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Reprint requests: Selim M. Arcasoy, MD, Lung Transplant Program, Columbia University College of Physicians & Surgeons, PH-14 East, Room 104, 622 West 168th Street, New York, NY 10032. Telephone: 212-305-6589. Fax: 212-342-5382.
Primary graft dysfunction (PGD) after lung transplantation results from an acute lung injury developing in the transplanted organ(s) in the immediate post-operative period. There is a wide spectrum of severity from subtle infiltrates on chest X-ray with mild impairment of gas transfer in an asymptomatic patient to a full-blown acute respiratory distress syndrome (ARDS)-like phenomenon manifest by diffuse infiltrates on chest X-ray and severe hypoxemic respiratory failure requiring a high level of critical care support.
There are many published data on the incidence and outcomes of PGD. However, interpretation of these data is limited by the lack of a widely accepted and concise definition of PGD, which has led to the use of different definitions in each individual study (Table 1). As a result, both the incidence and outcomes of this major early post-operative problem are significantly affected by differing definitions and patient populations included in studies of PGD.
Table 1Time and Severity Definitions of Primary Graft Dysfunction Used in Prevalence and Outcome Studies
Less stringent definitions will include milder cases, thereby increasing the incidence of PGD, decreasing its impact on outcomes, and interfering with the identification of variables that may be associated with adverse short- and long-term outcomes after severe forms of PGD. Conversely, more stringent definitions that include only the more severe cases of PGD will reduce its incidence, but will increase its apparent impact on clinical outcomes. Therefore, the interpretation of data on outcomes requires caution and consideration of the definition of PGD in individual studies. The topic of definition and classification of PGD is reviewed and a set of uniform definition criteria have been proposed in the accompanying article by Christie and colleagues.
Short- and long-term outcomes of PGD
The recognition of outcomes of PGD is essential for determination of the impact of this early post-operative problem on short- and long-term prognosis. PGD remains responsible for significant early morbidity and mortality after lung transplantation.
Relevant clinical outcomes of PGD include length of mechanical ventilation (LOV), ICU length of stay (LOS), hospital length of stay (hLOS), mortality, short- and long-term survival, cost, functional status and incidence of bronchiolitis obliterans syndrome (BOS). Most studies have shown that PGD leads to adverse short-term outcomes, including prolonged LOV, ICU LOS, hLOS, increased cost and increased short-term mortality (Table 2).
Conflicting short-term results are generally explained by the differences in the definition of PGD in individual studies and the resultant differences in severity of graft dysfunction in the patient population studied.
Grade 0–5 based on chest radiograph score. When patients with severe PGD (based on radiographic grade) are considered, LOV (6 vs 2 days), ICU LOS (10 vs 6 days) and hospital LOS (36 vs 20 days) are all significant compared to those without PGD. Such patients fit the more severe definition of PGD with Pao2:Fio2 ratio <200 mm Hg.
hypoxemia (Fio2≥0.3 to keep Pao2 ≥65 mm Hg), PAOP ≤12 mm Hg, no rejection or infection
a Grade 0–5 based on chest radiograph score. When patients with severe PGD (based on radiographic grade) are considered, LOV (6 vs 2 days), ICU LOS (10 vs 6 days) and hospital LOS (36 vs 20 days) are all significant compared to those without PGD. Such patients fit the more severe definition of PGD with Pao2:Fio2 ratio <200 mm Hg.
Although Kahn et al and Thabut et al did not report a difference in long-term survival between patients with and without PGD, the lack of a difference can be explained in part by the less stringent definitions of PGD in these studies, and by the relatively poor long-term survival rates in patients without PGD in the study by Kahn et al.
In general, the lower long-term survival rate appears to be secondary to increased early mortality. This was demonstrated in the study by Fisher et al, who found that long-term survival was comparable in patients with and without PGD when early deaths were excluded from their analysis (Figure 1).
There is a lack of data on long-term functional outcomes after severe PGD. Christie et al reported a decreased 6-minute walk distance and limited ambulatory status in a small group of survivors of PGD.
A possible association between PGD and the future development of BOS has been hypothesized for many years. The recognition that activation of the innate immune response plays an important role in the subsequent orchestration of the adaptive immune response is fundamental to the hypothesis that acute graft injury can predispose to chronic graft dysfunction in all solid organs.
The clinical significance of the “injury response” was first demonstrated in the renal transplant literature when delayed graft function in renal allografts was identified as an independent risk factor for the development of chronic rejection and graft loss in renal transplant recipients.
In lung transplantation, PGD represents a major activation of the innate immune response in the allograft. Experimentally, PGD increases MHC Class II expression in the lung, thereby increasing immunogenicity.
This finding provides additional support for the hypothesis that uncontrolled graft injury can begin with PGD, progress to acute rejection and lymphocytic bronchiolitis, and finally repair and remodeling to produce obliterative bronchiolitis.
In 2002, two large, single-center studies were published which investigated this hypothesis in lung transplant recipients.
reported a lack of association between PGD (defined by acute non-immune graft injury on early histopathologic examination) and the development of BOS when survival analysis was used. Because the larger of the two studies
used histologic evidence of diffuse alveolar damage as a marker of PGD and utilized survival analysis rather than uni- or multivariate analysis, it seems to provide a more robust assessment of likely differences in BOS incidence, especially since the risk of BOS increases with time after transplant. However, histologic findings to define PGD may also be subject to problems in the accurate identification of a patient population with this process in that sampling error during biopsy or inability to obtain biopsies from those with more severe degrees of PGD may result in an inability to capture all patients.
