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Year : 2007  |  Volume : 10  |  Issue : 1  |  Page : 19-26
Pathophysiology of congenital heart diseases

Department of Paediatric Cardiology, St. Stephens Hospital, Tis Hazari, Delhi., India

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How to cite this article:
Chowdhury D. Pathophysiology of congenital heart diseases. Ann Card Anaesth 2007;10:19-26

How to cite this URL:
Chowdhury D. Pathophysiology of congenital heart diseases. Ann Card Anaesth [serial online] 2007 [cited 2022 Nov 29];10:19-26. Available from:

Congenital heart disease occurs in 8 children for every 1000 liveborns. Out of these 50% are significant in the sense that they produce haemodynamic effects. The classification of congenital heart diseases is shown in [Table 1]. This article will focus on the pathophysiology of some of the commonly encountered congenital heart defects. In addition, physiology of some surgical procedures such as Glenn and Fontan will also be described.

   Left to Right Shunts Top

The commonest physiology that is seen in patients with congenital heart disease is left to right shunts. A physiological left to right shunt is when oxygenated blood returns back to the lungs to get re-oxygenated. This creates a redundancy in the circulation. In patients with left to right shunt, there is an increased venous return from the lungs via the pulmonary veins to the left atrium and the left ventricle (LV). This creates a volume overload on the LV. Thus, in a left to right shunt there is a volume overload to the LV, pulmonary circulation and a decreased systemic cardiac output.

The physiological alterations associated with left to right shunt lesions at the ventricular or great artery level are determined principally by the size of the defect and the post-natal changes in systemic (SVR) and pulmonary (PVR) vascular resistances. In the foetal stage a large defect has no major physiological effect as the PVR is high, which limits the blood flow to the lungs. With transition to extra-uterine circulation there is a decrease in the PVR with a simultaneous increase in SVR [Figure 1]. This usually happens between 2-6 weeks of life and causes manifestation of left to right shunts in the form of congestive heart failure. In addition, the physiological nadir in the haemoglobin that occurs in the first 3 months also exaggerates heart failure.

Any manoeuvres that decrease the PVR such as administration of oxygen, nitric oxide, or low arterial carbon dioxide tension and alkalosis will increase the left to right shunt. This increase in left to right shunt will be at the cost of decreased systemic output. With continued left to right shunting of blood, there is damage to the pulmonary vasculature ultimately causing hyperplasia of the vessel walls and pulmonary hypertension. Reversible pulmonary hypertension patho-physiologically indicates that with the discontinuation of the left to right shunt there will be return to normal PVR so the elevated PVR is secondary to increased pulmonary blood flow (Qp). Another circumstance, which is unrelated to left to right shunt where the PVR is reversible, is when the left atrial pressure is elevated as in mitral stenosis in the face of a normal Qp. In these patients relief of the mitral stenosis decreases the trans-pulmonary gradient and PVR returns to normal.

Irreversible pulmonary hypertension indicates that the elevated resistance is secondary to changes in the vessel wall (Heath Edward Class III or greater). In this circumstance the Qp can be same or just slightly above systemic flow but the mean pulmonary artery pressure is elevated or there is a high trans-pulmonary gradient. A PVR greater than 8 woods unit or a ratio of PVR:SVR > 0.5 is even more significant indicator of pulmonary vascular disease.

In an ASD, there is a left to right shunt at the atrial level. This results in dilatation of right atrium and right ventricle with increased pulmonary venous return to the left atrium. This causes a volume overload to the left atrium also. In an ASD there is therefore bi-atrial volume overload and a volume overload to the right ventricle (RV). With time there is a progression towards irreversibility of the PVR.


Saturation data: Since the left to right shunt is at the atrial level there is a step up of saturation from the superior vena cava (SVC) to the right atrium. The pulmonary artery saturation will be higher than the SVC saturation and will indicate the degree of left to right shunt. Higher the pulmonary artery saturation greater is the left to right shunt.

Pressure Data: The right atrial and left atrial pressures will be equal and normal. The RV pressure will be lower than the LV pressure, but may be slightly higher than normal. The RV pressure can be about 1/3 to 1/2 systemic pressure in a large ASD.

Ventricular septal defect (VSD, [Figure 3])

In VSD, there is a left to right shunt across the ventricular level. This shunting occurs during systole and blood from LV is ejected in systole to the pulmonary circulation and causes a volume overload to the left atrium and the LV. There is increased Qp. There is no volume overload to the RV as blood physiologically during systole makes it directly into the pulmonary circulation. Since in a VSD, the shunting occurs during systole (high pressure), the left to right shunt is haemodynamically more significant and the progression towards pulmonary vascular disease is sooner. It is important to recognize that in a large VSD both the right and the left ventricles are at systemic pressure, but the blood still shunts left to right due to lower PVR distally.


