Overview of Modified Blalock-Taussig-Thomas Shunt

Congenital heart disease (CHD) is the most common birth defect, affecting about 8 out of every 1000 babies born. In many developing countries, CHD may not be detected until after the baby is born or sometimes even later in life, like during childhood or adulthood. Almost 1 out of 4 babies born with a heart defect have a critical form of CHD that needs surgery or other procedures in the first year of life.

For patients with CHD, a common temporary procedure is the modified Blalock-Taussig-Thomas (mBTT) shunt. This procedure improves blood flow to the lungs, reduces bluish discoloration of the skin due to low oxygen (cyanosis), promotes lung artery growth, and helps maintain proper heart function. In developing countries, this procedure often happens before more permanent corrective surgery. But, it’s not done as often anymore in developed countries due to advances in surgery technology and experts who can perform complex surgeries.

Choosing the right surgical plan depends on a blend of experience and careful consideration of competing performance goals. For instance, the shunt (an artificial passage to divert fluid) used in this procedure needs to be large enough to ensure enough blood flow to the lung arteries for optimal breathing. However, too much blood flow can lead to complications like fluid buildup in the lungs (pulmonary edema) and heart failure. We also need to consider additional risks such as shunt narrowing or blood clotting (shunt stenosis or thrombosis), distortion of the pulmonary arteries, impaired blood flow to the heart muscle (coronary perfusion), and uneven growth of the pulmonary arteries.

The mBTT shunt is an improvement over the classical Blalock-Taussig (BT) shunt. This surgery is designed to treat diseases where there’s a drop in blood flow to the lung arteries that results in bluish skin (cyanotic heart diseases). The first BT shunt procedure took place at the Johns Hopkins Hospital in 1944, thanks to the concerted efforts of pediatric cardiologist Dr. Helen Taussig, cardiac surgeon Dr. Alfred Blalock, and lab assistant Mr. Vivien Thomas. In 2003, there was a request to acknowledge Mr. Vivien Thomas’s significant contributions by including his name as part of the term “Blalock-Taussig shunt”.

The mBTT shunt allows for enough blood flow to the pulmonary artery (the artery taking blood from the heart to the lungs), alleviates bluish discoloration while avoiding too much blood flow to the lungs. This procedure involves the use of a synthetic graft (a type of substitute for damaged or deficient tissue) to create a passage from the systemic circulation to the pulmonary circulation without having to sacrifice the artery that supplies blood to the upper body or any of its branches.

The benefits of the mBTT shunt over the original procedure include simplifying the removal process, preserving blood flow to the upper body, and allowing more precise control of the shunt flow by adjusting the diameter and length of the graft and choosing the best site for attachment.

Anatomy and Physiology of Modified Blalock-Taussig-Thomas Shunt

In the US, 1% of all babies born have a condition called Congenital Heart Disease (CHD), and out of those, around 25% have what is known as cyanotic congenital heart lesions.

There are several types of defects that fall under the category of cyanotic CHD, including:

  • Tetralogy of Fallot (TOF), a condition that combines four heart defects
  • Ebstein anomaly, a rare defect in the heart’s tricuspid valve
  • Hypoplastic left heart syndrome, where the left side of the heart doesn’t develop correctly
  • Total anomalous pulmonary venous return, a condition where the veins carrying blood from the lungs to the heart don’t attach in the normal position
  • Transposition of the great arteries, where the two main arteries of the heart are switched
  • Tricuspid atresia and other conditions where only one ventricle is properly formed
  • Truncus arteriosus, a condition where there’s only one large vessel instead of two separate ones to carry blood from the heart

There are different ways to classify these heart lesions, such as:

  • Right-to-left shunting, when the blood bypasses the lungs and doesn’t get oxygenated properly
  • Inadequate pulmonary blood flow, when there isn’t enough blood flow to the lungs
  • Common mixing lesions, when oxygen-rich and oxygen-poor blood mix together

A surgical procedure known as the mBTT shunt helps improve blood flow to the lungs in cyanotic babies who are not ready for a complete heart repair. There are other similar procedures but they’re rarely used due to high risks of complications. Another method, the Sano shunt, is used primarily for babies with hypoplastic left heart syndrome where it creates a direct passageway from the heart to the lungs. This procedure was first introduced in 1981.

