Blalock-Taussig shunt
Synonyms and keywords: Blalock Taussig Procedure, Procedure, Blalock-Taussig, Subclavian Pulmonary Artery Shunt, Blue Baby Operations, Modified Blalock-Taussig Procedure

# Overview
The Blalock-Taussig shunt is a palliative surgical procedure for cyanotic heart defects. This procedure is used to deviate blood flow to the lungs from systemic circulation to relieve cyanosis while a definitive corrective surgery can be performed at a later time. A connection is usually made between the subclavian artery and pulmonary artery. Usually, two types of shunts are used: classic or original Blalock-Taussig shunt and modified Blalock Taussig shunt. The classic/original BT shunt has been modified into modified Blalock-Taussig Shunt (mBTS). In this modified procedure, a graft from an innominate artery or subclavian artery is placed to the corresponding pulmonary artery. This type of BT shunt has superior prognostic value over the classical shunt leading to a greater rate of shunt patency in 3-5 years as compared to the classic or original BT Shunt.

# Historical Perspective

## Classic or Original Shunt
- The classic or original BT shunt procedure was named after a surgeon in Baltimore, Alfred Blalock (18991964), and a cardiologist in Baltimore/Boston, Helen B. Taussig (18981986), who along with Blalock's African American laboratory technician Vivien Thomas (19101985), developed and described the procedure.
- Taussig noticed that the children with cyanosis along with congenital heart disease accompanied by patent ductus arteriosus have a longer lifetime than those without PDA.
- In 1943, Dr. Taussig approached Blalock and Thomas in their Hopkins laboratory to work on this shunt as it was hypothesized that a shunt mimicking PDA can relieve the cyanosis and improve oxygenation in congential cyanotic diseases.

## Modified Blalock-Taussig Shunt
- In 1962, a modified method of inserting a shunt was developed by Klinner using teflon as a prosthetic graft material between the subclavian artery and the pulmonary artery in order to prevent scarring of the subclavian artery.

# Classification

## Classic/Original Blalock-Taussig Shunt
- For classic Blalock-Taussig shunt, an end-to-end anastomosis opposite to the aortic arch is performed between the subclavian and pulmonary arteries to minimize the kinking of subclavian artery as it crosses aortic prominence.

## Modified Blalock Taussig Shunt
- In 1962, it was first described and performed by Klinner et al.
- It is more commonly used procedure and holds a better prognosis compared to the classic procedure.
- An Interposition PTFE or Gore-Tex graft is placed between the subclavian artery and the pulmonary artery. As a result, there's no scarring of subclavian artery.

# Goals of BT shunt
- The ultimate endpoint goals of BT shunt are as follows:
To stabilize the preload, hence preventing hemodilution.
To maintain the systemic vascular resistance (afterload) hence, adequate diastolic pressure to maintain coronary perfusion.
Increase the blood flow to the pulmonary vasculature in a supervised manner.
- To stabilize the preload, hence preventing hemodilution.
- To maintain the systemic vascular resistance (afterload) hence, adequate diastolic pressure to maintain coronary perfusion.
- Increase the blood flow to the pulmonary vasculature in a supervised manner.

# Indications
- Following is a list of indications for Blalock-Taussig shunt:
Tricuspid atresia
Tetralogy of Fallot
Ebstein's Anomaly
Hypoplastic Left Heart syndrome
Pulmonary atresia
Inadequate pulmonary arteries
- Tricuspid atresia
- Tetralogy of Fallot
- Ebstein's Anomaly
- Hypoplastic Left Heart syndrome
- Pulmonary atresia
- Inadequate pulmonary arteries

# Risk Factors
- Following multiple risk factors have been identified to be associated with high morbidity and mortality in neonates:
Sternal approach for MBTS construction
Univentricular heart
Complicated surgery (CABG)
Weight <3 kg
Pre-operative ventilation support
Pre-operative acidosis and shock
Use of Innominate artery-PA shunt
Diagnosis of Ebstein's Anomaly
- Sternal approach for MBTS construction
- Univentricular heart
- Complicated surgery (CABG)
- Weight <3 kg
- Pre-operative ventilation support
- Pre-operative acidosis and shock
- Use of Innominate artery-PA shunt
- Diagnosis of Ebstein's Anomaly