There is a step up in the saturation from SVC to pulmonary artery. The RV pressure is determined by the size of the VSD- a large VSD (equal or larger than the size of aortic annulus) will have systemic RV pressure. In the face of a small VSD, the RV pressure can be normal. If there is pulmonary vascular disease then the RV pressure will be elevated. It is possible to have a VSD that is pressure restricted (RV pressure less than half systemic pressure) but still is not volume restricted (i.e Qp:Qs > 2:1).

Patent ductus arteriosus (PDA, [Figure 4])

In a PDA, there is a left to right shunt during systole and diastole from the aorta to the pulmonary artery. Compared to the aorta the pulmonary artery pressures are lower in both systole and diastole so there is a continuous shunt. This shunt creates a volume overload on the left atrium and LV. The shunting during diastole, particularly in a large PDA may produce steal of blood from coronary artery. Again in a large PDA, there is progression of pulmonary vascular disease. An aorto-pulmonary window physiologically is like a large PDA.


With left to right shunt there is a step up of saturation from the SVC to pulmonary artery. The RV pressure is normal unless there is pulmonary artery hypertension.

Truncus arteriosus

In truncus arteriosus, the pulmonary arteries are connected to the aorta. A decrease in PVR at birth causes a left to right shunt with evidence of congestive heart failure. These patients have a very high incidence of pulmonary hypertension and vascular disease.

Total anomalous pulmonary venous connection [Figure 5]

In this condition, the pulmonary venous return is to the right heart. There is complete mixing of blue and red blood. Some of this mixed blood crosses over to the left heart via an atrial level shunt and the patient has systemic cyanosis. There is a left to right shunt as the oxygenated blood returns back to the lungs to get oxygenated. However, the right atrium and RV are enlarged and the left heart appears small as there is paucity of blood returning to the left heart. Although the left heart is formed normally, it is very non-compliant and has diastolic dysfunction.

There is a very dynamic pulmonary vascular bed and these patients are very predisposed to pulmonary hypertensive crisis. In addition, it is important to recognize those patients that have pulmonary venous obstruction as in these patients, there is a mechanical obstruction to the return of pulmonary venous blood causing systemic to supra-systemic pulmonary pressures with an extremely low cardiac output.

Anomalous left coronary artery from pulmonary artery [Figure 6]

The left coronary artery arises from the pulmonary artery. With a decrease in PVR, there is left to right shunt created within the myocardial bed. The oxygenated blood from aorta goes to the right coronary artery, in the capillary network with the left coronary artery there is a left to right shunt, the blood from right coronary artery goes into the left coronary artery and then into the pulmonary artery. There is retrograde filling of the left coronary artery and at the same time there is myocardial steal causing myocardial ischaemia and infarction. This causes a dilated cardiomyopathy with mitral regurgitation.

   Right to Left Shunts Top

A physiological right to left shunt is when the deoxygenated blood that returns from the tissues returns back to the body without getting re­oxygenated.

Tetralogy of Fallot [Figure 7]

In a Tetrology of Fallot, due to presence of RV outflow tract obstruction, there is a right to left shunt across the large non-restricted VSD. The patient is cyanotic due to paucity of pulmonary blood flow. In the absence of additional sources of blood flow, the LV is smaller than RV as the pulmonary venous return is decreased.


The saturation in the pulmonary artery is the same as the superior vena cava. The saturation in the aorta is lower than the saturation in the pulmonary vein due to a right to left shunt at the ventricular level. The RV pressures are systemic due to the presence of a non-restrictive VSD and the pulmonary artery pressures are normal.

In a "Tet" spell, there is spasm of the infundibular muscle causing all the blood from the RV to shunt across the VSD to the systemic circulation. There is no pulmonary venous return to the left heart. The physiological treatment of "Tet" spell is to increase the SVR and to increase the preload of the right heart. These manoeuvres will decrease the right to left shunt and increase the flow across the pulmonary valve.

Transposition of great arteries (TGA, [Figure 8])

In transposition of great arteries there is ventriculo-arterial discordance; RV connected to aorta and LV to pulmonary artery. This creates a parallel circulation in contrast to a normal series circulation. The deoxygenated blood returning to the right heart returns back to the systemic circulation and the oxygenated blood from the pulmonary venous side returns back to the lungs. Therefore, in transposition, there is physiologically a complete left to right shunt and a complete right to left shunt. The only way for the patient to survive is, if there is anatomically a shunt from the right heart to the left heart and vice versa. This will allow some of the deoxygenated blood to reach the pulmonary circulation and oxygenated blood to the systemic circulation. So in a transposition physiology, there is no paucity of pulmonary blood flow, but there is poor "mixing" of blood. So manoeuvres to augment pulmonary blood flow like a shunt do not necessarily treat the cyanosis. To increase the atrial level mixing by creating a septostomy is a more effective way to treat the cyanosis.


