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Alternative Blood Supply to the Lungs in Complex Congenital Heart Disease

Introduction

Major Aortopulmonary Collateral Arteries — MAPCAs — are abnormal congenital blood vessels that arise from the aorta or one of its major branches and supply blood to the pulmonary circulation.

They are most strongly associated with congenital heart defects in which normal blood flow from the right ventricle to the lungs is severely reduced or absent, particularly:

  • Pulmonary Atresia with Ventricular Septal Defect — PA/VSD
  • severe forms of Tetralogy of Fallot
  • and other complex abnormalities of the right ventricular outflow tract.

MAPCAs are therefore best considered within the Blue Baby group, because they usually occur in conditions where pulmonary blood flow is inadequate and systemic oxygen saturation is reduced. (PubMed)

What is a MAPCA?

A MAPCA is a systemic artery supplying lung tissue.

Instead of blood reaching the lungs through the normal route:

Right ventricle → Pulmonary artery → Lungs

blood may reach part of the pulmonary circulation through:

Aorta → MAPCA → Pulmonary vessels → Lung

MAPCAs commonly originate from:

  • the descending thoracic aorta;
  • aortic arch branches;
  • subclavian arteries;
  • or other first-order systemic arterial branches. (PubMed)

Why Do MAPCAs Develop?

MAPCAs are thought to represent persistence of embryological systemic-to-pulmonary vascular connections that would normally regress during development.

If normal pulmonary arterial development is inadequate, these vessels may persist and provide alternative blood flow to the developing lungs. (PubMed)

They are therefore not simply abnormal vessels appearing randomly.

They represent an alternative method of supplying pulmonary tissue when the normal pulmonary arterial circulation is deficient.

The Classic Association — PA/VSD/MAPCAs

The condition most closely associated with MAPCAs is:

Pulmonary Atresia + Ventricular Septal Defect + MAPCAs

often abbreviated:

PA/VSD/MAPCAs

In this anatomy, there may be no effective connection between the right ventricle and the pulmonary arteries.

The VSD allows ventricular communication, while pulmonary blood flow is supplied through:

  • a patent ductus;
  • native pulmonary arteries;
  • MAPCAs;
  • or a combination of all three.

The pulmonary vascular anatomy can therefore vary enormously between patients. (PubMed Central (PMC))

Pulmonary Arteries May Be Tiny, Discontinuous or Absent

One of the defining features of PA/VSD/MAPCAs is the variability of the native pulmonary arteries.

They may be:

  • relatively well formed;
  • severely hypoplastic;
  • discontinuous;
  • present on one side but not the other;
  • or almost absent.

The less well developed the native pulmonary arteries are, the more important MAPCAs often become in supplying the lungs. (PubMed Central (PMC))

A Patchwork Pulmonary Circulation

MAPCAs create one of the most anatomically complex pulmonary circulations in congenital cardiac surgery.

Different regions of lung may receive blood from entirely different sources.

For example:

Right upper lobe → Native pulmonary artery

Right lower lobe → MAPCA from descending aorta

Left lung → Combination of native PA and two MAPCAs

A single lung segment may receive:

  • supply only from a native pulmonary artery;
  • supply only from a MAPCA;
  • or dual supply from both. (PubMed Central (PMC))

This means pulmonary blood flow is not uniform.

Single-Supply Versus Dual-Supply MAPCAs

This distinction is extremely important.

Single-Supply MAPCA

A MAPCA may be the only arterial supply to a segment of lung.

If that vessel is simply ligated or embolised, the lung segment may lose its blood supply.

These vessels generally need to be incorporated into the reconstructed pulmonary circulation if the supplied lung tissue is viable. (PubMed)

Dual-Supply MAPCA

Some lung segments receive blood from:

both a MAPCA and the native pulmonary artery.

If the native pulmonary arterial supply is adequate, the collateral vessel may be unnecessary and can sometimes be occluded.

Why Are These Babies Blue?

MAPCAs usually occur because normal pulmonary blood flow is inadequate.

If insufficient blood reaches ventilated lung tissue:

less blood becomes oxygenated.

Systemic venous blood may cross a VSD and enter the aorta with limited pulmonary passage.

