eComment. Lung size mismatch and graft dysfunction immediately after reperfusion

We read with great interest the manuscript by Mizota et al. describing the incidence and timing of primary graft dysfunction (PGD) in living donor lobar lung transplantation (LDLLT) and comparing this to cadaveric lung transplantation (CLT) [1]. The authors found that in LDLLT patients, the onset of severe PGD tended to be earlier and that severe PGD immediately after reperfusion was a significant predictor of postoperative morbidity and mortality.

Donor-to-recipient lung size mismatch in LDLLT is a common occurrence. We have conducted a series of investigations on the associations between donor-to-recipient lung size mismatch and outcomes after lung transplantation in CLT-recipients and have found the following:

(a) Undersizing (as assessed by the donor-to-recipient predicted total lung capacity ratio [pTLCratio]) is an independent risk factor for PGD in bilateral CLT [2];

(b) In a study of bilateral CLT, tidal volumes during mechanical ventilation (when tidal volumes were indexed to donor predicted body weight, as an estimate of the actual size of the allograft), were substantially higher (and potentially injurious), if the allograft was undersized compared to oversized allografts [2,3];

(c) Undersizing is an independent predictor of one-year mortality following CLT [4].

In the study by Mizota et al., lung size matching was assessed by calculating graft forced vital capacity. The majority of patients undergoing LDLLT with grade 3 PGD at reperfusion had a single-lung lobar transplant (55%) and were <15 years old (64%) [1]. The authors reported that there was no association between graft forced vital capacity and the development of grade 3 PGD at reperfusion in LDLLT. However, it is not clear how the authors accounted for the impact of single-lung LDLLT and paediatric age in their assessment of lung size mismatch. It would be helpful, if the authors could provide pTLCratio data (actual donor pTLC transplanted via lobar transplant/pTLC of the recipient) and the association between pTLCratio and PGD.

A consequence of ischaemia reperfusion injury is endothelial cell leak. Pulmonary oedema formation following reperfusion from this mechanism is transient and most apparent in the first 5–15 minutes of reperfusion. If an allograft is undersized below a critical threshold regarding the size of the pulmonary vascular bed, it is possible that this could lead to pulmonary oedema formation immediately following reperfusion. To avoid this, either a larger vascular surface area (larger allograft) would need to be transplanted, or an intra- and postoperative strategy that uses extracorporeal support to unload the undersized allograft would be needed, until the endothelial cell leak is resolved.

Furthermore, we recommend lung protective mechanical ventilation with low tidal volumes (6 ml/kg predicted body weight) and positive end expiratory pressure for the post-lung transplant period. In our opinion, the tidal volume needs to be based on donor-characteristics (donor predicted body weight, as a parameter of actual allograft size), rather than based on recipient characteristics to avoid potentially injurious ventilator settings for the recipients of undersized allografts [5].

Conflict of interest: none declared.