The blalock and taussig shunt revisited.

The systemic to pulmonary artery shunts are done as palliative procedures for cyanotic congenital heart diseases ranging from simple tetralogy of Fallots (TOFs)/pulmonary atresia (PA) to complex univentricular hearts. They allow growth of pulmonary arteries and maintain regulated blood flow to the lungs till a proper age and body weight suitable for definitive corrective repair is reached. We have reviewed the BT shunt with its anaesthtic considerations and management of associated complications.

Introduction

The systemic to pulmonary artery shunts are done as palliative procedures for a variety of complex cyanotic congenital heart diseases. The spectrum may range from simple tetralogy of Fallots (TOFs)/pulmonary atresia (PA) to complex univentricular hearts with major associated surgical problems. The systemic to pulmonary artery shunt provide the first line of management in these critically ill cyanotic neonates. They are less risky and provide regulated blood flow to the lungs allowing growth of pulmonary arteries for future complete repair. The infant also reaches a proper age and body weight suitable for definitive corrective repair.[1]

History

The credit for the evolution of the shunt goes to Helen Taussig, a pediatric cardiologist from John Hopkins. She noticed that several patients with right-sided obstruction, i.e., pulmonary stenosis or PA, cyanosis increased after the closure of the ductus arteriosus. She along with Alfred Blalock, chief surgeon at John Hopkins and Vivian Thomas, a surgical technician decided to use subclavian artery to pulmonary artery connection as a way to augment the pulmonary blood flow.

Since then many modifications to this original operation took place. In 1951, Dubost and Oeconomos reported used lyophilized human vessels as an interposed graft between the subclavian and pulmonary artery.[2] In 1960s, synthetic vascular prosthesis made of nylon, teflon or dacron, a free left subclacvian artery graft or an interposed azygos vein were used as modified Blalock-Taussig shunt (MBTS) when direct end-to-side anastomosis of subclavian and the pulmonary arteries was not possible.[3] In 1970s, the expandable polytetrafluoroethylene (PTFE) has been increasingly used as interposed graft and shown to be patent by clinical angiographic studies.[4]

The main aim of the palliative surgery is to increase pulmonary blood flow in a controlled manner to alleviate cyanosis and improve exercise tolerance in the patients.

Goals

Increase blood flow to the lungs in controlled manner – prostaglandin E1 (PGE1) infusion to maintain ductus arteriosus patency, decrease pulmonary vascular resistance (PVR) (avoid hypoxia, hypercarbia, acidosis, sympathetic stimulation)

Maintain adequate preload – prevent hemodilution

Maintain afterload, i.e., systemic vascular resistance (SVR) and hence the diastolic pressure to maintain coronary perfusion pressure.

Risk Factors

Recent reports regarding the outcomes of modified BT shunt suggest high morbidity and mortality in neonates. The mortality in single ventricle patients has been reported to be around 15% while in patients with two ventricles it is 3%–5%.[56]

Various risk factors have been identified which include:

Sternotomy[6]

Anatomic site[6]

Use of cardiopulmonary bypass (CPB)[6]

Weight <3 kg[5]

Preoperative ventilation[5]

Single ventricle palliation[5]

PA with intact ventricular septum[5]

Ebstein's anomaly[6]

In about one-third of patients, deaths occur in first 24 h postoperatively.[5]

Indications

Palliative shunts are not commonly done for most cases as initial corrective procedures are increasingly performed during infancy. It has mainly been possible due to improved extracorporeal circulation methods, modern perioperative supportive care, and refinements in operative technique.

Nevertheless, certain subsets of patients require this initial palliative surgery when cardiac disorders are not amenable to final corrective surgery. The indications can be broadly grouped as under:

To increase blood flow to the lungs

Tricuspid atresia and other univentricular conditions where bidirectional Glenn and Fontan shunts can only be performed after the decline in pulmonary vascular resistance which may take months

PA

TOF

Neonatal Ebstein's with functional PA-rare

Complete repair not possible

Anatomical-aberrant coronary anatomy left anterior descending coronary artery crosses the right ventricular (RV) outflow tract in TOF (approximately 5%)[789]

Physiological-prematurity, intracerebral bleed, or other contraindications to CPB

Hypoplastic left heart syndrome

To encourage growth of pulmonary arteries

PA

Ventricular septal defect (VSD) with aortopulmonary collaterals

Inadequate pulmonary arteries.

