventricle to the pulmonary artery. The connections establish the diagnosis. The anterior or posterior position of the aorta does not establish it independently. The usual conal arrangement is a subaortic conus with mitral to pulmonary fibrous continuity. The aortic root is generally anterior and often rightward of the pulmonary root. Subaortic muscle separates the aortic valve from the atrioventricular valves, while the left ventricular pathway to the pulmonary valve lacks a comparable muscular separation from the mitral valve. This becomes especially relevant during an arterial switch operation. The native pulmonary root becomes the neoaortic root, so the native left ventricular to pulmonary pathway becomes the systemic outflow. When mitral to pulmonary continuity is present, the resulting neoaortic outlet does not contain an intervening subpulmonary conal sleeve. It therefore lacks that particular substrate for muscular subaortic obstruction. This is not a guarantee against every possible form of postoperative outflow obstruction. TGA with bilateral conal tissue has muscle beneath the native pulmonary valve as well as beneath the aortic valve. The left ventricular to pulmonary pathway may be elongated or narrowed. After arterial switch, that same region becomes the subneoaortic pathway. Its length, calibre and muscular boundaries are therefore relevant to the risk of residual or later systemic outflow obstruction. The presence of muscle identifies an anatomical concern; it does not establish that obstruction or reoperation is inevitable. TGA with a large VSD and outlet septal malalignment requires additional analysis. In a usual arrangement, posterior deviation of the outlet septum may narrow the left ventricular pathway to the pulmonary artery. If pulmonary outflow obstruction precludes a straightforward arterial switch, a different reconstruction may be required. Conversely, narrowing of the native aortic channel should prompt careful examination of the aortic arch for associated hypoplasia, coarctation or interruption. 

Uncommon forms include TGA with a leftward aorta, bilateral conal deficiency, and TGA with a posterior aorta and a subpulmonary conus. A posterior aorta may create a deceptively normal external appearance despite discordant ventricular connections. The notation S,D,L with subaortic conus describes usual atrial arrangement, a D-loop ventricular topology and a leftward aortic relationship; it does not, by itself, indicate congenitally corrected TGA. Each connection must still be demonstrated.

Double outlet right ventricle

DORV describes a ventriculoarterial connection rather than a single anatomical or physiological entity. The label does not identify the VSD pathway, the degree of pulmonary obstruction, ventricular adequacy or the repair that will be feasible. Some hearts have physiology resembling a large VSD, others resemble tetralogy of Fallot or transposition, and others require a functionally univentricular strategy. The relationship of the VSD to the outlets is central because the interventricular communication usually provides the route by which the left ventricle empties. Rare cases with an intact ventricular septum and a severely hypoplastic left ventricle require a different description and should not be forced into the usual VSD-based scheme. Two complementary aspects of the VSD should be reported. One is its anatomical location and borders, including outlet, inlet or trabecular position, muscular boundaries and possible perimembranous extension. The other is its relationship to the arterial roots: subaortic, subpulmonary, doubly committed or noncommitted. A description such as outlet malalignment with perimembranous extension and subaortic commitment conveys more information than any of those terms alone. In many outlet defects, the communication is cradled between the limbs of the septomarginal trabeculation, also called the septal band. Different outlet septal attachments and arterial positions alter which root is accessible from that communication. The anatomy is better understood by relating the outlet complex to the defect than by imagining that an otherwise identical VSD simply moves from one artery to the other. A noncommitted or remote VSD is not necessarily confined to the inlet or trabecular septum. Anatomical studies also describe outlet communications that are distant from the arterial roots because of a broad muscular fold, a long outlet septum or marked arterial displacement. Thus, anatomical location and surgical accessibility must be assessed separately.

