Single ventricle defects and the hunt for the best shunt

In this case-based article, we’ll discuss single ventricle defects and their management (medical and surgical), then look at the two main shunt options during Stage 1 reconstruction.

Single ventricle defects

Many complex congenital heart defects have single ventricle physiology. This means that one ventricle is too small, weak or obstructed to pump effectively, leaving the other ventricle to supply both systemic and pulmonary circulations in parallel via a shunt such as the ductus arteriosus.

Hypoplastic left heart syndrome (HLHS) is the archetype, but there are many others, including tricuspid atresia, pulmonary atresia with an intact ventricular septum, double-outlet right ventricle and Ebstein’s anomaly. They are very rare, occurring in just 5/100,000 newborns. However, you should know about them given their severity, complexity and the frequency with which such patients present to healthcare facilities. Perhaps, most importantly, if you understand single ventricle physiology, you’ll understand the entire cardiorespiratory system.

Affected babies are often diagnosed antenatally when they should have a fetal echocardiogram and a plan for management after birth. But, they may present postnatally with cyanosis/hypoxaemia due to insufficient pulmonary blood flow, cardiogenic shock due to insufficient systemic blood flow, or both.

Managing single ventricle defects

Sometimes families consider palliation from birth if a severe abnormality is discovered antenatally. Usually, though, they will proceed to surgery. Whilst waiting for an operation, they need to be managed medically. A lot can be learned from the general principles underlying the initial management of these babies.

The primary management aim is the same as for all critical care – ensuring adequate systemic oxygen delivery to meet demands, i.e., avoiding hypoxia. In simple terms, oxygen delivery is the product of the oxygen content of the blood and the flow of that blood to where it needs to go. The former requires good pulmonary blood flow, whilst the latter requires good systemic blood flow. And herein lies our problem…  the single ventricle provides both pulmonary and systemic blood flow in parallel. More blood flow to the lungs means less blood flow to the body, and vice versa. Therefore, in addition to the usual measures for tackling hypoxia (maximising cardiac output, oxygenation and haemoglobin; minimising oxygen consumption), we must also carefully balance pulmonary and systemic blood flow.

Too much pulmonary blood flow and we will see well-oxygenated blood (SpO2 >85%) but insufficient systemic blood flow (pallor/mottling, cool peripheries, delayed capillary refill time, weak pulses, hypotension, narrow pulse pressure) and hypoxia (lactic acidosis).

Too much systemic blood flow (pink/flushed, warm peripheries, brisk capillary refill time, strong pulses, relative hypertension, wider pulse pressure) and we will see poorly oxygenated blood (SpO2 <75%) and, once again, hypoxia (lactic acidosis).

So how do we ‘balance the circulation’? We manipulate pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR). Not enough pulmonary blood flow? Reduce PVR and increase SVR. Not enough systemic blood flow? Increase PVR and reduce SVR. Sounds simple. The table below shows us the factors affecting PVR and SVR that we can play with. Practically, this involves adjusting ventilation strategy and vasoactive medications. Oxygenation is a particularly important variable, with the sweet spot being a SpO2 of 75-85%.

Medical management of babies with single ventricle physiology also means keeping open the shunt allowing parallel circulation. Before surgery, this shunt is usually the ductus arteriosus and an infusion of alprostadil or dinoprostone (‘Prostin’) is used to maintain its patency.

By now, it will be clear that babies with single ventricle physiology are extremely complex! Early discussion with specialists (paediatric cardiology, paediatric cardiac intensive care or a paediatric retrieval service) is vital – they will support you through the initial management phase. Ultimately, these babies require transfer to a paediatric cardiac intensive care unit for further evaluation and management.