# Preparation for BT Shunt
- To perform a BT shunt following pre-operative preparation is done:
Preoperative psychological counseling of the patient
Preoperative fasting
Preoperative medications
- Preoperative psychological counseling of the patient
- Preoperative fasting
- Preoperative medications
- Anesthesia induction is based on the following patient and surgery-related factors:

## Patient-Related Factors
- Diagnosis of a relative congenital cardiac anomaly
- Age of the patient
- Weight <3 kg leads to a high mortality and morbidity
- Premature infants
- Any non-cardiac coexisting congenital anomaly such as:
Tracheoesophageal fistula
Cleft lip and palate
Renal anomalies
Skeletal defects
- Tracheoesophageal fistula
- Cleft lip and palate
- Renal anomalies
- Skeletal defects
- Atlantoaxial instability or subluxation in Down syndrome should be considered while managing the airway.
- Fatigue, and/or dyspnea on feeding or any other factor showing exercise intolerance.
- Use of medications that can lead to an increase in operative mortality such as:
Aspirin
Warfarin
Anti-depressants
Diuretics
Angiotensin converting enzyme inhibitors (ACEI)
Antiarrhythmics
- Aspirin
- Warfarin
- Anti-depressants
- Diuretics
- Angiotensin converting enzyme inhibitors (ACEI)
- Antiarrhythmics
- Congenital conditions such as subglottic stenosis that can lead to difficulty in gaining access for intubation.
- Optimal right ventricular function

## Factors Related to Surgical Procedure
- Complex lesions lead to high overall and in-hospital mortality.
- Duration of the surgery
- Cardiovascular anomalies that can lead to complexity of the procedure such as aortic arch anomaly or presence of aberrant subclavian artery.
- Injury to the recurrent laryngeal nerve or phrenic nerve during the procedure.
- Nature and approach of the cardiac surgery e.g., cardiopulmonary bypass, sternotomy or thoracotomy.

## Approach
- Left lateral position is used for the right-sided modified Blalock-Taussig shunt (mBTS), and the right lateral position is used for left-sided mBTS.
- The approach used is usually thoracotomy.
- Sternotomy is performed for the central shunts.

# Complications
- The immediate post-operative complications include:
Development of chylothorax, chylopericardium and chylous ascites
Phrenic nerve resection leading to diaphragmatic paralysis
Anemia
Congestive heart failure resulting from excessive blood flow
Modified PTFE grafts can result in the formation of seroma or shunt occlusion due to hyperplasia of neo-intima
Stenosis of innominate artery
Shunt infection resulting in sepsis
- Development of chylothorax, chylopericardium and chylous ascites
- Phrenic nerve resection leading to diaphragmatic paralysis
- Anemia
- Congestive heart failure resulting from excessive blood flow
- Modified PTFE grafts can result in the formation of seroma or shunt occlusion due to hyperplasia of neo-intima
- Stenosis of innominate artery
- Shunt infection resulting in sepsis

## Management of Complications

### Shunt failure
- Failure of the shunt is the most immediate risk during the post-operative period which can lead to a drop in oxygen saturation secondary due to shunt thrombosis or kinking.
- Shunt failure is a surgical emergency and should be managed as follows:
Anticoagulation should be started promptly with heparin. It should be administered again if there is less risk of bleeding usually 4 hours post-op (when the drainage of the chest shows <3ml/kg/h and aPTT is <60s). Heparin induced thrombocytopenia can occur and should be managed accordingly.
Patient is put on aspirin which is usually started at 3-5 mg/kg (max. dose 75mg) OD.
Heparin should be continued until there is a second dose of aspirin.
- Anticoagulation should be started promptly with heparin. It should be administered again if there is less risk of bleeding usually 4 hours post-op (when the drainage of the chest shows <3ml/kg/h and aPTT is <60s). Heparin induced thrombocytopenia can occur and should be managed accordingly.
- Patient is put on aspirin which is usually started at 3-5 mg/kg (max. dose 75mg) OD.
- Heparin should be continued until there is a second dose of aspirin.

