Introduction

Heart transplantation is the most effective treatment for end-stage heart disease; however, it carries a substantial operative mortality risk of between 5 and 10 % related mainly to primary allograft failure [1]. Multiple donor, recipient, and procedural factors have been implicated in the pathogenesis of primary graft failure [2•]. Few of these risk factors are modifiable, but donor recipient size and gender matching are two risk factors that are. The major goal of donor–recipient matching is to avoid size discrepancies that adversely affect cardiac function and place the recipient at risk of primary graft failure. Current guidelines from the International Society of Heart and Lung Transplantation recommend the use of adult heart donors to within 30 % of recipient body weight, and further limit female to male donations to within 20 % [3•]. The guidelines state that a male donor of average weight (70 kg) can be safely used for any recipient regardless of weight [3•]; however, these recommendations are based on studies published more than 20 years ago [4–7]. Since then, there have been substantial changes in the characteristics of both donors and recipients, driven in part by the paucity of suitable donor organs relative to need and in part by the dramatic evolution of mechanical circulatory support for patients with end-stage heart failure. In light of these changes, we undertook a review of recent publications examining the impact of donor–recipient size and gender matching on post-transplant outcomes. The aim of this review was first to determine whether recent publications support the current recommendations regarding donor–recipient matching and, second, whether there may be alternative methods of donor–recipient matching that more accurately predict post-transplant outcomes.

Changing Characteristics of Donors and Recipients

The chronic shortage of standard criteria donor hearts has resulted in increased utilization of hearts from so called ‘marginal’ or ‘extended criteria’ donors [8]. These include hearts from older donors, and hearts with myocardial dysfunction, left ventricular hypertrophy, extended ischemic times, conduction abnormalities, or a combination of these [8–12]. While use of these donors has helped to maintain or increase heart transplant activity, their use is associated with an increased risk of primary graft failure and post-transplant mortality [2•]. As a consequence, there is considerable variability between individual transplant centers regarding acceptance of marginal donors [13], and also uncertainty as to whether the same donor–recipient size-matching and gender-matching criteria should be applied to these donors as for standard criteria donors.

The success of heart transplantation as a therapy for end-stage heart failure has resulted in a progressive broadening of recipient acceptance criteria, with the result that the potential pool of heart transplant recipients is dramatically larger now than it was 20 years ago. The changing characteristics of heart transplant recipients were highlighted in the latest report of the International Society for Heart and Lung Transplantation, which noted increasing age and co-morbidities of heart transplant recipients in the most recent era [8]. As with changing donor characteristics, changing recipient characteristics, such as advanced age and diabetes mellitus, are associated with an increased risk of primary graft failure [2•, 14•]. In addition, broadening acceptance criteria has increased the number of people waiting for heart transplantation. In the absence of a corresponding increase in donor organ availability, this has led to an increased reliance on acute and/or chronic mechanical circulatory support to bridge patients to heart transplantation [1, 8]. Recent publications from large observational registries demonstrate an increased risk of primary graft failure and mortality after transplantation in patients supported with extra-corporeal mechanical assist devices [15, 16]. However, post-transplant outcomes with long-term intra-corporeal pulsatile or continuous flow left ventricular assist devices (VADs) are more favorable, with post-transplant outcomes comparable to those of non-VAD patients [17, 18]. Nonetheless, mechanical circulatory support prior to heart transplantation is associated with increased cardiopulmonary bypass time, bleeding, and transfusion requirements at the time of transplantation [19]. These events may trigger both an acute pulmonary hypertensive reaction and a systemic inflammatory response syndrome with resultant vasoplegia [19], circumstances in which an undersized or otherwise marginal donor heart is likely to fail.

The Validity of Using Body Weight as a Means of Sizing Hearts

Currently, the most widely utilized measure to match cardiac donors to recipients is body weight ratio; however, the validity of using body weight to estimate heart size and as a basis for matching donors and recipients has been challenged recently by the findings of a number of large retrospective heart transplant registry studies [20, 21••, 22••, 23••, 24]. The details and major findings of these studies are summarized in Table 1.

