Left-to-Right Shunting #1: Basic Physiology

1. Core Concept

A left-to-right shunt occurs when oxygenated systemic blood recirculates into the pulmonary circulation through an abnormal communication.

Common examples include:

• Ventricular septal defect
• Atrial septal defect
• Patent ductus arteriosus
• Systemic-to-pulmonary shunt
• Aortopulmonary collateral arteries
• Partial anomalous pulmonary venous connection

The central physiology is increased pulmonary blood flow. This produces pulmonary overcirculation and volume loading of the cardiac chambers involved in the recirculation pathway.

The anatomic lesion defines the route of shunting, but the physiologic burden is determined by the magnitude of flow, the resistance relationship between the pulmonary and systemic circulations, and the compliance of the receiving chambers. In atrial septal defect and partial anomalous pulmonary venous connection, cardiovascular magnetic resonance data suggest that hemodynamic consequences are driven more by the degree of shunting than by anatomic subtype alone [1].

2. Normal Circulation Versus Left-to-Right Shunting

In normal circulation, systemic venous blood returns to the right heart, passes through the pulmonary circulation, returns oxygenated to the left heart, and is then delivered to the systemic circulation.

In left-to-right shunting, a portion of oxygenated blood that should enter the systemic circulation instead re-enters the pulmonary circulation. This creates an inefficient recirculation loop.

The consequences are:

• Increased pulmonary blood flow
• Increased pulmonary venous return
• Volume loading of selected cardiac chambers
• Increased cardiac workload
• Risk of pulmonary vascular remodeling if the shunt is large and persistent

The pulmonary-to-systemic flow ratio, Qp:Qs, is the standard physiologic expression of shunt magnitude. Qp:Qs does not itself cause the shunt; rather, it quantifies the net effect of anatomy, pressure gradients, chamber compliance, and pulmonary vascular resistance.

3. Basic Hemodynamic Sequence

The typical sequence is:

1. Oxygenated blood exits the systemic or left-sided circulation.
2. Blood crosses an abnormal communication into the right-sided or pulmonary circulation.
3. Pulmonary blood flow increases.
4. Pulmonary venous return increases.
5. The receiving cardiac chambers become volume loaded.
6. Chronic overcirculation may lead to pulmonary vascular remodeling.

A Qp:Qs greater than 1 indicates net left-to-right shunting. The higher the Qp:Qs, the larger the pulmonary overcirculation relative to systemic flow. Cardiovascular magnetic resonance can directly quantify pulmonary blood flow, systemic blood flow, and Qp:Qs, making it useful when echocardiography or catheterization data are incomplete or when surgical decision-making depends on precise flow assessment [2].

4. Ventricular Septal Defect as a Model Lesion

A ventricular septal defect is a useful model for understanding left-to-right shunt physiology.

During systole, left ventricular pressure usually exceeds right ventricular pressure. If a ventricular septal communication is present, blood flows from the LV to the RV and then into the pulmonary artery. This increases pulmonary blood flow. The excess pulmonary venous return returns to the left atrium and left ventricle, producing LA and LV volume overload.

Therefore, in a significant VSD, the dominant chronic volume-loaded structures are usually:

• Pulmonary vascular bed
• Left atrium
• Left ventricle

The right ventricle is exposed to the shunt jet and increased RV outflow tract flow, but the major chronic volume burden is usually on the pulmonary venous return side.

In children with VSD or PDA, left ventricular end-diastolic dimension Z-score correlates strongly with catheter-derived Qp:Qs and may serve as a non-invasive marker of pulmonary overcirculation and LV volume loading [3].

5. Determinants of Shunt Magnitude

The magnitude of a left-to-right shunt is not determined by defect size alone. It reflects the interaction of several physiologic variables.

5.1 Defect Size

A small restrictive defect may generate a high-velocity jet but a limited total shunt volume.

A large nonrestrictive defect allows greater flow and may permit pressure equalization between chambers or great vessels. In large VSDs, pulmonary artery pressure may approach systemic pressure if pulmonary vascular resistance is low or if pulmonary vascular disease develops.

