Paediatric perfusion

Compare the anatomy, physiology and pathophysiology of Tetralogy of Fallot(TOF) and Double Outlet Right Ventricle (DORV)

Congenital heart anomalies are described as any defect present at birth of the heart or the great vessels.  They are one of the most common causes of death in the newborn and are a major cause of distress to the neonate.  Causes of congenital heart defects (CHD) can be due to environmental factors such as exposure to radiation or maternal infection during pregnancy or they may be inherited.  Interactions of both environmental and inherited factors play a part in a fetus developing CHD due to arrested embryonic development.  The development of CHD can be generally classified as cyanotic and acyanotic.  Cyanotic defects reflect on the mixing of oxygenated and deoxygenated blood in the systemic circulation, while no mixing of oxygenated and deoxygenated blood occurs in acyanotic defects.  Congenital defects can alter the cardiac function due to the malformation of the cardiovascular system, in which increased pulmonary vascular resistance (PVR) and cardiac workload can occur, as well as inadequate cardiac output (CO) and decreased oxygen saturations in the cases of cyanotic malformations.  The severity, type and degree of the defects reflects on the physical symptoms of the pathophysiologic alterations that occur, where tissue hypoxia, decreased exercise tolerance, dyspnea, grown retardation, decreased exercise tolerance and recurrent respiratory infections are all some of the many symptoms that can be experienced (Friedman 1993)

Tetralogy of Fallot (TOF) and Double Outlet Right Ventricle (DORV) are two of the many cardiac defects that can arise.  TOF is one of the most common congenital defects that causes cyanosis and is characterised by four congenital abnormalities all occurring at once.  These four defects include a ventricular septal defect (VSD) where an abnormal pathway between the left ventricle (LV) and right ventricle (RV) occurs, over-riding aorta resulting in the aortic root overriding the VSD, right ventricular hypertrophy which is a thickening of the RV and pulmonary stenosis/atresia resulting in RV outflow obstruction.  DORV is a clinically significant congenital heart defect.  It is a ventriculoarterial connection in which both the great vessels being the Aorta and the pulmonary artery (PA) arise entirely or partially from the RV.

There are a number of differences between the normal fetal and post natal infant circulation.  The umbilical chord connects the fetus to the placenta, which is the organ that develops in the mother’s uterus during pregnancy.  The fetus receives all its oxygen and nutrition from the mother through the umbilical vein and sends all the waste products and carbon dioxide back to the mother to be eliminated through the umbilical arteries in the umbilical chord.  The nutrients and waste products diffuse through the placental membrane between the maternal and fetal blood as they come in close proximity within each other.

As the nourished blood travels to the fetus through the umbilical vein, it then further divides. One division passes through the liver, and the other bypasses the liver and enters a vessel called the ductus venosus (DV).  From there blood travels a short distance and joins the inferior vena cava (IVC) where it mixes with deoxygenated blood returning from the lower parts of the body.  From the IVC the blood enters the right atrium (RA), and then flows mostly to the left atrium (LA) through the foramen ovale (FO) which is a fetal opening between the RA and LA.  Back flow of blood from the LA to the RA is prevented by a one way valve (septum primum) located on the left side of the atrial septum which overlies the FO.  In the adult heart, blood flows from the RA to the RV, through the PA and to the lungs.  Because oxygen, carbon dioxide and nutrient exchange takes place in the placenta, the fetal lungs are non-functional and the blood largely bypasses them as they are collapsed and their blood vessels have a high resistance to flow (Friedman 1993, Jeon 2002)

The rest of the fetal blood that is not shunted from the RA to the LA including the blood returning from the head through the superior vena cava (SVC) flows to the RV and through the pulmonary trunk.  From the pulmonary trunk only a small volume of blood enters the pulmonary circuit primarily to sustain the lung tissue.  Most of the blood would travel from the pulmonary trunk bypassing the lungs into the ductus arteriosus (DA) which connects to the descending portion of the aortic arch.  By this mechanism, the blood with a low oxygen concentration that is returned from the head through the SVC does not travel through the aorta and to the head vessels.  The LA receives the more concentrated oxygen rich blood through the FO, which is then mixed with the small amount of blood returning from the pulmonary veins and it all enters the LV which is pumped into the aorta.  Some of the pumped blood enters the coronary arteries and reaches the myocardium, whereas the remaining volume of blood flows through the aorta to the carotids supplying the brain, and to various other parts of the lower regions of the body.  The rest of the blood is carried back to the umbilical arteries to be re-oxygenated through the internal iliac arteries.

