Patent Ductus Arteriosus #1: Pathophysiology

1. Overview

A patent ductus arteriosus (PDA) is persistent postnatal communication between the proximal descending aorta and the pulmonary artery, usually near the origin of the left pulmonary artery. The physiologic consequence is determined less by the mere presence of the ductus than by its effective diameter, length and morphology, the pressure difference between the systemic and pulmonary circulations, and the relative systemic and pulmonary vascular resistances.[1]

In the normal fetus, the ductus arteriosus is essential. High fetal pulmonary vascular resistance (PVR) directs much of the right ventricular output from the pulmonary artery through the ductus into the descending aorta. After birth, lung expansion, increased arterial oxygen tension, and reduction in circulating prostaglandins promote ductal constriction and ultimately anatomical closure.[2] Persistence of ductal patency after the normal transitional period establishes an abnormal systemic-to-pulmonary connection.

The characteristic postnatal physiology of a significant PDA is therefore:

Aorta → PDA → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta

The recirculated volume increases pulmonary blood flow (Qp) without directly increasing effective systemic flow (Qs). The principal downstream consequences are pulmonary overcirculation and left atrial/left ventricular volume loading.[1,3]

2. Transition From Fetal to Postnatal Circulation

The direction of ductal flow changes dramatically at birth.

During fetal life, PVR is high and systemic vascular resistance is relatively low because the placenta represents a low-resistance vascular bed. Ductal flow is therefore predominantly right-to-left, from the pulmonary artery into the descending aorta.

After delivery, several events occur almost simultaneously:

• Lung expansion markedly decreases PVR.
• Removal of the placenta increases systemic vascular resistance.
• Arterial oxygen tension rises.
• Prostaglandin concentrations fall.
• Ductal smooth muscle constricts.[2]

When the ductus fails to close, the new pressure and resistance relationship favors flow from the higher-pressure systemic circulation into the pulmonary circulation. The shunt consequently becomes predominantly left-to-right.[1,2]

Importantly, the magnitude of the shunt evolves over the first days and weeks of life as PVR continues to fall. A large PDA may therefore produce relatively modest shunting immediately after birth but substantial pulmonary overcirculation once pulmonary vascular resistance declines.

3. Determinants of Left-to-Right Shunting

The magnitude of PDA flow is governed principally by the pressure gradient across the ductus and its resistance to flow. In simplified physiologic terms:

Ductal flow ≈ systemic-to-pulmonary pressure gradient / ductal resistance

Ductal resistance is strongly influenced by luminal diameter. Consequently, a small restrictive ductus may generate high Doppler velocity yet transmit little total flow, whereas a large nonrestrictive ductus may produce substantial Qp despite a relatively low instantaneous velocity.

3.1 Ductal size and morphology

A small PDA limits flow anatomically. A large PDA offers little resistance and allows pulmonary artery pressure to approach systemic pressure, particularly during systole. Ductal length, configuration, and constriction at the pulmonary end further influence the effective resistance.[1]

3.2 Pulmonary vascular resistance

Declining PVR after birth increases the pressure gradient favoring systemic-to-pulmonary flow and thereby augments the shunt. Conversely, severe pulmonary vascular disease may progressively reduce left-to-right flow and eventually create bidirectional or right-to-left ductal flow.

3.3 Systemic vascular resistance

Higher systemic arterial pressure and systemic vascular resistance tend to favor increased left-to-right flow. Changes in systemic and pulmonary resistances therefore dynamically alter the Qp:Qs relationship.

4. Pulmonary Overcirculation

Blood entering the pulmonary artery through the PDA is added to normal right ventricular output. Thus:

Pulmonary blood flow = effective systemic venous return + ductal recirculation

When the PDA is hemodynamically significant, Qp may substantially exceed Qs. The pulmonary vascular bed is exposed to increased blood flow throughout the cardiac cycle.

Unlike most intracardiac shunts, ductal flow may persist during both systole and diastole because aortic pressure normally exceeds pulmonary arterial pressure throughout the cycle. This continuous runoff contributes to the classic continuous murmur of a restrictive PDA and, with a large ductus, may lower systemic diastolic pressure and produce bounding peripheral pulses and widened pulse pressure.[1,3]

Pulmonary overcirculation increases pulmonary venous return and may elevate pulmonary capillary hydrostatic pressure. Clinically important shunts can therefore produce:

• tachypnea and increased work of breathing;
• pulmonary edema or interstitial congestion;
• feeding intolerance and poor growth in infants;
• recurrent respiratory symptoms;
• exercise intolerance in older patients;
• heart failure when compensatory mechanisms are exceeded.[1,3,4]

The interaction is particularly important in premature infants, in whom lung disease, myocardial immaturity, ventilator dependence, and altered vascular regulation can amplify the consequences of ductal shunting.[3,4]

5. Left Atrial and Left Ventricular Volume Overload

A central feature of PDA physiology is that the recirculated pulmonary blood returns through the pulmonary veins to the left atrium and left ventricle.

