Systemic-Pulmonary Shunt #3: BTT Shunt vs RV-PA Shunt

1. Core Concept

In stage I single-ventricle palliation, especially the Norwood operation, pulmonary blood flow must be supplied by a controlled systemic-to-pulmonary pathway. Two major strategies are commonly compared:

1. Modified Blalock-Taussig-Thomas shunt
• A systemic arterial-to-pulmonary arterial shunt.
• Blood flows from the systemic arterial circulation into the pulmonary arteries.
2. Right ventricle-to-pulmonary artery shunt
• A conduit from the systemic right ventricle to the pulmonary arteries.
• Antegrade pulmonary blood flow is predominantly systolic.

The key distinction is not only the anatomic origin of the shunt, but the timing and hemodynamic source of pulmonary blood flow.

The modified BTT shunt supplies pulmonary blood flow during both systole and diastole. The RV-PA shunt reduces systemic diastolic runoff and provides mainly systolic pulmonary blood flow. This difference strongly influences diastolic blood pressure, coronary perfusion, Qp:Qs balance, and early postoperative stability.

2. Modified BTT Shunt: Continuous Systemic-to-Pulmonary Flow

The modified BTT shunt connects the systemic arterial circulation to the pulmonary arteries using a prosthetic graft. In Norwood physiology, the systemic arterial source is usually the reconstructed neo-aorta or one of its branches.

Because systemic arterial pressure exceeds pulmonary arterial pressure during both systole and diastole, flow through the shunt occurs throughout the cardiac cycle.

Physiologic Advantages

The continuous flow pattern may provide stable pulmonary perfusion and may support pulmonary artery growth by exposing the branch pulmonary arteries to persistent forward flow.

Potential advantages include:

• Continuous pulmonary blood flow during systole and diastole
• Avoidance of right ventriculotomy
• No direct incision into the systemic right ventricle
• Potentially favorable pulmonary artery flow exposure during the interstage period

Physiologic Disadvantages

The principal disadvantage is diastolic runoff from the systemic arterial circulation into the pulmonary vascular bed.

In Norwood physiology, the coronary arteries arise from the reconstructed neo-aorta. Therefore, systemic diastolic pressure is critical for coronary perfusion. If pulmonary runoff through the shunt is excessive, neo-aortic diastolic pressure may fall, reducing coronary perfusion pressure and increasing the risk of myocardial ischemia [1].

This creates a vulnerable physiology:

• Systemic arterial blood is diverted into the pulmonary arteries during diastole.
• Neo-aortic diastolic pressure may decrease.
• Coronary perfusion pressure may be compromised.
• Systemic output may become unstable if pulmonary blood flow is excessive.
• Qp:Qs balance may shift toward pulmonary overcirculation.

Clinically, this may present as low diastolic blood pressure, systemic hypoperfusion, metabolic acidosis, impaired myocardial performance, or difficulty maintaining a stable balance between pulmonary and systemic blood flow.

3. RV-PA Shunt: Predominantly Systolic Pulmonary Blood Flow

The RV-PA shunt connects the systemic right ventricle directly to the pulmonary arteries through a conduit.

Because pulmonary blood flow is driven primarily by ventricular contraction, antegrade flow occurs predominantly during systole. Compared with the modified BTT shunt, the RV-PA shunt reduces systemic arterial diastolic runoff.

Physiologic Advantages

The principal advantage is preservation of systemic diastolic pressure.

Because there is less diastolic runoff from the neo-aorta into the pulmonary vascular bed, neo-aortic diastolic pressure is better maintained. This may improve coronary perfusion pressure and early postoperative hemodynamic stability.

Early clinical series consistently showed higher diastolic blood pressure in patients receiving an RV-PA shunt compared with a modified BTT shunt after the Norwood operation [2–4].

Potential advantages include:

• Less systemic diastolic runoff
• Higher diastolic blood pressure
• Better preservation of coronary perfusion pressure
• Less diastolic pulmonary steal
• Potentially more stable early postoperative systemic perfusion
• Reduced risk of excessive pulmonary runoff during diastole

This physiology is especially important in neonates with a systemic right ventricle, where myocardial oxygen supply depends on adequate neo-aortic diastolic pressure.

Physiologic Disadvantages

The trade-off is the requirement for an incision into the systemic right ventricle.

This introduces a different set of risks:

• Right ventricular myocardial injury
• Ventricular dysfunction
• Scar-related arrhythmia
• RV-PA conduit stenosis or obstruction
• Pulmonary artery distortion at the distal conduit insertion
• Need for catheter-based or surgical conduit-related reintervention
• Potential adverse effect on late systemic ventricular function

Thus, the RV-PA shunt improves one part of the circulation—diastolic pressure and coronary perfusion—while introducing ventricular and conduit-related concerns.

