Tetraology of Fallot and BT shunts

Options include formation of a modified Blalock-Tausig (BT) shunt or definitive repair.

The occurrence of ‘tet spells’ is generally an indication for cardiac surgery, at least in the long term. Surgery may be palliative or corrective.

Definitive repair involves closure of the ventricular septal defect (VSD) and opening up of the obstructed right ventricular outflow tract. Elective repair in relatively asymptomatic children is usually performed at about 6 months of age. Definitive repair has about 95% survival at 25 years, with the majority of patients living unrestricted adult lives.

Fallot's Tetralogy Repair (from RCH Melbourne Cardiology - click on image)

The decision was made to perform a modified BT shunt in this child as the first stage pending definitive repair when the child is older.

The modified BT shunt is a surgically-created (goretex) conduit between a subclavian artery and the pulmonary artery.

It improves blood flow to the pulmonary circulation by providing a route for returning systemic blood that bypasses the right ventricular outflow obstruction. Following a modified BT shunt, arterial oxygen saturations of about 70 to 85% are optimal as they indicate relative balance between pulmonary and systemic blood flows.

Fallot's Tetralogy with modified BT Shunt (from RCH Melbourne Cardiology - click on image)

The original BT shunt operation was first performed in 1941 by Helen Tausig and Alfred Blalock. Marc De Leval later modified the procedure so that transection of the subclavian artery is not required.

As if being recognized as the founder of pediatric cardiology wasn’t enough, Helen Tausig went onto play a major role in limiting the thalidomide disaster in the United States by bringing European reports of phocomelia to the attention of the FDA.

• Check blood pressure and ventilatory parameters, correct as needed
• Rule out pulmonary disease (examination, ultrasound and CXR)
• Suspect BT shunt malfunction:

— Auscultate for a shunt murmur
— Arrange urgent echocardiography to assess shunt flow
— Inform the cardiac surgeon
— Consider a heparin infusion (e.g. 10-20 U/kg/h) if high risk of shunt occlusion
— Consider increasing vasopressors to increase SVR as a temporizing measure

• Check and correct ventilatory parameters (e.g. high inspired O2)
• Suspect ‘over-shunting’ (excessive pulmonary blood flow relative to systemic blood flow)  if:

— SaO2>85% at low FiO2
— pulmonary plethora or pulmonary edema (can be unilateral)
— cardiac failure
— low systemic diastolic pressure (due to excessive ‘run off’ into the pulmonary circulation)
— persistent metabolic acidosis in a ‘pink’ patient (due to decreased systemic blood flow)

• Manage’ over-shunting’ – if modest:

— restrict fluids
— consider diuretics and inotropes to assist the volume-loaded heart.

• if ‘over-shunting’ is more severe or refractory to the above measures:

— decrease pulmonary blood flow by increasing pulmonary vascular resistance:
— permissive hypercapnia to decrease pH
— lower FiO2
— Perform echocardiogapraphy
— define BT shunt flow
— assess for the presence of additional shunts such as a patent ductus arteriosus or major aortopulmonary collateral vessels (MAPCA)
— If acidosis and very low diastolic blood pressures persist despite the above measures then revision of the BT shunt (using a shunt of smaller diameter) may be needed.