The fetus must undergo many circulatory changes after birth and several cardiopulmonary adaptations must be made.  When the baby is born the placental circulation is switched off as the umbilical chord is clamped and cut off.  At the same time the baby takes its first breath and gas exchanges takes places in the neonatal lungs.  As the placental circulation is cut off, there is a significant fall in the blood flow through the DV and a decreased venous return to the IVC which also results in a fall in venous return to the RA.  As the baby takes its first breath, vasodilation of the pulmonary vessels occurs due to the increase in lung pressure resulting in a decrease in resistance to blood flow through the lungs, in addition, an increased volume of blood returns from the lungs through the pulmonary veins (PV) to the LA.  As the PA pressure decreases due to the functionality of the lungs, RA pressures are reduced due to the discontinuation of the placental circulation and LA pressures are raised due to the return of blood through the pulmonary veins, the LA pressure exceeds that of the RA pressure resulting in the closure of the FO as the septum primum is forced against the septum secundum.   This action completes the separation of the heart into two functional pumps.  The FO becomes the fossa ovalis.  The DA closes within one to two days after birth and is replaced with connective tissue known as ligamentum arteriosum, and the DV closes passively 3-10 days after birth (Friedman 1993, Jeon 2002).

Image above showing pre-natal circulation.   Retrieved on 18 May 2011 from: http://www6.ufrgs.br/favet/imunovet/molecular_immunology/circulation.html

Different hemodynamic changes occur in the fetus with congenital cardiac malformations which also differ from those that occur postnatally.  Congenital cardiac anomalies may affect the fetus by changing the pattern of blood flow through the cardiac chambers and the great blood vessels causing alterations in the delivery of oxygen to various organs and increasing the venous pressure of the fetus.  Myocardium development can be hindered by obstruction to cardiac outflow resulting in ventricular hypoplasia, inadequate oxygen or energy supply due to interference in blood flow and oxygen content may affect cerebral development and ductus arteriosus as well as pulmonary vascular responses can also be altered by oxygen content and inadequate supply to demand (Rudolph 2010).

Prenatal hemodynamics of TOF and DORV present no problems during intra-uterine life even with a significantly different cardiac anatomy and circulation.    Depending on the severity of obstruction to pulmonary flow in a fetus presenting with TOF, a larger proportion of blood will flow through the aorta to compensate.  If the pulmonary flow obstruction is severe, blood flow to the lungs is achieved via the DA.  The VSD is well tolerated in the fetus both with TOF and DORV as the RV and LV pressures are equal.

TOF is the most common form of cyanotic CHD.  Its overall incidence is 10% and a slight male to female predominance exists.  Sporadic deletion of chromosome 22q11 occurs in approximately 15% of patients which is tested with Fluroescence In Situ Hybridisation (FISH).  Affected subjects have a 50% chance of transmitting the deletion to their offspring, therefore, family screening and genetic counselling normally takes place (Gatzoulis 2011).

TOF is a complex of anatomic abnormalities arising from the underdevelopment of the right ventricular infundibulum with anterior and leftward displacement of the infundibular septum and its parietal extension. In 1888, Fallot described the anatomy as consisting of a subaortic VSD, right ventricular infundibular stenosis, aortic valve positioned to override the right ventricle, and right ventricular hypertrophy (RVH).

http://www.mountnittany.org/wellness-library/healthsheets/documents?ID=7385

During cardiac morphogenesis before the ventricular septum is closed, there is division of the RV ejection stream into a transeptal aortic stream and an infundibular pulmonary stream.  This division of the RV ejection stream is caused by obstruction to flow by a stenotic malformed pulmonary valve in almost all cases.  The transeptal aortic portion of the stream passes through the unclosed ventricular septum, maintains the patency of this communication and expands the VSD either behind or into the crista supraventricularis.  The decreased volume of flow through the RV outflow tract results in a small infundibulum which develops progressive stenosis in post natal life (Winn 1973).

The infundibular or subpulmonic obstruction in TOF is characterised by anterior and cephalad deviation of the infundibular, or outlet septum, which results in muscular subvalvular narrowing.  The obstruction is further enhanced by hypertrophy of the muscular outlet septum, the parietal RV free wall and components of the septomarginal trabeculations.  The anterocephalad deviation of the outlet septum, while resulting in muscular obstruction, also simultaneously gives rise to the large perimembranous VSD by virtue of the malalignment between the outlet and trabecular septum (Allen 2008).

