Heterotopic transplantation as a model to study functional recovery of unloaded failing hearts

Development of a cardiac loading device to monitor cardiac function during ex vivo graft perfusion

Background

Ex vivo heart perfusion systems, allowing continuous perfusion of the coronary vasculature, have recently been introduced to limit ischemic time of donor hearts prior to transplantation. Hearts are, however, perfused in an unloaded manner (via the aorta) and therefore, cardiac contractile function cannot be reliably evaluated.

Objectives

We aim to develop a ventricular loading device that enables monitoring of myocardial function in an ex vivo perfusion system. In this initial study, was to develop a prototype for rat experimentation.

Methods

We designed a device consisting of a ventricular balloon and a reservoir balloon, connected through an electronic check valve, which opens and closes in coordination with changes in ventricular pressure. All balloons were produced in our laboratory and their properties, particularly pressure-volume relationships, were characterized. We developed a mock ventricle in vitro test system to evaluate the device, which was ultimately tested in ex vivo perfused rat hearts.

Results

Balloon production was consistent and balloon properties were maintained over time and with use on the device. Results from in vitro and ex vivo experiments show that the device functions appropriately; hemodynamic function can be measured and compares well to measurements made in an isolated, working (loaded) rat heart preparation.

Conclusions

Our cardiac loading device appears to reliably allow measurement of several left ventricular hemodynamic parameters and provides the opportunity to control ventricular load.

Rat Heterotopic Heart Transplantation Model to Investigate Unloading-Induced Myocardial Remodeling

Unloading of the failing left ventricle in order to achieve myocardial reverse remodeling and improvement of contractile function has been developed as a strategy with the increasing frequency of implantation of left ventricular assist devices in clinical practice. But, reverse remodeling remains an elusive target, with high variability and exact mechanisms still largely unclear. The small animal model of heterotopic heart transplantation (hHTX) in rodents has been widely implemented to study the effects of complete and partial unloading on cardiac failing and non-failing tissue to better understand the structural and molecular changes that underlie myocardial recovery. We herein review the current knowledge on the effects of volume unloading the left ventricle via different methods of hHTX in rats, differentiating between changes that contribute to functional recovery and adverse effects observed in unloaded myocardium. We focus on methodological aspects of heterotopic transplantation, which increase the correlation between the animal model and the setting of the failing unloaded human heart. Last, but not least, we describe the late use of sophisticated techniques to acquire data, such as small animal MRI and catheterization, as well as ways to assess unloaded hearts under "reloaded" conditions. While giving regard to certain limitations, heterotopic rat heart transplantation certainly represents the crucial model to mimic unloading-induced changes in the heart and as such the intricacies and challenges deserve highest consideration. Careful translational research will further improve our knowledge of the reverse remodeling process and how to potentiate its effect in order to achieve recovery of contractile function in more patients.

Clinical, molecular, and genomic changes in response to a left ventricular assist device

The use of left ventricular assist devices in treating patients with end-stage heart failure has increased significantly in recent years, both as a bridge to transplantation and as destination therapy in those who are ineligible for cardiac transplantation. This increase is based largely on the results of several recently completed clinical trials with the new second-generation continuous-flow devices that showed significant improvements in survival, functional capacity, and quality of life. Additional information on the use of the first- and second-generation left ventricular assist devices has come from a recently released report spanning the years 2006 to 2009, from the Interagency Registry for Mechanically Assisted Circulatory Support, a National Heart, Lung, and Blood Institute-sponsored collaboration between the U.S. Food and Drug Administration, the Centers for Medicare and Medicaid Services, and the scientific community. The authors review the latest clinical trials and data from the registry, with tight integration of the landmark molecular, cellular, and genomic research that accompanies the reverse remodeling of the human heart in response to a left ventricular assist device and functional recovery that has been reported in a subset of these patients.

Taking pressure off the heart: the ins and outs of atrophic remodelling

Our work on atrophic remodelling of the heart has led us to appreciate the simple principles in biology: (i) the dynamic nature of intracellular protein turnover, (ii) the return to the foetal gene programme when the heart remodels, and (iii) the adaptive changes of cardiac metabolism. Although the molecular mechanisms of cardiac hypertrophy are many, much less is known regarding the molecular mechanisms of cardiac atrophy. We state the case that knowing more about mechanisms of atrophic remodelling may provide insights into cellular consequences of metabolic and haemodynamic unloading of the stressed heart. Overall we strive to find an answer to the question: 'What makes the failing heart shrink and become stronger?' We speculate that signals arising from intermediary metabolism of energy-providing substrates are likely candidates.