This distinguishes PDA physiologically from an atrial-level shunt, in which the dominant volume load falls on the right heart.

With a substantial PDA:

1. Blood is ejected from the LV into the aorta.
2. A fraction passes through the PDA back into the pulmonary circulation.
3. That blood traverses the lungs.
4. It returns to the LA through the pulmonary veins.
5. The LV must eject both effective systemic output and the recirculated ductal volume.

The result is progressive LA and LV dilation, increased LV stroke volume, and eccentric volume loading.[1,3]

The LV can initially compensate through increased preload and stroke volume. This compensation may maintain apparently adequate systemic arterial pressure despite substantial recirculation. However, increased total LV output should not be confused with increased effective systemic blood flow; a significant proportion may simply be cycling repeatedly through the pulmonary circulation.

Echocardiographic LA and LV enlargement therefore provide important evidence that a PDA is physiologically consequential rather than merely anatomically present.

6. Systemic Runoff and the Concept of Ductal Steal

A large PDA provides a low-resistance pathway from the systemic arterial circulation into the pulmonary vascular bed, including during diastole. This may produce diastolic runoff from the descending aorta.

In neonates—particularly premature infants—marked runoff may be associated with reduced diastolic flow or flow reversal in the descending aorta and systemic arteries. This phenomenon is often termed ductal steal.[3]

The physiologic concern is that excessive pulmonary flow occurs partly at the expense of systemic organ perfusion. Cerebral, coronary, mesenteric, and renal perfusion may therefore be affected in vulnerable infants, although the causal relationship between PDA and specific neonatal morbidities is complex and remains incompletely established.[2,3]

This distinction is important: association of a PDA with adverse neonatal outcomes does not necessarily demonstrate that routine closure improves those outcomes. Contemporary management of PDA in extremely premature infants consequently remains substantially more controversial than management of a hemodynamically important PDA in an older infant or child.[9,10]

7. Pulmonary Arterial Pressure and Pulmonary Vascular Disease

Pulmonary arterial hypertension in PDA can arise through two related mechanisms.

Increased pulmonary flow

In a moderate or large PDA, pulmonary artery flow is increased. When the ductus is relatively nonrestrictive, systemic arterial pressure can also be transmitted directly into the pulmonary artery.

Progressive pulmonary vascular remodeling

Prolonged exposure to excessive pulmonary flow and pressure can produce pulmonary arteriolar medial hypertrophy, intimal proliferation, fibrosis, and progressive elevation of PVR.[8]

As PVR rises, the left-to-right gradient across the PDA decreases. A seemingly paradoxical reduction in Qp may therefore represent progressive pulmonary vascular disease rather than improvement.

When PVR approaches systemic vascular resistance, ductal flow becomes bidirectional. If PVR exceeds systemic resistance, right-to-left flow may predominate, producing Eisenmenger physiology.[8]

Because the PDA usually joins the descending aorta distal to the origin of the left subclavian artery, advanced right-to-left ductal shunting may produce differential cyanosis: relatively preserved oxygen saturation in the right upper extremity with lower saturation in the lower extremities.

Established irreversible pulmonary vascular disease fundamentally changes management, because closure of the ductus may remove a necessary pressure-relief pathway for the right ventricle and can be contraindicated.[6,8]

8. Echocardiographic Assessment of Hemodynamic Significance

The question is not simply whether a PDA is present, but whether it imposes a meaningful hemodynamic burden.

Echocardiography should evaluate:

• ductal diameter and morphology;
• direction and pattern of ductal flow;
• estimated pulmonary artery pressure;
• left atrial size;
• LV end-diastolic size and volume loading;
• ventricular systolic function;
• pulmonary artery dimensions;
• diastolic flow in the descending aorta when relevant;
• associated congenital heart disease.

A small restrictive PDA generally demonstrates high-velocity continuous left-to-right flow with little chamber enlargement. In contrast, a large PDA may show lower velocity because pulmonary artery pressure is elevated and the systemic-to-pulmonary pressure gradient is reduced.[1]

Thus, Doppler velocity alone should not be used as a surrogate for shunt magnitude.

When pulmonary hypertension is substantial or operability is uncertain, cardiac catheterization may be necessary to define pulmonary artery pressures, Qp:Qs, pulmonary vascular resistance, and—when appropriate—the pulmonary vascular response to changes in loading or vasodilator conditions.[5,6]

9. Rationale for PDA Closure

The fundamental purpose of PDA closure is to eliminate pathological systemic-to-pulmonary recirculation.

Closure immediately removes the abnormal pathway:

Aorta → PDA → PA

and consequently reduces pulmonary blood flow and pulmonary venous return. Over time, LA and LV volume loading generally regress.