Why do People Need Modified Blalock-Taussig-Thomas Shunt

In the past, palliative shunt procedures were more common for certain heart defects. These methods have become less common with advancements in medical technology like circulatory support machines, neonatal intensive care units, and improved surgical techniques allowing more immediate corrective procedures. However, some newborns with specific birth heart defects still need initial palliative surgery since some conditions cannot be completely repaired immediately. In these cases, a boost to blood flow to the lungs is necessary to support the development of the pulmonary artery, which carries blood from the heart to the lungs.

In addition, newborns that have had an intracerebral bleed (a type of stroke happening inside the brain), or those who cannot have cardiopulmonary bypass – a procedure that handles the function of the heart and lungs during surgery, might be suitable candidates for a palliative shunt, also known as an mBTT shunt. Also, extremely premature newborns with heart lesions might require an mBTT shunt until they are old enough for definitive cardiac surgery.

There are several types of heart conditions that prevent complete repair but can be treated with an mBTT shunt. These include Hypoplastic left heart syndrome, where an mBTT shunt is a part of the corrective procedure; ‘corrected’ transposition of the great arteries, where the large vessels that carry blood from the heart to the lungs and body are in the wrong position; and certain coronary artery anomalies, that affect a small percentage of newborns.

The mBTT shunt can be used in various conditions, particularly those affecting a single ventricle, where certain types of shunts can’t be established until the newborn’s pulmonary vascular resistance decreases. Other heart conditions that require a boost of blood flow to the lungs to support pulmonary artery development include Pulmonary atresia, Ebstein’s anomaly with functional pulmonary atresia, or inadequate pulmonary arteries. In these cases, mBTT shunt might be the preferred treatment method.

However, it’s important to note that making decisions about these surgical procedures is often complex and requires balancing a number of factors. For example, a large conduit (a tube in the body to redirect blood flow) could lead to an excess of blood flow to the lungs, causing complications like fluid in the lungs or heart failure. Equally, the dynamics of the shunt could lead to other complications like narrowing or clotting of the shunt, distortion of the pulmonary arteries, reduced blood flow to the heart muscle, and uneven growth of the pulmonary arteries.

When a Person Should Avoid Modified Blalock-Taussig-Thomas Shunt

If a person is suffering from a lung disease known as pulmonary hypertension, where there is high blood pressure in the blood vessels that supply the lungs, and if their body is struggling to maintain normal blood flow in these vessels, known medically as elevated pulmonary vascular resistance, it is generally advised against (or “contraindicated”) to set up an mBTT shunt. An mBTT shunt is a medical procedure where a channel is created to allow blood flow between two different locations in the body.

Equipment used for Modified Blalock-Taussig-Thomas Shunt

If your doctor needs to cool and stop your heart during a procedure called an mBTT shunt placement, they might have you on standby with a machine that can take over for your heart and lungs (cardiopulmonary bypass) even though it’s not always needed. If the doctor decides to make an incision in the center of your chest (median sternotomy), they will use an electric saw designed for this purpose and wires made of stainless steel to close it afterward.

On the other hand, if your doctor opts for a less invasive approach, a side chest surgical incision (minimally invasive thoracotomy), they will use tools to gently spread your ribs (Finochietto or Tuffier rib spreader) and other specialized long-handled tools intended for children due to their smaller size. The standard tools used in heart surgery will also be part of the procedure.

In both cases, the doctor will use PTFE grafts which are type of tubes used in surgical procedures. These grafts are chosen in advance based their diameter to ensure they are a perfect fit, allowing for a successful connection (anastomosis) and optimal blood flow.

Who is needed to perform Modified Blalock-Taussig-Thomas Shunt?

For this procedure known as mBTT shunt placement, a specific team of health care professionals is needed, all working together to keep you safe and make sure everything goes smoothly.

The team includes a pediatric cardiothoracic surgeon, a special type of doctor who is trained to perform heart and lung surgeries in children. A surgical first assistant is there to aid the surgeon during the procedure. A cardiac anesthesiologist, an expert in pain management, is present to ensure you remain comfortable and safe during the procedure. The surgical technician or operating room nurse prepares the surgical area and assists during the operation. Also, there’s a circulating or operating room nurse who manages the equipment and supplies and communicates with the rest of the surgical team.

