Left-to-Right Shunting #2: Representative Lesions

Left-to-right shunting refers to recirculation of oxygenated systemic blood back into the pulmonary circulation. The physiologic impact is determined by the anatomic level of the communication, the pressure gradient across the defect, defect size, pulmonary vascular resistance, ventricular compliance, and associated atrioventricular valve or outflow tract abnormalities.

Representative lesions include:

• Patent ductus arteriosus
• Atrial septal defect
• Ventricular septal defect
• Atrioventricular septal defect

Although these lesions all increase pulmonary blood flow, their chamber-loading patterns and pulmonary vascular consequences differ substantially.

1. Classification by Shunt Level

Left-to-right shunts can be understood by the level at which systemic blood re-enters the pulmonary circulation.

The distinction between pre-tricuspid and post-tricuspid shunting is clinically important. ASD is usually a low-pressure pre-tricuspid shunt. VSD and PDA are post-tricuspid or arterial-level shunts that may expose the pulmonary vascular bed to both increased flow and increased pressure. AVSD combines atrial-level and ventricular-level shunting with abnormal atrioventricular valve anatomy.

2. Atrial Septal Defect

An atrial septal defect is a communication between the atria. Major subtypes include secundum ASD, primum ASD, sinus venosus ASD, and unroofed coronary sinus.

ASD is typically a low-pressure atrial-level shunt. The direction and magnitude of shunting are determined largely by the relative compliance of the ventricles rather than by pressure alone. Because the right ventricle is usually more compliant than the left ventricle, blood preferentially flows from the left atrium to the right atrium [1].

Typical ASD physiology includes:

• Left-to-right shunting at the atrial level
• Right atrial enlargement
• Right ventricular volume loading
• Increased pulmonary blood flow
• Dilated main and branch pulmonary arteries
• Relative left ventricular underfilling
• Usually normal pulmonary artery pressure in childhood

This explains why ASD often remains clinically subtle in infants and young children. The pulmonary circulation is exposed to excess flow, but not usually to systemic ventricular pressure. Therefore, pulmonary vascular disease generally develops later than in large VSD, PDA, or complete AVSD.

3. Patent Ductus Arteriosus

A patent ductus arteriosus is persistence of the fetal ductal connection between the descending aorta and pulmonary artery.

After birth, systemic vascular resistance rises and pulmonary vascular resistance falls. This creates left-to-right flow from the aorta into the pulmonary artery. A significant PDA functions as an aortopulmonary shunt.

Typical PDA physiology includes:

• Aortic-to-pulmonary arterial shunting
• Increased pulmonary blood flow
• Increased pulmonary venous return
• Left atrial and left ventricular volume loading
• Diastolic runoff from the systemic circulation
• Wide pulse pressure when the ductus is large
• Pulmonary edema and congestive heart failure when severe

PDA and VSD both cause increased pulmonary blood flow and left heart volume loading, but they are not physiologically identical. In a comparative cineangiocardiographic study of 58 patients with VSD and 25 patients with PDA, patients with PDA had greater left ventricular end-diastolic pressure, left ventricular end-diastolic stress, and impaired left ventricular distensibility compared with patients with VSD of equivalent shunt magnitude [2]. This reflects the additional hemodynamic effect of aortic runoff and continuous arterial-level shunting in PDA.

4. Ventricular Septal Defect

A ventricular septal defect is a communication between the left and right ventricles. VSDs may be perimembranous, muscular, inlet, or outlet in location.

The physiology depends primarily on:

• Defect size
• Pulmonary vascular resistance
• Right ventricular and pulmonary artery pressure
• Associated outflow obstruction
• Associated aortic valve prolapse or regurgitation
• Presence of additional lesions

A small restrictive VSD limits flow and produces a high-velocity jet. A large nonrestrictive VSD allows near-equalization of ventricular pressures and can transmit systemic pressure to the right ventricle and pulmonary arteries.

Large VSD physiology includes:

• High-flow left-to-right shunting
• Pulmonary overcirculation
• Increased pulmonary venous return
• Left atrial and left ventricular volume loading
• Pulmonary artery pressure elevation
• Congestive heart failure
• Failure to thrive
• Recurrent respiratory symptoms
• Risk of pulmonary vascular obstructive disease

The left ventricle is volume loaded because shunted blood passes through the pulmonary circulation and returns to the left atrium and left ventricle. This differs from ASD, where the dominant volume load is on the right atrium and right ventricle.

5. Atrioventricular Septal Defect

Atrioventricular septal defect is characterized by deficiency of the atrioventricular septal structures and abnormal development of the atrioventricular valves.

Complete AVSD includes:

• Primum atrial septal defect
• Inlet ventricular septal defect
• Common atrioventricular junction
• Common atrioventricular valve
• Variable common AV valve regurgitation

AVSD therefore combines atrial-level shunting, ventricular-level shunting, and atrioventricular valve regurgitation. The result is often earlier and more severe pulmonary overcirculation than isolated ASD.

Typical complete AVSD physiology includes:

• Left-to-right shunting at both atrial and ventricular levels
• Increased pulmonary blood flow
• Pulmonary artery pressure elevation
• Common AV valve regurgitation
• Left and right heart volume loading
• Early congestive heart failure
• Accelerated risk of pulmonary vascular obstructive disease

Moderate-to-large VSDs and complete balanced AVSDs generally require closure to prevent pulmonary vascular obstructive disease and progressive cardiac dysfunction. Early repair is especially important in complete AVSD because the pulmonary vascular bed may be exposed to both increased flow and increased pressure, often with additional volume load from common AV valve regurgitation [3].