If it is true that the recipients with PGD who survive >3 months do not have an increased risk of subsequent BOS, why should this be different in lung transplantation compared with renal transplantation? There are several possible explanations: First, those who survive PGD are likely to have suffered a milder lung injury than those who died in the early post-operative period. As a result, there may not have been a sufficient injury stimulus to increase graft immunogenicity in these recipients. Second, the degree of allograft injury needed to increase the risk of chronic rejection in renal transplantation may not be compatible with survival in lung transplantation. The patient with a dysfunctional renal allograft may be supported with dialysis; thus, severe renal allograft injury has more opportunity to resolve than a comparable degree of injury in a lung allograft.
Predictors/scores of outcomes of PGD
Although there is more information on the prediction of development of PGD, there are limited data on the prediction of outcomes of PGD once it is present. Certain cellular, molecular, physiologic and clinical variables have been associated with outcomes in PGD in studies that typically included a small number of patients.
Fisher et al reported a significant association between high donor IL-8 concentration in bronchoalveolar lavage fluid and the development of early graft dysfunction and increased early mortality in recipients.
Lung tissue IL-8 levels at 2 hours after reperfusion correlated negatively with graft oxygenation (Pao2:Fio2 ratio) and ICU-free days in the first 30 days, and positively with APACHE score and mean airway pressure at various time-points in the first 24 hours.
Ware et al reported the presence of increased alveolar-capillary barrier permeability by measurement of the pulmonary edema fluid to plasma protein ratio in 6 of their 8 patients with PGD.
Although the degree of permeability did not correlate with clinical parameters (A-a gradient, severity of infiltrates, LOV, ICU LOS), the preservation of net alveolar epithelial fluid clearance was associated with better clinical outcomes. Four of 6 patients with intact transport had good results, with rapid resolution of infiltrates and hypoxemia. In contrast, 2 patients with no net fluid transport had prolonged hypoxemia and infiltrates with a trend toward longer ICU stay and LOV. These data suggest that the preservation of alveolar epithelial transport capacity is critical for timely resolution of the inflammatory edema in acute lung injury, and the rate of fluid transport may be an important prognostic indicator in PGD.
The prediction of outcomes of PGD using clinical variables has recently been reported by Thabut et al,
who retrospectively studied 259 lung transplant recipients. One hundred thirty-one (50.6%) patients had PGD, which was defined as radiographic infiltrates developing over 72 hours along with a Pao2:Fio2 ratio <300 in the absence of other identifiable causes. The authors also evaluated the association between PGD and early hemodynamic failure (EHF) and graded the severity of EHF based on vasopressor requirements. They noted a 29% ICU mortality rate among those who developed PGD and 10.9% for those who did not. In addition, they tested multiple variables in a univariate analysis and found that 5 variables were associated with ICU mortality: ischemic time; severe EHF; bilateral lung transplantation; Pao2:Fio2 ratio; and age. These variables were entered into a multivariate logistic regression, and 4 variables were found to be independent predictors of ICU mortality: age; ischemic time; Pao2:Fio2 ratio; and severe EHF. The coefficient for each variable was multiplied by a factor of 10, and all the variables were added to derive an ischemia-reperfusion injury severity score (IRISS). The group used 67% of their cohort to develop this model and IRISS; they then validated the model and score using the other 33% of the cohort, with good results. An IRISS point-scale system was provided, and a probability of ICU mortality was determined from this score (Figure 2). It is important to note that the objective of the study was not to develop a scoring system to predict an individual patient’s probability of mortality, but rather to develop a system that would allow for better assessment of severity of PGD and would ensure comparability between groups in the studies of PGD.
Using a similar approach, Sekine et al retrospectively analyzed donor, recipient, operative factors and immediate post-operative physiologic parameters in 122 lung transplant recipients to identify risk factors for 30-day mortality and prolonged ICU LOS.
They found that the use of marginal donors, cardiopulmonary bypass, recipient body mass index (BMI) >25 kg/m2 and recipient diagnosis of a pulmonary hypertensive disorder were significant risk factors for 30-day mortality and prolonged ICU stay. On multivariate analysis, recipient BMI >25 kg/m2 and certain physiologic post-operative parameters, including APACHE II score, systolic pulmonary artery pressure, Pao2:Fio2 ratio and oxygenation index trend, were identified as risk factors for 30-day mortality. A recipient BMI >25 kg/m2, use of cardiopulmonary bypass, APACHE II score, and oxygenation index trend were risk factors associated with prolonged ICU stay. Furthermore, they developed a scoring system, referred to as the risk quantification for lung transplantation (RQLT), to predict outcomes and found that the RQLT correlated very well with early post-transplantation outcomes. It is of interest to note that both of the aforementioned articles sought to predict outcomes after transplantation and to develop a scoring system that would objectively quantify the severity of early graft dysfunction and predict early outcomes, although they found relatively different results. Clearly, further studies are necessary to corroborate the results of these studies. Ideally, future studies should be prospective, multicenter trials that include a large number of patients, in which a proportion of the cohort can be used to develop a model and the remainder used for its validation.
In summary, severe PGD leads to adverse short-term outcomes. The relationship with long-term outcomes, including BOS, is not well defined. A uniform definition of the spectrum of PGD as well as a definition of outcomes will allow for an accurate assessment of the impact of this problem after lung transplantation. A reliable prediction model for outcomes of PGD could then be developed and validated across multiple lung transplant centers. More work is needed in this area to collect short- and long-term outcome data across multiple lung transplant centers, focusing on functional outcomes as well as survival and risk of chronic graft dysfunction.