BACKGROUND: Preoperative anemia is associated with adverse outcomes in cardiac surgery, yet it remains unclear what proportion of this association is mediated through red blood cell (RBC) transfusions. METHODS: This is a historical observational cohort study of adults undergoing coronary artery bypass grafting or valve surgery on cardiopulmonary bypass at an academic medical center between May 1, 2008, and May 1, 2018. A mediation analysis framework was used to evaluate the associations between preoperative anemia and postoperative outcomes, including a primary outcome of acute kidney injury (AKI). Intraoperative RBC transfusions were evaluated as mediators of preoperative anemia and outcome relationships. The estimated total effect, average direct effect of preoperative anemia, and percent of the total effect mediated through transfusions are presented with 95% confidence intervals and P-values. RESULTS: A total of 4117 patients were included, including 1234 (30%) with preoperative anemia. Overall, 437 of 4117 (11%) patients went on to develop AKI, with a greater proportion of patients having preoperative anemia (219 of 1234 [18%] vs 218 of 2883 [8%]). In multivariable analyses, the presence of preoperative anemia was associated with increased postoperative AKI (6.4% [4.2%–8.7%] absolute difference in percent with AKI, P < .001), with incremental decreases in preoperative hemoglobin concentrations displaying greater AKI risk (eg, 11.9% [6.9%–17.5%] absolute increase in probability of AKI for preoperative hemoglobin of 9 g/dL compared to a reference of 14 g/dL, P < .001). The association between preoperative anemia and postoperative AKI was primarily due to direct effects of preoperative anemia (5.9% [3.6%–8.3%] absolute difference, P < .001) rather than mediated through intraoperative RBC transfusions (7.5% [−4.3% to 21.1%] of the total effect mediated by transfusions, P = .220). Preoperative anemia was also associated with longer hospital durations (1.07 [1.05–1.10] ratio of geometric mean length of stay, P < .001). Of this total effect, 38% (22%, 62%; P < .001) was estimated to be mediated through subsequent intraoperative RBC transfusion. Preoperative anemia was not associated with reoperation or vascular complications. CONCLUSIONS: Preoperative anemia was associated with higher odds of AKI and longer hospitalizations in cardiac surgery. The attributable effects of anemia and transfusion on postoperative complications are likely to differ across outcomes. Future studies are necessary to further evaluate mechanisms of anemia-associated postoperative organ injury and treatment strategies.

中文翻译：

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心脏手术的术前贫血和术后结果：评估术中输血暴露的中介分析

背景：术前贫血与心脏手术的不良后果相关，但目前尚不清楚这种关联中有多少是通过红细胞（RBC）输注介导的。方法：这是一项历史观察性队列研究，研究对象为 2008 年 5 月 1 日至 2018 年 5 月 1 日期间在学术医疗中心接受冠状动脉旁路移植术或体外循环瓣膜手术的成年人。使用中介分析框架来评估术前贫血和术后结果，包括急性肾损伤（AKI）的主要结果。术中红细胞输注被评估为术前贫血和结果关系的中介因素。估计的总效应、术前贫血的平均直接效应以及通过输血介导的总效应的百分比以 95% 置信区间和 P 值表示。结果：共纳入 4117 例患者，其中 1234 例（30%）患有术前贫血。总体而言，4117 名患者中的 437 名 (11%) 继续发展为 AKI，其中术前贫血的患者比例更高（1234 名患者中的 219 名 [18%] vs 2883 名患者中的 218 名 [8%]）。在多变量分析中，术前贫血的存在与术后 AKI 增加相关（与 AKI 的百分比绝对差异为 6.4% [4.2%–8.7%]，P < .001），术前血红蛋白浓度的逐渐降低显示 AKI 风险更大（例如，与参考值 14 g/dL 相比，术前血红蛋白为 9 g/dL 时 AKI 概率绝对增加 11.9% [6.9%–17.5%]，P < .001）。术前贫血和术后 AKI 之间的关联主要是由于术前贫血的直接影响（5.9% [3.6%–8.3%] 绝对差异，P < .001），而不是通过术中 RBC 输注介导（7.5% [−4.3% 到输血介导的总效应的 21.1%，P = .220）。术前贫血还与较长的住院时间相关（几何平均住院时间的比率为 1.07 [1.05–1.10]，P < .001）。在这一总效应中，估计有 38%（22%、62%；P < .001）是通过随后的术中红细胞输注介导的。术前贫血与再次手术或血管并发症无关。结论：术前贫血与 AKI 发生率较高和心脏手术住院时间较长相关。贫血和输血对术后并发症的影响可能因结果而异。未来的研究有必要进一步评估贫血相关术后器官损伤的机制和治疗策略。