6. Flow Alone Versus Flow Plus Pressure

The pulmonary vascular consequences of left-to-right shunting are not determined by flow alone.

A low-pressure, high-flow lesion such as ASD usually causes right heart dilation with relatively preserved pulmonary artery pressure in childhood. In contrast, large VSD, PDA, and complete AVSD may expose the pulmonary vasculature to both increased blood flow and increased pressure.

Experimental models support this distinction. In ovine congenital heart disease models, increased pulmonary blood flow combined with increased pulmonary arterial pressure produced more severe pulmonary vascular remodeling than increased pulmonary blood flow alone [4]. This provides a mechanistic explanation for why post-tricuspid and arterial-level shunts are more likely to produce early pulmonary vascular disease than isolated ASD.

Pulmonary vascular disease in congenital heart disease is driven by interacting factors, including pulmonary blood flow, pulmonary arterial pressure, left atrial pressure, and pulmonary vascular resistance [5]. Once pulmonary vascular resistance becomes fixed and markedly elevated, shunt closure may no longer be beneficial and may become harmful.

7. Progression Toward Pulmonary Vascular Disease

Large left-to-right shunts follow a predictable pathophysiologic sequence if untreated:

1. Pulmonary blood flow increases.
2. Pulmonary venous return rises.
3. Cardiac chambers dilate according to the shunt level.
4. Pulmonary artery pressure may increase.
5. Pulmonary vascular remodeling develops.
6. Pulmonary vascular resistance rises.
7. Shunting becomes bidirectional.
8. Right-to-left shunting may develop.
9. Eisenmenger physiology may occur.

This sequence is particularly relevant in large VSD, PDA, and complete AVSD. ASD may also lead to pulmonary hypertension in later life, but the usual childhood pattern is right-sided volume overload with normal pulmonary artery pressure.

8. Intervention Principles

The goal of intervention is not simply to close an anatomic hole. The goal is to eliminate an abnormal circulatory loop before irreversible chamber remodeling, atrioventricular valve deterioration, pulmonary hypertension, or pulmonary vascular obstructive disease occurs.

General intervention principles are:

• ASD: closure is indicated when there is significant right atrial and right ventricular volume loading, usually with adequate rims for device closure or surgical closure depending on anatomy.
• PDA: closure is indicated when the ductus is hemodynamically significant, causing left heart volume loading, pulmonary overcirculation, pulmonary hypertension, or other clinical consequences.
• VSD: closure is indicated for large or moderate defects causing left heart dilation, symptoms, elevated pulmonary artery pressure, significant Qp/Qs, aortic valve prolapse, or failure to thrive.
• AVSD: complete balanced AVSD generally requires early complete repair to close the atrial and ventricular components and reconstruct the atrioventricular valve.

Large registry data show that ASD closure is most commonly performed for right ventricular volume overload, whereas PDA closure is commonly performed for left ventricular volume overload, endocarditis prevention, or pulmonary hypertension [6]. This mirrors the fundamental physiologic difference between atrial-level and arterial-level shunting.

9. Key Comparison

10. Key Takeaway

PDA, ASD, VSD, and AVSD are all representative left-to-right shunt lesions, but they are not physiologically equivalent.

ASD is usually a low-pressure atrial-level shunt that causes right atrial and right ventricular volume loading with relatively normal pulmonary artery pressure in childhood.

PDA and VSD produce post-tricuspid or arterial-level left-to-right shunting, causing pulmonary overcirculation, increased pulmonary venous return, and left atrial and left ventricular volume loading.

AVSD combines atrial-level and ventricular-level shunting with abnormal atrioventricular valve anatomy, making pulmonary overcirculation and pulmonary vascular disease more likely to develop early.

The essential clinical task is to identify the level of shunting, define chamber loading, estimate pulmonary vascular exposure, and intervene before irreversible pulmonary vascular remodeling develops.

References

[1] Torres AJ. Hemodynamic assessment of atrial septal defects. J Thorac Dis. 2018. doi:10.21037/jtd.2018.02.17.

[2] Jarmakani MM, Graham TP, Canent RV, Spach MS, Capp MP. Effect of site of shunt on left heart-volume characteristics in children with ventricular septal defect and patent ductus arteriosus. Circulation. 1969;40(3):411. doi:10.1161/01.CIR.40.3.411.

[3] Rao PS, Harris AD. Recent advances in managing septal defects: ventricular septal defects and atrioventricular septal defects. F1000Res. 2018. doi:10.12688/f1000research.14102.1.

[4] Kameny RJ, Datar SA, Boehme JB, Morris CA, Zhu T, Goudy BD, et al. Ovine models of congenital heart disease and the consequences of hemodynamic alterations for pulmonary artery remodeling. Am J Respir Cell Mol Biol. 2019. doi:10.1165/rcmb.2018-0305MA.

[5] Wacker J, Joye R, Genecand L, Lador F, Beghetti M. Pulmonary vascular disease as a complication of pediatric congenital heart diseases. Transl Pediatr. 2023. doi:10.21037/tp-23-64.

[6] O’Byrne ML, Kennedy KF, Rome JJ, Glatz AC. Variation in practice patterns in device closure of atrial septal defects and patent ductus arteriosus: an analysis of data from the IMPACT Registry. Am Heart J. 2017. doi:10.1016/j.ahj.2017.10.018.