If a method called cardiopulmonary bypass is used, where a machine temporarily takes over the function of the heart and lungs during the surgery, a perfusionist will be present. The perfusionist is skilled in using this machine, ensuring oxygen and blood flow through your body during the operation.

Every one of these healthcare team members plays an important role in your procedure, all working together to make sure you are safe and that the surgery is successful.

Preparing for Modified Blalock-Taussig-Thomas Shunt

If a newborn baby has a bluish color, known as cyanosis, it is vital for doctors to evaluate the baby thoroughly before and after birth. Part of this involves asking about any history with the baby and physically examining the infant. The blue color of a newborn’s skin might be due to heart conditions that the baby was born with, which could come on their own or alongside other medical issues. So, it’s crucial to spot every single anomaly.

Doctors use a medical imaging technique known as a transthoracic echocardiogram, which a pediatric heart doctor must read. This test gives a lot of information on the large blood vessels leaving the heart: the ascending aorta, aortic arch, and subclavian vessels. In some cases, special techniques like coronary angiography or computed tomography angiography may be needed for a complete check-up.

When checking a newborn with cyanosis, it’s very important to note if the baby has any history of repeated lung infections. This could show signs of improper blood flow in their lungs. If a baby or infant has to undergo a minor heart surgery, known as the mBTT shunt placement, and they already have a lung infection, this can increase the chance of issues occurring during or after their surgery. Both, premature and full-term infants, who weigh less than 3 kg, are more prone to these issues. They might require special care during and after the surgery to support their heart and lungs.

Before surgery, doctors need to calculate the exact size of a synthetic material, known as a PTFE graft, which will be placed inside of the baby’s heart. To do this, they use a formula that involves a few factors, such as the flow rate of blood, pressure difference, diameter and length of the graft, and the “stickiness” of the blood. Here, it is to note that a minor change in graft diameter can greatly affect blood flow.

To ensure adequate blood flow in newborns who weigh 2.5 to 4.0 kg, a PTFE graft which measures 3.0 to 3.5 mm in diameter is usually used. For those who weigh less than 2.5 kg, a smaller graft under 3.0 mm is suggested, while ones who weigh more than 4.0 kg may require a larger graft with a 4.0 mm diameter.

How is Modified Blalock-Taussig-Thomas Shunt performed

The modified Blalock-Taussig-Thomas (mBTT) shunt procedure is a surgical technique used to improve blood flow in people with certain types of heart defects. Instead of accessing the heart from the side as in the original method, the heart is now accessed by making an incision down the middle of the chest, a technique called a median sternotomy. This method offers many benefits such as enabling the shunt to be placed on either side of the chest, better visibility for the surgeons, and easier access for emergency medical support if needed. It also lowers the risks of certain kinds of nerve injuries during and after surgery.

However, this surgical approach can interfere with blood flow from the internal chest arteries, which can make future heart procedures more challenging. Despite this, a median sternotomy is often the preferred method for mBTT shunt placement because it allows doctors to also fix other heart abnormalities at the same time if necessary.

After the heart is accessed, the doctors would usually tie off a certain vein early in the operation to simplify future heart surgeries. This kind of surgery also requires part of the thyroid gland to be removed. Additionally, an examination of the larynx (voice box) is done before starting the operation.

In this procedure, the doctor creates a detour or bypass around the blocked parts of the two main blood vessels that supply blood to the lungs. This is done using a graft, which is a tube made of synthetic material. The graft is sewn to the blood vessels in a way that suits the patient’s individual anatomy and ensures there’s no twisting or tension.

In people who are dependent on a certain duct in their heart for blood flow, this duct is sealed off only after confirming that the newly created shunt is functioning properly. Doctors can monitor the shunt’s efficiency by checking changes in the patient’s oxygen levels and blood flow.

Once the operation is completed and the chest is closed up, the patient would need to stay in intensive care for at least a week. Some patients might need additional postoperative support.

Finally, at some point after the initial surgery, a follow-up procedure, known as a ‘takedown’, is required to disconnect the shunt and prevent abnormal blood flow. This is important for the patient’s overall health and to allow the heart to function properly as the child grows. In some cases, non-surgical methods can also be used to disconnect the shunt, but these carry their own risks and are not commonly used.