Systemic arterial oxygen saturation therefore falls.

Cyanosis results.

The exact oxygen saturation depends on:

  • total pulmonary blood flow;
  • number and size of MAPCAs;
  • native pulmonary artery development;
  • pulmonary vascular resistance;
  • stenosis within the MAPCAs;
  • and associated intracardiac anatomy.

But MAPCAs Can Also Produce Too Much Pulmonary Blood Flow

One of the unusual features of MAPCAs is that different lung segments can simultaneously receive:

too little blood

and

too much blood.

A large unrestricted MAPCA arises from the systemic circulation.

It therefore receives blood at systemic arterial pressure.

If it supplies a lung segment without significant narrowing, that segment can be exposed to:

  • excessive pulmonary flow;
  • systemic-level arterial pressure;
  • and early pulmonary vascular disease.

Meanwhile another lung segment supplied through a stenotic collateral may be severely underperfused. (PubMed Central (PMC))

Segmental Pulmonary Hypertension

This creates an important concept:

segmental pulmonary hypertension.

One area of the lung may develop very high pulmonary arterial pressure while another remains relatively protected.

The patient therefore may not have one uniform pulmonary vascular resistance.

Instead, the pulmonary vascular bed can consist of multiple territories with very different:

This is one reason PA/VSD/MAPCAs cannot be understood simply by measuring one pulmonary artery pressure.

MAPCA Stenosis

MAPCAs frequently develop areas of narrowing.

Stenosis may occur:

  • at their origin from the aorta;
  • along the vessel;
  • at branch points;
  • or where they connect with the pulmonary vascular bed.

A stenotic MAPCA can protect the downstream pulmonary circulation from systemic pressure.

But if the stenosis is severe, pulmonary blood flow to that lung segment may be inadequate.

Thus, the narrowing can be both:

protective

and

pathological.

Natural History

Without reconstruction, patients may face competing problems.

If pulmonary flow is inadequate:

  • cyanosis worsens;
  • exercise tolerance is limited;
  • and tissue oxygen delivery remains poor.

If pulmonary flow is excessive:

  • pulmonary vascular disease develops;
  • pulmonary hypertension may become irreversible;
  • and later repair becomes more difficult or impossible.

The aim of treatment is therefore not simply to increase pulmonary blood flow.

It is to create a balanced, low-pressure and centrally supplied pulmonary vascular bed.

Diagnosis

The first step is to map the entire pulmonary circulation.

This requires identifying:

  • all MAPCAs;
  • their origins;
  • their size;
  • areas of stenosis;
  • which lung segments they supply;
  • whether supply is single or dual;
  • and the anatomy of the native pulmonary arteries. (PubMed)

Echocardiography

Echocardiography defines the intracardiac anatomy.

It is particularly useful for demonstrating:

  • Pulmonary Atresia;
  • VSD;
  • right ventricular anatomy;
  • aortic override;
  • native pulmonary artery origins;
  • and ventricular function.

However, echocardiography usually cannot map every MAPCA completely.

CT Angiography

CT angiography is extremely valuable.

It provides detailed three-dimensional information about:

  • number of MAPCAs;
  • their origins from the aorta;
  • course through the thorax;
  • pulmonary artery connections;
  • stenotic segments;
  • and the lung segments supplied. (PubMed)

Three-dimensional reconstruction is particularly useful for surgical planning.

Cardiac Catheterisation

Cardiac catheterisation can provide both anatomical and physiological information.

It may determine:

  • MAPCA pressure;
  • pulmonary artery pressure;
  • degree of stenosis;
  • oxygen saturation;
  • pulmonary blood flow;
  • and suitability for intervention.

It also allows catheter treatment during the same procedure.

Treatment Goals

The ideal long-term objective is:

Right ventricle → Pulmonary arteries → Both lungs

with as much normal pulmonary vascular bed incorporated as possible.

The surgical challenge is to take multiple separate blood vessels arising from the systemic circulation and combine them into a single pulmonary arterial system.

This process is known as:

Unifocalisation

What is Unifocalisation?

Unifocalisation is the surgical process of gathering MAPCAs and native pulmonary arteries into one central pulmonary arterial circulation.