Surgical management of single ventricle defects – Stage 1 reconstruction

Single ventricle defects require surgical management unless families elect for a primary palliative strategy. Definitive surgical repair, creating a biventricular circulation, is preferable but is often impossible (for example, they may only have a rudimentary ventricle). In these cases, they need a staged reconstruction. Life expectancy is still significantly limited to about 40 years, which is still considered a palliative approach. Staged reconstructions are a topic (click here for a nice summary by Children’s Hospital of Philadelphia), but here we focus on Stage 1.

Stage 1 is performed in the first days of life. A sternotomy with a cardiopulmonary bypass is usually required. The underlying heart defect determines the precise operation performed – you might have heard of the Norwood procedure, an example used for HLHS specifically. Regardless of the precise operation, there are THREE key characteristics that any circulation must have upon completion of the Stage 1 procedure:

1. Unrestricted atrial septum

If needed, an atrial septostomy is performed to create a common atrium where free mixing can occur between blood from the pulmonary veins (oxygenated) and venae cavae (deoxygenated).

2. Unobstructed systemic blood flow

They need a clear pathway for mixed oxygenated/deoxygenated blood to flow from the single/common ventricle to the aorta. This is often done by combining the pulmonary artery and aorta to create a neo-aorta.

3. Appropriate pulmonary blood flow

There should be a permanent pathway for mixed oxygenated/deoxygenated blood to flow from a relatively high-pressure system (either the systemic arterial circulation or the right ventricle) to the pulmonary arteries. The ductus arteriosus is usually closed when this alternative pathway is established.

This is the most nuanced aspect. Resistance provided by this pathway will have a marked bearing on how well-balanced the circulation is – it is the primary determinant of PVR. This pathway can take various forms: stented ductus arteriosus, major aortopulmonary collateral arteries, central aortopulmonary shunt, modified Blalock-Taussig shunt (mBT shunt) or right ventricular-pulmonary artery conduit (RV-PA conduit), commonly referred to by its eponym, the Sano shunt. These days, the choice is primarily between the mBT shunt or RV-PA conduit.

The mBT shunt is an artificial graft connecting the right brachiocephalic/subclavian artery (high pressure) to the pulmonary arteries.

The RV-PA conduit is an artificial graft connecting the right ventricle (lower pressure) to the pulmonary arteries – akin to the native right ventricular outflow tract but usually valveless.

The table below compares the primary advantages and disadvantages of each. The major complication to be aware of is shunt blockage. Thrombosis, or any other obstruction, of the shunt is an emergency that requires prompt recognition. Patients may be hypoxaemic, with a loss of shunt murmur. An urgent echo can confirm the diagnosis, requiring urgent therapeutic anticoagulation, thrombolysis and/or a surgical/endovascular procedure.

An editorial note on names: It’s commonly called the Blalock-Taussig shunt after Helen Taussig and Alfred Blalock. Surgical assistant, Vivien Thomas, had as much a role to play in the operation as Blalock, but because he was just a lab assistant and, perhaps more importantly, because he was Black, it took another 30 years before he received any official recognition for the groundbreaking work he had done.

Let’s look at some cases…

Case 1: modified Blalock-Taussig shunt

A baby is born at term with an antenatally diagnosed HLHS. After delivery, they are excessively hypoxaemic, with oxygen saturations below 75%. They are rapidly intubated, sedated and ventilated, achieving target oxygen saturations between 75 and 85%. The team started an alprostadil infusion to keep the ductus arteriosus open, and they were admitted to PICU.

After a few stable days, the pulmonary pressures drop.

The baby has a Stage 1 reconstruction – a Norwood procedure with a modified Blalock-Taussig shunt. The procedure is uncomplicated, but there is significant diastolic run-off post-operatively – their diastolic blood pressure is really low in the mid-20s. Six hours post-operatively, they become more mottled, with weak pulses – signs of poor systemic perfusion. Their lactate starts to climb.