Subaortic VSD and Fallot-type DORV

With a subaortic VSD, the left ventricular outlet is preferentially directed toward the aorta. The outlet septum often attaches toward the anterior limb of the septal band. The great arteries may retain a spiralling relationship, although this appearance alone does not determine VSD commitment. When the pulmonary outlet is unobstructed, the physiology may resemble a large VSD with substantial pulmonary blood flow. When the subpulmonary pathway is narrowed, the morphology and physiology can resemble tetralogy of Fallot. The amount of subaortic muscle helps distinguish the route to the aortic valve in otherwise similar-looking hearts. In usual tetralogy of Fallot with aortomitral continuity, the aortic valve is closely related to the superior margin of the VSD. Patch closure directs the left ventricle toward the aorta without creating a long pathway through an intervening subaortic conal sleeve. In Fallot-type DORV with substantial subaortic conal muscle, the arterial valve may be considerably farther from the communication. Repair then creates a longer tunnel whose boundaries include muscle. The terminology applied to these hearts varies between institutions. Some deliberately distinguish tetralogy from Fallot-type DORV on the basis of subaortic muscle, while others emphasise the extent of aortic commitment. For operative planning and follow-up, the actual distance to the valve, muscular boundaries and tunnel geometry are more informative than the label alone. Late subaortic obstruction is unusual after conventional tetralogy repair, but it is not impossible. A fibrous membrane or other acquired narrowing can occur. Conversely, a muscular tunnel in DORV is a substrate for obstruction rather than a prediction that every patient will develop it.

Subpulmonary VSD and Taussig Bing morphology

With a subpulmonary VSD, the left ventricular stream is preferentially directed toward the pulmonary outlet. The outlet septum commonly has a different orientation and attachment from that seen in subaortic commitment, and the arteries often run in parallel. This arrangement can produce transposition-like physiology. The term Taussig Bing is commonly used for DORV with a subpulmonary VSD, although historical descriptions place additional emphasis on bilateral conal tissue and the relationship of the arterial roots. The full anatomy should therefore accompany the term. Associated narrowing of the native aortic outflow and obstruction of the aortic arch should be sought. Atrioventricular valve straddling or chordal attachment across the proposed tunnel can also influence repair. The presence or absence of a subpulmonary conus is particularly informative. When the pulmonary valve is in fibrous continuity with the mitral valve, the route from the VSD to the pulmonary root can be relatively short. When a substantial subpulmonary conus is present, the valve is farther from the communication and the pathway may have a longer muscular component. In a repair combining left ventricular routing to the native pulmonary root with an arterial switch, this pathway becomes the systemic outlet. Native subpulmonary conal tissue then occupies the sub-neoaortic region. Preoperative assessment must therefore identify the pathway that will become systemic, as well as the structures currently associated with the aorta. Both reconstructed outflows require evaluation. Native subaortic muscle becomes part of the neopulmonary side after arterial switch, and residual narrowing there can obstruct right ventricular egress. Conal analysis must follow the final circulation rather than retaining preoperative names without adjustment.

Doubly committed defects and the importance of septal orientation

A doubly committed communication provides access toward both arterial roots. One familiar arrangement has marked deficiency of the muscular outlet septum and fibrous continuity between the aortic and pulmonary valves. In this setting, both semilunar valves may form part of the superior boundary of the defect. A second arrangement is when an identifiable muscular outlet septum oriented approximately perpendicular to the crest of the ventricular septum and the plane of the interventricular communication. Its central attachment can leave the defect related to both arterial outlets. This can occur when the arteries have an anterior to posterior relationship. Direct inspection of both pathways is needed to recognise it. These arrangements explain why double commitment cannot be inferred solely from the presence or absence of outlet septal muscle. Fibrous continuity between the arterial valves does not invariably make a DORV communication doubly committed; the attachment of the remaining fibrous partition may direct it toward one root. Equally, the presence of muscular outlet septal tissue does not exclude access to both roots. Terminology should be precise. The familiar term doubly committed and juxta-arterial VSD usually emphasises arterial valve continuity and deficiency of the muscular outlet septum. A communication committed to both outlets in a complex DORV heart can have a different arrangement. Reporting the borders, septal orientation and individual pathways prevents these patterns from being conflated. Surgical planning depends on the calibre and geometry of each route, semilunar valve suitability, coronary anatomy and associated defects. A baffle to one root, a repair incorporating arterial switch or a more complex reconstruction may be considered. The label doubly committed does not select the operation on its own.