In simple terms:

many separate sources → one reconstructed pulmonary arterial tree.

The MAPCAs are dissected from their systemic origins and connected together with native pulmonary arteries wherever appropriate.

The objective is to create:

  • a central right pulmonary artery;
  • a central left pulmonary artery;
  • and a pulmonary vascular bed capable of receiving blood from the right ventricle. (PubMed)

Why Unifocalisation Matters

If a lung segment has only MAPCA supply, that vessel may represent the only useful pulmonary artery to that portion of lung.

It cannot simply be removed.

It must be recruited into the reconstructed pulmonary circulation.

Modern surgical series strongly support incorporating single-supply MAPCAs where possible. (PubMed)

RV-to-PA Conduit

Once a usable pulmonary arterial tree has been constructed, a connection can be created between:

Right ventricle → Pulmonary arteries

using an:

RV-to-PA conduit.

This restores a more conventional source of pulmonary blood flow.

Closing the VSD

The VSD may then be closed so that right ventricular blood is directed into the reconstructed pulmonary circulation.

However, VSD closure is only safe if the reconstructed pulmonary vascular bed can accept the entire right ventricular output without producing excessive RV pressure.

If the pulmonary vascular bed is too small or resistant, immediate VSD closure may overload the right ventricle.

The decision is therefore based on anatomy and haemodynamic testing rather than simply completing the repair at all costs. (PubMed Central (PMC))

Complete Repair

A complete repair typically involves:

  • unifocalisation of suitable MAPCAs;
  • reconstruction of the pulmonary arteries;
  • RV-to-PA continuity;
  • and VSD closure.

The goal is to establish:

Right ventricle → reconstructed pulmonary arteries → lungs

and:

Left ventricle → aorta → body

thereby converting the circulation towards a conventional two-ventricle arrangement.

Staged Repair

Not every patient is suitable for complete repair in one operation.

Some require staged reconstruction.

This may involve:

  • promoting growth of native pulmonary arteries;
  • creating systemic-to-pulmonary shunts;
  • staged unifocalisation;
  • later RV-to-PA conduit placement;
  • and delayed VSD closure.

The treatment must be individualised because pulmonary anatomy varies so greatly. (PubMed Central (PMC))

Pulmonary Artery Rehabilitation

Another surgical strategy has historically aimed to encourage growth of the native pulmonary arteries before definitive reconstruction.

This can involve:

  • surgical shunts;
  • central pulmonary artery reconstruction;
  • or other methods of increasing flow through the native pulmonary circulation.

Modern practice varies between specialist centres, although recent surgical literature strongly supports unifocalisation particularly for lung segments dependent upon single-supply MAPCAs. (PubMed)

Catheter Intervention

MAPCAs can also be treated in the catheter laboratory.

Procedures can include:

  • balloon angioplasty;
  • stent insertion;
  • coil embolisation;
  • and vascular plug occlusion. (PubMed)

The treatment depends entirely upon what role the individual collateral plays.

Coil Embolisation

Some MAPCAs become unnecessary after surgical reconstruction.

If a lung segment already has adequate native pulmonary arterial supply, continued MAPCA flow may create excessive pulmonary blood flow.

Such collateral vessels can sometimes be occluded using coils or vascular plugs. (PubMed)

This reduces unwanted systemic-to-pulmonary flow.

Why You Cannot Simply Embolise Every MAPCA

A MAPCA may be the only arterial supply to a region of lung.

Closing it without first understanding the pulmonary vascular map could effectively infarct or severely underperfuse that segment.

The fundamental question is therefore:

What does this vessel supply?

before deciding:

Should it be incorporated, enlarged, left alone or occluded?

Theatre and Surgical Considerations

PA/VSD/MAPCAs surgery can be among the most demanding procedures in congenital cardiac surgery.

MAPCAs may lie:

  • posterior to the heart;
  • alongside the oesophagus;
  • behind the bronchi;
  • or across the mediastinum.

Extensive dissection may be required to mobilise each vessel sufficiently for reconstruction.

Cardiopulmonary Bypass

Definitive intracardiac repair and pulmonary arterial reconstruction generally require cardiopulmonary bypass.

