Year : 2014 | Volume
: 17 | Issue : 3 | Page : 197--199
Modified Blalock Taussig shunt: Comparison between neonates, infants and older children
Department of Anaesthesiology and Critical Care, Narayana Hrudayalaya Hospitals Bengaluru, Karnataka, India; International Professor, University of Minnesota, Minneapolis, USA
Narayana Hrudayalaya Hospitals, Bengaluru, Karnataka, India
|How to cite this article:|
Muralidhar K. Modified Blalock Taussig shunt: Comparison between neonates, infants and older children.Ann Card Anaesth 2014;17:197-199
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Muralidhar K. Modified Blalock Taussig shunt: Comparison between neonates, infants and older children. Ann Card Anaesth [serial online] 2014 [cited 2019 Jul 15 ];17:197-199
Available from: http://www.annals.in/text.asp?2014/17/3/197/135849
Modified Blalock Taussig shunt (MBTS) is a palliative procedure indicated in a subset of children with cyanotic heart disease not amenable to primary repair at the time of presentation. Though simple in concept, Blalock Taussig (BT) shunt may be associated with significant morbidity and mortality. Major problems are over-shunting and shunt thrombosis. In this issue of Annals of Cardiac Anesthesia, in a retrospective study, Singh et al., have categorized 134 children who underwent MBTS into three groups- neonates, infants, and older children and conclude that age <30 days, weight <3 kg, packed red blood cells (RBCs) transfusion >6 ml/kg, mechanical ventilation >24 h and postshunt increase in PaO 2 (P Diff ) <25% of baseline PaO 2 are independent predictors of mortality. 
There are several important considerations which need to be kept in mind:
1. Differences in clinical behavior and outcome in different pathophysiological states needing a BT shunt for e.g. biventricular versus univentricular physiology:
A. Tetralogy of fallot: BT shunt/systemic-pulmonary artery (PA) shunts:
Classic BT shunt: The subclavian artery is anastomosed to the ipsilateral pulmonary artery (PA). This procedure is usually performed in infants older than 3- months; a right-sided shunt is performed in patients with left aortic arch; a left sided shunt is performed for right aortic arch.  Modified Blalock-Taussig shunt: A Gore-Tex interposition graft is placed between the subclavian artery and the ipsilateral PA. This is the most popular procedure for any age, especially for small infants younger than 3 months of age. A left-sided shunt is preferred for patients with a right aortic arch. The surgical mortality rate is 1% or lessThe Waterston shunt: Anastomosis is performed between the ascending aorta and the right PA; this shunt is no longer performed because of high incidence of surgical complications, which include too large a shunt leading to congestive heart failure (CHF) or pulmonary hypertension, or both, and narrowing and kinking of the right PA causing difficulty at the time of corrective surgeryThe Pott-shunt operation: Anastomosis is performed between the descending aorta and the left PA; this shunt is no longer performed. It may result in CHF or pulmonary hypertension, as in the Waterston operation. Moreover, left thoracotomy is required to close the shunt during corrective surgery.
B. Ebstein's anomaly: A BT shunt can be life-saving in patients of Ebstein's anomaly if there are obstructive lesions between the right ventricle and PA or if the tricuspid valve is stenotic/atretic. Good left ventricle (LV) size and function is needed for the patient with Ebstein's anomaly to survive the procedure. A Fontan-type operation is performed later.
C. Tricuspid atresia: The BT shunt is the most frequently performed first-stage operation in this subset of patients. Most patients who have tricuspid atresia with decreased pulmonary blood flow need a BT shunt soon after the birth when pulmonary vascular resistance is still high. In these patients, LV supplies both systemic and pulmonary circulations and the shunt can result in volume load of the LV. Thus the shunt should be relatively small and should not be left alone too long.
D. Single ventricle: BT shunt is necessary for patients with cyanosis in single ventricle physiology associated with pulmonary stenosis/atresia. A sternotomy approach is preferable to a thoracotomy approach as the former results in a lower rate of distortion of the PA. Shunt to right PA is preferable as any distortion of right PA can be incorporated into Fontan anastomosis, or can be repaired at the time of correction of Tetralogy of Fallot.
2. Sternotomy versus thoracotomy approach for BT shunt: The purpose of a BT shunt is to create a reliable path of blood flow to the lungs when the pulmonary blood flow is restricted and the patient is not suitable for total repair. During the last few years, a midline approach is preferred for systemic-to-PA shunt in many centers. , The advantages of sternotomy approach are preservation of subclavian artery, ease of take down and relative ease of construction of the shunt. There are two other advantages with sternotomy approach (i) if the child desaturates alarmingly or becomes hemodynamically unstable, the procedure can be done under CPB support (ii) if the child suffers a cardiac arrest, cardio pulmonary resuscitation is facilitated. Access to patent ductus arteriosus for ligation to remove a source of competitive flow is also possible with sternotomy approach. Thoracotomy approach can cause distortion of the distal PA, which may interfere with total correction when the patient presents at a later date.
3. Size of shunt: A MBTS of 3.5 mm is the optimal size to use in a term neonate >3.0 kg. If a 3.0 mm shunt is used, the risk of sudden thrombosis and acute obstruction in the post shunt period is increased. Early institution of anticoagulation with low dose heparin 10-20 units/kg/h is important once the hemostasis is secured. Another significant issue with small shunt is the likelihood of the child outgrowing the shunt size causing progressive hypoxemia and cyanosis. On the other hand, if a large shunt is used in a neonate (>4 mm), ensuing excessive pulmonary blood flow may compromise systemic perfusion, cause LV volume overload, failure and prolonged Intensive Care Unit and hospital stay.
4. Autologous withdrawal of blood: Cyanotic children undergoing MBTS may benefit from hemodilution due to improved blood flow attributable to lower viscosity.  However, this benefit must be weighed against the possibility of precipitating a spell in children who are unstable or have a dynamic right ventricular outflow tract obstruction.
5. Postoperative bleeding: Baseline coagulation defects have been demonstrated in children with congenial heart disease. Bleeding tendency in cyanotic patients is associated with prolongation of prothombin time, activated partial thromboplastin time, decreased levels of fibrinogen and coagulation factors and thrombocytopenia. In a prospective study, three independent factors responsible for significant bleeding were low preoperative body weight, cyanotic heart disease and increased time required for chest closure.  Coagulation and hemostasis in children with congenital heart disease is influenced by various factors, important ones being, patient's age, pathophysiology of heart disease. Two other factors are considered responsible for bleeding. Firstly, bleeding dyscrasias are common in cyanotic children that are attributable to erythrocytosis due to the effects of hypoxemia. Secondly, neonates and infants bleed more than bigger children and excessive bleeding may occur if the body weight is lower than 6.5 Kg. Point of care testing for coagulation and stringent trigger points for RBC, platelet and fresh frozen plasma transfusion are recommended to reduce blood loss and transfusion requirements and to fine-tune management of bleeding patients irrespective of age group.
6. Use of inotropic agents: Use of inotropic support may be indicated to maintain adequate systemic perfusion pressure during and after the surgery to prevent early shunt thrombosis in MBTS. Additionally, inotropic agents help to mitigate left ventricular failure in case of overflow situation. And a diastolic pressure of >30 mmHg is recommended to maintain coronary perfusion.
7. Fast tracking: In this series, neonates were ventilated postoperatively for a mean duration of 51 h versus infants and children for 14 and 12 h respectively. Patients who tolerate the procedure well and exhibit improved oxygenation at the end of the shunt may be subjected to early extubation and fast tracking.
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