Double outlet left ventricle and unusual ventricular arrangements

Double outlet left ventricle (DOLV) is rare. Both arterial roots connect entirely or predominantly to the morphological left ventricle. Bilateral conal deficiency is a recognised configuration, but other conal patterns occur. As with DORV, the diagnosis does not determine physiology or surgical strategy. It is important to carefully "reconstruct", before surgery, where blood from each ventricle will travel after a patch is placed. Root position and a diagnostic label are insufficient substitutes for this assessment. Superior to inferior ventricular arrangements create a related imaging challenge. The ventricular septum may be closer to horizontal than expected, making familiar parasternal assumptions unreliable. A root that appears above a particular chamber in one image cannot be assigned confidently without following its connection through orthogonal planes. Conal tissue, the interventricular communication and atrioventricular valve attachments must be incorporated into the same three-dimensional understanding.

Surgical rerouting and late outflow obstruction

In a Rastelli-type repair, blood is directed from the left ventricle through the VSD to the native aorta, while a separate right ventricular to pulmonary arterial pathway is established, commonly using a conduit. The VSD becomes part of the reconstructed left ventricular outlet. Its size, muscular margins and relationship to the aortic root therefore remain important after successful repair. There are risks of obstruction after this type of reconstruction. Several mechanisms can contribute to obstruction: a restrictive original VSD, a long or angulated baffle, residual outlet septal tissue, muscular hypertrophy, fibrous tissue formation and postoperative changes in ventricular geometry. The pathway can therefore narrow even if the immediate postoperative study is satisfactory. Changes in ventricular geometry and VSD dimensions after intraventricular repair have been documented. Long-term Rastelli series show that many patients remain free of major left ventricular outflow obstruction. Surgical series also suggest that appropriately selected VSD enlargement and infundibular septal resection can reduce recurrence. The extent of intervention must account for the conduction system, atrioventricular valve apparatus and the anatomy of the opposite outflow. The relevant counselling is therefore anatomical and individualised. Families can be told that a muscular or surgically constructed outflow may require later reassessment or intervention. A specific probability should come from outcomes applicable to the patient's lesion and repair, rather than from the mere presence of conal muscle.

Echocardiographic assessment and reporting

Evaluation should begin with sequential identification of the atria, morphological ventricles, atrioventricular connections and great arteries. Each artery should then be traced back to its ventricular support. This provides the framework for assessing the conal anatomy without assuming the connections from arterial position alone. For each arterial root, imaging should establish whether there is fibrous continuity with an atrioventricular valve or intervening muscle. Where conal tissue is present, its length, thickness and circumferential extent should be described, together with the narrowest part of the outlet. The conal septum should be assessed separately for its composition, orientation, attachment and effect on either channel. The VSD should be examined in multiple planes to identify its anatomical borders, effective dimensions, relation to each root and any chordal attachments that could limit a tunnel. The shortest visible route is not automatically the safest surgical route; the final partition must preserve both ventricular exits and atrioventricular valve function. Associated arch obstruction, ventricular hypoplasia and coronary relationships may change the interpretation or operative options. Subcostal, parasternal and apical sweeps provide complementary perspectives. Colour Doppler helps identify the direction and location of flow acceleration, while spectral Doppler characterises an obstruction within the anatomical pathway. Doppler findings must be interpreted with flow conditions in mind: substantial anatomical narrowing can have a modest gradient when flow through that route is limited.

Three-dimensional echocardiography can help display the VSD margins and the outlet as viewed along its axis. This is particularly useful for understanding circumferential muscle, a potential baffle route or the orientation of the outlet septum. CT or cardiovascular magnetic resonance can complement echocardiography when acoustic access or spatial complexity prevents a confident reconstruction. The modality should resolve a specific unanswered anatomical question. After repair, the examination should trace the entire systemic and pulmonary pathways and describe the site and mechanism of any narrowing. A single peak velocity does not explain whether obstruction arises at the VSD, within a muscular tunnel, near a valve or in a conduit. Serial changes in calibre, muscle thickness, ventricular response and flow help distinguish a stable pathway from progressive obstruction. A useful report therefore states the ventriculoarterial connections; the spatial relationship of the roots; the presence and extent of subaortic and subpulmonary conal tissue; the composition and alignment of the outlet septum; the VSD borders and commitment; and any obstruction or valve attachment affecting the proposed or existing routes. This allows the diagnostic label to be connected to the patient's actual anatomy and the circulation created by repair.

