Introduction
There are a number of conditions or clinical situations in which it is not possible or safe to correct the circulation. These can be divided into two groups:
1. Anatomies in which biventricular repair is not possible. This includes a whole variety of conditions that can be summarized as ‘functionally univentricular circulations’. To be born with a truly single ventricle is vanishingly rare, and most of these conditions have a small, underdeveloped or hypoplastic ventricle on either the right or left side. This mixed bag of anatomical variants accounts for 3 to 4 per cent of all congenital heart disease but for 15 per cent of all congenital cardiac surgeries (because they all generally need a series of staged procedures).
2. Anatomies in which biventricular repair will be possible at a later age but which present in a neonate or small infant with inadequate pulmonary blood supply (e.g. pulmonary atresia or severe tetralogy of Fallot).
Since the list of anatomical variants is virtually endless, each patient needs to be assessed on an individual basis, and the morphology and anatomical connections must be carefully analyzed with echocardiography and clinical assessment. The key to managing any neonate with a functionally univentricular circulation is to achieve a balanced circulation: i.e. unobstructed venous inflow into the heart, unobstructed systemic outflow and a balanced pulmonary blood flow. In terms of the latter, anatomies that have pulmonary atresia or severe pulmonary/sub-pulmonary stenosis will require some form of securing additional pulmonary blood flow (a shunt), whereas anatomies with too much pulmonary blood flow will require a limiting band on the pulmonary artery.
Arterial Shunts
Most cases can be stabilized with the use of prostaglandin E2 (Prostin) to establish ductal patency – and hence are referred to as ‘duct-dependent circulations’. However, the ductal flow is not truly secure (completely dependent on intravenous Prostin and can still gradually close or vary despite this) and is uncontrolled and can lead to pulmonary over-circulation and heart failure. Thus the aim of initial palliation in these situations is to provide a secure and controlled pulmonary blood supply. The choice of technique is essentially guided by pulmonary vascular resistance: a neonate will have high pulmonary vascular resistance (PVR; close to systemic at birth and gradually falling over the first five to six months of life to normal values) and so requires a source of blood flow at relatively high pressure – an arterial shunt. The duct is usually ligated at the same procedure to avoid over-circulation and/or competitive flow with the shunt (but it can be left open, expecting it to close after the prostaglandin is stopped).
Blalock-Taussig Shunt.
Described in the 1940s, before the era of cardiopulmonary bypass or prosthetic grafts, the classical Blalock-Taussig (BT) shunt involved sacrificing the subclavian artery and turning it down to connect it directly into the ipsilateral pulmonary artery. Sacrificing the artery in a neonate or young infant is a relatively safe procedure in that the collateral supply to the arm is very rich. The modern operation uses an interposition Gore-Tex graft between the subclavian (or innominate) artery and the pulmonary artery (the ‘modified BT shunt’; Figure 4.1) – this avoids having to sacrifice the artery but also provides more controlled pulmonary blood flow, the Gore-Tex tube providing a fixed resistance in the circulation. The procedure was traditionally performed via a thoracotomy and can be performed on either the left or right side. The right side is usually preferred as this allows the shunt to be taken from the distal innominate artery rather than the slightly smaller subclavian and also because the shunt will sit more medially (almost immediately behind the superior vena cava (SVC)), which makes it much easier to find and control at the subsequent procedure. If the aorta is a right arch, then the shunt is usually placed on the left side as the innominate artery is left sided.
The BT shunt is now more commonly performed via a median sternotomy. The advantages of this are mainly to avoid a thoracotomy scar for the child and easy access to the ductus to ligate it. The approach also allows for the ready use of cardiopulmonary bypass should the haemodynamics be unstable during the procedure. However, it is preferable to avoid bypass, if possible.
The choice of shunt size has evolved through trial and error over the years, and most neonates would receive a 3.5-mm shunt. A 3-mm shunt can be used for smaller neonates, and occasionally a 4-mm shunt is used in older neonates or for more distal shunts (such as those placed via thoracotomy).
In some anatomies, an alternative to the BT shunt can be to place a small (4–5 mm) right ventricle–to–pulmonary artery (RV-to-PA) conduit to provide pulmonary blood flow. This has the advantage of sustaining diastolic pressure (analogous to use of the RV-to-PA shunt in the Norwood operation; see Chapter 20), and in the setting of a biventricular anatomy such as pulmonary atresia with VSD, it will also deliver predominantly desaturated blood into the lungs – rather than partially oxygenated blood as in the case of a BT shunt. This is only feasible in the right ventricle is of adequate size and has the disadvantage of requiring cardiopulmonary bypass and the need to create a ventriculotomy. Nevertheless, the more stable haemodynamics have given this approach increased popularity.
Central Shunt
An alternative method of delivering additional pulmonary blood flow is to construct a shunt directly from the aorta rather than from one of its branches. If the origin of the shunt is from the aorta, then it is referred to as a ‘central shunt’ (the term does not refer to a median sternotomy approach, since the shunt could be performed via sternotomy or thoracotomy). These shunts have the advantage of delivering higher flow and can be used if the head and neck artery anatomy is not suitable (e.g. no innominate artery or small vessels). Classically, central shunts were performed as direct anastomoses between the back of the ascending aorta and the right pulmonary artery (Waterston shunt; Figure 4.2) or between the front of the descending aorta and the left pulmonary artery (Potts shunt; Figure 4.3). These are now of historical interest only, although adult patients may still be encountered who had these procedures as children (meaning that the aorta and pulmonary artery will be fused together at these points). The current technique for central shunts is to use an interposition Gore-Tex graft from the ascending aorta to the most appropriate pulmonary artery. Again, these are ideally performed without cardiopulmonary bypass, although the need to place a side-biting clamp on the aorta may not be well tolerated and may require bypass support. As these shunts tend to be high flow and of shorter length, diameters are smaller than those chosen for a BT shunt, usually 3 or 3.5 mm.