5.2 Pressure Gradient

Flow across a communication depends on the pressure gradient between the connected chambers or vessels.

In VSD, the systolic LV-to-RV pressure gradient drives shunting.

In PDA, the aorta-to-pulmonary artery gradient drives flow, especially during systole and diastole.

In ASD, the instantaneous pressure gradient is small, and shunting is strongly influenced by ventricular compliance.

If right-sided pressure or pulmonary artery pressure rises, the left-to-right gradient decreases. With severe pulmonary vascular disease, shunt flow may become bidirectional or right-to-left.

5.3 Ventricular Compliance

Compliance is critical, especially in atrial-level shunts.

In ASD, the right ventricle is usually more compliant than the left ventricle, allowing left atrial blood to pass into the right atrium and right ventricle. This produces RA and RV volume overload.

If RV compliance is reduced, as in severe pulmonary hypertension, restrictive RV physiology, or significant RV outflow obstruction, left-to-right shunting may decrease or reverse.

5.4 Pulmonary Vascular Resistance

Pulmonary vascular resistance is one of the most important determinants of pulmonary blood flow.

Low PVR favors left-to-right shunting and pulmonary overcirculation.

Elevated PVR reduces pulmonary blood flow and may mask the apparent size of the shunt. In advanced disease, irreversible pulmonary vascular remodeling can lead to Eisenmenger physiology, with bidirectional or right-to-left shunting and cyanosis.

For patients with pulmonary arterial hypertension associated with congenital left-to-right shunts, operability depends on the relationship between shunt magnitude, pulmonary vascular resistance, pulmonary vascular reactivity, and response to targeted therapy. Cardiac catheterization remains central for evaluating baseline hemodynamics and determining whether closure, fenestrated closure, staged therapy, or non-closure is appropriate [4].

6. Pulmonary Overcirculation

Pulmonary overcirculation means excessive pulmonary blood flow relative to systemic blood flow.

Clinically, this may produce:

• Tachypnea
• Increased work of breathing
• Feeding difficulty
• Poor weight gain
• Pulmonary edema
• Recurrent respiratory symptoms
• Cardiomegaly
• Ventricular dilation
• Heart failure symptoms

The key concept is recirculation. Oxygenated blood repeatedly returns to the lungs instead of contributing efficiently to systemic perfusion. The heart must pump the same oxygenated blood multiple times through the pulmonary circuit.

This is why a large left-to-right shunt may produce high total ventricular output but inefficient effective systemic output.

7. Chamber Volume Loading Depends on the Level of the Shunt

Different lesions volume-load different chambers.

7.1 VSD

A significant VSD increases pulmonary blood flow and pulmonary venous return.

Typical volume-loaded chambers:

• Left atrium
• Left ventricle

7.2 ASD

An ASD usually produces right-sided volume overload.

Typical volume-loaded chambers:

• Right atrium
• Right ventricle
• Pulmonary arteries

Because the pressure gradient across the atrial septum is low, ASD shunt magnitude is strongly influenced by ventricular compliance rather than pressure gradient alone.

7.3 PDA

A PDA produces runoff from the aorta into the pulmonary artery.

Typical consequences:

• Increased pulmonary blood flow
• Increased pulmonary venous return
• LA and LV volume overload
• Diastolic runoff from the systemic circulation
• Potential systemic hypoperfusion if the ductus is large

PDA management is especially nuanced in premature infants because spontaneous closure, heterogeneous physiology, and variable treatment response complicate interpretation of the evidence. The physiologic relevance of the ductus depends on its impact on pulmonary overcirculation, systemic perfusion, respiratory status, and ventricular loading rather than ductal patency alone [5].

7.4 Systemic-to-Pulmonary Shunt

A systemic-to-pulmonary shunt is intentionally created to provide pulmonary blood flow in cyanotic congenital heart disease.

