Systemic-Pulmonary Shunt #1–4: Modified BTT Shunt, Surgical Technique, Hemodynamics, and Alternative Configurations

1. Role of the Systemic-to-Pulmonary Artery Shunt

A systemic-to-pulmonary artery shunt provides a controlled source of pulmonary blood flow when native antegrade flow through the right ventricular outflow tract or pulmonary valve is absent or inadequate. The operation remains relevant in selected patients with tetralogy of Fallot, pulmonary atresia with or without ventricular septal defect, pulmonary atresia with intact ventricular septum, complex double-outlet right ventricle, and single-ventricle malformations with restricted pulmonary blood flow.

The original Blalock-Taussig shunt, introduced in 1945, connected a systemic artery—usually the subclavian artery—to a pulmonary artery, establishing the principle that surgically augmenting pulmonary blood flow could relieve severe cyanosis [1]. Contemporary terminology increasingly recognizes the contribution of Vivien Thomas as the Blalock-Taussig-Thomas (BTT) shunt.

The modified BTT shunt replaces the direct arterial anastomosis with an interposition prosthetic conduit, typically expanded polytetrafluoroethylene (PTFE), between the subclavian or innominate artery and a branch pulmonary artery. The modification preserves continuity of the systemic artery and permits greater control over shunt diameter, length, and geometry [2].

Although technically straightforward in concept, a systemic-to-pulmonary shunt creates a circulation in which pulmonary blood flow depends on the interaction among systemic arterial pressure, pulmonary vascular resistance, shunt resistance, and competing sources of pulmonary flow. Consequently, both insufficient flow and excessive flow can become immediately life-threatening.

2. Physiologic Objectives

The principal objective is to establish dependable pulmonary blood flow sufficient to improve systemic arterial oxygenation without producing pulmonary overcirculation.

The physiologic consequences extend beyond arterial saturation. Increased pulmonary blood flow increases pulmonary venous return and therefore atrial and ventricular preload. In selected staged strategies, particularly when ventricular filling has previously been limited by reduced pulmonary flow, this additional preload may support growth or recruitment of downstream cardiac structures. This effect is anatomy-dependent and should not be regarded as a universal indication for shunt placement.

A functioning shunt may therefore accomplish several objectives:

• Increase pulmonary blood flow and systemic oxygen saturation.
• Promote pulmonary arterial growth before later biventricular repair or cavopulmonary palliation.
• Increase pulmonary venous return and ventricular preload where this is physiologically desirable.
• Provide a bridge to definitive repair or the next stage of palliation.

The desired result is not maximal oxygen saturation. Increasing shunt flow also increases ventricular volume load and may reduce systemic perfusion. Optimal palliation therefore represents a balance between pulmonary and systemic circulations.

3. Hemodynamics of the Modified BTT Shunt

The modified BTT shunt connects a systemic artery directly to the pulmonary arterial circulation. Because systemic arterial pressure exceeds pulmonary arterial pressure during both systole and diastole, the conduit generally produces continuous systolic-diastolic pulmonary blood flow.

This continuous flow is advantageous for maintaining pulmonary perfusion and may promote pulmonary arterial growth. However, it also creates diastolic runoff from the systemic arterial circulation into the pulmonary arteries.

The magnitude of shunt flow can be conceptualized as:

Qshunt≈Psystemic−PPARshuntQ_{\text{shunt}} \approx \frac{P_{\text{systemic}}-P_{\text{PA}}}{R_{\text{shunt}}}

where shunt resistance is strongly influenced by conduit diameter and length. Because resistance changes markedly with small alterations in internal diameter, apparently modest differences in graft size can substantially alter pulmonary blood flow.

An oversized shunt may produce excessive pulmonary flow, pulmonary edema, ventricular volume loading, systemic hypoperfusion, and reduction in systemic diastolic pressure. Conversely, an undersized or stenotic shunt produces persistent cyanosis and increases susceptibility to thrombosis.

Contemporary series continue to demonstrate this narrow therapeutic window. In one cohort of 95 children undergoing modified BTT shunting, pulmonary overcirculation occurred in 30%, whereas shunt thrombosis or stenosis occurred in 23% [3]. Other series similarly identify a larger graft-to-body-weight relationship as a marker of adverse outcomes, emphasizing that graft selection cannot be reduced to a single universal size formula [3,4].

4. Surgical Technique for the Modified BTT Shunt

4.1 Exposure and shunt configuration

The operation can be performed through median sternotomy or thoracotomy depending on cardiac anatomy, aortic arch sidedness, anticipated subsequent operations, pulmonary arterial anatomy, and institutional strategy. Contemporary neonatal practice frequently uses sternotomy, particularly when central pulmonary arterial exposure or concomitant intracardiac or pulmonary arterial procedures are required [3].

The systemic inflow is usually derived from the subclavian or innominate artery, and the distal graft is anastomosed to the ipsilateral or central branch pulmonary artery.

