Segmental analysis utilizes clinical assessment, plain x-ray of the chest & heart, and echocardiography, among other tools.
- First define the status of the viscera: stomach, liver, bronchi & lungs.
- Then define the status of the abdominal aorta (AO) & inferior vena cava (IVC).
- And finally the heart: atria, great veins, atrioventricular (AV) arrangement, ventricles, great arteries, and ventriculoarterial (VA) arrangement.
Define visceral situs:
The patient is asked to drink water while auscultating the upper abdomen; the water gurgle heard defines the site of the stomach.
The stomach fundus air bubble can also be seen in plain x-ray.
Right-sided liver and left-sided stomach.
The stomach air bubble is seen in plain x-ray as a fluid level at the fundus of the stomach. It defines the site of the stomach, while the liver should be on the other side.
Normally; there is:
- Tri-lobed right lung with short right main bronchus which is seen in plain x-ray more in-line with the trachea,
- And bi-lobed left lung with longer left main bronchus which is seen in plain x-ray more angulated with the trachea.
Tri-lobed right lung with short right main bronchus and bi-lobed left lung with longer left main bronchus
The air shadow of the trachea and main bronchi
Normal and mirror-image arrangement of organs.
Inferior vena cava and abdominal aorta:
Echocardiography from the Epigastric view normally shows the IVC to the right & the abdominal AO to the left. The IVC is usually smaller, non-pulsating & shows a venous-type Doppler signal. The abdominal AO is larger, pulsating & shows an arterial-type Doppler signal. Branches of the abdominal AO & tributaries of the IVC are seen in 2D imaging. Color Doppler also differentiates between them by the direction of blood flow in the two vessels, branches & tributaries.
Inferior vena cava to the right and abdominal aorta to the left.
There is very close correlation between the situs of the atria of the heart and the situs of:
- The IVC & the abdominal AO.
- The bronchi & lungs.
- If the IVC is right-sided & the abdominal AO is left-sided,
- And/or, if the bronchi & lungs are in the normal arrangement.
The atria are differentiated anatomically by:
- Atrial appendages.
- Great veins attached to the atria.
- The RA appendage (RAA) is triangular, while the LA appendage (LAA) is tubular (worm-like).
- The RAA has a wide communication with the RA cavity, while the LAA has a narrow communication with the LA cavity.
- The RAA has rough surface due to extensive pectinate muscles, while the LAA has little pectinate muscles.
- Presence of Eustachian valve and/or Chiari network identifies the anatomically RA.
Right atrial appendage (RAA) is triangular in shape with a wide base communicating with RA cavity. It is located upwards at the right anterolateral side of the ascending aorta.
Position of the right atrial appendage (RAA): at the right anterolateral side of the root of the ascending aorta (AA). Thus the right border of the heart is formed by the superior vena cava (SVC), ascending aorta (AA), right atrial appendage (RAA), and its lowest part is formed by the right atrium itself (RA).
Right atrial appendage seen in TEE biatrial (bicaval) view
The left atrial appendage is tubular in shape
left atrial appendage in TEE short-axis view
Once the anatomically-right and the anatomically-left atria are identified, look at their positions:
Situs solitus (normal): the anatomically-right atrium is to the right and anterior, while the anatomically left atrium is to the left and posterior.
Situs inversus (reversed): the anatomically-right atrium is to the left and posterior, while the anatomically left atrium is to the right and anterior.
Great veins normally attached to the heart include:
- Veins attached to the anatomically-right atrium: inferior vena cava (IVC) & superior vena cava (SVC).
- Veins attached to the anatomically-left atrium: the 4 pulmonary veins.
Inferior vena cava and hepatic veins seen in subcostal view, within the shadow of liver parenchyma
Superior vena cava lying side-by-side with the ascending aorta.
Once the atrium receiving the SVC & the IVC is identified, it is almost always the anatomically RA. Rarely the IVC may be attached to the anatomically LA or to the Azygos or hemi-Azygos system.
- If the anatomically RA is anterior in position to the other atrium, situs solitus is diagnosed.
- On the other hand, if the anatomically RA is posterior in position, situs inversus is diagnosed.
The four pulmonary veins draining into the left atrium.
Once the atrium receiving the pulmonary veins is identified, it is almost always the anatomically LA. Rarely there is anomalous connection of the pulmonary veins to the anatomically RA.
- If the anatomically LA is left sided & posterior to the other atrium, situs solitus is diagnosed.
- On the other hand, if the anatomically LA is right-sided & anterior, situs inversus is diagnosed.
Identification of the ventricles:
- The Right ventricular (RV) apex has coarse trabeculations, while the left ventricular (LV) apex has fine trabeculations.
- The presence of moderator band is characteristic of the anatomically RV.
- The tricuspid valve (TV) is always part of RV. It has three papillary muscles, and some of its chordae tendineae are connected to the interventricular septum (IVS). The tricuspid septal leaflet is more apical than the nitral anterior leaflet.