What is an Ideal Shunt?

Features of an ideal shunt are as follows:

It should be technically simple to perform

It should provide adequate but not excessive pulmonary blood flow, hence minimize the risk of congestive cardiac failure and pulmonary hypertension

It should provide good long-term patency

It should be technically easy to close when repair is completed

It should result in no residual cardiopulmonary abnormalities after closure.

Types of Shunts

Classic Blalock-Taussig shunt

The classic Blalock-Taussig shunt (CBTS) was the original shunt described by Alfred Blalock and Helen Taussig. The end-to-side anastomosis between the subclavian (or the innominate) and pulmonary arteries is performed. As described by them, the operation was performed on the side opposite to the aortic arch to minimize kinking of the subclavian artery as it crosses over the aortic prominence. In addition, the longer innominate artery reduces kinking of the pulmonary artery.

Surgical approach: Thoracotomy

Advantages:

Predictability of blood flow, because the subclavian artery diameter prevents excessive blood flow to lungs and hence, congestive cardiac failure

Unlikely to cause pulmonary vascular disease

Ease of closure during corrective surgery

Potential adaptive growth of the anastomosis.

Disadvantages:

Thrombosis of the shunt due to its size

Subclavian or the innominate artery have to be sacrificed although it did not appear to have a clearly significant clinical effect[10]

Extensive dissection[10]

Significant arm ischemia from subclavian artery division was uncommon, although the pulse in the ipsilateral arm was often not felt for days after the operation

Neurologic disadvantages are rare and include risk of recurrent laryngeal nerve injury, phrenic nerve injury, and Horner syndrome

Various modifications of systemic-to-pulmonary artery shunt have been described. Among them, the MBTS is the most common type currently in use.

Modified Blalock-Taussig shunt

It is currently most commonly performed procedure. It was first described by Klinner et al. in 1962.[11] An interposition PTFE (or Gore-Tex) graft between the subclavian artery and the pulmonary artery is used to prevent sacrificing of the subclavian artery. PTFE conduit is considered ideal.[12] PTFE was found to be superior to Dacron because it has a smaller pore size that limits the tissue ingrowth but allows for fibroblastic incorporation to bind it to surround structures.[13]

Approach: Thoracotomy/midline

Advantages:

Subclavian artery need not be sacrificed

MBTS is useful when CBTS cannot easily be performed, such as on the same side as the aortic arch

Less distortion of pulmonary artery than CBTS

Future shunt takedown is relatively easy

Patency rate >90% at the age of 2 years[13]

Gazzaniga et al. advocate using shunts of at least 5 mm, even in small infants as they found that the orifice of the subclavian artery serves to regulate blood flow through the shunt.

Complications:

Thrombosis

Leakage of serous fluid through the PTFE in the chest

Pseudoaneurysm, which can cause massive fatal hemoptysis.[14]

Central shunt

It is the anastomosis between the ascending aorta and the main pulmonary artery using a short PTFE conduit. A central shunt may be especially useful with bilateral small branch pulmonary arteries, a concomitant procedure requiring a median sternotomy, or both.

Surgical approach: Midline

Pericardium is incised. Aortotomy is done with an aortic punch device using a side-biting clamp. A short PTFE conduit is used to anastomose the ascending aorta and the main/branch pulmonary artery.

The advantages of this technique are as follows:

Applicability to small children with small peripheral vessels

Prevention of distortion of pulmonary arteries

Provision of equal pulmonary blood flow to both lungs

Lower occlusion rate (compared with the CBTS or MBTS techniques)

Avoidance of subclavian artery steal

Ease of closure during corrective repair.

Disadvantages

Entry into the pericardium

Inapplicability for patients without a patent ductus arteriosus (PDA) or other source of pulmonary blood flow.

Immediately postoperatively, patients may be anticoagulated with heparin for 24-72 h. Patients may be placed on long-term aspirin therapy for shunt patency maintenance.