Table 1 Recent large-scale registry studies on the impact of size and gender matching on outcomes post-heart transplantation
Full size table

Post-Transplant Outcomes with Undersized Hearts

Although Smits et al. [25] reported that a donor–recipient weight ratio <0.8 was associated with inferior post-transplant survival in the Eurotransplant Heart Transplant Registry, this was not found to be the case in a larger United Network for Organ Sharing (UNOS) Registry study reported by Patel et al. [22••]. In the UNOS Registry Study, donor–recipient weight ratio <0.8 was not a significant predictor of mortality post-heart transplantation, with the exception that recipients with high pulmonary vascular resistance (PVR) who received an undersized heart had poorer survival. In addition, males with elevated PVR receiving hearts from female donors had significantly worse survival than males who received hearts from male donors [22••]. In a subsequent UNOS Registry study, Jayarajan et al. [20] reported that donor–recipient weight ratios down to <0.6 were not associated with any increase in mortality in male-to-male, female-to-female, and male-to-female heart transplants; however, donor recipient weight ratios <0.9 were associated with increased mortality in female-to-male transplants [20]. In the most recent and largest analysis of the UNOS Registry published to date, Reed et al. [23••] analyzed post-transplant survival in 31,634 heart transplant recipients stratified by donor–recipient weight ratio. A total of 86 % of donors were within 30 % of the recipient weight, reflecting current guidelines. When stratified into septiles from the most undersized to the most oversized weight-matches, they observed no difference in post-transplant survival across the seven categories.

Post-Transplant Outcomes with Oversized Hearts

Recent large UNOS Registry analyses have also compared the post-transplant outcomes of transplants performed with oversized hearts compared with size-matched hearts [22••, 23••]. Both analyses found no survival benefit or risk in adult heart transplants performed with oversized hearts compared with optimally sized hearts, including in recipients with elevated PVR. In the most recent UNOS report, male recipients of oversized female hearts were uncommon, representing <1 % of the study cohort, whereas the majority of male-to-female transplants were oversized [23••]. Interestingly, Reed et al. [23••] observed that treatment for rejection during the first year post-transplant was significantly reduced in oversized heart transplants and increased in undersized hearts compared with ideally sized matched heart transplants.

Oversized cardiac allografts are often implanted in small children and infants for whom optimally sized donors are scarce [26]. Recipients of oversized allografts are at risk of ‘big heart or hyperperfusion syndrome’, which usually manifests as an acute neurological syndrome with headache, drowsiness, and occasionally fitting in the immediate post-transplant period [27]. Otherwise, post-transplant morbidity and long-term post-transplant survival appear to be similar in pediatric recipients of oversized compared with optimally sized hearts [26].

The Interaction Between Size Matching and Gender Matching

Multiple observational registry studies have highlighted the increased mortality risk associated with the transplantation of an undersized female donor heart into a male recipient, particularly in the context of recipient pulmonary hypertension [20, 22••, 24, 25]. The first large multicenter registry study to specifically examine the impact of donor and recipient gender on post-transplant outcomes was that of Weiss et al. [28•]. These authors undertook a retrospective review of 18,240 heart transplant recipients in the UNOS Registry. They confirmed the adverse outcomes associated with transplantation of female donors into male recipients. They also reported that female recipients experienced an increased mortality relative to male recipients. There was a trend towards increased mortality in male donor to female recipient (M:F) transplants compared with female donor to female recipient transplants (F:F), but this was non-significant [28•]. The largest study to examine the impact of donor–recipient gender mismatching on post-transplant outcomes was that conducted by Khush et al. [21••]. These authors undertook a retrospective review of 60,584 heart transplant recipients enrolled in the International Society for Heart and Lung Transplantation Registry. They reported that donor–recipient gender mismatched transplants (both F:M and M:F) were associated with a statistically significant 10 % increase in adjusted mortality compared with gender-matched transplants (M:M and F:F). These differences persisted after adjusting for donor–recipient weight ratio, which led the authors to conclude that the findings were related to gender differences rather than size differences; however, inspection of the survival curves suggests different mechanisms of death depending on the type of gender mismatch. Whereas the excess mortality in F:M transplants was evident in the immediate post-transplant period consistent with death due to primary graft failure, the excess mortality in M:F transplants only developed years after transplantation, suggesting an alternative explanation for the increased mortality. Similar differences in post-transplant mortality trends between F:M and M:F transplants were reported in the study by Weiss et al. [28•].

Reed et al. [23••] also examined the interaction between donor size mismatch and gender mismatch. Consistent with previous reports, they found that, in unadjusted analyses, F:M transplants (but not M:F) were associated with a significant increase in early post-transplant mortality. After adjusting for multiple co-variates, including predicted heart mass, they observed the opposite trends – no difference in survival in F:M transplants but an increased mortality in M:F transplants. The authors concluded that survival differences in F:M heart transplants were altered by and largely attributable to differences in predicted heart mass. Conversely, the excess mortality in M:F recipients could not be explained by size differences but rather pointed to a different cause of death such as increased rate of rejection [29, 30] or coronary allograft vasculopathy [31, 32].