In addition to subpulmonic obstruction, areas of stenosis are common at the valvular and supravalvular levels.  In TOF, pulmonary valves can be found to be bicuspid or unicuspid in a majority of patients and the PA can also have diffused or focal obstruction or hypoplasia.

The VSD is enclosed anteriorly and postero-inferiorly between the limbs of the septal band, and superiorly by the conal septum and the junction of the anterior limb of septal band and RV free wall.  Absence of the conal septum and associated hypoplasia of the subpulmonary infundibulum and hypertrophy of the septoparietal band results in a variant of TOF with doubly committed subarterial VSD, overriding aorta and pulmonary annular hypoplasia and stenosis (Lai 2009).

The VSD most frequently has fibrous continuity between the tricuspid and aortic valve, hence may be considered a true perimembranous defect.  The VSD is usually large and non-restrictive but restrictive VSDs can occur due to overlying abnormal or accessory tissue associated with the tricuspid valve.  In addition to the isolated subarterial defect, additional ventricular septal defects also may be present occasionally.  There may be inlet extension of the subarterial defect, or, in some patients, there may be an associated complete atrioventricular septal defect (Allen 2008, Lai 2009).

RVH in TOF is a compensatory response to the pressure load on the RV resulting in increased wall thickness.  The description of the over-riding aorta in TOF is controversial as to distinguishing the anatomy of TOF to the anatomy of DORV.  In TOF the aorta partially over-rides the VSD and is committed to both the RV and the LV.  Some authors use the term DORV when the PA arises from the RV and more than 50% of the aorta rises from the RV.  Other authors only use the term DORV when the pulmonary artery arises from the RV and 90% or more of the aorta arises from the RV and still others use the term DORV only when there is absence of fibrous continuity between the aortic and mitral valves.  DORV is defined as a type of ventriculoarterial connection in which both great vessels arise predominantly from the RV (Jacobs 2000).

Coronary artery anatomy in patients with TOF can vary.  There may be a single coronary ostium that supplies both the left and right coronary arteries which arises either anteriorly or posteriorly in a small amount of cases that exist, and the left anterior descending artery can occasionally arise from the right coronary artery.

There is a diverse range of physiology seen in TOF.  The degree of cyanosis depends upon the degree of malalignment of the infundibular septum and the severity of the consequent RV outflow tract obstruction.  Some patients exhibit severe cyanosis because of profound right to left ventricular level shunting and mixing of deoxygenated blood with oxygenated blood, whereas other patients may be fully saturated and have net left to right shunt which occurs in the cases of mild pulmonary obstruction as there is no significant restriction to the flow of blood into the pulmonary arteries (Allen 2008).

After birth, the effect of the DA will also depend on the severity of the RV outflow obstruction.  If the obstruction is severe, the neonatal circulation is said to be “duct-dependent” and the DA closure will lead to severe cyanosis as pulmonary flow is not achieved or dramatically hindered.  Re-establishment of the ductal flow by means of prostaglandin infusion is an important intervention to stabilise the neonates and achieve flow through the pulmonary circuit.  After birth the circulatory effects of a VSD are dependent on the size of the defect and the balance between the PVR and SVR.  In neonates with a large VSD, as SVR rises and PVR falls, a significant left to right shunt through the VSD becomes apparent.  As PVR continues to fall during the first weeks of life, this shunt increases leading to congestive heart failure.  Partial or limited pulmonary stenosis allows limited flow to the pulmonary artery and into the lungs, however, this flow will not be sufficient enough to sustain ultimate gas and nutrient exchange, by that, pulmonary vasculature (Major Aorta-Pulmonary Collateral Arteries or MAPCAs) develop prenatally where arteries arise from the descending aorta to pulmonary vasculature.  This will help increase pulmonary flow after the baby is born.

Consistent hemodynamic features include RV hypertension because of the large VSD, with normal or low PA pressures.  The low distal PA pressures are maintained due to the various levels of pulmonic obstruction (Allen 2008).

The amount of obstruction to the right ventricular outflow is the determinant of the severity of the shunting that occurs through the VSD.  Thus, the difference in resistance between the pulmonary vascular bed and the systemic vascular bed will determine the balance between the aortic and pulmonary blood flow.  Due to the nature of the non-restrictive VSD, this balance between both aortic and pulmonary blood flow can be achieved.  Given that the ventricular pressures are essentially equal, the RVH is in proportion to the LV mass.  In the presence of a restrictive VSD, RV pressures would be higher than that of LV pressures hence increased RVH (Allen 2008).