The ideal conduit should be:

short, smoothly oriented, free of tension, and resistant to kinking or compression.

A conduit that is too short may place traction on the pulmonary artery or systemic vessel; an excessively long graft can buckle after chest closure or changes in lung volume.

4.2 Proximal anastomosis

After systemic heparinization, the selected systemic artery is controlled. An appropriately sized arteriotomy is created, and the PTFE conduit is implanted end-to-side.

The anastomosis should be broad enough to prevent an inflow stenosis but should not distort the native systemic artery. Particular attention is required when using a small neonatal subclavian artery because an oversized arteriotomy or unfavorable graft angle can compromise native arterial flow.

4.3 Distal pulmonary arterial anastomosis

The branch pulmonary artery is controlled and opened longitudinally. The graft is then sewn end-to-side to the pulmonary artery.

The distal anastomosis is particularly important because distortion at this site may compromise not only the shunt but also future pulmonary arterial architecture. Historical and contemporary studies have documented branch pulmonary arterial stenosis or deformation following systemic-to-pulmonary shunts [5].

Technical objectives include:

1. A sufficiently wide pulmonary arteriotomy.
2. Symmetric incorporation of the pulmonary arterial wall.
3. Avoidance of purse-stringing.
4. Absence of graft tension or torsion.
5. Preservation of unobstructed flow toward both central and distal pulmonary arteries whenever anatomically possible.

Before completing the anastomosis, the graft should be thoroughly de-aired. Flow is then restored gradually while systemic pressure, arterial oxygen saturation, ventricular performance, and pulmonary blood flow are assessed.

5. Completion Assessment and Early Postoperative Physiology

A technically patent shunt should demonstrate continuous flow by Doppler and, when directly accessible, a palpable continuous thrill. Echocardiography should assess the proximal and distal anastomoses, branch pulmonary arteries, ventricular function, atrioventricular valve regurgitation, and alternative sources of pulmonary blood flow.

The immediate postoperative circulation should be evaluated as an integrated physiologic system rather than by oxygen saturation alone.

Excessive shunt flow may present with unexpectedly high saturation, low systemic diastolic pressure, widened pulse pressure, pulmonary edema, rising lactate despite adequate oxygenation, ventricular volume loading, or systemic hypoperfusion.

Restricted shunt flow may produce persistent or abrupt cyanosis, loss of the shunt murmur, increasing lactate, or hemodynamic collapse. Acute thrombosis should be presumed when severe unexplained desaturation occurs until proven otherwise.

Low body weight, particularly below approximately 3 kg in historical database studies, has been associated with increased morbidity and mortality [6]. In the Society of Thoracic Surgeons Congenital Heart Surgery Database analysis of 1,273 neonatal modified BTT shunts, discharge mortality was 7.2%, with particularly high risk among patients with pulmonary atresia with intact ventricular septum and functionally univentricular hearts [6].

Antiplatelet therapy is commonly used once surgical hemostasis permits. In a prospective multicenter observational study of 1,004 infants with systemic-to-pulmonary shunt physiology, aspirin use was associated with substantially lower shunt thrombosis risk, although the nonrandomized study design prevents attribution of definitive causality [7].

6. Modified BTT Shunt Versus RV-to-PA Conduit

The distinction between a systemic arterial shunt and a right ventricle-to-pulmonary artery (RV-PA) conduit is particularly important in the Norwood circulation.

With a modified BTT shunt, pulmonary blood flow continues during diastole, producing systemic arterial runoff. In the reconstructed Norwood circulation, this can lower neo-aortic diastolic pressure and potentially impair coronary perfusion.

An RV-PA conduit instead originates from the systemic right ventricle. Pulmonary flow is therefore predominantly generated by ventricular systole, eliminating the direct systemic arterial-to-pulmonary arterial diastolic runoff characteristic of the modified BTT shunt. This generally produces a higher systemic diastolic pressure and more favorable coronary perfusion pressure. The trade-off is a right ventriculotomy, together with potential conduit stenosis, ventricular scar, dysfunction, and arrhythmogenic substrate.

The Pediatric Heart Network Single Ventricle Reconstruction randomized trial assigned 549 analyzable infants undergoing Norwood palliation to an RV-PA or modified BTT shunt. Transplant-free survival at 12 months was 74% with the RV-PA conduit versus 64% with the modified BTT shunt; however, the RV-PA group experienced more unintended cardiovascular interventions and complications. With longer early follow-up, the transplantation-free survival difference was no longer statistically significant [8].

Thus, the RV-PA conduit should not simply be regarded as a universally superior shunt. Its physiologic advantages and ventricular consequences are specific to the Norwood circulation and must be interpreted within that context.

7. Alternative Systemic-Pulmonary Shunt Configurations

Several configurations preceded or coexist with the modified BTT shunt.