- While the mitral valve (MV) is always part of LV. Its chordae are not connected to IVS, but to two separate papillary muscles connected to the LV free wall.
Moderator band in the right ventricle
Moderator band; seen in an apical 4-chamber view from a patient with transposition of the great arteries, where the RV is the systemic ventricle. The view also shows the more apical attachment of the tricuspid septal leaflet than the mitral anterior leaflet
Some chordae from the tricuspid valve are attached to the septum, while the mitral chordae are not
- Normally the anatomically RV is to the right. This situation is called: d-loop of the ventricles.
- If the anatomically RV is to the left, this situation is called: l-loop of the ventricles.
For the heart: the term Situs is used to indicate the position of the atria, while the term Loop is used to indicate the position of the ventricles:
All combinations are possible: situs solitus with either loop or situs inversus with either loop.
Atrioventricular (AV) connection:
Once the anatomically RA, LA, RV & LV are identified, the AV connection should be established:
- AV concordance: means that the anatomically RA is connected to the anatomically RV, and the anatomically LA to the anatomically LV.
- If the anatomically RA-RV are right-sided & the anatomically LA-LV are left-sided; this condition is called: situs solitus with AV concordance.
- If the anatomically RA-RV are left-sided & the anatomically LA-LV are right-sided; this condition is called: situs inversus with AV concordance.
- AV discordance: the anatomically RA is connected to the anatomically LV & the anatomically LA to the anatomically RV.
- If the anatomically RA-LV are right-sided & the anatomically LA-RV are left-sided; this condition is called: situs solitus with AV discordance.
- If the anatomically RA-LV are left-sided & the anatomically LA-RV are right-sided; this condition is called: situs inversus with AV discordance.
Situs solitus with AV discordance can also be called: situs solitus, l-loop.
Situs inversus with AV concordance can also be called: situs inversus, l-loop.
Situs inversus with AV discordance can also be called: situs inversus, d-loop.
The term situs is not only used to determine the atrial positions, but also used to indicate the position of the abdominal & thoracic organs:
- Situs solitus of the viscera: the abdominal & thoracic organs are in the correct position.
- Situs inversus of the viscera: the abdominal & thoracic organs are in the wrong position.
- Situs solitus of the atria.
- Situs solitus of the thoracic organs.
- The IVC to the right & the abdominal aorta to the left.
- Situs solitus of the abdominal organs.
Situs inversus totalis:
This is exactly the reverse of the normal anatomy (mirror image).
- Situs inversus of the atria.
- Situs inversus of the thoracic organs.
- Situs inversus of the abdominal viscera.
Isolated situs inversus:
- Situs inversus of the atria, almost always associated with situs inversus of the thoracic organs and reversed abdominal aortic & IVC positions.
- But with: Situs solitus of the abdominal organs.
The atrial situs is not always simple as described before.
- Single atrium: the atrial septum may be absent or very rudimentary.
- Right atrial isomerism: both atria have the anatomical features of the RA.
- Left atrial isomerism: both atria have the anatomical features of the LA.
- With RA isomerism: The spleen is usually absent, a condition called: asplenia.
- With LA isomerism: there are usually multiple spleens, a condition called: polysplenia.
We may describe the cardiac structure as: right or left atrial isomerism with d-loop or l-loop of the ventricles, according to the ventricular positions.
The bronchial situs, as well, is not always simple as described before.
Beside situs solitus & situs inversus, two other situations exist:
- Right bronchial isomerism: the two bronchi have the anatomical features of the right bronchus.
- Left bronchial isomerism: the two bronchi have the anatomical features of the left bronchus.
The ventricular morphology may also be complex.
Beside the d-loop & l-loop of the ventricles, the following situations are possible:
- Single ventricle: one ventricle is only present, i.e. the ventricular septum is absent. One or two valves may be present between the single ventricle & the atria. The single ventricle may show the anatomical features of either the RV or the LV, or may be an indeterminate ventricle: with no definite RV or LV anatomic features.
- Dominant ventricle: which is anatomically RV or LV or indeterminate. The other ventricle is present but rudimentary. One or two valves may be present between the dominant ventricle & the atria.
The AV connection may also be complex.
According to the presence of two ventricles, a single ventricle or a dominant ventricle, the AV connection may be described as:
- Bi-ventricular AV connection: in presence of two atrioventricular (AV) valves each of them is draining dominantly into a separate ventricle.
- Uni-ventricular AV connections, which are classified into:
- Double-inlet ventricle: if > 50 % of both AV valves open into the same ventricle (the 50 % rule), even if another ventricle is present,
- One of the AV valves is atretic and the other AV valve opens into a single or a dominant ventricle,
- Or there is a single AV valve opening into a single or a dominant ventricle.
AV valve abnormalities include:
- Stenotic valve: reduced orifice area.
- Atretic valve: total obstruction of the valve orifice by fibrous tissue.
- Regurgitant valve: blood regurgitates in the opposite direction due to incompetent closure.