There is almost always a patent foramen ovale associated with TOF.  Occasionally this can enlarge to become a secundum atrial septal defect (ASD) and allow a left to right shunt posteriorly to also balance the pressures of the right and left chambers with the occurrence of RV outflow obstruction.

Infants with TOF may become gradually cyanosed as they grow and appear pink in colour at birth. Some infants initially may present with symptoms of left to right shunts and minimal obstruction to pulmonary flow, however, gradually the increase in resistance to flow of blood into the lungs due to increased pulmonary stenosis can occur after a variable period of time, resulting in an increased PVR.  Eventually PVR becomes higher then the SVR and right to left shunting will result and the patient becomes cyanosed as deoxygenated blood mixes with oxygenated blood and delivered to the brain and body.

Infants can present with “cyanotic attacks” where they become irritated and cry as though in pain, becoming increasingly cyanosed and breathless resulting in loss of consciousness.  This attack is a result of spasm of the infundibulum preventing flow of blood through the PA to the lungs, therefore, blood diverts through the VSD and into the aorta.  As the child loses consciousness, the infundibulum relaxes and blood flow is reinitiated to the lungs and recovery gradually occurs.  Some children become more limited to physical activity and become more cyanosed with increased physical exertion.  A child with TOF may be observed to squat with knees up to chest with increased cyanosis, and becomes gradually less cyanosed and less breathless, then resumes their activity.  As a patient with TOF exercises, oxygen debt increases as supply is less than demand, the muscular vascular beds are dilated and there is an increase in extraction of oxygen locally so that the amount of oxygen in the systemic venous blood is reduced more than usual.  Such a reduction of venous oxygen levels in patients with TOF causes a further decreased in arterial oxygen levels as blood is shunned from the RV to the LV and into the aorta.  The SVR is further lowered due to vasodilation of the muscle vasculature during exercise resulting in a lowered systemic blood pressure favouring an increase in blood flow from the RV into the aorta.  As patients become more cyanosed with increased physical exercise and squat, this results in reduced blood flow to the legs which helps maintain the SVR so that more blood enters the lungs.  Squatting also results in a reduction in venous blood return from the legs, therefore, oxygen debt after exercise can be paid off over a longer period of time which corrects the sudden fall in arterial blood oxygen saturations (Jordan 1989).

Patients may remain virtually asymptomatic if little or no right to left shunting occurs, but as they hit middle age, polycythaemia tends to develop and thromboembolism may occur.  The increase in hematocrit levels in patients with TOF correlates with the severity of the arterial desaturation and pulmonary stenosis.

Selected anatomy, physiology and pathophysiology of TOF can be very similar to that of DORV.  DORV is a term that refers to any cardiac malformation resulting in both the aorta and the pulmonary artery originating predominantly or entirely from the RV.  In this situation the LV has no direct outlet and therefore ejects through an interventricular communication or VSD.  A wide spectrum of malformations can occur with DORV with some similar to that of TOF.  Clinical outcomes of patients with DORV can vary widely depending on the combination of abnormalities present.

DORV occurs due to an error in formation of the outlet part of the ventricular loop during early embryonic life, probably between 5 and 6 weeks of gestational age.  The occurrence of DORV like TOF is also sporadic and can also occur due to deletion of chromosome 22q11.  Other chromosomal abnormalities associated with DORV include trisomies 13 and 18.  Mutations that have been described in association with DORV phenotype include genes related to the neural crest, genes controlling literality, genes controlling transcriptional factors and genes controlling cell proliferation and transformation from endothelial to cell communication as well as outflow tract myocardialisation and polarisation.  ***10

Several developmental factors are involved with DORV formation including developmental arrest at the embryonic stage when the conotruncus relates exclusively to the RV, disruption in the development of the proximal cushions, and abnormal ventricular development with or without an abnormal conus (Lai 2009).

DORV has a substantial and wide description of a large range of anatomical variations which are reflected in many different clinical presentations.  Atrial and atrioventricular connections must be ascertained correctly.  The rise of the two great arteries from the RV must be well identified including any obstructive nature that may be present and their inter-relationship.  The morphology, size and placement of the VSD is also very important to describing DORV malformations particularly in relation to the arterial outlets.  Attention must also be paid to the anatomy and function of the mitral and tricuspid valves and the presence of any other cardiac defect that may be associated (Wilkinson 2005).