Oxygen saturation is around 90%, and a chest x-ray shows plethoric, wet lungs. This is probably a combination of pulmonary over-circulation and post-bypass low cardiac output syndrome (LCOS). The intensivists adjust the ventilation to increase PVR and decrease SVR, but this has limited effect. Cardiac output is boosted with careful IV fluid boluses, low-dose adrenaline and milrinone infusions and the baby is sedated. Frank blood is aspirated from the endotracheal tube, indicating a significant pulmonary haemorrhage, and the baby suffers a cardiac arrest at 20 hours postoperatively.

The team starts CPR, which is emergently converted to extra-corporeal cardiopulmonary resuscitation via the open sternotomy wound. Extra-corporeal membrane oxygenation (ECMO) is continued for 48 hours, during which the pulmonary arteries are banded to prevent over-circulation. The baby is successfully weaned off ECMO.

The ventilator, vasoactive medications and sedation are weaned over the next few days, and the baby receives aggressive diuresis. The sternum is closed, and chest/peritoneal drains are removed.

After ten days in intensive care, they are transferred to the ward, breathing spontaneously in air and on regular captopril to reduce their afterload. They are discharged home with regular Paediatric Cardiology follow-ups.

They go on to have Stage 2 reconstruction when they’re six months old.

Case 2: RV-PA conduit

Imagine the same baby is born with HLHS, stabilised and transferred to a Paediatric Cardiac Intensive Care Unit. After a few days, they proceed to Stage 1 reconstruction but with an RV-PA conduit.

Soon after the operation, the baby is noted to have junctional ectopic tachycardia (JET). This is well tolerated initially but is compounded over the next few hours by post-bypass LCOS associated with low blood pressure and rising lactate.

The LCOS is treated with careful IV fluid boluses and low-dose adrenaline. The JET is treated by deepening sedation (to minimise exogenous and endogenous catecholamines), optimizing electrolytes and active mild hypothermia. Ultimately, an amiodarone infusion is needed to control the rate. The rhythm reverts to sinus over the next few days, and the LCOS improves. Ventilation, vasoactive medications and sedation are weaned, and it’s time for aggressive diuresis again. The chest is closed, and chest/peritoneal drains are removed.

After five days in intensive care, the baby is transferred to the ward, breathing spontaneously in air. They are discharged home with regular Paediatric Cardiology follow-ups. Oxygen saturations are persistently borderline-low at 75%. Their chest x-ray demonstrates reduced pulmonary vasculature, and an echo shows limited pulmonary blood flow with some obstruction at the level of the RV-PA conduit. They go to the cath lab and undergo balloon dilation of the conduit, with some improvement.

The baby progresses to Stage 2 reconstruction when they are four months old.

What is the best option for repair?

These cases illustrate some common issues associated with each shunt type, but what does the evidence say about outcomes?

The seminal trial addressing this in the Norwood population was The Single Ventricle Reconstruction Trial, published in 2010. This randomised controlled trial included 549 patients across 15 North American centres. Transplantation-free survival at 12 months was significantly higher with the RV-PA conduit than with the mBT shunt (74% vs 64%, p = 0.01). However, the RV-PA conduit was associated with a higher rate of unintended cardiovascular interventions (primarily balloon dilatation or stent placement of the shunt, pulmonary arteries or neo-aorta) and complications (respiratory, neurological and infectious). Subsequent follow-up at three years found the RV-PA conduit group no longer had significantly superior transplantation-free survival (67% vs 61%; p = 0.15). Furthermore, the RV-PA conduit group had slightly worse right ventricular ejection fractions and underwent more catheter interventions.

Researchers also looked at neurodevelopmental status three years after the operation. Scores were below average, but shunt type was not associated with the degree of impairment nor quality of life measures.

Six years later, about 15% of children had developed severe heart failure post-Norwood procedure. Again, shunt type did not affect risk.

Several studies have compared suspected RV-PA conduit complications to mBT shunt complications. An observational cardiac MRI study confirmed that ventricular scarring and volume-loading lead to increased ventricular dilation and decreased myocardial strain (an indication of systolic function). Another study suggested an increased risk of ventricular arrhythmias (which were largely <1min in duration) because of this scarring in the RV-PA conduit group (29% vs. 14%; p = 0.049). What this means is unclear.