Contractile function is preserved in unloaded hearts despite atrophic remodeling

OBJECTIVE

Recent studies have shown that mechanically unloading a failing heart may induce reverse remodeling and functional improvement. However, these benefits may be balanced by an unloading-related remodeling including myocardial atrophy that might lead to decrease in function. Using a model of heterotopic heart transplantation, we aimed to characterize the myocardial changes induced by long-term unloading.

MATERIAL AND METHODS

Macroscopic as well as cellular and functional changes were followed in normal hearts unloaded for a 3-month period. Microscopic parameters were evaluated with stereologic methodology. Myocardial contractile function was quantified with a Langendorff isolated, perfused heart technique.

RESULTS

Atrophy was macroscopically obvious and accompanied by a 67% reduction of the myocyte volume and a 43% reduction of the interstitial tissue volume, thus accounting for a shift of the myocyte/connective tissue ratio in favor of noncontractile tissue. The absolute number of cardiomyocyte nuclei decreased from 64.7 +/- 5.1 x 10(7) in controls to 22.6 +/- 3.7 x 10(7) (30 days) and 21.6 +/- 3.1 x 10(7) (90 days) after unloading (P < .05). The numeric nucleic density in the unloaded myocardium, as well as the mean cardiomyocyte volume per cardiomyocyte nucleus, remained constant throughout the 90 days of observation. Functional data indicated an increase in ventricular stiffness, although contractile function was preserved, as confirmed by unaltered maximal developed pressure and increased contractility (maximum rate of left ventricular pressure development) and relaxation (minimum rate of left ventricular pressure development).

CONCLUSION

Atrophic remodeling involves both the myocyte and interstitial tissue compartment. These data suggest that although there is decreased myocardial volume and increased stiffness, contractile capacity is preserved in the long-term unloaded heart.

Current State of the Art in Myocardial Tissue Engineering

Myocardial tissue engineering aims to repair, replace, and regenerate damaged cardiac tissue using tissue constructs created ex vivo. This approach may one day provide a full treatment for several cardiac disorders, including congenital diseases or ventricular dysfunction after myocardial infarction. Although the ex vivo construction of a myocardium-like tissue is faced with many challenges, it is nevertheless a pressing objective for cardiac reparative medicine. Multidisciplinary efforts have already led to the development of promising viable muscle constructs. In this article, we review the various concepts of cardiac tissue engineering and their specific challenges. We also review the different types of existing biografts and their physiological relevance. Although many investigators have favored cardiomyocytes, we discuss the potential of other clinically relevant cells, as well as the various hypotheses proposed to explain the functional benefit of cell transplantation.

The reduction of hemodynamic loading assists self-regeneration of the injured heart by increasing cell proliferation, inhibiting cell apoptosis, and inducing stem-cell recruitment

OBJECTIVES

Mitotic cardiomyocytes and cardiac stem cells have been identified recently in adult hearts, and both have been found to be increased in acute infarcted myocardium. Although these findings suggest potential self-repair of the heart after injury, obvious self-regeneration of the injured heart has never been observed clinically. We hypothesized that hemodynamic loading impairs myocardial repair.

METHODS

Myocardial infarction was induced in C57BL/6 mice by ligating the left anterior descending artery. After 60 minutes, either the infarcted heart was transplanted heterotopically into a healthy recipient C57BL/6 mouse to remove the ventricular hemodynamic loading (unloading group) or it was left as an infarcted heart under normal hemodynamic loading conditions in the same mouse (loading group). The infarcted hearts were dissected for histologic analysis after 3, 7, 14, and 28 days.

RESULTS

Histologic analysis showed that the wall thickness of the infarcted left ventricle was significantly greater and the area of infarction was significantly smaller in the unloading group than in the loading group. Immunostaining analysis revealed significantly more Ki-67-positive cells and significantly fewer apoptotic cells in the infarcted myocardium in the unloading group than in the loading group. There were also significantly more c-kit- and Sca-1-positive stem cells in the infarcted myocardium in the unloading group than in the loading group.

