In this issue of Liver Transplantation, Kim et al. share their experience in performing living donor liver transplantation with right lobe grafts, and they demonstrate that in well-selected donors, remnant liver volumes < 30% can sustain donors' lives with limited morbidity and no mortality. The message seems to be that the standard remnant volume cutoff of 30% is arbitrary, and a lower donor remnant volume can be accepted under certain conditions, such as a young donor age, little steatosis, and exclusion of the middle hepatic vein with the graft. The authors are to be congratulated for their meticulous technique, careful patient management, and excellent outcomes. However, a word of caution is needed for those performing living donor liver transplantation.
The narrative of the article suggests that lower limits for the donor remnant volume (<30%) are acceptable under certain circumstances. Donor safety is paramount, but the message of this article runs counter to this principle and shifts more of the risk to the donor for the benefit of the recipient. This is a potentially dangerous proposition and might even be considered unacceptable by some because donor mortality with right lobe grafts is approximately 0.2% to 0.5% with the currently accepted donor remnant volume (>30%). As a result of the significant morbidity and mortality associated with right lobe donation, the current sentiment is switching back toward shifting the risk of living donor liver transplantation to the recipient rather than the donor.
As we gain a better understanding of the interplay of recipient disease severity, portal hypertension, and portal and arterial flow phenomena as the cause of graft dysfunction rather than simply graft size, an opportunity exists for us to rethink the use of smaller left lobe grafts for a wide range of recipients with surgical inflow modification and pharmacological manipulation. Such grafts should prove to be adequate for the recipient and safer for the donor. An excellent article by Roll et al. evaluates this issue of double equipoise and provides a framework for proceeding with smaller grafts in recipients rather than compromising donor safety.
In their report, Kim et al. admit that their initial cutoff for the donor remnant volume was actually 30%, but undoubtedly because of errors in the calculation of the SLV as well as deviations from the intended resection plane, some donors were left with significantly less remnant volume than intended (20%-30%). Fortunately, all the donors in this group eventually did well. This can be credited not only to the program's clinical expertise but also to good fortune.
The Achilles' heel of remnant liver volume estimation in donors is the proper evaluation of the SLV. In a recent report from our group evaluating all of the different equations for estimating SLV and comparing them to software-assisted actual measurement, we found that the SLV varied by 59%, and this depended on the equation used. Therefore, the choice of the equation for estimating the SLV can have a profound impact on the projected graft size and the eventual remnant volume in the donor. In this report, the authors estimated the SLV with Vauthey et al.'s equation, which uses body weight. This equation is a reasonably good choice for estimating the SLV and, in our experience, has a percentage error of +6.5% in comparison with software-assisted SLV estimates. The overestimation of the SLV with Vauthey et al.'s equation may have provided some buffer of safety in the present study through an overestimation of the initial SLV. The choice of another equation with considerably higher percentage errors for the SLV could have been disastrous for the donors and the recipients.
The present study by Kim et al. pushes the limit of the donor remnant volume to new extremes and suggests that remnant volumes as low as 23% can be tolerated in a healthy living liver donor. The removal of this much liver can be easily justified for a patient with a tumor in an attempt at a cure but should probably not be tolerated in donors in an attempt to improve recipient outcomes. The time has come to revisit smaller segmental grafts that shift the risk back to the recipient. Refinements and more accurate equations for estimating and measuring the SLV exist, and at the same time, we have gained a better understanding of the pathophysiology of segmental graft dysfunction (it is not all about size). Surgical innovations and pharmacological manipulations have emerged that may help to prevent graft dysfunction (so-called small-for-size syndrome) despite a small graft size. The current study demonstrates that good outcomes can be achieved by experienced centers even when they push the physiological limits of liver resection. A major concern is that new programs and less experienced centers performing living donor liver transplantation may not have such good fortune when they tempt these extremes.