A modification of central shunt was described by Waterson et al. for patients with PA, VSD, and hypoplastic pulmonary arteries.[15] An end-to-side anastomosis was done between a transected main pulmonary artery and the side of the ascending aorta. They believed that it provides for adequate pulmonary artery growth in patients with small central pulmonary arteries. This shunt has also been termed the Mee shunt after the senior surgeon involved in its description.

Potts shunt

Potts shunt was developed as an alternate to CBTS in neonates. In this, a connection is made between the descending aorta and left pulmonary artery.

Advantages:

Easier to perform because does not involve vessels of small caliber, i.e., the subclavian artery

Lesser incidence of shunt thrombosis and occlusion.

Disadvantages:

High incidence of subsequent pulmonary hypertension

Preferential blood flow to one lung with kinking and distortion of the pulmonary artery

Technical difficulties during takedown

Congestive cardiac failure (20%).[16]

Because of above-mentioned problems, pott's shunt is not routinely performed.

Waterston shunt

It is the anastomosis between the ascending aorta and right pulmonary artery.

Disadvantages are similar as the Potts shunt

Excessive pulmonary blood flow,

Risk of pulmonary hypertension

Congestive cardiac failure (20%).[16]

Therefore, it is also not routinely performed.

Cooley shunt

A newer approach using intrapericardial anastomosis from the ascending aorta to the right pulmonary artery was described.

Surgical approach – Right anterolateral thoracotomy

Advantages:

Avoidance of mediastinal dissection (decreased risk of bleeding from collaterals)

Complete repair could be done in through the same incision

Less technical difficulties during future takedown.

Disadvantages:

Complex technique

Improper size anastomosis could lead to heart failure and pulmonary congestion

Invasion of pericardium leads to higher risk of adhesions in future cardiac repair.

Sano shunt

An anastomosis between the right ventricle–to–pulmonary artery shunt was done to ameliorate disadvantages of BT shunts.[17] A 4-mm nonvalved PTFE graft was used in infants <2.5 kg and a 5.0 mm nonvalved PTFE graft in infants weighing >2.5 kg. It was reported that mortality improved from 62% to 89% after the stage 1 surgeries.[1819] Sano shunt was preferred for univentricular conditions such as in Norwood operation.

Disadvantages:

Early obstruction (approximately 3 months) as compared to MBTS[17]

To overcome complications of ventriculotomy in a systemic ventricle, a valved conduit from the right ventricle to the pulmonary artery was attempted. It was abandoned following high failure rates.

Preoperative Considerations

Patients presenting for surgery may be grouped into:

Nonoperated cyanotic patient for BT shunt

Patients presenting for BT shunt takedown and subsequent total correction

Patients presenting with pulmonary overcirculation for PA banding

Patients presenting with shunt thrombosis.

For each of these, the anesthetic management will depend on:

Patient-related Factors

Anatomical diagnosis

Age

Weight <3 kg

Prematurity

Coexisting diseases-mostly noncardiac congenital: Tracheoesophageal fistula, anorectal malformation, cleft lip and palate, renal and skeletal pathologies. Down syndrome patients may have atlantooccipital subluxation that warrants airway management precautions

Respiratory infections: Effects are more deleterious in patients with pulmonary hypertension or cavopulmonary anastomosis. Risk-benefit ratio is to be discussed with the surgeon when delaying the surgery

Exercise tolerance indicated by fatigue, dyspnea on feeding, irritability, and inability to gain weight

Medications such as aspirin, warfarin, antidepressants, diuretics, angiotensin converting enzyme (ACE) inhibitors, antiarrhythmics

Previous surgeries and length of Intensive Care Unit stay-indicates possible difficult intravenous (IV) access and difficult intubation (? subglottic stenosis)

RV function patients with high pulmonary blood flow may present with tachycardia, tachypnea, irritability, cardiomegaly, and hepatomegaly.

Surgical Factors

The complexity of the heart lesion determines the duration of the surgery affecting the postoperative outcome. Details such as aortic arch anatomy and possibility of aberrant subclavian artery are important

Possible injury to nearby structures during dissection which further complicates the procedure

The use of CPB, sternotomy, or thoracotomy changes the anesthetic plan.

