Finding a donor heart is not simply a matter of finding the next person on a waiting list. When a heart becomes available, the transplant system compares important medical and logistical information about the donor and potential recipients to determine which patients can safely receive that heart.
For someone waiting for a heart transplant surgery, understanding the donor matching process can help explain why a suitable heart may not become available immediately, why some patients receive an offer sooner than others, and why a particular donor heart may ultimately not be accepted.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
What Is Heart Transplant Donor Matching?
Heart transplant donor matching is the process of determining whether a donated heart is medically and practically suitable for a person waiting for transplantation. Several factors are considered together, including:
- Blood group compatibility
- Donor and recipient body size
- The recipient's medical urgency
- Antibodies that could react against the donor heart
- Distance between the donor and transplant centre
- How long the recipient has been waiting, where applicable under the local allocation system
- The condition and function of the donor heart
- Other donor-specific and recipient-specific medical factors
Is a Heart Transplant Waiting List Just a Queue?
No. A heart transplant waiting list is generally not simply a first-come, first-served queue. When a donor heart becomes available, the transplant system generates a new set of potential recipients based on the applicable allocation rules. Patients are then prioritized according to factors such as medical urgency, compatibility and logistical considerations.
For example, NHS Blood and Transplant explains that patients are prioritized according to urgency, blood group, donor-recipient size, antibodies, geography and waiting time. In the United States, heart allocation similarly considers medical urgency, distance and pediatric status under the current allocation framework.
Step 1: A Donor Heart Becomes Available
The matching process begins when a heart from a deceased donor becomes available for transplantation. The donor is carefully assessed before the heart is offered.
Medical teams evaluate the donor's health, the condition and function of the heart, blood group, body size and other relevant information.
Testing may also look for infections and other conditions that could affect the safety or suitability of transplantation. For example, donor assessment can include an echocardiogram and other investigations of heart function.
Step 2: The Recipient's Information Is Compared With the Donor's
Once the donor information is available, the transplant allocation system identifies potential recipients whose medical characteristics make them possible candidates for that heart.
The system considers information already collected during the recipient's transplant evaluation, including blood group, body measurements, urgency status and antibody information.
This process can happen quickly because a donor heart cannot be preserved indefinitely. The heart therefore has to be allocated, transported and transplanted within a limited period.
Blood Group Compatibility
Blood group is one of the fundamental factors considered when matching a donor heart with a recipient. The main blood groups are A, B, AB and O. In adults, a donor heart generally needs to have a compatible ABO blood group because an incompatible blood group can trigger a serious immune reaction.
As a simplified example:
- A recipient can generally receive a heart from an A or O donor.
- B recipients can generally receive from B or O donors.
- AB recipients can generally receive from A, B, AB or O donors.
- O recipients generally receive from O donors.
Why Does Donor and Recipient Size Matter?
The donor heart needs to be capable of supplying enough blood for the recipient's body. For this reason, the transplant team considers the donor's and recipient's body size, including height and weight and, increasingly, measures designed to estimate the heart's size relative to the recipient.
A heart that is too small for the recipient may not be able to provide adequate circulation, while significant size differences can increase the technical or physiological challenges of transplantation.
Size matching is particularly important when the recipient has certain conditions, such as significant pulmonary hypertension, because the transplanted right ventricle may have to work against increased pressure in the lungs. Donor selection guidelines therefore consider recipient pulmonary pressures alongside donor size and other characteristics.
What Role Do Antibodies Play in Heart Transplant Matching?
Another important part of matching involves the recipient's immune system. Some people waiting for transplantation have antibodies that can recognize proteins found on another person's cells. These are commonly referred to as anti-HLA antibodies.
If a recipient has antibodies that react strongly against the donor heart, the transplanted organ may be at increased risk of antibody-mediated rejection. Therefore, transplant teams assess the recipient's antibody profile and compare it with the donor's relevant tissue markers.
This is particularly important in people who are highly sensitized. Previous transplantation, blood transfusions and pregnancy can be among the events associated with developing antibodies, although sensitization can have other causes as well.
What Is a Crossmatch?
A crossmatch is a laboratory assessment used to determine whether the recipient's antibodies are likely to react against the donor's cells or tissues.
In heart transplantation, antibody information may be assessed before the donor heart is accepted. Depending on the circumstances and the transplant centre, this can involve a virtual crossmatch based on previously available HLA information or a physical laboratory crossmatch.
The purpose is to identify potentially harmful donor-specific antibodies and reduce the risk of a serious immune reaction after transplantation.
For highly sensitized patients, this part of donor selection can significantly reduce the number of donor hearts that are considered suitable.
Why Does Distance Between the Donor and Recipient Matter?
A donor heart has a limited preservation time outside the body. Once the heart is removed from the donor, it needs to reach the recipient and be transplanted as quickly and safely as possible.
For this reason, geographic distance and transportation time can influence donor allocation. A heart may be medically suitable for more than one recipient, but a patient located closer to the donor hospital may sometimes have an advantage because reducing transportation time can help limit the period during which the heart is without its normal blood supply.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Does Waiting Time Determine Who Gets the Heart?
Waiting time can matter, but it is not necessarily the most important factor. A patient who has waited longer does not automatically receive every compatible heart.
Medical urgency, blood group, size, antibody compatibility, geography and other allocation criteria can affect priority. In other words, the transplant waiting list is better understood as a dynamic matching system rather than a simple numbered queue.
What About the Donor Heart's Health?
Matching a patient with a donor is not only about whether their blood group and size are compatible. The donor heart itself must be assessed carefully. The transplant team may review:
- Heart pumping function
- Heart structure and valve function
- Donor medical history
- Potential infections
- Other donor health conditions
- Age and other donor characteristics
- Expected preservation and transportation time
A donor heart may meet basic matching criteria but still be declined if further assessment suggests that it is unlikely to provide an acceptable outcome for the intended recipient.
Who Makes the Final Decision?
A computerized allocation or matching system can identify and prioritize potential recipients, but the final decision to accept a particular donor heart involves the transplant team.
When an organ offer is made, the transplant clinicians review the donor information alongside the recipient's current medical condition and determine whether proceeding is appropriate.
This is important because the recipient's condition may change while waiting, and the suitability of a donor heart has to be assessed in the context of that patient's specific medical circumstances.
What Happens When a Suitable Heart Is Found?
When a donor heart appears suitable for a recipient, the transplant centre is contacted and the medical team reviews the offer.
The recipient may be asked to come to the transplant centre urgently. Additional tests are performed to confirm that the patient remains fit for surgery and that the donor heart is still considered suitable.
If the transplant team accepts the heart and all final checks are satisfactory, the patient proceeds to the operating room for transplantation.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
The process can happen very quickly because both the donor heart and recipient must be managed within strict time limits.
The Bottom Line
Heart transplant donor matching is a carefully coordinated process that considers much more than blood type. Blood group, donor and recipient size, medical urgency, antibodies, geography, waiting time and the condition of the donor heart can all influence whether a particular heart is suitable.
The process is designed to balance two important goals: giving the donor heart to a patient who can benefit from it while making sure the organ can be transplanted safely within the limited time available.
For patients waiting for transplantation, this is why receiving a donor offer can take time and why not every available heart will be suitable. A patient may remain on the transplant list while the system continues looking for a donor heart that meets the necessary medical and logistical requirements.