The operation may include:

  • VSD closure;
  • RV-to-PA conduit placement;
  • pulmonary artery reconstruction;
  • and unifocalisation.

The precise procedure depends upon the individual anatomy.

Bleeding Risk

MAPCAs arise from the systemic arterial circulation.

They can therefore carry blood at systemic pressure.

Dissection of multiple collateral vessels creates a significant potential for bleeding.

Previous operations and collateral development may make repeat surgery particularly challenging.

Bronchial Relationships

MAPCAs often travel close to the tracheobronchial tree.

Understanding their relationship to:

  • bronchi;
  • oesophagus;
  • descending aorta;
  • and pulmonary arteries

is important during surgical mobilisation.

This is one reason detailed preoperative CT mapping is so valuable.

Anaesthetic Considerations

The anaesthetic challenge is that pulmonary blood flow may be highly heterogeneous.

Changes in:

  • systemic vascular resistance;
  • pulmonary vascular resistance;
  • ventilation;
  • oxygen concentration;
  • and systemic arterial pressure

can alter flow through the collateral circulation.

Because MAPCAs originate from the systemic arterial system, a fall in systemic pressure can significantly reduce pulmonary blood flow through them.

Oxygen Saturation

Oxygen saturation alone gives only a limited picture.

A patient may have:

  • reasonable overall saturation;
  • while some lung segments are overcirculated;
  • and others are profoundly underperfused.

Management therefore depends upon understanding the actual anatomy rather than simply treating the saturation number.

Postoperative Management

Following unifocalisation and reconstruction, important concerns include:

  • right ventricular pressure;
  • pulmonary artery pressure;
  • conduit function;
  • residual MAPCAs;
  • branch pulmonary artery stenosis;
  • pulmonary reperfusion injury;
  • bleeding;
  • and ventricular function.

Reintervention

Reconstructed MAPCAs can later develop:

  • stenosis;
  • obstruction;
  • or occlusion.

Long-term surveillance is therefore essential.

A contemporary series found that most unifocalised MAPCAs remained patent, but a substantial minority developed stenosis or occlusion requiring continued surveillance and sometimes reintervention. (PubMed)

Catheter ballooning, stenting or further surgery may be required.

MAPCAs and Tetralogy of Fallot

MAPCAs can also occur in severe Tetralogy of Fallot where pulmonary blood flow is markedly restricted.

When pulmonary atresia is present with TOF-type anatomy, the condition is often described as:

Tetralogy of Fallot with Pulmonary Atresia and MAPCAs

or within the broader:

PA/VSD/MAPCAs spectrum.

These are among the most complex forms of TOF.

MAPCAs Are Not the Same as a PDA

Both may carry blood from the systemic circulation towards the lungs, but they are anatomically different.

PDA

A PDA is persistence of a normal foetal connection between:

Aorta and pulmonary artery.

MAPCA

A MAPCA is an abnormal systemic collateral artery supplying part of the pulmonary vascular bed.

There may be several MAPCAs, each supplying different lung segments.

MAPCAs Are Not the Same as Bronchial Arteries

Normal bronchial arteries provide a small systemic arterial supply to the airways and supporting lung tissue.

MAPCAs are much larger congenital vessels providing significant blood flow to the pulmonary vascular bed.

They can therefore become an essential component of gas-exchange circulation.

Long-Term Outcome

Modern surgical treatment has dramatically changed the outlook for patients with PA/VSD/MAPCAs.

Many can ultimately achieve two-ventricle repair with a reconstructed pulmonary circulation. (PubMed Central (PMC))

However, long-term follow-up is essential because patients may develop:

  • branch pulmonary artery stenosis;
  • MAPCA stenosis;
  • conduit failure;
  • pulmonary hypertension;
  • right ventricular dysfunction;
  • residual VSD;
  • and need for repeated catheter or surgical intervention.

MAPCAs in One Sentence

MAPCAs are congenital systemic arteries that arise from the aorta or its branches to supply lung tissue when the normal pulmonary arterial circulation is inadequate, particularly in Pulmonary Atresia with VSD.

The central surgical question is:

Which pulmonary segments are being supplied by which vessels, and how can those separate sources be reconstructed into one low-pressure pulmonary circulation?