Possible Complications of Modified Blalock-Taussig-Thomas Shunt

Even with progress in surgery, intensive care, and modern medical equipment, operations that connect the body’s main artery (systemic) to the lung artery (pulmonary) still have a relatively high risk of causing death during the hospital stay. The rate can vary from 2.3% up to 16%. Two factors have been identified that increase this risk: having a body weight less than 4.25 kg and receiving the surgery as an emergency. Specifically, body weight under 4.25 kg amplifies this risk by almost 21 times, and emergency surgery boosts it by 3.5 times. However, there isn’t specific research exploring how the timing of the surgery impacts death risk.

One disadvantage of using PTFE (a man-made plastic) to construct the mBTT shunt (a device used to improve blood flow) is that fluid occasionally leaks through this synthetic material. This can lead to swelling in the nearby area and prolong the need for a chest drain tube to remove fluid.

One of the most severe complications of the mBTT shunt operation is too much blood flowing to the lungs shortly after surgery. This can occur when the resistance to blood flow in the body is high and in the lungs is low, or when too much blood is being diverted from the body to the lungs. This can cause low blood pressure, an overworked heart, fluid in the lungs, and a condition called acidosis, which is too much acid in body fluids. If not addressed, it can lead to cardiac arrest and death. This imbalance can also increase the risk of heart failure and insufficient blood flow to vital organs, possibly causing damage to the intestines.

On the other hand, if the shunt gets distorted or forms a clot, it can result in not enough blood reaching the lungs, low oxygen levels, and blue discoloration of the skin. To prevent clot formation, heparin (a medication that helps prevent blood clotting) is given right after surgery at a rate of 10 units/kg/h, and adjusted based on blood clotting test results. Aspirin is also given at a dose of 3 to 5 mg/kg per day (with a maximum limit of 75 mg/day) and continued for life.

If there’s a blockage in the mBTT shunt due to a clot, a complicated procedure called transcatheter intervention may be used to restore normal blood flow. However, this procedure carries significant risk of complications. It’s crucial to continuously monitor the patient’s circulatory system after surgery and be prepared to act swiftly if reoperation is needed.

The mBTT shunt’s functionality can be evaluated by touch and listening. After surgery, if a vibration can be felt on the side of the shunt radiating to underneath the collarbone, it generally indicates the shunt is working. This can be confirmed by listening over that area where a continuous sound indicates effective shunt function. An ultrasound of the heart can measure the amount of blood going through the shunt, while a CT scan can give a detailed assessment of the blood flow through the shunt.

What Else Should I Know About Modified Blalock-Taussig-Thomas Shunt?

A large study conducted over six decades at Johns Hopkins Hospital noted a decline in the use of a specific heart device called the mBTT shunt. Interestingly, they found an increase in the use of this device in treating a specific type of heart ailment called univentricular heart lesions. However, most of the shunts studied were a different type called classic BT shunts, which makes it hard to apply these findings to the mBTT type directly. Generally speaking, the mBTT shunt is regarded as safer than other types such as the Potts or Waterston shunt.

A different study – the Single Ventricle Reconstruction trial – found that there were better survival rates at 12 months (without the need for a heart transplant) for patients who had undergone a type of heart operation called the Norwood procedure with a device called a right ventricle-to-pulmonary artery shunt, as compared to those who received the mBTT shunt. However, the long-term survival rates of both methods were similar, and those in the Norwood procedure group experienced a significant decline in the overall function of their right ventricle. Also, at 6 years, there wasn’t a significant difference between the 2 groups in terms of death rates, the need for a transplant, or other medical interventions.

Another study found that patients with the mBTT shunt had a greater reduction in their weight-for-age score compared to those with a different type of shunt called a Sano shunt.

In comparing the mBTT shunt with newer, less invasive, and more cost-effective methods such as a technique called ductal stenting, a study by McMullan et al reported a higher rate of complications related to the procedure and distal branch pulmonary artery narrowing in the mBTT group. However, both groups had similar rates of being free from intervention. Another less invasive option called right ventricular outflow tract stenting has shown to be safer and just as effective as the mBTT shunt in treating newborns with complex TOF lesions.

