Balancing Pulmonary Flow & Cardiac Function

In congenital heart disease, surgical treatment is not always directed immediately toward complete anatomical repair. In many patients—especially neonates, infants, and patients with single-ventricle physiology—the first surgical objective is physiologic optimization. The circulation must be adjusted so that pulmonary blood flow is sufficient for oxygenation, but not so excessive that it compromises ventricular performance or systemic perfusion.

This concept is central to surgical palliation.

Pulmonary blood flow and cardiac function behave like a seesaw. When pulmonary blood flow increases, systemic oxygen saturation may improve, but ventricular volume load and pulmonary venous return also increase. When pulmonary blood flow decreases, ventricular loading may improve, but systemic oxygenation worsens. The goal of palliation is therefore not to maximize pulmonary blood flow, but to establish the right amount of pulmonary blood flow for the patient’s anatomy, physiology, and staged surgical pathway.

Theoretical modeling in hypoplastic left heart syndrome has shown that systemic oxygen availability is not maximized by excessive pulmonary flow. Rather, an optimal pulmonary-to-systemic flow relationship often requires a controlled Qp/Qs, with excessive pulmonary flow reducing systemic oxygen delivery despite apparently acceptable oxygen saturation [1]. This principle explains why oxygen saturation alone is an incomplete marker of circulatory success.

1. The Core Balance: Oxygenation Versus Cardiac Workload

The pulmonary and systemic circulations are tightly linked. Any change in pulmonary blood flow affects not only oxygen saturation, but also ventricular preload, systemic output, pulmonary vascular exposure, and end-organ oxygen delivery.

A balanced circulation requires:

1. Adequate pulmonary blood flow

Enough blood must reach the lungs to maintain acceptable systemic oxygen saturation.

2. Acceptable ventricular volume load

Pulmonary venous return must not overload the ventricle.

3. Preserved systemic perfusion

Cardiac output must support the body, not be disproportionately diverted into the pulmonary circuit.

4. Protection of the pulmonary vascular bed

The pulmonary arteries must be protected from chronic excessive flow and pressure.

5. Compatibility with the next surgical stage

Palliation should create favorable conditions for future definitive repair, Glenn, Fontan completion, or biventricular conversion when feasible.

In univentricular hearts, this balance is particularly important because the single functional ventricle must support both systemic and pulmonary circulations before staged cavopulmonary reconstruction. Surgical decision-making therefore depends on anatomy, pulmonary vascular resistance, ventricular function, atrioventricular valve competence, systemic outflow, and the expected next stage of palliation [2].

2. Optimal Pulmonary Blood Flow

In the optimal state, pulmonary blood flow and cardiac function are matched.

Physiologic Features

• Systemic oxygen saturation is acceptable.
• Ventricular preload is not excessive.
• Systemic perfusion is stable.
• Pulmonary vascular resistance remains low.
• There is no significant cyanosis or clinical heart failure.

In this state, pulmonary blood flow is sufficient for oxygen uptake, but not so high that it causes pulmonary overcirculation or ventricular volume overload.

Conceptually:

Adequate pulmonary blood flow + preserved ventricular performance

= stable systemic oxygen delivery without heart failure.

This is the physiologic target of many palliative strategies.

3. Excessive Pulmonary Blood Flow

When pulmonary blood flow is excessive, arterial oxygen saturation may look reassuring. However, this apparent improvement may occur at the expense of cardiac efficiency.

Excessive pulmonary blood flow increases pulmonary venous return, producing ventricular volume overload. In parallel circulations, such as the early single-ventricle circulation, excessive Qp may also reduce systemic blood flow by “stealing” output into the pulmonary circuit.

Typical Lesions

This physiology may occur in:

• Large ventricular septal defect
• Large patent ductus arteriosus
• Complete atrioventricular septal defect
• Truncus arteriosus
• Double-outlet right ventricle with unrestricted pulmonary blood flow
• Single-ventricle physiology with excessive pulmonary blood flow
• Hybrid or shunt-dependent circulations with excessive pulmonary flow

Clinical Consequences

Excessive pulmonary blood flow may cause:

• Tachypnea and increased work of breathing
• Pulmonary edema
• Feeding difficulty and failure to thrive
• Ventricular dilation
• Atrioventricular valve regurgitation secondary to dilation
• Elevated ventricular end-diastolic pressure
• Systemic hypoperfusion despite acceptable oxygen saturation
• Progressive heart failure
• Pulmonary vascular remodeling if prolonged

The key point is that good oxygen saturation does not always mean good circulation. A patient may appear well saturated but still have inadequate systemic oxygen delivery because too much cardiac output is directed toward the lungs.

4. Pulmonary Artery Banding: Reducing Excessive Pulmonary Flow

When pulmonary blood flow is excessive and complete repair is not suitable or not yet optimal, pulmonary artery banding may be used to restrict pulmonary blood flow.

