Introduction
Although nearly half of all of children undergoing heart transplantation at present have developed end-stage cardiac failure due to intrinsic disease of the myocardium, the proportion undergoing transplantation for congenital heart disease (CHD) has gradually declined as techniques of repair have improved. A slowly increasing percentage of young adults with corrected or palliated CHD will, however, ultimately develop end-stage cardiac failure for which the only long-term therapy at present remains heart transplantation. Limited donor availability limits the application of this therapy and requires increasingly complex strategies to bridge these young patients to transplantation and maximize the use of available organs. There is growing concern that the increasing number of young adults with palliated single-ventricle circulations will exacerbate the shortage of donor organs with few available alternatives to improve their quality of life. Transplantation in young patients with a failing Fontan circulation provides a formidable challenge to the transplant team, not least because of the deleterious effects of long-term raised venous pressure and cyanosis on hepatic, haematological and renal function but also the impact that cardiac debilitation can have on the course immediately after transplantation. Multiple previous surgeries often lead to severe mediastinal adhesions and immune sensitization, which increase both the operative and logistical complexities of the transplantation procedure.
Anatomy
Very few anatomical barriers remain to preclude transplantation in CHD. Perhaps the most challenging, situs inversus, can be overcome by careful preoperative planning and the harvest of as much additional donor tissue as possible to aid reconstruction. Limiting the graft warm ischaemic time similarly requires careful planning, completing in-situ reconstruction of the great veins and arteries where required prior to the onset of warm ischaemia. Perhaps the only anatomical features that render transplantation not feasible are extensive stenoses of the pulmonary arteries or veins beyond the area amenable to surgical repair. Heart-lung transplantation is often the only viable therapy under these circumstances. The small number of paediatric heart-lung transplants performed worldwide each year (only five to 10 paediatric heart-lung transplants are reported to the International Society for Heart and Lung Transplantation every year) reflects the extreme scarcity of suitable donor organ blocks and the relatively lower survival in this patient group (see ‘Outcomes’).
Immunology
As with all other cardiac transplants, blood type and weight are the major determinants of donor availability. In children younger than 2 years of age, however, the development of antibodies to the major ABO blood group antigens is incomplete, and during this period, ABO-incompatible cardiac transplantation has been undertaken with good results and the development of long-term immune tolerance to the non-self ABO antigens. Full blood-volume exchange transfusion is usually undertaken intra-operatively, and close postoperative monitoring for the development of increasing levels of anti-graft antibodies and antibody-mediated rejection is instituted. Previous blood transfusions and homograft material used in cardiac and vascular reconstructions frequently cause sensitization to HLA antigens in patients with CHD. The panel-reactive antibody (PRA) test is undertaken at transplant assessment to ascertain the levels of antibodies to common HLA antigens. Patients sensitized to more than 75 per cent of common major HLA antigens often face very protracted waiting periods on the transplant list and the possibility of not finding a suitable donor in time. The use of anti-CD20 antibody serum seems to reduce the development of activated plasma cells producing injurious antibodies. The logistics of donor preparation and organ procurement, along with the relatively short tolerated ischaemic time of the cardiac allograft, prevents prospective cross-matching to reduce the deleterious effects of a positive cross-match on rejection and graft longevity.
Mechanical Circulatory Support (MCS)
Internationally, around a third of paediatric heart transplant patients reported to the International Society for Heart and Lung Transplantation (ISHLT) are bridged to transplantation by MCS. The relatively small number of children requiring treatment for end-stage cardiac failure and the wide variation in required pump output for different-sized children have resulted in the development of paediatric MCS lagging behind that for adults by a considerable margin. Pulsatile extra-corporeal devices have been abandoned in adult practice in favour of continuous-flow pumps with a resulting marked reduction in morbidity and the emergence of durable support options that enable destination therapy in those not suitable for transplantation. In children, however, the Berlin Heart extra-corporeal pump remains the only device presenting options for support of patients from neonatal to adult size for more than 30 days through a range of pumps from 10 to 50 mL (Figure 26.1). This has proven considerably more durable than ECMO, which until the 1980s was the only modality suitable to provide MCS to children. The modern ECMO devices such as the rotating levitated magnetic systems have proven to be well tolerated for prolonged periods of extra-corporeal support as a bridge to transplantation and are increasingly used as a short-term bridge in acute situations (Figure 26.2). In recent years, implantable continuous-flow pumps have increasingly been used in children with improving results. This has proven feasible in paediatric patients down to a weight of 13 kg and in many anatomical substrates, including the systemic right ventricle and failing single-ventricle circulation. Smaller, more durable pumps are eagerly awaited, with the Jarvik 2015 pump and Heartware devices recently re-entering clinical trials (Figure 26.3). These systems still require an external drive line, but tunnelling these lines has reduced the risk of line infection, and rapid improvement in battery technology has provided increasingly portable battery packs.