Original BTT shunt. The native subclavian artery is divided and its proximal end anastomosed directly to the pulmonary artery [1]. Although physiologically effective, sacrifice of the native arterial continuity and limitations imposed by vessel size contributed to the development of the modified technique.

Potts shunt. A direct side-to-side connection is created between the descending thoracic aorta and left pulmonary artery. Although historically important for cyanotic congenital heart disease, its uncontrolled flow and difficult takedown substantially limited its role.

Waterston shunt. The ascending aorta is connected side-to-side to the right pulmonary artery. Significant late problems included excessive pulmonary blood flow, preferential perfusion of the right lung, distortion or obstruction of the right pulmonary artery, and impaired growth of the contralateral pulmonary arterial tree [9,10].

Central shunt. A prosthetic conduit connects the ascending aorta to the main or central pulmonary artery. Central shunts remain useful in selected anatomy, particularly when bilateral pulmonary arterial distribution is desired or branch systemic arteries are unsuitable. However, flow control is critical because the high-pressure aortic source can generate substantial pulmonary overcirculation.

These historical operations illustrate a central principle: the success of a systemic-to-pulmonary shunt depends not simply on establishing flow, but on controlling its volume, distribution, geometry, and long-term effect on pulmonary arterial architecture.

8. Key Surgical Principles

• Define the physiologic target before selecting the graft. The objective is adequate—not maximal—pulmonary blood flow.
• Individualize graft diameter and length. Patient size, pulmonary arterial anatomy, systemic pressure, native antegrade flow, and expected pulmonary vascular resistance all matter.
• Prioritize conduit geometry. Avoid tension, redundancy, torsion, compression, and acute angulation.
• Protect the pulmonary artery. A technically patent shunt that distorts a branch pulmonary artery may compromise later repair or cavopulmonary palliation.
• Assess systemic perfusion as carefully as oxygen saturation. High saturation accompanied by low diastolic pressure or rising lactate may indicate excessive pulmonary runoff.
• Treat abrupt postoperative desaturation as possible shunt obstruction or thrombosis until excluded.
• Recognize the shunt as temporary physiology. Surveillance must address patency, pulmonary arterial growth, ventricular loading, and timing of definitive repair or subsequent palliation.

References

1. Blalock A, Taussig HB. The surgical treatment of malformations of the heart: in which there is pulmonary stenosis or pulmonary atresia. JAMA. 1945. doi:10.1001/JAMA.1945.02860200029009.
2. de Leval MR, McKay R, Jones M, Stark J, Macartney FJ. Modified Blalock-Taussig shunt: use of subclavian artery orifice as flow regulator in prosthetic systemic-pulmonary artery shunts. J Thorac Cardiovasc Surg. 1981. doi:10.1016/S0022-5223(19)37668-8. PMID: 6450303.
3. Sasikumar N, Hermuzi A, Fan CS, et al. Outcomes of Blalock-Taussig shunts in current era: a single center experience. Congenit Heart Dis. 2017. doi:10.1111/chd.12516. PMID: 28736841.
4. Dirks V, Prêtre R, Knirsch W, et al. Modified Blalock Taussig shunt: a not-so-simple palliative procedure. Eur J Cardiothorac Surg. 2013;44:1096–1102. doi:10.1093/ejcts/ezt172. PMID: 23539419.
5. Godart F, Qureshi SA, Simha A, et al. Effects of modified and classic Blalock-Taussig shunts on the pulmonary arterial tree. Ann Thorac Surg. 1998. doi:10.1016/S0003-4975(98)00461-5. PMID: 9725394.
6. Petrucci O, O'Brien SM, Jacobs ML, Jacobs JP, Manning PB, Eghtesady P. Risk factors for mortality and morbidity after the neonatal Blalock-Taussig shunt procedure. Ann Thorac Surg. 2011. doi:10.1016/j.athoracsur.2011.02.030.
7. Li JS, Yow E, Berezny KY, et al. Clinical outcomes of palliative surgery including a systemic-to-pulmonary artery shunt in infants with cyanotic congenital heart disease: does aspirin make a difference? Circulation. 2007. doi:10.1161/CIRCULATIONAHA.106.652172. PMID: 17592082.
8. Ohye RG, Sleeper LA, Mahony L, et al. Comparison of shunt types in the Norwood procedure for single-ventricle lesions. N Engl J Med. 2010;362:1980–1992. doi:10.1056/NEJMoa0912461. PMID: 20505177.
9. Levin DC, Fellows KE, Sos TA. Angiographic demonstration of complications resulting from the Waterston procedure. AJR Am J Roentgenol. 1978;131:431–437. doi:10.2214/AJR.131.3.431. PMID: 98985.
10. Tay DJ, Engle MA, Ehlers KH, Levin AR. Early results and late developments of the Waterston anastomosis. Circulation. 1974;50:220–229. doi:10.1161/01.CIR.50.2.220. PMID: 4846629.