In the 1940s, the classical BT shunt procedure, known then as “the blue baby operation,” gained international attention and has since saved the lives of millions of children. However, modern clinical practice has seen a shift away from these procedures in favor of avoiding complications that could arise from these interventions.

Frequently asked questions

1. What are the potential risks and complications associated with the modified Blalock-Taussig-Thomas (mBTT) shunt procedure? 2. How will the mBTT shunt improve blood flow to my lungs and alleviate cyanosis? 3. Are there any alternative treatment options to the mBTT shunt for my specific heart condition? 4. How will the size and diameter of the synthetic graft used in the mBTT shunt be determined for my individual anatomy? 5. What is the expected recovery process after the mBTT shunt procedure, and will I need any follow-up surgeries or interventions in the future?

The Modified Blalock-Taussig-Thomas (mBTT) shunt is a surgical procedure that helps improve blood flow to the lungs in cyanotic babies who are not ready for a complete heart repair. It is used in cases where there is right-to-left shunting, inadequate pulmonary blood flow, or common mixing lesions. The mBTT shunt is one of the methods used to improve blood flow to the lungs, but there are other similar procedures that are rarely used due to high risks of complications.

You would need a Modified Blalock-Taussig-Thomas (mBTT) shunt if you are suffering from pulmonary hypertension and your body is struggling to maintain normal blood flow in the blood vessels supplying the lungs. The mBTT shunt is a medical procedure that creates a channel to allow blood flow between two different locations in the body.

A person should not get a Modified Blalock-Taussig-Thomas Shunt if they have pulmonary hypertension and their body is struggling to maintain normal blood flow in the vessels supplying the lungs, as this procedure is contraindicated in such cases.

The recovery time for a Modified Blalock-Taussig-Thomas (mBTT) shunt procedure can vary, but typically, the patient will need to stay in the intensive care unit (ICU) for at least a week after the surgery. Some patients may require additional postoperative support. A follow-up procedure, known as a 'takedown', is also necessary at some point after the initial surgery to disconnect the shunt and allow the heart to function properly as the child grows.

To prepare for a Modified Blalock-Taussig-Thomas (mBTT) shunt, the patient should undergo a thorough evaluation before and after birth, including a physical examination and medical imaging tests like a transthoracic echocardiogram. It is important to note any history of repeated lung infections and to assess the baby's weight, as this can impact the size of the synthetic graft used in the procedure. The surgical approach for the mBTT shunt involves making an incision down the middle of the chest, known as a median sternotomy, and the patient may need to stay in intensive care for at least a week after the surgery.

The complications of Modified Blalock-Taussig-Thomas Shunt include a relatively high risk of death during the hospital stay, especially in patients with a body weight less than 4.25 kg and those undergoing emergency surgery. The use of PTFE to construct the shunt can lead to fluid leakage and swelling, requiring a chest drain tube. Too much blood flow to the lungs after surgery can cause low blood pressure, an overworked heart, fluid in the lungs, acidosis, and potentially cardiac arrest and death. Distortion or clot formation in the shunt can result in insufficient blood reaching the lungs, low oxygen levels, and blue discoloration of the skin. Blockage due to a clot may require a complicated procedure called transcatheter intervention. Continuous monitoring of the patient's circulatory system is necessary, and reoperation may be needed. The functionality of the shunt can be evaluated by touch and listening, and further assessment can be done using ultrasound and CT scan.

Symptoms that require Modified Blalock-Taussig-Thomas Shunt include specific birth heart defects that cannot be completely repaired immediately, intracerebral bleed (a type of stroke happening inside the brain), inability to have cardiopulmonary bypass, extremely premature newborns with heart lesions, and certain heart conditions such as Hypoplastic left heart syndrome, 'corrected' transposition of the great arteries, and certain coronary artery anomalies. These conditions may require a boost of blood flow to the lungs to support pulmonary artery development, making the mBTT shunt the preferred treatment method in these cases.

The safety of the Modified Blalock-Taussig-Thomas (mBTT) shunt in pregnancy is not mentioned in the provided text. Therefore, it is not possible to determine the safety of this procedure in pregnancy based on the given information. It is recommended to consult with a healthcare professional for specific advice regarding the safety of the mBTT shunt in pregnancy.

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