aka Cardiovascular Curveball 009
Despite the therapeutic measures described in Pediatric Perplexity 003 (Q6 and Q7), the 26 day-old boy with Tetralogy of Fallot and severe hypercyanotic spells is in dire straits. He has been intubated and requires mechanical ventilation in conjunction with a phenylephrine infusion in order to achieve arterial oxygenation compatible with life.
The decision is made to perform life-saving cardiac surgery.
Q1. What are the surgical options for Tetralogy of Fallot?
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.
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.
Q2. What is a modified Blalock-Tausig shunt?
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.
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.
Q3. Describe the ICU management of excessive hypoxia (e.g. SO2 60%) following insertion of a Blalock-Tausig shunt?
- 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
Q4. Describe the ICU management of excessive oxygenation (e.g. SO2 95%) following insertion of a Blalock-Tausig shunt?
- 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.
- Duncan A, Croston E. Guidelines for Intensive Care Management of Infants and Children after Congenital Heart Surgery. Pediatric Intensive Care Unit, Princess Margaret Hospital, 2008.
- Libby P, et al. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine (8th edition), Saunders Elsevier, 2008.