- Over-riding valve: there is high ventricular septal defect (VSD) & the valve is over-riding the interventricular septum (IVS) & facing both ventricles.
- Straddling valve: an over-riding valve with its chordae inserted into both ventricles.
Straddling tricuspid valve
The great arteries:
The aorta is identified by:
- Its shape: a long arching vessel.
- Its branches: It gives rise into the two coronary arteries & the three big branches of the arch: Innominate, left common carotid & left subclavian arteries.
The pulmonary trunk is identified by:
- Its shape: a short almost straight vessel.
- Its branching: no branches arise from the trunk until it bifurcates at the end into the right (RPA) & left (LPA) pulmonary arteries.
Echocardiography: Parasternal short-axis view: The pulmonary trunk is a short vessel that immediately bifurcates into the right and left pulmonary arteries.
Normally there is a separate aorta arising from LV and a separate pulmonary trunk arising from RV.
In the anomaly of persistent truncus arteriosus (PTA), a single trunk, called the truncus arteriosus, arises from the heart facing both ventricles, overriding a big ventricular septal defect.
- The pulmonary trunk may arise from the lateral aspect of the truncus arteriosus, then bifurcate into the right & left PA (PTA type 1),
- or the 2 PA arise closely from the back of the truncus arteriosus (PTA type 2),
- or the 2 PA arise separately from the right & left sides of the truncus arteriosus (PTA type 3),
- or the pulmonary trunk as well as the right & left PA are all absent (PTA type 4), while the pulmonary blood flow is supplied by collaterals from the descending thoracic aorta..
Types of Persistent Truncus Arteriosus (PTA).
Ventriculoarterial (VA) connections:
Normally; the aorta arises from the anatomically LV, and the pulmonary trunk from the anatomically RV.
The pulmonary valve is a little higher & to the right of the aortic valve, but the P. trunk soon passes in-front of the aorta, then to its left side, and finally bifurcates into the right & left PA, at the concavity of the aortic arch.
The ascending aorta ascends upwards behind, then to the right of the P. trunk, then continues posteriorly as the aortic arch, which finally descends downwards as the descending thoracic aorta.
When the P. trunk crosses from right-to-left in-front of the ascending AO, their long axes are almost perpendicular to each other. Thus when one vessel is imaged in long axis the other is imaged in short axis & vice versa.
Relation of the ascending aorta to the pulmonary trunk
When one vessel is imaged in long axis the other one is imaged in short axis & vice versa.
Normally the aorta is imaged to the right & posterior to the pulmonary trunk, the pulmonary trunk being to the left and anterior.
The rightward position of the aorta relative to the P. trunk is called:
d-position of the great vessels.
Normal and abnormal VA connections:
- VA concordance: this is the normal VA connection; the aorta (AO) arises from the anatomically LV, and the pulmonary (P.) trunk from the anatomically RV. According to atrioventricular (AV) connection, 2 conditions are possible:
- AV concordance with VA concordance: normal anatomy: LA-LV-AO and RA-RV-P. trunk.
- AV discordance with VA concordance: this anomaly is called: isolated ventricular inversion: LA-RV-P. trunk and RA-LV-AO.
- VA discordance: AO arises from the anatomically RV & P. trunk from the anatomically LV. According to AV connection, 2 possibilities occur:
- AV concordance with VA discordance: LA-LV-P. trunk and RA-RV-AO: an anomaly called: complete transposition of the great arteries (complete TGA).
- AV discordance with VA discordance: LA-RV-AO and RA-LV-P. trunk: an anomaly called: anatomically-corrected TGA.
Physiology and Pathophysiology:
In the normal physiology, the systemic venous return is delivered to the pulmonary circulation for oxygenation, and the pulmonary venous return is delivered to the systemic circulation for tissue supply allover the body, i.e. the systemic and pulmonary circulations work in series. Such scenario is essential for normal health.
In corrected TGA, the circulation is normal if there is no communications such as atrial septal defect (ASD), ventricular septal defect (VSD), or patent ductus arteriosus (PDA). Most of those patients, however, have such communications and there is mixing of the 2 circulations, with various degrees of cyanosis and/or congestive heart failure (CHF).
All other mentioned transposition/mal-position complexes also have such communications with mixing of the 2 circulations, otherwise the situation will be incompatible with life.
If the 2 circulations are completely separated in these complexes, the situation will be incompatible with life.
Other combinations of VA connections include:
- Persistent truncus arteriosus.
- Presence of two trunks, but one of them is atretic:
- Aortic atresia,
- Pulmonary atresia.
- For survival, mixing of the 2 circulations via ASD, VSD and/or PDA is essential.
- Define the position of the abdominal & thoracic viscera, abdominal AO & IVC.
- Define the position of the anatomically RA, LA, or single atrium.
- Define the atrial & visceral situs.
- Define the position of the anatomically RV, LV or single ventricle.
- Define the position of the aorta, pulmonary trunk, or persistent truncus arteriosus.
- Establish the AV connection.
- Establish the VA connection.
- Conclude the cardiac and great vessel structure.