However, excessive shunt flow can cause:

• Pulmonary overcirculation
• Low diastolic pressure
• Coronary and systemic runoff
• Systemic hypoperfusion
• Ventricular volume loading

If pulmonary vascular resistance is elevated, patients may become borderline for shunt closure or complete repair. In selected patients with elevated PVR, preoperative pulmonary vasodilator therapy combined with fenestrated or partial closure has been associated with improved functional status and hemodynamics [6].

7.5 Aortopulmonary Collateral Arteries

Aortopulmonary collateral arteries can provide additional pulmonary blood flow, particularly in pulmonary atresia with VSD and major aortopulmonary collateral arteries.

Their physiologic impact depends on:

• Number of collateral vessels
• Size of each collateral
• Segmental pulmonary artery supply
• Native pulmonary artery architecture
• Pulmonary vascular resistance in each lung segment
• Degree of competitive or excessive pulmonary blood flow

In PA/VSD/MAPCAs, pulmonary blood flow is anatomically and physiologically heterogeneous. Surgical strategy often requires unifocalization, staged assessment of pulmonary vascular bed adequacy, and careful timing of VSD closure [7].

8. Qp:Qs and Clinical Interpretation

Qp:Qs is the pulmonary-to-systemic blood flow ratio.

• Qp:Qs = 1:1 indicates balanced pulmonary and systemic blood flow.
• Qp:Qs > 1 indicates left-to-right shunting.
• Qp:Qs < 1 indicates net right-to-left shunting or reduced pulmonary blood flow.

A Qp:Qs of 2:1 means pulmonary blood flow is twice systemic blood flow. This does not mean the patient has twice the effective systemic cardiac output. It means that a large portion of cardiac output is recirculating through the lungs.

Qp:Qs should be interpreted together with:

• Symptoms
• Chamber dilation
• Ventricular function
• Pulmonary artery pressure
• Pulmonary vascular resistance
• Oxygen saturation
• Age and growth
• Lesion-specific anatomy
• Operative risk

A high Qp:Qs with chamber dilation usually indicates a significant physiologic burden. A lower Qp:Qs in the setting of elevated PVR may indicate advanced pulmonary vascular disease rather than a benign lesion.

9. Pulmonary Vascular Resistance and Operability

Large unrepaired left-to-right shunts expose the pulmonary vascular bed to increased flow and pressure. Over time, this can produce pulmonary vascular remodeling.

The progression may be conceptualized as:

1. High pulmonary blood flow with low PVR
2. Pulmonary vascular remodeling
3. Rising PVR
4. Reduced left-to-right shunt magnitude
5. Bidirectional shunting
6. Right-to-left shunting and cyanosis

Once PVR becomes severely elevated and fixed, complete closure of the shunt may be dangerous because the defect may function as a decompressive pathway for the right heart.

Therefore, intervention timing is guided by:

• Magnitude of shunting
• Evidence of chamber volume overload
• Symptoms and growth failure
• Pulmonary artery pressure
• PVR and PVR/SVR relationship
• Vasoreactivity or response to pulmonary vasodilator therapy
• Feasibility of complete versus fenestrated closure
• Expected postoperative ventricular adaptation

In borderline patients, a “treat-and-repair” strategy may be considered when pulmonary vasodilator therapy reduces PVR and preserves a significant left-to-right shunt, suggesting residual pulmonary vascular reserve [4,6].

10. Surgical Relevance

For congenital heart surgeons, the indication for intervention is not based solely on the presence of a hole or communication. It is based on the physiologic burden created by that communication.

Key surgical questions include:

• Is the shunt restrictive or nonrestrictive?
• Which chambers are volume loaded?
• Is pulmonary blood flow excessive?
• Is systemic output compromised?
• Is pulmonary vascular resistance still acceptable?
• Is complete closure safe?
• Is fenestrated closure safer?
• Is staged repair required?
• Are there associated lesions that modify the physiology?

For example, in staged repair of PA/VSD, shunt size and pulmonary artery growth influence progression to complete repair. Smaller systemic-to-pulmonary shunts have been associated with earlier progression to complete repair in staged PA/VSD cohorts, emphasizing that shunt design affects downstream physiology and surgical timing [8].

