Late donor cardiectomy after paediatric heterotopic cardiac transplantation

To hit a home run as a heterotopic heart recipient-living with two hearts for over three decades: a case report

Heterotopic heart transplantation (HHT) is an alternative to the orthotopic technique in selected patients with terminal heart failure. We report the case of the longest survival after HHT, with an uneventful follow-up for over three decades after transplantation. At the age of 25 years, endomyocardial fibrosis following myocarditis rendered the patient's native heart unable to maintain the body's needs. An allograft provided a second chance at life. The HHT technique was favoured due to severe pulmonary hypertension. The patient had an uneventful follow-up since then. The scarcity of donors and the revolutionary advances in the mechanical circulatory device field restricted the utilization of the HHT technique, but it has the potential to provide an excellent prognosis with a good quality of life.

Neonatal injury models: integral tools to decipher the molecular basis of cardiac regeneration

Myocardial injury often leads to heart failure due to the loss and insufficient regeneration of resident cardiomyocytes. The low regenerative potential of the mammalian heart is one of the main drivers of heart failure progression, especially after myocardial infarction accompanied by large contractile muscle loss. Preclinical therapies for cardiac regeneration are promising, but clinically still missing. Mammalian models represent an excellent translational in vivo platform to test drugs and treatments for the promotion of cardiac regeneration. Particularly, short-lived mice offer the possibility to monitor the outcome of such treatments throughout the life span. Importantly, there is a short period of time in newborn mice in which the heart retains full regenerative capacity after cardiac injury, which potentially also holds true for the neonatal human heart. Thus, in vivo neonatal mouse models of cardiac injury are crucial to gain insights into the molecular mechanisms underlying the cardiac regenerative processes and to devise novel therapeutic strategies for the treatment of diseased adult hearts. Here, we provide an overview of the established injury models to study cardiac regeneration. We summarize pioneering studies that demonstrate the potential of using neonatal cardiac injury models to identify factors that may stimulate heart regeneration by inducing endogenous cardiomyocyte proliferation in the adult heart. To conclude, we briefly summarize studies in large animal models and the insights gained in humans, which may pave the way toward the development of novel approaches in regenerative medicine.

Better together: a reappraisal of heterotopic heart transplantation

Heterotopic heart transplantation (HHT) is rare in the modern era. When used as a biologic left ventricular assist, HHT provides pulsatile flow, supports the left ventricle with a physiologic cardiac output, responds to humoral stimuli, and with modern immunosuppression may offer long-term untethered survival. This study was undertaken to compare survival of HHT with orthotopic heart transplantation (OHT) to assess its viability in the modern era. In the United Network for Organ Sharing database, from January 1999 to December 2020, there were 27691 bicaval OHT, 13836 biatrial OHT, 1271 total OHT, and 51 HHT with sufficient follow-up. Survival was analyzed using restricted mean survival time (RMST) through 4 years as the outcome. In the first 4 years after transplant, compared with HHT, differences in RMST were 0.1 years (99% CI: -0.4 to 0.5 years) for bicaval OHT, 0.0 years (99% CI: -0.4 to 0.5 years) for biatrial OHT, and 0.0 years (99% CI: -0.5 to 0.4 years) for total OHT. In this cohort, survival was indistinguishable between HHT and OHT recipients in the first four years. Thus, HHT might be a viable alternative to durable mechanical circulatory assist particularly with size mismatched grafts or for patients with refractory pulmonary hypertension.

Heart regeneration: beyond new muscle and vessels

The most striking consequence of a heart attack is the loss of billions of heart muscle cells, alongside damage to the associated vasculature. The lost cardiovascular tissue is replaced by scar formation, which is non-functional and results in pathological remodelling of the heart and ultimately heart failure. It is, therefore, unsurprising that the heart regeneration field has centred efforts to generate new muscle and blood vessels through targeting cardiomyocyte proliferation and angiogenesis following injury. However, combined insights from embryological studies and regenerative models, alongside the adoption of -omics technology, highlight the extensive heterogeneity of cell types within the forming or re-forming heart and the significant crosstalk arising from non-muscle and non-vessel cells. In this review, we focus on the roles of fibroblasts, immune, conduction system, and nervous system cell populations during heart development and we consider the latest evidence supporting a function for these diverse lineages in contributing to regeneration following heart injury. We suggest that the emerging picture of neurologically, immunologically, and electrically coupled cell function calls for a wider-ranging combinatorial approach to heart regeneration.