4. Coronary Perfusion as the Central Physiologic Difference

The most important physiologic distinction between the two shunts is their effect on neo-aortic diastolic pressure.

Modified BTT Shunt

The modified BTT shunt allows systemic arterial-to-pulmonary arterial flow during diastole.

This may reduce:

• Neo-aortic diastolic pressure
• Coronary perfusion pressure
• Systemic vascular reserve
• Myocardial oxygen supply

This is the classic mechanism of diastolic runoff or pulmonary steal in Norwood physiology.

RV-PA Shunt

The RV-PA shunt provides predominantly systolic antegrade pulmonary blood flow.

This tends to preserve:

• Diastolic blood pressure
• Coronary perfusion pressure
• Systemic output
• Early postoperative hemodynamic reserve

For this reason, the RV-PA shunt is often described as a strategy that protects coronary perfusion at the cost of a right ventriculotomy.

5. Evidence From Hemodynamic Studies

Early observational studies suggested that RV-PA shunts improve postoperative hemodynamics compared with modified BTT shunts.

Mair and colleagues reported higher diastolic blood pressure, a lower Qp:Qs ratio, and improved early survival in patients receiving an RV-PA conduit compared with a modified BTT shunt after the Norwood operation [2]. Pizarro and colleagues similarly reported improved hospital outcome and simplified postoperative management with the RV-PA conduit [3]. Azakie and colleagues also found higher diastolic blood pressure in the RV-PA group, although overall Norwood survival did not differ between shunt groups in that small cohort [4].

However, the hemodynamic advantage is not absolute across all markers. Bradley and colleagues found equivalent postoperative systemic oxygen delivery markers between modified BTT and RV-PA shunts during the early postoperative period [5]. Edwards and colleagues reported higher diastolic blood pressure with the RV-PA conduit, but no significant difference in pulmonary blood flow markers, systemic blood flow markers, or estimated Qp:Qs [6]. A small randomized hemodynamic comparison by Ghanayem and colleagues found higher diastolic blood pressure with RV-PA, but no overall hemodynamic benefit and more variable pulmonary blood flow in the RV-PA group [7].

Therefore, the most consistent physiologic finding is higher diastolic blood pressure with RV-PA shunting, while the effect on global systemic oxygen delivery and overall postoperative physiology is more variable.

6. Survival and Interstage Outcomes

The strongest comparative evidence comes from the Pediatric Heart Network randomized trial, which assigned 549 infants undergoing the Norwood procedure to either a modified BTT shunt or an RV-PA shunt.

At 12 months, transplant-free survival favored the RV-PA shunt:

• RV-PA shunt: 74%
• Modified BTT shunt: 64%

This represented a significant early survival advantage for the RV-PA strategy [8].

However, this benefit came with trade-offs. The RV-PA group required more unintended cardiovascular interventions and had more complications [8]. In addition, when follow-up extended beyond the early endpoint, the survival advantage became less clear, with no statistically significant difference over longer mean follow-up in the same trial report [8].

This evidence supports a practical interpretation:

RV-PA shunting may improve early transplant-free survival after the Norwood operation, but it does not eliminate interstage risk and may increase the burden of reintervention.

7. Late Ventricular Function and Long-Term Trade-Offs

The long-term concern with the RV-PA shunt is the effect of right ventriculotomy on the systemic right ventricle.

Graham and colleagues evaluated longer-term outcomes after Norwood palliation and found no clear survival advantage between shunt types at an average follow-up of 6.8 years. However, patients who had received an RV-PA shunt demonstrated poorer late ventricular function before Fontan completion, and cardiac transplantation was more frequent in the RV-PA group, although the transplant difference did not reach conventional statistical significance [9].

This finding reinforces the central surgical trade-off:

• Modified BTT shunt: avoids ventriculotomy but risks diastolic runoff and coronary steal.
• RV-PA shunt: preserves diastolic pressure early but creates a ventricular incision and potential late ventricular consequences.

The decision is therefore not simply about early survival. It must also consider interstage management, reintervention burden, pulmonary artery anatomy, conduit behavior, and late systemic ventricular function.

8. Pulmonary Artery Growth and Reintervention

Pulmonary artery growth depends on multiple interacting factors:

• Shunt size
• Flow volume
• Flow timing
• Pulmonary vascular resistance
• Branch pulmonary artery anatomy
• Anastomotic geometry
• Distal shunt or conduit insertion site
• Catheter-based or surgical reintervention during the interstage period

The modified BTT shunt provides continuous pulmonary blood flow, which may support pulmonary artery growth, but excessive flow can worsen pulmonary overcirculation and systemic steal.

The RV-PA shunt provides predominantly systolic pulmonary blood flow and may better preserve systemic diastolic pressure, but the conduit can cause stenosis, branch pulmonary artery distortion, or reintervention at the distal anastomosis.