Preanesthesia checkup

A detailed history regarding the onset of symptoms, presence of cyanosis, feeding difficulties, sweating on the forehead should be elicited. Medication history must be elicited. Patients with previous BT shunt may be on aspirin, warfarin, diuretics, ACE inhibitors, and antiarrhythmics. The vast majority of newborns have PGE1 infusions running on presentation to avoid the possibility of duct related cardiovascular collapse. The usual dose is 0.01–0.1 mcg/kg/min. Laboratory investigations should be tailored accordingly.

Risk assessment

Increased risk of morbidity (mechanical support, reoperation, low cardiac output): Weight <3 kg and preoperative shock

Increased risk of mortality: Preoperative acidosis, mechanical support (ventilatory or circulatory), renal impairment, PA with intact ventricular septum and weight <3 kg.

Preoperative orders

Psychological preparation: It should be started right after the patient comes to the hospital for surgery. In case of children, counseling of parents is important to:

Reduces the child's distress on separation to the operating room

Increased child compliance

Reduced child anxiety

Reduced need for preoperative sedatives

Increased parental satisfaction.

Preoperative fasting

Fasting time orders should be clearly written with timing if possible as per ASA guidelines. Dehydration should be avoided in cyanotic patients. If the timing of surgery is uncertain, then an IV line should be placed, and fluids started.

Premedication

To produce sedation, ease separation anxiety, and facilitate induction of anesthesia (<6 months: Not required)

To supplement analgesia and reduce the requirements for general anesthetic drugs

To block harmful vagal reflexes

To dry secretions in the respiratory tract.

Drugs Used in Premedication

Midazolam

Produces sedation and anxiolysis

Increases cooperation

Does not prolong recovery time

Smoothens postoperative recovery

Routes

Oral: 0.25–0.33 mg/kg (syrup form)

IV: 0.1 mg/kg

Rectal: 0.4–0.5 mg/kg

Nasal: 0.2–0.3 mg/kg.

Ketamine

Produces sedation, anxiolysis, and facilitates induction of anesthesia

Oral: 5–10 mg/kg

Rectal: 5–10 mg/kg

Nasal: 3 mg/kg

Intramuscular: 2 mg/kg.

Fentanyl

Oral transmucosal fentanyl citrate

15–20 ug/kg: Onset in 20–30 min with 30 min duration of action

4. Continue all cardiac medications on the morning of surgery except ACE inhibitors and diuretics due to their hypotensive effects during anesthetic induction as seen in adult cases

5. Sedation: Sympathetic stimulation due to the crying of an anxious and distressed child can increase oxygen consumption and myocardial work; this might be poorly tolerated in a child with limited cardiac reserve. Midazolam is the preferred premedication to reduce oxygen consumption in the dose of 0.5 mg/kg orally half an hour before surgery. If IV line is present, then, incremental doses of 0.1–0.25 mg/kg midazolam can be given, provided airway and breathing issues are addressed accordingly. Promethazine (5 mg/ml) can also be given in dose of 0.5 mg/kg.

Investigations

Full blood count: Polycythemia increases blood viscosity which leads to thrombosis and infarction in cerebral, renal and pulmonary regions. White blood cell (WBC) count may be increased in cases of respiratory infections. Spuriously, low platelet counts may be observed in cases of polycythemia

Coagulation profile: Prothrombin time and partial thromboplastin time (PTT) are usually deranged in a polycythemic patient. Coagulation abnormalities also occur due to platelet dysfunction, hypofibrinogenemia, and factor deficiencies. Preoperative phlebotomy is performed in symptomatic hyperviscosity and hematocrit more than 65%. However, dehydration must be corrected first before deciding about phlebotomy. On the other hand, WBC count and C-reactive protein measurement provide potential diagnosis of infection

Serum electrolytes as mentioned earlier should be checked in patients receiving diuretics

Electrocardiogram (ECG) may show ventricular strain or hypertrophy

Echocardiogram is used for Doppler and color flow mapping, while catheterisation is used for information about pressures in different chambers, magnitude of shunt and coronary anatomy

The chest X-ray shows the heart position and size, atelectasis, acute respiratory infection, vascular markings, and elevated hemidiaphragm.

Cardiologist evaluation

The need for cardiologist evaluation depends on the complexity of the lesion. In a patient with a simple or moderately complex lesion that has been completely corrected and is well compensated, a standard preanesthetic visit without cardiology consultation is acceptable. Moderately complex lesions accompanied with the inability to compensate will warrant cardiologist evaluation and optimization, but clearance must always be given by anesthesiologist.