References

• Dr Meryl S. Cohen — Conus and infundibulum: watch the source lecture. The anatomical drawings, specimen demonstrations and dynamic imaging provide the visual context for the concepts discussed here.

• Dr Meryl S. Cohen — Double Outlet Right Ventricle: Imaging Assessment for Biventricular Repair: watch the full lecture. Selected teaching segments: surgical imaging checklist (19:10); VSD-to-outlet relationships (21:30); three-dimensional model (36:09); and continuous two-dimensional sweeps (53:21).

• Continue on NeoCardioLab: Morphological approach · Double-outlet right ventricle · Transposition of the great arteries.

• Van Praagh S, Davidoff A, Chin A, Shiel FS, Reynolds J, Van Praagh R. Double outlet right ventricle: anatomic types and developmental implications based on a study of 101 autopsied cases. Coeur. 1982;8:389-440.

• Bharucha T, Hlavacek AM, Spicer DE, Theocharis P, Anderson RH. How should we diagnose and differentiate hearts with double-outlet right ventricle? Cardiology in the Young. 2017;27(1):1-15. doi:10.1017/S1047951116001190

• Walters HL III, et al. Congenital Heart Surgery Nomenclature and Database Project: double outlet right ventricle. Ann Thorac Surg. 2000;69:S249–S263. PubMed · DOI: 10.1016/S0003-4975(99)01247-3.

• Ebadi A, et al. Double-outlet right ventricle revisited. J Thorac Cardiovasc Surg. 2017;154:598–604. PubMed · DOI: 10.1016/j.jtcvs.2017.03.049.

• Aiello VD, et al. The independence of the infundibular building blocks in the setting of double-outlet right ventricle. Cardiol Young. 2017;27:825–836. PubMed · DOI: 10.1017/S1047951117000452.

• Pasquini L, et al. Conal anatomy in 119 patients with d-loop transposition of the great arteries and ventricular septal defect: an echocardiographic and pathologic study. J Am Coll Cardiol. 1993;21:1712–1721. PubMed · DOI: 10.1016/0735-1097(93)90392-E.

• Pushparajah K, et al. A systematic three-dimensional echocardiographic approach to assist surgical planning in double outlet right ventricle. Echocardiography. 2013;30:234–238. PubMed · DOI: 10.1111/echo.12037. 

• Oladunjoye O, et al. Repair of double outlet right ventricle: Midterm outcomes. J Thorac Cardiovasc Surg. 2020;159:254–264. PubMed · DOI: 10.1016/j.jtcvs.2019.06.120.

• Anderson RH and colleagues. Development of the arterial roots and ventricular outflow tracts. Journal of Anatomy. 2024;244:497–513. https://doi.org/10.1111/joa.13973  

• Aiello VD, Spicer DE, Anderson RH, Brown NA, Mohun TJ. The independence of the infundibular building blocks in the setting of double-outlet right ventricle. Cardiology in the Young. 2017. https://doi.org/10.1017/S1047951117000452 

• Beekman RP and colleagues. The morphologic nature of noncommitted ventricular septal defects in specimens with double-outlet right ventricle. Journal of Thoracic and Cardiovascular Surgery. 2002;124:984–990. https://pubmed.ncbi.nlm.nih.gov/12407383/

• Early changes in ventricular geometry and ventricular septal defect size following Rastelli operation or intraventricular baffle repair for conotruncal anomaly. A cause for development of subaortic stenosis. Circulation. 1994;90(5 Pt 2):II13–II19. https://pubmed.ncbi.nlm.nih.gov/7955240/

• Cleveland DC and colleagues. Late Left Ventricular Outflow Tract Obstruction Following the Rastelli Operation: Expectations Out to 20 Years. World Journal for Pediatric and Congenital Heart Surgery. 2016;7:605–610. https://pubmed.ncbi.nlm.nih.gov/27587496/

• Alsoufi B and colleagues. The Rastelli procedure for transposition of the great arteries: resection of the infundibular septum diminishes recurrent left ventricular outflow tract obstruction risk. Annals of Thoracic Surgery. 2009;88:137–142. https://pubmed.ncbi.nlm.nih.gov/19559212/