CONCLUSION

Our findings suggest that hemodynamic unloading assists self-regeneration of the injured heart by increasing cell proliferation, inhibiting cell apoptosis, and inducing stem-cell recruitment.

Improved assessment of graft function by echocardiography in cynomolgus monkey recipients of hDAF-transgenic pig cardiac xenografts

BACKGROUND

The current practice of evaluating heterotopic heart xenografts by palpation allows only detection of severe graft dysfunction, which indicates terminal graft failure. Therefore, we evaluated whether echocardiography is a better method of detecting early graft dysfunction as a marker of rejection in abdominal pig heart xenografts in cynomolgus monkeys.

METHODS

Six cynomolgus monkeys received heterotopic heart transplants from pig donors transgenic for human decay-accelerating factor (hDAF). Induction therapy consisted of either cyclophosphamide or rabbit anti-thymocyte globulin. Maintenance therapy consisted of cyclosporine or tacrolimus, steroids, and sodium mycophenolate or mycophenolate mofetil, GAS914 (alphaGal oligosaccharide containing glycoconjugate), and for some animals TP10 (soluble complement receptor type 1). Echocardiography was performed immediately after transplantation and 3 times a week after surgery. We scored contractility and measured left ventricular wall thickness. Impaired contractility or increased wall thickness were considered graft dysfunction and were treated with pulse steroids. Palpation score was recorded daily. We also obtained myocardial biopsy specimens.

RESULTS

Palpation score remained at 4 out of 4 in all animals until 2 to 5 days before final graft failure, whereas echocardiography detected several episodes of impaired graft function, either decreased left ventricular contractility or increased left ventricular wall thickness before graft failure. Treatment with pulse steroids improved graft function only during early episodes of graft impairment. Final graft failure was steroid resistant and caused by severe vascular rejection.

CONCLUSIONS

Echocardiography is a better method of assessing graft dysfunction than is palpation. Therefore, echocardiography may detect early rejection episodes of heterotopic heart xenografts in non-human primates.

Morphological and functional magnetic resonance imaging after heterotopic heart transplantation

In patients with severe congestive heart failure, a marked elevation in pulmonary vascular resistance limits the success of orthotopic cardiac transplantation, providing the rationale for heterotopic transplantation. In the case reported, cardiac anatomy and function of two hearts in the same chest were imaged using magnetic resonance imaging (MRI). MRI offers a high-resolution, three-dimensional, noninvasive technique to visualize the complex anatomy after heterotopic heart transplantation, providing information of morphologic and functional parameters at the same time. The challenge of sufficient electrocardiogram triggering, hindered by two hearts with electrophysiological activity in the same chest, can be overcome using new real-time techniques.

Demonstrating time sequence and extent of sustained decrease in native heart ejection fraction after heterotopic transplantation

BACKGROUND

In this study we investigate the time sequence and extent of the sustained decrease in native heart ejection fraction (EF) after heterotopic heart transplantation (HHTx) when using gated cardiac blood pool scanning (GCBPS) and transthoracic echocardiography (TTE) One case report of 2 patients used post-operative GCBPS and TTE and found a significant deterioration in native heart EF post-operatively over the course of several years. Comparison with pre-operative measurements using these techniques in a series of patients has not been performed previously.

METHODS

Thirteen of 16 HHTx patients with adequate pre- and post-operative GCBPS follow-up were included in this study. All patients also underwent TTE post-operatively and the GCBPS results were correlated with the TTE findings.

RESULTS

GCBPS demonstrated a marked (21.1 +/- 4.7% vs 10.5 +/- 3.7%, p < 0.0001) decrease in native EF post-HHTx. Spontaneous echo contrast in the native left ventricle and/or poor opening of the mitral/aortic valves was noted at Day 1 in 4 of 5 patients who had a TTE at this stage. No further decline was noted between the first and last post-operative GCBPS (10.8 +/- 3% vs 8.6 +/- 2.1%, p = NS).

CONCLUSIONS

A dramatic decrease in native heart EF post-HHTx occurs as early as Day 1 post-transplant. Dissociation of ventricular contraction is the most likely cause. Studies have demonstrated that paced linkage (counterpulsation) between the ventricles results in improved hemodynamics. This may have clinical implications as to the timing of ejection of blood from a left ventricular assist device (LVAD) and for providing the best hemodynamics for the ventricle being assisted and for optimizing its chances of long-term recovery.