Perspectives on Directions and Priorities for Future Preclinical Studies in Regenerative Medicine

The myocardium consists of numerous cell types embedded in organized layers of ECM (extracellular matrix) and requires an intricate network of blood and lymphatic vessels and nerves to provide nutrients and electrical coupling to the cells. Although much of the focus has been on cardiomyocytes, these cells make up <40% of cells within a healthy adult heart. Therefore, repairing or regenerating cardiac tissue by merely reconstituting cardiomyocytes is a simplistic and ineffective approach. In fact, when an injury occurs, cardiac tissue organization is disrupted at the level of the cells, the tissue architecture, and the coordinated interaction among the cells. Thus, reconstitution of a functional tissue must reestablish electrical and mechanical communication between cardiomyocytes and restore their surrounding environment. It is also essential to restore distinctive myocardial features, such as vascular patency and pump function. In this article, we review the current status, challenges, and future priorities in cardiac regenerative or reparative medicine. In the first part, we provide an overview of our current understanding of heart repair and comment on the main contributors and mechanisms involved in innate regeneration. A brief section is dedicated to the novel concept of rejuvenation or regeneration, which we think may impact future development in the field. The last section describes regenerative therapies, where the most advanced and disruptive strategies used for myocardial repair are discussed. Our recommendations for priority areas in studies of cardiac regeneration or repair are summarized in Tables 1 and 2 .

Postnatal Cardiac Development and Regenerative Potential in Large Mammals

The neonatal capacity for cardiac regeneration in mice is well studied and has been used to develop many potential strategies for adult cardiac regenerative repair following injury. However, translating these findings from rodents to designing regenerative therapeutics for adult human heart disease remains elusive. Large mammals including pigs, dogs, and sheep are widely used as animal models of humans in preclinical trials of new cardiac drugs and devices. However, very little is known about the fundamental cardiac cell biology and the timing of postnatal cardiac events that influence cardiomyocyte proliferation in these animals. There is emerging evidence that external physiological and environmental cues could be the key to understanding cardiomyocyte proliferative behavior. In this review, we survey available literature on postnatal development in various large mammal models to offer a perspective on the physiological and cellular characteristics that could be regulating cardiomyocyte proliferation. Similarities and differences between developmental milestones, cardiomyocyte maturational events, as well as environmental cues regulating cardiac development, are discussed for various large mammals, with a focus on postnatal cardiac regenerative potential and translatability to the human heart.

Pregnancy after heterotopic heart transplant removal

Heterotopic heart transplants were introduced in 1974. The technique allows the patient's native heart to be preserved in situ, alongside the transplanted heterotopic donor heart. We present the case of a nulliparous woman who underwent heterotopic heart transplant in infancy, and subsequent explantation of the donor heart eleven years later, when her native heart function recovered. In adulthood the patient attended pre-pregnancy counselling and was awaiting cardiac magnetic resonance imaging when she presented pregnant at 6 weeks-of-gestation. She attended the joint cardiac obstetric and anaesthetic clinic, where she was reviewed monthly and had bi-monthly echocardiograms. At 35 weeks-of-gestation she was admitted to hospital with preeclampsia. After blood pressure control and steroid administration, a category 3 caesarean delivery under spinal anaesthesia was performed. To our knowledge this is the first case report describing pregnancy in a patient with a removed heterotopic heart transplant.

Cardiac regeneration based on mechanisms of cardiomyocyte proliferation and differentiation

Cardiomyocyte proliferation and progenitor differentiation are endogenous mechanisms of myocardial development. Cardiomyocytes continue to proliferate in mammals for part of post-natal development. In adult mammals under homeostatic conditions, cardiomyocytes proliferate at an extremely low rate. Because the mechanisms of cardiomyocyte generation provide potential targets for stimulating myocardial regeneration, a deep understanding is required for developing such strategies. We will discuss approaches for examining cardiomyocyte regeneration, review the specific advantages, challenges, and controversies, and recommend approaches for interpretation of results. We will also draw parallels between developmental and regenerative principles of these mechanisms and how they could be targeted for treating heart failure.

Heterotopic heart transplantation: where do we stand?

Orthotopic heart transplantation (OHT) is a well established and commonly utilized procedure for end-stage heart failure patients. Heterotopic heart transplantation (HHT) is a surgical procedure that allows the graft to be connected to the native heart in a parallel fashion. The main advantage of HHT is to assist the patient's native heart and to maintain circulation in the cases of severe acute rejection. HHT has also been proposed to overcome pulmonary hypertension, to increase the size of the donor pool and to decrease waiting times without increasing morbidity caused by the procedure. However, only a few papers have reported the short- or long-term results of HHT, and most of these studies have included <30 cases. OHT remains the standard technique and is preferable whenever the patient meets the current criteria and a suitable organ is available. HHT is far less useful than in the past because of the major advances in immunosuppression therapy and the development of long-term mechanical circulatory support. This study reviews the origin of HHT and discusses clinical developments, including their advantages and disadvantages.

[Progress in pediatric cardiology, congenital heart disease in adults, and heart surgery for congenital heart disease]

The field of pediatric cardiology is continually developing and now covers not only congenital and acquired heart disease in children but also congenital heart disease in adults and the prenatal diagnosis and prevention of heart disease. This review highlights new findings in the field of genetics, selected articles on the use of magnetic resonance imaging and multislice CT in diagnosis, and recent publications on electrophysiology and the surgical treatment of children and adults with congenital heart disease. In addition, the increasingly advanced use of mechanical assist devices as a bridge to heart transplantation in children is also discussed.