Pulmonary artery banding is a calibrated palliative operation. Its purpose is not simply to narrow the main pulmonary artery, but to create a controlled resistance that reduces pulmonary overcirculation, unloads the ventricle, and protects the pulmonary vascular bed [3].

Goals of Pulmonary Artery Banding

• Reduce excessive pulmonary blood flow
• Decrease pulmonary venous return
• Relieve ventricular volume overload
• Improve systemic cardiac output
• Reduce symptoms of heart failure
• Protect the pulmonary vasculature from chronic high-flow exposure
• Prepare the patient for later staged repair

This is particularly relevant in patients with large left-to-right shunts, complex biventricular lesions requiring delayed repair, or single-ventricle physiology with unrestricted pulmonary blood flow.

Hemodynamic Concept

A good pulmonary artery band is neither too loose nor too tight.

• Too loose: persistent pulmonary overcirculation, heart failure, and pulmonary vascular exposure.
• Too tight: cyanosis, inadequate pulmonary blood flow, and excessive ventricular pressure load.

For biventricular physiology, one commonly described target is a distal pulmonary artery pressure approximately 30–50% of systemic pressure, although the ideal target depends on anatomy, age, ventricular function, and oxygen saturation [3]. In univentricular physiology, the target is often more restrictive because the pulmonary vascular bed must be prepared for future cavopulmonary circulation, where low pulmonary artery pressure and low pulmonary vascular resistance are essential.

Early exposure of the pulmonary vascular bed to abnormal flow and pressure may contribute to vascular remodeling, particularly in complex congenital heart disease and single-ventricle physiology [4]. Therefore, timely control of excessive pulmonary blood flow is not only symptomatic treatment; it is also a strategy for preserving future surgical options.

5. Technical and Outcome Considerations in Pulmonary Artery Banding

Pulmonary artery banding appears conceptually simple, but physiologically it is highly precise. Classic clinical experience demonstrated that pulmonary artery banding can be effective in controlling excessive pulmonary blood flow, but outcomes depend strongly on patient selection, band adequacy, and the subsequent surgical pathway [5].

Later outcome studies emphasized that an inadequate band can adversely affect long-term results. Inadequate banding may lead to persistent heart failure, continued pulmonary vascular exposure, or the need for earlier reintervention. Conversely, an overly tight band can cause cyanosis, branch pulmonary artery distortion, or excessive ventricular pressure load [6].

Important Intraoperative and Postoperative Parameters

Assessment usually integrates:

• Systemic oxygen saturation
• Distal pulmonary artery pressure
• Systemic arterial pressure
• Ventricular function
• Atrioventricular valve regurgitation
• Evidence of pulmonary overcirculation
• Lactate and systemic perfusion
• Echocardiographic gradient across the band
• Future surgical pathway

Thus, pulmonary artery banding should be understood as physiologic titration, not merely anatomical narrowing.

6. Insufficient Pulmonary Blood Flow

The opposite problem is insufficient pulmonary blood flow. In this setting, the ventricle may be relatively protected from volume overload, but the lungs receive too little blood for adequate oxygenation.

Typical Lesions

This physiology may occur in:

• Tetralogy of Fallot with severe pulmonary stenosis
• Pulmonary atresia
• Critical pulmonary stenosis
• Ductal-dependent pulmonary circulation
• Single-ventricle physiology with pulmonary outflow obstruction
• Complex pulmonary artery discontinuity or severe branch pulmonary artery hypoplasia

Clinical Consequences

Reduced pulmonary blood flow may cause:

• Cyanosis
• Low systemic oxygen saturation
• Limited systemic oxygen delivery
• Hypercyanotic episodes in selected physiology
• Dependence on ductal flow
• Acidosis and end-organ hypoxia when severe
• Poor pulmonary artery growth over time

In this physiology, the problem is not ventricular volume overload but inadequate oxygen delivery.

7. Systemic-to-Pulmonary Shunt: Increasing Pulmonary Blood Flow

When pulmonary blood flow is insufficient, a systemic-to-pulmonary shunt can provide a controlled source of blood flow to the pulmonary arteries.

The classic concept is the Blalock–Taussig shunt, and in modern practice this is commonly performed as a modified systemic-to-pulmonary shunt using a prosthetic graft between a systemic artery and the pulmonary artery.

Goals of a Systemic-to-Pulmonary Shunt

• Increase pulmonary blood flow
• Improve systemic oxygen saturation
• Stabilize cyanotic physiology
• Promote pulmonary artery growth
• Provide time before definitive repair or the next staged procedure
• Maintain pulmonary blood flow when ductal flow is unreliable or insufficient

However, shunt physiology also requires balance. A shunt that is too small will not correct cyanosis. A shunt that is too large may produce pulmonary overcirculation, systemic diastolic runoff, ventricular volume overload, and systemic hypoperfusion.