Intraoperative Considerations

Induction

Most induction agents are well tolerated depending on the rate and dose of the drug and whether it is inhalational or IV.

Inhalation induction

It is most commonly used in the absence of IV access. Sevoflurane is the most commonly used inducing agent. However, patients with poor cardiac function, who require inotropes preoperatively, may not tolerate inhalational induction and favor the use of ketamine.

Intravenous

SVR and PVR balance should be considered when using IV agents. Ketamine in the dose of 1–2 mg/kg is the most commonly used inducing agent.

Monitoring

The outcome data in these patients prescribe need for invasive monitoring in addition to routine monitoring so proceed as for a procedure on cardiopulmonary bypass.

ECG: Particular attention to be paid to baseline ECG trace to detect rate, arrhythmias and ST changes due to myocardial ischemia

Arterial line: It is to be placed on the side opposite to the side of proposed shunt in case of radial artery (i.e., right-sided shunt, so left radial artery and vice versa) or femoral route can be taken. Upper and lower limb pressures can be compared with the use of noninvasive blood pressure to diagnose coarctation that may have been missed. The diastolic pressures are important as they give indication of any coexisting PDA as well as coronary steal resulting in myocardial compromise

SpO2 probe: One should be aware that in right to left shunts pulse oximetry overestimates arterial oxygen saturation as saturation decreases, end-tidal carbon dioxide readings underestimate PaCO2, and discrepancy worsens with hypoxemia. When in doubt, obtain an arterial blood gas (ABG) preoperatively for baseline

Near infrared spectroscopy

Central venous line: To administer volume and inotropes if required

EtCO2 monitoring

Temperature probe

Baseline ABG and activated clotting time are obtained

Urine output.

Position

Thoracotomy: For right sided modified Blalock-Taussig shunt, left lateral position and for left sided modified Blalock-Taussig shunt right lateral positions are required

Sternotomy: For central shunts.

Perioperative Management

A balanced anesthetic technique, combining narcotic, and inhalational agents supplemented with muscle relaxant is usually undertaken. The ordinary routines such as the use of 100% oxygen, hyperventilation and excessive inhalational agents may decrease PVR, increase Qp: Qs and decrease diastolic pressure with resultant concerns for myocardial ischemia and systemic perfusion. Particular attention to be paid for redo cases as sternotomy may result in excessive blood loss. Adequate blood should always be arranged beforehand.

Ventilatory concerns

These patients may already have preoperative respiratory infections resulting in high airway pressures and low saturation due to pulmonary congestion and bronchospasm. Further insults are added when surgery requires thoracotomy in right lateral or left lateral positions resulting in one lung ventilation with retractors applied on the exposed lung for a better view of the surgical field. Hyperventilation and high FiO2 to keep EtCO2 and saturation to acceptable levels may be required during this time.

Before proceeding for anastomosis, heparin is given 3 min before clamping in a dose of 1–1.5 mg/kg IV. ACT is to be maintained >250 s. For BT shunt first, the pulmonary artery branch is clamped. Desaturation is quite frequent during this anastomosis. It requires vigilant monitoring of the blood pressure, any bradycardia occurring due to hypoxia and etCo2 values (low due to decreased pulmonary blood flow). Paradoxically, this may be less of an issue with thoracotomy, as clamping the PA will reduce ventilation-perfusion mismatch.

After the PA end is opened, the clamp is applied on systemic artery-subclavian/innominate of the proposed side or aorta in case of central shunt when the partial aortic clamp is applied. Subclavian artery may easily be confused with PDA. The absence of arterial trace in femoral artery can diagnose this condition. Hemodynamic instability may occur after the shunt is opened due to blood loss, systemic runoff to the PA and subsequent acidosis and ischemia-reperfusion injury occurring from release of metabolites from ischemic limb. Inotropes may be required. As per our institutional protocol, before the release of systemic artery clamp, we start inotropes and administer sodium bicarbonate prophylactically. It is common to see low saturations initially due to high PVR, and pulmonary vein desaturation secondary to lung collapse, hypoxic pulmonary vasoconstriction, and atelectasis. Saturation improves subsequently. At this time, low FiO2 is preferable. A rise in EtCo2 is noted after the shunt is open. Hemodilution is to be avoided. It is prudent to check ABGs at this time.