The positional relationship of the aorta and the pulmonary artery are variable in hearts with DORV.  The normal interrelationship between these two arteries is normal or close to normal with the aorta located posteriorly and on the rightward relative to the pulmonary artery.  Less often the aorta can be located to the right of the pulmonary artery and both being side by side.  Transposition of the great arteries results from abnormal rotation and septation of the arterial truncus during embryogenesis, where the aorta arises from the RV and the pulmonary artery from the LV.  Hearts with DORV may exhibit D-malposition, with the aorta being anterior and directly to the right of the pulmonary artery.  Occasional L-malposition occurs in patients with DORV with the aorta lying to the left but being side by side with the pulmonary artery (Kirklin 2003).

The most clinically useful tool to classification of DORV is the spatial description of the VSD in relation to the great arteries.  When present, the VSD may fall into one of four categories including: subaortic VSD, subpulmonary VSD, doubly committed VSD or noncommitted VSD.  In most cases of DORV, the VSD is located between the antero-superior and postero-inferior limbs of the septal band.  The spatial relationship of the VSD to the mitral and tricuspid valves is characterised by the presence, size and position of the subarterial conus and deficiency of adjacent septal segments such as the AV canal septum and conal septum.  The size of the VSD is characterised by the distance between the VSD to either one of the semilunar valves (Lai 2009).

The ventriculoarterial relationship known as DORV is distinguished from other lesions such as TOF because the VSD in DORV forms an integral part of the LV outflow tract.  The VSD in most cases in DORV is non-restrictive with its diameter being equal to or larger than the diameter of the aortic annulus.  However, in 10% of the cases, a restrictive VSD can be found (Walters 2000).

In rare occasions of DORV, a VSD may not be present which results in an extremely hypoplastic LV and mitral valve, and a small atrial septal defect (ASD) serves as the only source of left to right shunt.  The location of the VSD is rather constant being conoventricular as they lie within the anterior and posterior limbs of the septal band.  When the VSD is located in the inlet septum they are not classified as conoventricular as they lie in the trabecular portion of the muscular interventricular septum or when a perimembranous VSD extends inferiorly to occupy the inlet septum (Walters 2000).

Subaortic VSDs are the most common type of VSD that occurs in DORV.  It is located below the aortic valve and is snug between the limbs of the septal band in the postoinferior position.  When there is continuity between the aortic and mitral valve due to the absence of the subaortic conus, the left cusp of the aortic valve or the base of the anterior leaflet of the mitral valve form the actual poterosuperior margin of the VSD.  This type of defect is termed a malalignment VSD or conoventricular septal defect as the conal septum is not aligned with the muscular septum below it.  There is occasional separation from the posterior margin of the VSD from the tricuspid valve base by a rim of muscular tissue.  By this, the posterior limb of the septal band and the ventriculoinfundibular band are fused together.

DORV can be associated with L-malposition of the great arteries. In that case, the VSD is subaortic in the majority of cases, also snug within the septal band, but lays more superiorly and anteriorly in respect to the muscular interventricular septum than it does when the aorta is in its normal position.  In these cases, the aortic valve or subaortic conus represents the superior margin of the VSD.  In DORV with D-malposition of the great arteries, the conal septum is attached to the antero-superior limb of the septal band and is rotated out of the plane of the ventricular septum.  The degree of deviation of the anterior and leftward deflection of the conal septum is reflected with the severity of the RV outflow tract obstruction which is often present (Allen 2008, Lai 2009, alters 2000).

Subpulmonary VSD is the second most common type of VSD present in DORV.  It is also known as the Tussig-Bing anomaly.   The great arteries are located side by side but the pulmonary truck is widely dilated due to the absence of pulmonary stenosis.  The location of the VSD is immediately adjacent is anterosuperior to the pulmonary valve and is usually non-restrictive.  Subpulmonary VSDs are similar to those described to that for DORV with L-malposition.  As the VSD is more anterior than the subaortic VSD, the conal septum shields the aorta as it attaches to the ventriculo-infundibular fold that is more posteriorly located rather than the antero-superior limb of the septal band.  By that, since the aorta is shielded, the only exit from the LV is through the pulmonary trunk.  The pulmonary valve is separated from the anterior leaflet mitral valve by the pulmonary conus.  Patients seen with subpulmonary VSDs could also have coarctation of the aorta and aortic arch interruptions.  The distinction between DORV and transposition of the great arteries is by the alignment of the pulmonary valve with the ventricles below it.  In DORV, the pulmonary valve is entirely related to the RV, while, in transposition of the great arteries, the pulmonary valve relates nearly wholly to the LV.  Although DORV with subpulmonary VSD may be considered as the Tussig-Bing anomaly, the original description was set out as DORV with subpulmonary VSD, side by side great arteries and bilateral conus.  Patients with the Tussig-Bing anomaly can have varying degrees of aortic stenosis and aortic arch interruptions due to the hypertrophy of the subaortic conal free wall and hypertrophy and rightward deviation of the conal septum (Lai 2009, Walters 2000).

