Chapter 21 – Cavo-pulmonary Shunts and the Fontan Circulation




Chapter 21 Cavo-pulmonary Shunts and the Fontan Circulation



David J. Barron



Introduction


A truly univentricular heart is virtually unknown. However, there are a wide variety of different conditions in which there is functionally only one ventricle – typically one dominant ventricle with a second vestigial or underdeveloped ventricle. These ‘functionally ventricular morphologies’ account for 3 to 4 per cent of all congenital heart disease, but because they all usually require a series of palliative procedures to balance the circulation, they account for 15 to 20 per cent of all congenital heart surgeries. The myriad conditions that fall into this category are summarized in Table 21.1, but regardless of the precise morphology, the principles of management are always the same: stabilize the circulation by ensuring unobstructed venous return, unobstructed systemic outflow and a balanced pulmonary-systemic circulation. Many of these principles are discussed in detail in Chapters 4 and 20 and apply to the neonatal management of all these conditions.




Table 21.1 The Fontan Circulation Is the Common Pathway for All Conditions with a Functionally Single Ventricle





























Dominant left ventricle Dominant right ventricle
Tricuspid atresia Hypoplastic left heart syndrome
Double-inlet left ventricle (DILV) DORV with mitral atresia
Pulmonary atresia with intact septum Unbalanced AVSD (dominant right)
Unbalanced AVSD (dominant left)
Transposition/VSD with small RV
Indeterminate dominant ventricle
Left and right isomerism


Note: The dominant ventricle can be of either left or right morphology, and this table summarizes the commonest conditions in each group.


Once the circulation has been stabilized as a neonate, all these hearts converge on a common pathway towards what is referred to as the ‘Fontan circulation’. This describes the situation in which the systemic and pulmonary circulations are placed in series, driven by a single ‘pump’ – in contrast to the normal circulation, in which the systemic and pulmonary circulations are in parallel, driven by two ‘pumps’. This ingenious concept enables the single ventricle to support the entire circulation without any mixing of the bloodstreams and so is (in principle) acyanotic and depends on passive venous pressure to drive the cardiac output through the pulmonary vascular bed. Thus, the Fontan circulation is completely dependent on a low pulmonary vascular resistance (PVR) – and all management of the child prior to Fontan completion is to strive towards protecting the pulmonary vascular bed. These procedures and circulations are referred to as ‘palliative’ in that they do not achieve a fully ‘repaired’ circulation – nevertheless, they are the definitive procedure for that individual and can provide a remarkably good quality of life. Thus, the rather negative connotation of the word ‘palliative’ in the more commonly used sense of end-of-life care makes this a rather unfortunate description and should be avoided where possible.



The Cavo-pulmonary Shunt


Conversion from the newborn circulation to the Fontan circulation is a gradual process essentially because of the changes that occur within the lung vasculature and somatic growth during the first months and years of life. PVR is systemic at birth and only comes down to adult levels by the age of six months. The microvasculature of the lungs is also not fully developed for several months, and this is why a systemic shunt is required in a newborn to drive pulmonary blood flow.


However, once a child has reached four to six months of age, the PVR has dropped sufficiently that a high-pressure source of blood flow is no longer required. Instead, the SVC blood can be connected directly into the pulmonary arteries – the cavo-pulmonary shunt. This is a passive shunt that depends entirely on a head of pressure in the systemic veins to drive blood through the pulmonary vasculature as continuous, non-pulsatile flow. This has major advantages over the systemic shunt because it does not place a volume load on the circulation, and it also delivers fully deoxygenated blood to the lungs rather than partially deoxygenated blood. Thus, at the time of cavo-pulmonary shunt, any systemic shunt is usually disconnected, thus removing a volume load from the ventricle and allowing it to function at better loading conditions.



Technique.


The procedure is named after William Glenn, who described the procedure in the 1950s as an end-to-end connection between the SVC and the right PA (i.e. flow to only one lung). This has been superseded by an end-to-side anastomosis, as shown in Figure 21.1, and referred to as the ‘bidirectional Glenn’ shunt to emphasize that flow is to both lungs contrary to the historical depiction (Figure 21.1). The procedure is typically performed at four to six months of age via median sternotomy on cardiopulmonary bypass. It is essential that any stenosis or hypoplasia of the central pulmonary arteries is addressed at the same time to optimize flow to the lungs. The azygous vein must be ligated to prevent SVC blood escaping to the lower-pressure system in the lower half of the body. The SVC is disconnected from the heart at the SVC-RA junction, taking care to preserve the sino-atrial (S-A) node. The procedure can usually be performed with the heart beating unless it needs to be combined with any additional intra-cardiac procedures. Generally, any additional source of pulmonary blood flow is interrupted to create optimal loading conditions for the ventricle. However, there are some schools of thought that think it necessary to leave some additional flow (such as flow through a tightly banded PA) in an attempt to delay the need for the completion operation. Monitoring of the SVC pressure is essential as it will naturally rise to overcome the PVR. Pressures of 15 to 18 mmHg are typical, but greater than 20 mmHg may raise concerns that the PVR is too high. Oxygen saturation SaO2 of 80 to 85 per cent would be expected. Postoperative management is focused on lowering PVR with oxygen therapy and adequate ventilation. The child should be nursed head up and avoid high positive end-expiratory pressure (PEEP). The procedure is generally low risk, with a 2 to 3 per cent operative mortality, most of which is related to poor underlying ventricular function or high PVR.


Jan 16, 2021 | Posted by in CARDIOLOGY | Comments Off on Chapter 21 – Cavo-pulmonary Shunts and the Fontan Circulation
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