This secondary analysis of a prospective cohort of critically ill patients indicates that pRBC transfusion is associated with an increased risk of developing ARDS. This link is independent of multiple variables, including other important potential confounders such as severity of illness and reason for ICU admission. Furthermore, there appears to be a dose-response relationship between pRBC use and development of ARDS; receiving more pRBC units raises the likelihood of subsequent ARDS. In addition, progression to ARDS in the ICU substantially prolonged the duration of mechanical ventilation and was associated with a poor prognosis.
Three prior studies have focused expressly on ARDS as it relates to pRBC transfusion practice [8
]. Hébert and colleagues [8
] randomly assigned critically ill patients to withhold transfusions at or above a hemoglobin level of 7 versus 10 mg/dl. Despite receiving fewer units of pRBCs, those in the conservative hemoglobin arm (that is, 7 mg/dl) had similar overall survival rates compared to those in the comparator group. With respect to ARDS, 7.7% of those randomly assigned to the lower hemoglobin threshold developed ARDS versus 11.4% of those in the higher target cohort (p
= 0.06). Kahn and colleagues [10
] noted that pRBC transfusion independently more than doubled the probability of ARDS among a group of individuals suffering a subarachnoid hemorrhage. Gong and colleagues [9
], in an analysis similar to ours, studied a mixed population of 688 critically ill patients at risk for ARDS. ARDS developed in approximately one third of the population. Adjusting for several covariates indicated that pRBC transfusion was significantly related to a diagnosis of ARDS and, importantly, was also associated with a greater mortality rate.
Our observations build on these earlier reports and confirm a relationship between pRBC exposure and ARDS. In contrast to Hébert and colleagues [8
], Kahn and colleagues [10
], and Gong and colleagues [9
], we were able to control for multiple processes of care beyond transfusion practice alone. For example, we could adjust for use of continuous sedation. Likewise, we were able to consider severity of illness not only at ICU entry (APACHE II score), but also immediately prior to the onset of ARDS (SOFA score). This is important because as patients remain in the ICU their severity of illness changes. Failure to evaluate the evolution in disease severity might lead one to find a relationship where, in fact, the observation is simply a marker for alterations in severity of illness. Moreover, unlike the studies by Kahn and colleagues [10
] and Gong and colleagues [9
], ours is derived from a multi-institutional registry, and this underscores the likely generalizability of our data. An additional strength of our study stems from the large sample size. Having nearly 5,000 subjects to analyze afforded us statistical power to assess multiple covariates that other reports could not take into consideration.
Among other factors that we found were correlated with ARDS, several merit comment. Prior analyses of 'risk factors' for ARDS have not investigated either continuous sedation or early enteral feeding as potential process-of-care issues that could promote ARDS [2
]. The impact of continuous sedation likely arises from the fact that this strategy (as compared to intermittent sedation protocols) prolongs the time on mechanical ventilation and hence the risk for ventilator-associated lung injury. On the other hand, since continuous sedation remained an independent predictor of ARDS after the period of observation was controlled for, it could be that this is simply a surrogate marker for severity of illness which is not captured by traditional scoring tools. The correlation between early enteral feeding could reflect the fact that this approach to nutrition heightens the probability of either gross aspiration or microaspiration. Both of these variables merit scrutiny in future analyses. As such, our work serves to generate hypotheses for future research.
Why might transfusion correlate with the development of ARDS? In fact, what is diagnosed as ARDS in the critically ill population may represent TRALI. Thus, the proposed mechanisms for TRALI – (a) alloimmunization of the recipient white cells by the donor anti-leukocyte [6
] and monocyte antibodies [23
] and (b) a response to biologically active lipids originating in donor plasma [24
], resulting in an oxidative burst leading to degranulation of neutrophils after some priming event – could underlie the more general relationship we note between pRBC use and ARDS. Alternatively, several studies have shown that pRBCs contain multiple pro-inflammatory cytokines that, when infused into a susceptible subject, could potentially tip the balance and lead to a dysregulation of multiple cascades that have their clinical manifestation as acute lung injury [28
]. Thus, transfusion promotes inflammation directly as demonstrated in studies that measure serial levels of interleukin-6 in the recipient following pRBC administration [12
]. On the other hand, residual donor white blood cells could promote T-cell activation [29
], which, in turn, could result in subtle changes in the host's immune status, predisposing him or her to infection. Whatever the mechanism, it may vary from individual to individual based on genetics and proteomics.
Our findings should give physicians pause when considering transfusion in persons at risk for ARDS. Evidence continues to mount that transfusion increases the risk for multiple adverse consequences ranging from bloodstream infections to nosocomial pneumonia [18
]. A recent prospective study by Taylor and colleagues [32
] explored the impact of pRBC use on subsequent rates of nosocomial infection. Although they pooled all types of infection into a common endpoint, they concluded that transfusion independently increases the risk for infection. In other analyses looking at distinct forms of nosocomial infection, such as pneumonia or bloodstream infection [18
], researchers have reached similar conclusions regarding the potential deleterious effects of transfusion. All of these reports, including our own, are necessarily limited in that they can demonstrate only association rather than causation. However, given the consistent theme observed in multiple datasets, our results should help to shift the burden against assuming that pRBC exposure is free of substantial risk. Bolstering this recommendation to discard the assumption that transfusion is a relatively 'safe' endeavor is the fact that the relationship between pRBCs and ARDS in the present report follows a dose-response relationship. Even small transfusion volumes (for example, 1 to 2 units) convey an increased probability for the development of ARDS.
Our study has a number of significant limitations. First, although the data were collected prospectively, this report represents a retrospective analysis. As such, it is exposed to multiple forms of bias. Second, as our analysis describes observational data and does not derive from a randomized study, we can conclude only that transfusion is associated with the development of ARDS. Causation, therefore, cannot be inferred from our analytic approach. Third, the diagnosis of ARDS was based on a prospectively chosen definition, but these criteria were applied by a diverse group of researchers. Inter-observer variability in the diagnosis of ARDS [39
], which has been documented in prior studies, could confound our findings. The large sample size, however, should limit the impact of this variability. Fourth, we lacked information on transfusions given prior to ICU admission. Thus, it is possible that we may have misclassified at least some of the exposure information. However, if present, this misclassification of exposure would be nondifferential and, if anything, would have resulted in a weaker association between transfusions and the development of ARDS than one that actually exists. In that same vein, we did not record information regarding the use of other forms of blood products. Fifth, there may be further variables we did not investigate or record that could have affected our findings. Finally, the Crit trial was conducted prior to the implementation of leukoreduction. Leukoreduction is thought to decrease the immunomodulatory effects of pRBC transfusion. Despite theoretical reasons to hypothesize that leukoreduction might prevent serious infectious and non-infectious complications in critically ill patients, clinical evidence of the benefit of leukoreduction is sparse [41
]. Nonetheless (and notwithstanding these limitations), our observations are consistent with an emerging literature indicating that transfusion is not benign.