Image above showing the anatomy of DORV with subaortic VSD and DORV with subpulmonary VSD and D-transposition.  AO – Aorta, PA – Pulmonary Artery, PV – Pulmonary Valve, AOV – Aortic Valve, VSD – Ventricular Septal Defect, RA – Right Atrium, RV – Right Ventricle, LA – Left Atrium, LV – Left Ventricle.   Retrieved on 16 May 2011 from:

http://heartdefectsforeveryone.blogspot.com/search/label/Double%20Outlet%20Right%20Ventricle

Doubly committed (subaortic and subpulmonary) VSDs on the other hand are a result of an absent or deficient conal septum.  The VSD is typically large and lies within the divisions of the septal band superiorly in the interventricular septum and directly below the leaflets of the semilunar valves.  Both semilunar valves form the superior borders of this VSD and are generally contiguous as the infundibular septum is absent or deficient.  As the VSD is snug between the limbs of the septal bands, the right ventriculo-infundibular fold along with this part of the septal band form the entire postero-inferior margins of the defect.  A single conus may exist between the aortic and pulmonary valves or bilateral deficient coni with an absent or deficient infundibular septum (Lai 2009, Walters 2000).

Noncommitted or remote VSDs are the least occurring.  These VSDs do not have perimembranous extensions or a trabecular interventricular septum.  The VSD is not snug between the septal bands unlike the other VSDs described above, but rather located along the inlet septum along the posterior aspect of the membranous septum, hence these VSDs are remotely far from both semilunar valves (Lai 2009, Walters 2000).

Image above showing the relationship of great arteries and locations of the VSDs in a study of 70 patients with DORV.  A – Aorta, P – Pulmonary Artery, d-MGA – D-Malposition of the great arteries, l-MGA – L-Malposition of the great arteries (Allen 2008).

In most varieties of DORV, the position of the great arteries will influence the origin of the coronary arteries.  Normal coronary artery anatomy is similar to most varieties of DORV except for the position of the aortic sinuses where they are rotated in a clockwise direction resulting in the left coronary artery lying posteriorly and the right coronary artery lying anteriorly.  As in TOF, in a small amount of cases with DORV there may be a single coronary ostium that supplies both the left and right coronary arteries which arises either anteriorly or posteriorly.  The left anterior descending artery can occasionally arise from the right coronary artery (Kirklin 2003).

Image above illustrating the four categories of conal morphology: (a) bilateral conus, (b) absent subaortic conus, (c) absent subpulmonary conus, (d) bilaterally absent conus.  Ao – Aorta, CS – Conal Septum, MV – Mitral Valve, PA – Pulmonary Artery, TV – Tricuspid Valve (Lai 2009).

Irrespective of the great artery relationship in DORV, the pathophysiology and hemodynamic effects of the defect varies, mostly depending on the location of the VSD as it influences the streaming of blood from the ventricles into the great vessels and the presence or absence of pulmonary valve stenosis.  Patients with DORV and significant pulmonary stenosis, cyanosis becomes severe and the clinical presentations and features are similar to that of patients with TOF.  The amount of obstruction to the right ventricular outflow is the determinant of the severity of the shunting that occurs through the VSD.  Thus, the difference in resistance between the pulmonary vascular bed and the systemic vascular bed will determine the balance between the aortic and pulmonary blood flow.  Due to the nature of the non-restrictive VSD, this balance between both aortic and pulmonary blood flow can be achieved.

Patients with DORV and no pulmonary stenosis can have a faster onset of pulmonary vascular disease than patients with a simple large VSD, particularly in DORV with subpulmonary VSD.  This is due to the reduction in the pulmonary flow which has a marked influence on the arterial saturations than in patients with a simple VSD as it reduces