Current comparative studies emphasize survival and early hemodynamics more than detailed pulmonary artery growth patterns. Therefore, pulmonary artery development should not be attributed to shunt type alone. It reflects the interaction between surgical geometry, flow distribution, patient-specific pulmonary vascular physiology, and interstage surveillance.

9. Practical Surgical Interpretation

The choice between a modified BTT shunt and an RV-PA shunt is a balance between vascular physiology and ventricular preservation.

Modified BTT Shunt

Best understood as:

Continuous pulmonary blood flow without a ventricular incision, but with the risk of diastolic runoff, reduced neo-aortic diastolic pressure, and compromised coronary perfusion.

RV-PA Shunt

Best understood as:

Predominantly systolic pulmonary blood flow with better preservation of diastolic pressure and coronary perfusion, but with the cost of a right ventricular incision and conduit-related complications.

Neither strategy is physiologically perfect. Each solves one problem while creating another.

10. Clinical Summary

The modified BTT shunt and RV-PA shunt provide pulmonary blood flow through fundamentally different mechanisms.

The modified BTT shunt supplies continuous systemic arterial-to-pulmonary arterial flow. This may support pulmonary artery flow exposure and avoids ventriculotomy, but diastolic runoff can lower neo-aortic diastolic pressure and compromise coronary perfusion.

The RV-PA shunt supplies predominantly systolic pulmonary blood flow. This better preserves diastolic pressure and may improve early postoperative hemodynamics and 12-month transplant-free survival, but it requires a right ventricular incision and is associated with more unintended interventions, conduit-related complications, and concern for impaired late ventricular function.

The central teaching point is:

The shunt is not merely a tube for pulmonary blood flow. It defines the entire circulatory physiology after stage I palliation.

The surgical decision is therefore a physiologic trade-off:

continuous pulmonary blood flow and ventricular preservation versus improved diastolic pressure and coronary perfusion at the cost of ventriculotomy.

References

[1] Ohye RG, Ludomirsky A, Devaney EJ, Bove EL. Comparison of right ventricle to pulmonary artery conduit and modified Blalock-Taussig shunt hemodynamics after the Norwood operation. Ann Thorac Surg. 2004. doi:10.1016/S0003-4975(03)01386-9.

[2] Mair R, Tulzer G, Sames E, Gitter R, Lechner E, Steiner J, et al. Right ventricular to pulmonary artery conduit instead of modified Blalock-Taussig shunt improves postoperative hemodynamics in newborns after the Norwood operation. J Thorac Cardiovasc Surg. 2003. doi:10.1016/S0022-5223(03)00389-1.

[3] Pizarro C, Malec E, Maher KO, Januszewska K, Gidding SS, Murdison KA, et al. Right ventricle to pulmonary artery conduit improves outcome after stage I Norwood for hypoplastic left heart syndrome. Circulation. 2003. doi:10.1161/01.cir.0000087390.94142.1d.

[4] Azakie A, Martínez D, Sapru A, Fineman JR, Teitel DF, Karl TR. Impact of right ventricle to pulmonary artery conduit on outcome of the modified Norwood procedure. Ann Thorac Surg. 2004. doi:10.1016/J.ATHORACSUR.2003.10.002.

[5] Bradley SM, Simsic JM, McQuinn TC, Habib DM, Shirali GS, Atz AM. Hemodynamic status after the Norwood procedure: a comparison of right ventricle-to-pulmonary artery connection versus modified Blalock-Taussig shunt. Ann Thorac Surg. 2004. doi:10.1016/J.ATHORACSUR.2004.04.016.

[6] Edwards L, Morris K, Siddiqui A, Harrington D, Barron D, Brawn W. Norwood procedure for hypoplastic left heart syndrome: BT shunt or RV-PA conduit? Arch Dis Child Fetal Neonatal Ed. 2006. doi:10.1136/adc.2006.094664.

[7] Ghanayem N, Jaquiss R, Cava J, Frommelt P, Mussatto K, Hoffman G, Tweddell J. Right ventricle-to-pulmonary artery conduit versus Blalock-Taussig shunt: a hemodynamic comparison. Ann Thorac Surg. 2006. doi:10.1016/J.ATHORACSUR.2006.05.103.

[8] Ohye RG, Sleeper LA, Mahony L, Newburger JW, Pearson GD, Lu M, et al. Comparison of shunt types in the Norwood procedure for single-ventricle lesions. N Engl J Med. 2010. doi:10.1056/NEJMoa0912461.

[9] Graham EM, Zyblewski SC, Phillips JW, Shirali GS, Bradley SM, Forbus GA, Bandisode VM, Atz AM. Comparison of Norwood shunt types: do the outcomes differ 6 years later? Ann Thorac Surg. 2010. doi:10.1016/j.athoracsur.2010.03.078.