The bottom line

Single ventricle defects are rare and complex but important to understand.

Single ventricle defects are rare and complex but important to understand.

The primary goal of critical care is the same as always: ensuring oxygen delivery is sufficient to meet demands and avoiding hypoxia.

The need to balance the parallel pulmonary and systemic circulations makes this particularly challenging – get help!

Stage 1 repair requires a shunt to establish pulmonary blood flow: either an mBT shunt or RV-PA conduit.

Stage 1 repair requires a shunt to establish pulmonary blood flow: either an mBT shunt or RV-PA conduit.

About PICSTAR

PICSTAR is a trainee-led research network open to all doctors, nurses and allied health trainees within Paediatric Intensive Care.  We are the trainee arm of the Paediatric Critical Care Society – Study Group (PCCS-SG) and work with them on research, audit and service evaluation.

If you would like to join PICSTAR and get involved in projects, have ideas you would like to propose or get advice/mentorship via PCCS-SG, don’t hesitate to contact us at picstar.network@gmail.com. See their website for more: https://pccsociety.uk/research/picstar/

References

Children’s Hospital of Philadelphia – Staged Reconstruction Heart Surgery. https://www.chop.edu/treatments/staged-reconstruction-heart-surgery

Goldberg CS, Lu M, Sleeper LA, et al. Factors associated with neurodevelopment for children with single ventricle  lesions. J Pediatr. 2014;165(3):490-496.e8. doi:10.1016/j.jpeds.2014.05.019

Hall EJ, Smith AH, Fish FA, et al. Association of Shunt Type With Arrhythmias After Norwood Procedure. Ann Thorac Surg. 2018;105(2):629-636. doi:10.1016/j.athoracsur.2017.05.082

Mahle WT, Hu C, Trachtenberg F, et al. Heart failure after the Norwood procedure: An analysis of the Single Ventricle  Reconstruction Trial. J Hear lung Transplant  Off Publ  Int Soc Hear Transplant. 2018;37(7):879-885. doi:10.1016/j.healun.2018.02.009

Newburger JW, Sleeper LA, Frommelt PC, et al. Transplantation-free survival and interventions at 3 years in the single ventricle reconstruction trial. Circulation. 2014;129(20):2013-2020. doi:10.1161/CIRCULATIONAHA.113.006191

Ohye RG, Sleeper LA, Mahony L, et al. Comparison of shunt types in the Norwood procedure for single-ventricle lesions. N Engl J Med. 2010;362(21):1980-1992. doi:10.1056/NEJMoa0912461

Schilling C, Dalziel K, Nunn R, et al. The Fontan epidemic: Population projections from the Australia and New Zealand  Fontan Registry. Int J Cardiol. 2016;219:14-19. doi:10.1016/j.ijcard.2016.05.035

Single Ventricle Defects | Boston Children’s Hospital. https://www.childrenshospital.org/conditions-and-treatments/conditions/s/single-ventricle-defects

Wong J, Lamata P, Rathod RH, et al. Right ventricular morphology and function following stage I palliation with a  modified Blalock-Taussig shunt versus a right ventricle-to-pulmonary artery conduit. Eur J cardio-thoracic Surg  Off J Eur  Assoc Cardio-thoracic Surg. 2017;51(1):50-57. doi:10.1093/ejcts/ezw227

PICSTAR is a trainee-led research network open to all doctors, nurses and allied health trainees within Paediatric Intensive Care.  We are the trainee arm of Paediatric Critical Care Society – Study Group (PCCS-SG) and work together with them on research, audit and service evaluation. If you would like to join PICSTAR and get involved in projects, have ideas you would like to propose or get advice/mentorship via PCCS-SG please contact us on picstar.network@gmail.com. See our website for more: https://pccsociety.uk/research/picstar/