The Interaction Between Size Matching and Other Risk Factors for Primary Graft Failure

Several studies have highlighted the existence of multiple independent risk factors that contribute to primary graft failure and how these risk factors may interact to magnify the risk [2•, 14•, 33]. In another large retrospective UNOS Registry analysis, Russo et al. [34] reported an adverse interaction between increasing donor age and increasing ischemic time. In that study, there was no detectable adverse effect of ischemic time on survival after heart transplantation when the donor was less than 20 years of age. In contrast, when the donor age increased above 20 years, a prolonged ischemic time had a significant negative impact on survival. This effect became even more marked when the donor age exceeded 35 years. More recently, Segovia et al. [14•] developed a risk prediction score for primary graft failure based on the presence of six independent risk factors, including donor age, ischaemic time, and recipient age [14•]. In another study of patients enrolled in the Cardiac Transplant Research Database, Stehlik et al. [24] reported that donor–recipient weight differences interacted with age and gender. In that study, weight differences of up to 30 % did not affect survival so long as the donor was aged 30 years or younger. Donor and recipient gender further modified the degree of risk: risk was higher in female donors and when recipients were male. Considered together, these studies suggest that optimal donor–recipient size matching is particularly important when other risk factors for primary graft failure (such as increased donor age, older recipient age, or prolonged ischemic time) are also present.

Alternative Approaches to Donor–Recipient Size Matching

A number of authors have suggested that alternative approaches to donor–recipient size matching [23••, 35, 36] may be more predictive of post-transplant outcome and have postulated that alternative approaches to donor–recipient matching could potentially expand opportunities for transplantation that may otherwise be overlooked due to concern about undersizing [21••, 22••, 23••]. Reed et al. [23••] calculated predicted heart mass using formulae derived from studies using cardiac magnetic resonance imaging (MRI) in normal adults and that incorporate height, weight, age, and sex [23••, 37, 38]. Using this model of predicted heart mass (pHM), size mismatch was found to be a powerful independent predictor of survival, with recipients of the most undersized hearts (donor pHM >10–15 % below recipient pHM) having a 25 % increase in mortality and a 33 % increase in acute rejection at 1 year compared with ideally matched donor recipient pairs. Strikingly, the increase in mortality in the most undersized hearts was greatest in the first 30 days, with a 36 % increase in relative risk of death during this period. Although cause of death was not specifically analyzed in the study, this finding suggests that an increased rate of primary graft failure in the most undersized cohort was the most likely explanation. As suggested by the authors, adoption of donor–recipient pHM ratio to match donors and recipients is unlikely to increase the number of heart transplants performed, but it is likely to lead to greater utility by a redistribution of donor hearts to recipients who are at lower risk of primary graft failure. Another potential benefit is an expanded opportunity for female recipients through better recognition of acceptable size matching with small male donors who could otherwise be overlooked due to concerns of undersizing based on body weight differences [23••].

Some authors have also suggested the use of simply obtained echocardiographic measurements to assess total cardiac volume. This may be particularly valuable in pediatric heart transplantation where use of markedly oversized donors may necessitate delayed chest closure to prevent hemodynamic compromise of the newly transplanted heart. Camarda et al. [35] reported that left ventricular end-diastolic volume measured by a modified Simpson’s rule from a four-chamber view of the heart correlated extremely well with MRI-determined total cardiac volume. Zuckerman et al. [36] demonstrated a highly significant linear correlation between body height and a novel measurement from the superior vena cava-right atrium junction to inferior vena cava-right atrium junction and suggested that use of these parameters would provide better donor recipient size matching than the use of body weight. As yet, no studies have examined post-transplant outcomes using these echocardiographic measurements, and it remains to be seen whether either method provides a more accurate prediction of post-transplant outcomes compared with matching based on body weight.

Future Trends and Conclusions

Matching potential heart donors with suitable recipients is a complex process that requires consideration of issues of utility, equity, and logistics. The changing characteristics of both donors and recipients, combined with an increased risk of primary graft failure, have led to a renewed focus on donor heart preservation and donor–recipient matching. Novel approaches to donor organ preservation and retrieval such as ex vivo machine perfusion of donor hearts are exciting developments; however, it remains to be seen whether these approaches lower the risk of primary graft failure and improve post-transplant outcomes. Regardless of developments in donor heart preservation, optimal donor–recipient size matching will remain an essential step in ensuring the success of heart transplantation. Traditional methods of size matching for heart transplantation based on donor and recipient body weight have been found to be poorly predictive of post-transplant outcomes in the current era. Newer methods of donor–recipient size matching that take into account age, gender, and height in addition to weight appear to be more reliable in predicting heart transplant outcomes and, if adopted, should enhance the safety of this life-saving procedure.