Reducing the Wait: Total Cardiac Volume Can Expand the Donor Pool for Heart Transplant Candidates

Abstract

A 16-year-old with new-onset dilated cardiomyopathy underwent VAD placement, later complicated by low flow from outflow graft kinking. To expedite heart transplantation, total cardiac volume was calculated and compared with 141 normal patients pinpointing the upper weight threshold. He was transplanted 2 days later within the expanded weight range with no post-transplant complications.

Case Presentation

A 16-year-old previously healthy male presented to an outside hospital in cardiogenic shock and was found to have new-onset dilated cardiomyopathy. Echocardiography showed severe LV chamber dilation and dysfunction with an ejection fraction of 10%. He was started on inotropic support and transferred to our center’s cardiac intensive care unit. A cardiac catheterization showed increased LV end-diastolic pressure of 30 mmHg with estimated pulmonary vascular resistance of 0.8 indexed Wood’s units.

The decision was made to undergo left ventricular assist device (VAD) placement as a bridge-to-transplantation. A virtual fit was performed and the HeartMate III (HM3; Abbott) was determined to be best for this patient. The patient underwent HM3 placement without complication. Following VAD placement, the patient was discharged home for cardiac and nutritional rehabilitation before listing for transplant.

Eight weeks following VAD implant, he presented with low flow, palpitations and chest pain. HM3 device interrogation revealed increased intermittent low flow and pulsatility index events that were only responsive to fluid therapy. A cardiac computed tomography (CT) showed the HM3 outflow cannula had shifted rightward creating a sharp turn over the right chest which confirmed the mechanism for VAD flow restriction. It was decided to list him for transplant with the hope that his VAD could be managed to donor acceptance. He was listed status 1a at a dry weight of 64 kg (height 177 cm, BSA 1.79 m²) with an accepted donor weight range of 60 to 80 kg, equivalent to a donor recipient weight ratio (DRWR) between 0.9 and 1.3. Given his worsening clinical status, the team sought to expand the donor weight range to potentiate offers. A total cardiac volume (TCV) was calculated at 1164 cm³ (Figure 1) from the cardiac CT, using Mimics 3D modeling software (Materialise, Belgium)^1,2. TCV is a 3D estimation of the myocardial and chamber volumes obtained by segmenting the intracardiac blood pool, atrial myocardial borders, ventricular myocardial borders, and the borders of the typical heart transplant anastomotic sites (systemic veins, pulmonary veins, pulmonary artery, and aorta). The recipient TCV was then compared to a virtual TCV donor pool of 146 normal subjects from our center (Figure 2B). Based on the patient’s recipient TCV, the upper limit of donor weight was increased from 80 kg to 100 kg. Within 2 days of increasing the listed weight, a suitable donor heart was offered in a patient weighing 92 kg (height 182 cm, BSA 2.13 m²)(Figure 2). The DRWR was 1.43. There were no intraoperative or postoperative complications related to oversizing. He was extubated on post-operative day (POD) 1 and transferred from the CICU on POD 8. He was discharged home on POD 12. At follow-up 7 months later, the patient continues to do clinically well.

Figure 1:

The TCV segmentation is shown in the axial plane (A), coronal plane (B), and 3-dimensional reconstruction (C). TCV mass is in Red and HMIII is in green. Abbreviations: TCV; Total Cardiac Volume, HMIII; Heartmate III.

Figure 2:

The recipient TCV segmentation (red) overlaid with predicted donor heart TCV (yellow) is shown in multiple planes (A), and in a 3D reconstruction. Other structures are the implanted HM3 device (green), superior vena cava (blue), pulmonary arteries in purple. (B) A best fit line of weight vs. TCV in the normal population (black dots) with the 95% confidence interval (dotted line). The measured recipient TCV (red diamond) is larger than all normal TCVs and the predicted donor TCV (yellow square).

Discussion

Weight-based size matching in pediatric heart transplantation is the current standard, however weight-based organ matching has not consistently been shown to impact outcomes^3-7. There are no guidelines for weight ranges and therefore significant practice variation exists in transplant listing, indicating a need for guidance on size matching to enable optimal utilization of the available organs for transplant³. Furthermore, the most recent guidelines by the International Society of Heart and Lung Transplantation offer no specific guidance for constraining weight range⁸. In adult heart transplantation, weight ratio has long been the standard of care though the adult heart transplant field has been moving toward use of predicted heart mass (PHM)⁹. PHM is a predictive model of right and left ventricular myocardial mass calculated from sex, age, weight, and height¹⁰.

Lacking a heart size measure like PHM in children, TCV-based size matching has recently emerged as a method for more precise organ matching, and has demonstrated on numerous occasions the potential to expand the donor pool for listed patients^1,2,6,11,12. This type of targeted size matching allows avoidance of complications related to oversizing, such as bronchial compression, delayed chest closure, and prolonged length of stay^13,14.

Searching the United Network for Organ Sharing (UNOS) database reveals that it is uncommon for adult-sized patients to undergo heart transplantation when the DRWR is greater than 1.4. The UNOS database (1987-2020) contains 55,816 unique heart transplants with recipient’s age 16 years or older and over 60 kg. For these transplants, the mean DRWR was 0.98 and only 4.2% (n= 2,458) had a DRWR greater than 1.4.

This case exemplifies the benefit of using TCV to inform listed weight ranges for heart transplant candidates. As demonstrated, the wait-time for a donor was likely reduced by increasing the acceptable weight range by 20 kg. Comparing the patient’s actual TCV to our virtual TCV donor pool assured our team that the patient had lower risk for complications related to graft oversizing. While our center has historically been conservative on DRWR, some transplant centers select a less restrictive DRWR of >2 for pediatric patients⁶. However, the actual transplanted DRWR average is 1.64 for these patients and not significantly higher than those with a lower listing DRWR. Our center is cognizant of our conservative DRWR approach, and thus, have recently begun using CT-derived TCV to inform listing behaviors with this case being the first example.

Inclusion of height, age, sex, and Chest X-ray heart diameter has been shown to improve the prediction of donor TCV¹². Further study is needed regarding supplemental imaging, such as echocardiography, to improve TCV prediction. The integration of patient imaging and anthropometrics can enable the field to move toward a more accurate system of TCV-based size matching. Advanced 3D modeling of patient-specific imaging can allow for targeted weight ranges that expand the donor pool and reduce waitlist time, all while providing a data-driven and informed approach for safe size matching practices.

To decrease the time and labor burden of these measurements, future studies may aim to use deep learning convolutional neural networks (DL-CNN) to perform automatic TCV calculation. There has been some progress in the development of DL-CNN autosegmentation algorithms using cardiac CT and magnetic resonance imaging (MRI); however, this is specific to intracardiac blood pool segmentation only and not transplant-focused TCV segmentation as described above.¹⁵. Additionally, future investigation may determine whether TCV methods are beneficial in adult heart transplantation.

In conclusion, size matching in pediatric heart transplantation listing is predominantly determined by comparing donor and recipient weights; yet, there is insufficient evidence to guide the acceptable upper and lower limits of donor weight. Total cardiac volume-based size matching holds potential for more precise organ matching in pediatric heart transplantation.

Abbreviations

CT

Computed Tomography

DRWR

Donor recipient weight ratio

HM3

Heartmate 3

POD

Post-operative day

TCV

Total Cardiac Volume

UNOS

United Network for Organ Sharing

VAD

Ventricular Assist Device

Data Availability Statement:

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.