Hemodynamic Concept

• Too little shunt flow: persistent cyanosis.
• Excessive shunt flow: heart failure, pulmonary overcirculation, and systemic steal.
• Appropriate shunt flow: improved oxygenation with preserved systemic perfusion.

Recent work on systemic-to-pulmonary shunt banding has attempted to make control of pulmonary blood flow more reproducible. Formula-based approaches using in vitro and clinical measurements have shown that shunt flow can be mechanically modified to reduce excessive pulmonary blood flow in selected shunt-dependent circulations [7].

8. Contemporary Perspective: Flow Restriction and Hybrid Palliation

The traditional surgical tools for adjusting pulmonary blood flow are pulmonary artery banding and systemic-to-pulmonary shunting. However, contemporary congenital heart care increasingly uses hybrid and catheter-based strategies in selected high-risk neonates.

For example, pulmonary artery flow restrictors have emerged as a catheter-based method to reduce pulmonary overcirculation in selected neonates with complex congenital heart disease. These approaches may serve as a bridge in patients considered high risk for conventional neonatal surgery, although patient selection, institutional experience, device-related complications, and long-term outcomes remain important considerations [8].

This newer direction reinforces the same central principle: regardless of whether the technique is surgical, hybrid, or catheter-based, the physiologic objective is still controlled pulmonary blood flow.

9. The Seesaw Model of Surgical Palliation

The concept can be summarized as follows:

This table highlights a key surgical principle:

The goal is not the highest saturation.

The goal is the best balance between oxygenation, ventricular workload, and systemic perfusion.

10. Practical Clinical Interpretation

When assessing a patient with complex congenital heart disease, the surgeon should not ask only, “What is the oxygen saturation?”

A more complete physiologic assessment includes:

1. Is pulmonary blood flow excessive, insufficient, or balanced?
2. Is the ventricle volume-loaded, pressure-loaded, or failing?
3. Is systemic perfusion adequate?
4. Is the pulmonary vascular bed protected?
5. Is pulmonary artery growth adequate for the next stage?
6. Will the current strategy support future repair, Glenn, Fontan, or biventricular conversion?
7. Does the patient need more pulmonary flow, less pulmonary flow, or definitive anatomical correction?

This framework prevents overreliance on a single number such as oxygen saturation. In congenital heart surgery, saturation must be interpreted together with ventricular performance, pulmonary vascular resistance, systemic perfusion, and the long-term surgical plan.

Key Message

Pulmonary blood flow and cardiac function must be balanced.

Excessive pulmonary blood flow improves oxygenation but may produce ventricular volume overload, pulmonary overcirculation, heart failure, and pulmonary vascular remodeling. Insufficient pulmonary blood flow reduces ventricular volume load but worsens cyanosis and systemic oxygen delivery. Surgical palliation aims to position the circulation at the most stable point between these two extremes.

The objective is not maximum pulmonary blood flow.

The objective is balanced pulmonary blood flow that supports oxygenation without sacrificing cardiac function.

References

[1] Barnea O, Austin EH, Richman B, Santamore WP. Balancing the circulation: theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome. J Am Coll Cardiol. 1994;24(5):1376-1381.

[2] Davies RR, Pizarro C. Decision-making for surgery in the management of patients with univentricular heart. Front Pediatr. 2015;3:61.

[3] Sharma R. Pulmonary artery banding: rationale and possible indications in the current era. Ann Pediatr Cardiol. 2012;5(1):40-43.

[4] Fratz S, Fineman JR, Görlach A, Sharma S, Oishi PE, Schreiber C, Kietzmann T, Adatia I, Hess J, Black SM. Early determinants of pulmonary vascular remodeling in animal models of complex congenital heart disease. Circulation. 2011;123(8):916-923.

[5] Albus RA, Trusler GA, Izukawa T, Williams WG. Pulmonary artery banding. J Thorac Cardiovasc Surg. 1984;88(5 Pt 1):645-653.

[6] Pinho P, von Oppell UO, Brink J, Hewitson J. Pulmonary artery banding: adequacy and long-term outcome. Eur J Cardiothorac Surg. 1997;11(1):105-111.

[7] Atlin CR, Haller C, Honjo O, Jegatheeswaran A, Van Arsdell GS. Balancing pulmonary blood flow: theory, in vitro measurements, and clinical correlation of systemic-to-pulmonary shunt banding. J Thorac Cardiovasc Surg. 2016;152(5):1343-1352.e2.

[8] Sperotto F, Lang SM, Farias M, et al. Transcatheter palliation with pulmonary artery flow restrictors in neonates with congenital heart disease: feasibility, outcomes, and comparison with a historical hybrid stage 1 cohort. Circ Cardiovasc Interv. 2023;16(12):e013383.