Key Points

MAPCA stands for:

Major Aortopulmonary Collateral Artery.

They arise from the:

aorta or major systemic arterial branches

and connect to the pulmonary vascular bed. (PubMed)

They are strongly associated with:

Pulmonary Atresia + VSD.

They can also occur with severe Tetralogy of Fallot and other lesions with reduced pulmonary blood flow. (PubMed Central (PMC))

A lung segment may have:

  • native pulmonary artery supply;
  • MAPCA supply;
  • or both.

MAPCAs can cause either:

pulmonary under-circulation

or

pulmonary over-circulation and segmental pulmonary hypertension. (PubMed Central (PMC))

Detailed CT and catheter mapping is essential.

Unifocalisation combines suitable MAPCAs and native pulmonary arteries into a central pulmonary arterial circulation. (PubMed)

Selected unnecessary MAPCAs may be embolised.

MAPCAs supplying a lung segment exclusively must not simply be occluded without providing an alternative pulmonary arterial supply.

Definitive repair may include:

Unifocalisation + RV-to-PA conduit + VSD closure.

Glossary of Terms

Aortopulmonary Collateral

An abnormal systemic artery carrying blood from the aorta or one of its branches towards the pulmonary circulation.

Balloon Angioplasty

Catheter treatment in which a balloon is inflated within a narrowed blood vessel to enlarge it.

Blue Baby

An informal teaching term describing congenital cardiac physiology associated with reduced systemic arterial oxygenation and cyanosis.

Coil Embolisation

A catheter procedure in which metallic coils are placed within a vessel to intentionally occlude it.

Collateral Circulation

An alternative vascular pathway supplying tissue when the normal route is absent, restricted or insufficient.

Dual-Supply MAPCA

A MAPCA supplying a lung segment that also receives blood from the native pulmonary arteries.

MAPCA

Major Aortopulmonary Collateral Artery.

A congenital systemic-to-pulmonary arterial vessel supplying lung tissue.

Native Pulmonary Arteries

The pulmonary arteries arising from the normal embryological pulmonary arterial circulation rather than from systemic collateral vessels.

PA/VSD/MAPCAs

Pulmonary Atresia with Ventricular Septal Defect and Major Aortopulmonary Collateral Arteries.

A complex congenital cardiac condition characterised by absent or severely impaired normal pulmonary outflow and variable alternative pulmonary blood supply.

Pulmonary Atresia

Complete absence of an open connection from the right ventricle through the pulmonary valve into the pulmonary artery.

Pulmonary Hypertension

Abnormally elevated pressure within the pulmonary arterial circulation.

Pulmonary Over-Circulation

Excessive blood flow through the lungs.

Unrestricted MAPCAs can produce regional pulmonary over-circulation.

Pulmonary Under-Circulation

Insufficient blood flow through the pulmonary vascular bed.

This contributes to cyanosis.

RV-to-PA Conduit

A surgically implanted conduit connecting the right ventricle to the pulmonary arteries.

Segmental Pulmonary Hypertension

Pulmonary hypertension affecting particular regions of lung rather than the whole pulmonary vascular bed uniformly.

It can occur when individual lung segments are supplied by unrestricted systemic-pressure MAPCAs.

Single-Supply MAPCA

A MAPCA providing the only arterial blood supply to a particular lung segment.

Stenosis

Abnormal narrowing of a blood vessel or valve.

Systemic Arterial Pressure

The pressure generated within the systemic arterial circulation.

MAPCAs may expose pulmonary vessels to systemic pressure.

Tetralogy of Fallot — TOF

A congenital cardiac defect characterised by VSD, overriding aorta, right ventricular outflow obstruction and right ventricular hypertrophy.

Severe forms with Pulmonary Atresia may be associated with MAPCAs.

Unifocalisation

Surgical reconstruction in which multiple MAPCAs and native pulmonary arterial branches are combined into a unified central pulmonary arterial system.

Ventricular Septal Defect — VSD

An abnormal opening between the right and left ventricles.

A large VSD is a defining feature of PA/VSD/MAPCAs.

Vascular Plug

A catheter-delivered device used to intentionally occlude a blood vessel.