Further intraoperative course is guided by ruling out any anatomical issues, i.e., appropriate shunt size and whether prone to any kinking or narrowing. If there are no anatomical issues, a review of the physiological issues should be done-adequate preload, sinus rhythm, good contractility (i.e., good myocardial perfusion), maintenance of SVR and appropriate PVR. Remember that shunt is not the sole determinant of PVR. PVR also depends on changes in lung volume, alveolar oxygenation, pH, and pCO2. Use of inotropes results in improvement in hemodynamic parameters and oxygen delivery but at the cost of increased consumption.

Postoperative Management

Postoperative pain is commonly managed with continuous narcotic infusion usually fentanyl at 1 mcg/kg/h

The decision regarding extubation: This has been attributed to a number of factors:

Physiologic vulnerability of the neonates particularly with congenital heart disease

There are changes in PVR, SVR, and Qp which occur with lung re-expansion in combination with emergence from anesthesia, transition to postoperative pain regimens, possibility of bleeding, etc. These contribute to the postoperative risk of cardiac arrest.

The incidence of failure to extubate neonates following MBTS was investigated by Gupta et al.[20] They found that 27% patients required either reintubation or the instigation of noninvasive ventilation within 96 h. Decreased ventricular function was strongly associated with failure. There was a 13% incidence of diaphragmatic paralysis, all in the group that failed. Postextubation hemodynamics also changed, with an increased heart rate and systemic blood pressure in successful subjects.

Management of Complications

Shunt failure

The immediate postoperative period is the time when the incidence of shunt failure is high. This can present acutely with precipitously dropping saturations. It might be due to acute shunt failure secondary to the shunt clotting or kinking. This is an emergency and should be addressed immediately.[212223]

BT shunt thrombosis has been reported to be around 12%.[24] Various studies suggest the use of aspirin or heparin to prevent shunt thrombosis. However, histopathology of shunts electively taken down found 25% had a 50% stenosis at a median age of 8 months.[125]

Anticoagulation Management

Heparin should be started as soon as possible in the postoperative period. Patients are usually given heparin during clamping of the artery. Heparin should be restarted if bleeding is not an issue usually after 4 h (chest drains <3 ml/kg/h and postop aPTT is <60 s).

Management of heparin therapy

Standard heparin regime for prophylaxis:

Prepare infusion of heparin at 1000 U/kg and start infusion at 10 units/kg/h (0.5 ml/h).[2627]

Monitoring

Clotting profile (apTT, PT, and fibrinogen) is to be monitored

Before commencement of heparin

4–6 h after starting

1 h after syringe change

Platelet count if drop >50%

Consider HIT 5–10 days after institution of the treatment

Therapeutic range – aPTT (60–90 s)

Loading dose not required if recently returned from OR

If APTT <50 s bolus of 50 U/kg over 10 min

If APTT 50–90 s, infusion at 200 U/kg/h and APTT to be checked after 4 h.

Aspirin

Aspirin is to be started

Chest is closed, and intracardiac lines removed

Pacing wires removed

Start at 3–5 mg/kg (maximum 75 mg) once daily

Continue heparin until the second dose of aspirin is given.

Management of Blocked Blalock-Taussig Shunt

This is an emergency.

Diagnosis: Any patient with a significant drop in saturation with systemic to pulmonary cardiac shunt or inaudible shunt murmur.

If there is any concern regarding surgical repair or deteriorating child despite above interventions, return to the OR should not be delayed.

Management of Pulmonary Overcirculation

BT shunt too big in size leads to excessive pulmonary blood flow and high saturation described as pulmonary overcirculation. This is common if ductus arteriosus is patent and resolves only after its closure.

Diagnosis

Relatively high saturation

Chest X ray-edematous lungs

Low mixed venous saturation

Rising lactate

Increase in base deficit

Relatively low mean blood pressure

Signs of right heart failure.

Treatment

Mild form: Fluid restriction, diuretics

Severe form: Manipulation of SVR and PVR

Surgical: Pulmonary artery band/clipping.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.