There is a step up of saturation from the SVC to the aorta if there is mixing of blood at the atrial or ventricular level or great artery level. The pulmonary artery saturations are increased as the pulmonary venous blood returns to the pulmonary artery. The saturation data is extremely difficult to interpret in a TGA. If there is an anatomical shunt from the RV to LV, then only there is a decrease in pulmonary artery saturation. The RV pressures remain systemic, however, the LV pressures decrease in association with a decrease in PVR, unless there is another cause of maintaining high LV pressure like a large non-restrictive VSD or a large non-restrictive PDA.

Double outlet right ventricle (DORV)

In a patient with DORV depending on the relationship of the great arteries and the VSD, there can be manifestation of one of the 3 physiology: VSD physiology with left to right shunt and congestive heart failure, Tet physiology in the face of pulmonary stenosis causing paucity of pulmonary blood flow causing cyanosis, or a transposition physiology causing cyanosis with congestive heart failure.

Single ventricle [Figure 9]

There is complete mixing of blood and the quantity of pulmonary flow will be governed by degree of pulmonary stenosis. In the face of no pulmonary stenosis the Qp is far greater than Qs, as the PVR will be much lower than SVR, causing increased left to right shunt and congestive heart failure. In case of severe stenosis, there is a necessity of a ductus or a shunt to maintain adequate pulmonary blood flow. Also, there can be cases with just enough pulmonary stenosis that may create a balanced circulation where Qp:Qs is between 1:1 and 2:1.

In any of the above scenarios the single ventricle is pumping cardiac output Q, which is equal to Qs plus Qp (Q = Qp + Qs). This creates a volume load on the single ventricle.

Single ventricle physiology with severe pulmonary stenosis or pulmonary atresia will require a systemic to pulmonary shunt (Blalock­Taussig shunt) as there is paucity of pulmonary blood flow.

Superior vena cava to pulmonary artery connection

In the steps of palliation of a single ventricle towards a Fontan operation, a SVC to pulmonary artery connection is made (Glenn shunt). This now directs the venous return from the SVC as the sole venous return to the lungs. This return will be dependent on upper body circulation, especially the circulation to brain in children and distally on the PVR. There is no pumping chamber incorporated in this connection, so the flow is completely passive. Manoeuvres that decrease venous return from brain will decrease the venous return to the lungs. Hyperventilation decreases cerebral perfusion and hence will decrease the venous return to the SVC and decrease Glenn flow. However, hyperventilation will also decrease PVR and may augment the passive venous return to the lungs in a Glenn circulation. So it is important that there be a balance of ventilation to avoid extreme situations.

After a Glenn shunt, the single ventricle receives the inferior vena cava (IVC) blood, so the systemic circulation still receives cyanotic blood, but now the ventricle is pumping only one cardiac output; Q is equal to Qs only, Qp is exclusively from passive flow from the SVC. Thus a Glenn shunt takes the volume load off from the single ventricle.

Fontan physiology

In a patient with Fontan repair, the IVC and SVC blood is directed passively to the pulmonary circulation. The only source of blue blood to the systemic circulation is from the coronary sinus. The blue and the red blood are thus separated. The venous return from the liver is also directed to the pulmonary circulation, which then allows the regression or prevention of formation of pulmonary arteriovenous malformations. Since the Fontan circulation does not incorporate a pumping chamber, there is a passive flow in the circuit. It is very important that the PVR be low to allow forward flow of blood in the pulmonary circulation. Positive pressure ventilation increases the intra­thoracic pressure and hinders the Fontan circulation. It is prudent to discontinue positive pressure ventilation in this circulation as soon as possible. Also, hyperventilation by providing rapid multiple breaths also hinders the circulation, as this does not allow for the blood to go through the Fontan circulation. A Fontan physiology requires normal number of breaths in the face of positive pressure ventilation.

Sometimes patients have elevated pressures in the Fontan baffle and in these patients to avoid a low output state, often a fenestration is created between the Fontan baffle and the right atrium, which allows for a "pop-off" and maintains cardiac output but at the cost of cyanosis.

The current strategy in placing the Fontan baffle is to place it outside the heart to minimize atrial surgery and future risks of arrhythmias - this is then called an extra-cardiac Fontan.

In conclusion, there is a complex cardiac and respiratory physiology that is created by congenital heart disease. An understanding of the pathophysiology is extremely important for safe anaesthetic management of these patients.[Figure 2],[Figure 7],[Figure 9],[Figure 10],[Figure 11],[Figure 12][2]

   References Top

1.Garson Jr. A, Bricker JT, Fisher DJ, Neish SR, Eds. The Science and Practice of Pediatric Cardiology, 2nd edition, William & Wilkins, Baltimore 1998.  Back to cited text no. 1    
2.Rudolph AM, Ed. Congenital disease of the heart: clinical-physiological considerations, 2nd edition, Blackwell Publishing, 2001.  Back to cited text no. 2    

Correspondence Address:
Devyani Chowdhury
Department of Paediatric Cardiology, St. Stephens Hospital, Tis Hazari, Delhi.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0971-9784.37920

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]

  [Table 1]

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