### Blockage of BT shunt
- Blockage or shunt thrombosis is another surgical emergency with a reported incidence of 12%.
- If there's any recent onset murmur or a significant drop in oxygen saturation, it must be investigated.
- Operative management is usually required and the shunt is usually repaired or replaced if necessary.

## High pulmonary blood flow
- The appropriate size of BT shunt is very important to prevent the long term complications regarding high pulmonary blood flow or high oxygen saturation.
- Chest X Ray usually shows edematous lungs with low mixed venous saturation, rising lactate levels or signs of right heart failure, systemic diastolic pressure may be low, persistent metabolic acidosis leading to "pink patient" and pulmonary hemorrhage.
- High FiO2
- PDA still open leading to excessive blood delivery to the lungs
- Large shunt diameter
Following treatment is instituted:
- For mild to moderate symptoms, fluid resuscitation and diuretics are given. Permissive hypercapnia and lowering of FiO2 is done.
- For severe symptoms, systemic vascular resistance and pulmonary vascular resistance is adjusted.
- Cases refractory to medical management are usually treated with a surgical approach that involves pulmonary artery banding or clamping.

# Prognosis
- Modified Blalock-Taussig shunt has a superior prognostic value over classic Blalock-Taussig Shunts. Following prognostic factors are compared between the two procedures in multiple studies:
The increase in saturation of oxygen is greater in the modified shunt as compared to the classic shunt.
Modified shunt has an 88.8% patency rate in 3-5 years. Whereas for the classic shunt, the patency rate is 90% in the first year, 62% in two years, and 78.0% in 3 years.
The risk of early shunt failure is 20.8% in modified and 51.7% in the classic shunt.
Post-shunt increase in pulmonary arterial index (mm2/m2) is 158 +/- 21 versus 117 +/- 52 in classic Blalock-Taussig shunt.
- The increase in saturation of oxygen is greater in the modified shunt as compared to the classic shunt.
- Modified shunt has an 88.8% patency rate in 3-5 years. Whereas for the classic shunt, the patency rate is 90% in the first year, 62% in two years, and 78.0% in 3 years.
- The risk of early shunt failure is 20.8% in modified and 51.7% in the classic shunt.
- Post-shunt increase in pulmonary arterial index (mm2/m2) is 158 +/- 21 versus 117 +/- 52 in classic Blalock-Taussig shunt.

# Alternative Shunts

## Central Shunt
- Central shunt is made by creating an anastomosis between ascending aorta and main pulmonary artery.
- A short PTFE conduit for this purpose is used.
- It is also known as Mee's shunt. 
- Internal mammary artery is used for this purpose and to create a systemic to pulmonary conduit after there has been a failure of previously used BT shunt.
- It leads to adaptation of growth, and flow, and it also reduces the risk for graft infection.

## Potts shunt
- Potts shunt can be used as an alternative to the classic BT shunt.
- To avoid the consequences of right heart failure, it has been used as an alternative to offload the right ventricle.
- This improves the cardiac output but the oxygen saturation is lowered and there is decreased oxygen delivery to lower extremities.
- Potts shunt includes a connection that is made between descending aorta and left pulmonary artery.

## Waterston shunt
- It is made by connecting the ascending aorta and right pulmonary artery.
- It has not been clinically performed now due to increased risk of congestive heart failure, pulmonary hypertension, and kinking of the pulmonary artery.

## Cooley Shunt
- It is an intrapericardial anastomosis between ascending aorta and the right pulmonary artery.
- Right anterolateral thoracotomy approach is used.