Left Modified Blalock-Taussig Shunt (LMBTS) in VSD with Pulmonary Atresia
The left modified Blalock-Taussig shunt (LMBTS) is a palliative surgical procedure that creates a systemic-to-pulmonary artery connection using a synthetic conduit (typically Gore-Tex) between the left subclavian artery and the left pulmonary artery to provide pulmonary blood flow in patients with pulmonary atresia and ventricular septal defect who lack adequate native pulmonary circulation. 1
Purpose and Mechanism
The LMBTS serves as a bridge to definitive repair in patients with PA-VSD by:
- Establishing reliable pulmonary blood flow when there is no direct communication between the right ventricle and pulmonary arterial tree 2
- Promoting pulmonary artery growth to enable future biventricular repair, with significant increases in pulmonary artery z-scores documented after shunt placement 1
- Improving oxygen saturation in severely cyanotic patients, typically raising saturations from the 60-70% range into the 80s 3
Clinical Context and Patient Selection
When LMBTS is Indicated
High-risk neonates with PA-VSD should be considered for initial palliation rather than primary complete repair, particularly those with: 1
- Low birth weight (<2.5 kg) or prematurity 1
- Small, discontinuous, or hypoplastic pulmonary arteries 1, 4
- Major genetic syndromes or extracardiac anomalies 1
- Serious comorbidities (intracranial hemorrhage, sepsis, end-organ compromise) 1
Transcatheter vs. Surgical Palliation
Transcatheter interventions (ductal stenting or RVOT stenting) are now preferred over surgical shunts like LMBTS in high-risk symptomatic neonates at experienced centers (Class IIb, LOE B-NR). 1, 5 However, when transcatheter approaches fail or are not feasible, the modified Blalock-Taussig shunt remains a reliable surgical option.
Outcomes and Effectiveness
Mortality and Survival
- Operative mortality for modified Blalock-Taussig shunts ranges from 3-4% in standard-risk patients 1
- In PA-VSD specifically, hospital mortality is approximately 14%, higher than in tetralogy of Fallot (0%) but reflecting the more complex anatomy 6
- Overall survival to next surgery (shunt takedown) is 86.1% when accounting for both in-hospital (8.7%) and interstage mortality (5.1%) 6
Pulmonary Artery Growth
The LMBTS effectively promotes bilateral pulmonary artery growth, not just on the ipsilateral side: 7
- Combined pulmonary artery area index increases from 121 mm²/m² pre-shunt to 167 mm²/m² post-shunt in PA-VSD patients (p<0.002) 7
- No significant difference exists between left-sided and right-sided shunts in promoting pulmonary artery development 7
- Growth occurs in both the origin and distal pulmonary arteries equally 7
However, patients with initially diminutive pulmonary arteries (<2-3mm) may show inadequate growth despite shunting, and these patients may require additional interventions such as early palliative RVOT enlargement. 4
Comparison with Antegrade Flow
The presence of antegrade pulmonary blood flow (as in extreme tetralogy of Fallot) does not provide additional benefit over LMBTS alone in PA-VSD: 8
- Pulmonary artery growth (delta Nakata index) is similar whether antegrade flow is present or absent (112±102 vs. 107±66 mm²/m², p=0.89) 8
- Shunt thrombosis rates are comparable (15% with antegrade flow vs. 10% without, p=0.63) 8
Complications and Risk Factors
Shunt-Related Complications
Shunt complications occur in approximately 18% of patients and include: 6
- Acute thrombosis (5%) 6
- Shunt stenosis (2%) 6
- Overshunting causing pulmonary overcirculation (5%) 6
- Pulmonary artery stenosis at anastomotic sites (7%) 6
Critical Risk Factors for Poor Outcomes
Multivariate analysis identifies two key predictors of worse shunt-dependent survival: 6
- Shunt complication occurring in a univentricular circulation (HR 4.10,95% CI 1.05-17.43, p=0.01) 6
- Increased shunt size-to-weight ratio (HR 2.72,95% CI 0.80-9.18, p=0.04) 6
In PA-VSD specifically, shunt thrombosis is a negative outcome predictor when unifocalization procedures are required (p=0.05). 6
Technical Considerations
Shunt Sizing
Smaller shunt diameters (≤3.5 mm) are associated with: 9
- Earlier progression to complete repair (p=0.006) 9
- Increased interstage morbidity (HR 4.5,95% CI 1.1-18.5, p=0.039) 9
- No difference in mortality or ability to achieve complete repair 9
This creates a clinical tension: smaller shunts accelerate definitive repair but increase complications, while larger shunts may delay repair without mortality benefit.
Surgical Backup Requirements
Immediate surgical backup must be available during any transcatheter palliation attempt, as procedural failure occurs in approximately 15% of cases and may necessitate emergency surgical shunt placement. 1
Long-Term Strategy
The LMBTS is a temporizing measure, not definitive treatment: 1, 9
- Complete repair should be performed between 3-6 months of age in stable patients to optimize long-term outcomes 1
- 85% of patients with PA-VSD managed with systemic-to-pulmonary shunts achieve complete repair at a median age of 1 year (IQR 8-20 months) 9
- Long-term survival after staged repair is 81% at 10 years 9
Common Pitfalls
- Do not delay intervention in symptomatic neonates; while prostaglandin E1 can temporize, timely palliation or repair is essential 1
- Avoid oversizing the shunt, as increased shunt size-to-weight ratio compromises shunt-dependent survival 6
- Recognize that initially small pulmonary arteries may not grow adequately despite shunting and may require additional RVOT procedures 4
- In adult patients with chronic shunt occlusion, percutaneous interventions (rotational thrombectomy and stenting) can successfully restore patency when surgical risk is prohibitive 3