Systemic-to-pulmonary shunts also carry lesion-specific complications. Acute shunt thrombosis is a serious event associated with worse outcomes, greater blood product use, and increased resource utilization [9]. Thus, shunt physiology must be balanced: insufficient flow causes cyanosis, while excessive flow causes pulmonary overcirculation and systemic runoff.

11. Key Takeaways

Left-to-right shunting is recirculation of oxygenated blood into the pulmonary circulation.

The primary consequences are increased pulmonary blood flow, pulmonary overcirculation, and volume loading of the involved cardiac chambers.

Qp:Qs quantifies shunt magnitude but does not fully explain the mechanism. The mechanism depends on defect size, pressure gradient, chamber compliance, and pulmonary vascular resistance.

The volume-loaded chamber depends on the level of shunting.

In VSD and PDA, LA and LV volume overload are typical.

In ASD, RA and RV volume overload are typical.

Persistent large left-to-right shunts can produce pulmonary vascular remodeling and pulmonary hypertension.

Intervention timing depends on symptoms, Qp:Qs, chamber dilation, pulmonary vascular resistance, and expected postoperative physiology.

In borderline pulmonary vascular disease, catheterization and response to targeted therapy are central to determining operability.

References

[1] Jacquemyn X, Van de Bruaene A, Budts W, Dresselaers T, Bogaert J. Anatomy versus shunt fraction: hemodynamic consequences in atrial septal defects and partial anomalous pulmonary venous connection—a comprehensive CMR study. Eur Heart J Cardiovasc Imaging. 2025. doi:10.1093/ehjci/jeaf001.

[2] Honjo O. Integrating novel physiologic data into decision-making in congenital heart surgery. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2022. doi:10.1053/j.pcsu.2022.02.001.

[3] Sumitomo NF, Kodo K, Maeda J, Miura M, Yamagishi H. Echocardiographic left ventricular Z-score utility in predicting pulmonary-systemic flow ratio in children with ventricular septal defect or patent ductus arteriosus. Circ J. 2021. doi:10.1253/circj.CJ-21-0559.

[4] D’Alto M, Romeo E, Argiento P, Vergara A, Caiazza E, Orlando A, Franzese R, Scognamiglio G, Sarubbi B, Dimopoulos K. Pulmonary arterial hypertension with left to right shunts: when to treat and/or close? Int J Cardiol Congenit Heart Dis. 2024. doi:10.1016/j.ijcchd.2024.100526.

[5] de Waal K, Prasad R, Kluckow M. Patent ductus arteriosus management and the drift towards therapeutic nihilism—what is the evidence? Semin Fetal Neonatal Med. 2021. doi:10.1016/j.siny.2021.101219.

[6] Linder AN, Hsia J, Krishnan S, Bacha E, Crook S, Rosenzweig E, Krishnan U. Management of systemic to pulmonary shunts and elevated pulmonary vascular resistance. ERJ Open Res. 2023. doi:10.1183/23120541.00271-2023.

[7] Deng MX, Zahiri Y, Honjo O, Barron DJ. Staged approach: the role of delayed repair following complete unifocalization of major aortopulmonary collateral arteries with ventricular septal defect and pulmonary atresia. World J Pediatr Congenit Heart Surg. 2025. doi:10.1177/21501351241297710.

[8] Macalister SJ, Buratto E, Naimo PS, Ye X, Fulkoski N, Weintraub R, Brizard C, Konstantinov I. Long-term outcomes of staged complete repair of pulmonary atresia with ventricular septal defect. Ann Thorac Surg. 2022. doi:10.1016/j.athoracsur.2022.09.022.

[9] Dutta P, Emani S, Ibla J, Emani S, Nathan M. Clinical implications of acute shunt thrombosis in paediatric patients with systemic-to-pulmonary shunt re-interventions. Cardiol Young. 2022. doi:10.1017/S1047951122001548.