In infants and children beyond the premature neonatal setting, closure is well established when the PDA produces symptoms, pulmonary overcirculation, or left-heart enlargement attributable to the shunt.[5] Adult congenital heart disease guidelines similarly support closure when there is significant left-to-right shunting with LA/LV enlargement and pulmonary vascular physiology remains suitable for intervention.[6]

A very small PDA without symptoms, chamber enlargement, or other hemodynamic consequences represents a different clinical situation. The historical practice of closing essentially all audible ducts partly reflected concern regarding infective endarteritis; contemporary decision-making is more individualized for truly trivial or incidentally detected ducts.[1,5]

When intervention is indicated, transcatheter closure is the predominant approach for most anatomically suitable patients, whereas surgery remains important when catheter closure is unsuitable, unavailable, or associated with unacceptable anatomic risk.[7] Surgical closure is commonly achieved through a left thoracic approach with direct ligation or division of the ductus, although contemporary practice varies according to patient size and clinical circumstances.

10. Physiology After Closure

Eliminating the ductal shunt produces an abrupt loading change.

Preload falls because pulmonary recirculation and pulmonary venous return decrease. At the same time, the LV no longer ejects into the low-resistance parallel pathway represented by the PDA, effectively increasing systemic afterload.

Consequently, transient reduction in LV fractional shortening or ejection fraction may occur after closure, particularly after elimination of a large shunt. This usually represents altered loading conditions rather than intrinsic myocardial injury.

Pulmonary arterial flow and pressure decrease when pulmonary vascular disease is reversible, while LA and LV dimensions remodel over subsequent weeks to months.

The key physiologic endpoint is therefore not simply anatomical ductal occlusion, but restoration of a circulation in which:

Qp approaches Qs, pulmonary overcirculation resolves, and LV output is directed primarily toward effective systemic perfusion.

Key Clinical Principles

• The postnatal PDA is principally a systemic-to-pulmonary shunt. Falling PVR after birth establishes predominant aorta-to-pulmonary artery flow.
• Hemodynamic significance depends on flow, not merely ductal diameter. Ductal anatomy, systemic and pulmonary pressures, and vascular resistances all determine shunt magnitude.
• The dominant cardiac volume load is left-sided. Increased pulmonary venous return produces LA and LV dilation.
• A large PDA may compromise systemic perfusion through diastolic runoff, particularly in premature infants.
• Falling ductal flow is not always reassuring. Rising PVR may reduce Qp as pulmonary vascular disease progresses.
• Closure is most clearly indicated when the PDA causes symptoms, pulmonary overcirculation, or left-heart volume overload.
• Advanced irreversible pulmonary vascular disease requires specific operability assessment before closure.
• Premature neonatal PDA is a distinct clinical problem. Although significant ductal shunting can be physiologically important, the optimal threshold and timing for intervention remain areas of ongoing investigation.

Figure. Hemodynamic consequences of PDA

After birth, falling pulmonary vascular resistance establishes a predominant left-to-right shunt from the aorta through the PDA into the pulmonary artery. Increased pulmonary blood flow returns through the pulmonary veins, producing left atrial and left ventricular volume overload. Closure interrupts this recirculating pathway and restores a more balanced relationship between pulmonary and systemic blood flow.

References

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2. Clyman RI. Mechanisms regulating the ductus arteriosus. Neonatology. 2006. doi:10.1159/000092870. PMID: 16770073.
3. Giliberti P, De Leonibus C, Giordano L, Giliberti P. The physiopathology of the patent ductus arteriosus. J Matern Fetal Neonatal Med. 2009. doi:10.1080/14767050903198215. PMID: 19718587.
4. Eilers L, Kyle W, Allen HD, Qureshi A. Patent ductus arteriosus. Pediatr Rev. 2021. doi:10.1542/pir.2020-004564. PMID: 34725225.
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6. Stout KK, Daniels CJ, Aboulhosn JA, et al. 2018 AHA/ACC Guideline for the Management of Adults With Congenital Heart Disease. Circulation. 2019. doi:10.1161/CIR.0000000000000603. PMID: 30586767.
7. Baruteau AE, Hascoët S, Baruteau J, et al. Transcatheter closure of patent ductus arteriosus: past, present and future. Arch Cardiovasc Dis. 2014. doi:10.1016/j.acvd.2014.01.008. PMID: 24560920.
8. Vongpatanasin W, Brickner ME, Hillis LD, Lange RA. The Eisenmenger syndrome in adults. Ann Intern Med. 1998. doi:10.7326/0003-4819-128-9-199805010-00008. PMID: 9556469.
9. Baruteau AE, Méot M, Benbrik N, et al. Device closure of hemodynamically significant patent ductus arteriosus in premature infants. JACC Adv. 2024. doi:10.1016/j.jacadv.2024.101211. PMID: 39263415.
10. Seyam O, Frishman WH, Aronow WS. Contemporary management strategies for patent ductus arteriosus. Cardiol Rev. 2026. doi:10.1097/CRD.0000000000001204. PMID: 41680985.