Reversal of anemia with allogenic RBC transfusion prevents post-cardiopulmonary bypass acute kidney injury in swine

Abstract

Anemia during cardiopulmonary bypass (CPB) is strongly associated with acute kidney injury in clinical studies; however, reversal of anemia with red blood cell (RBC) transfusions is associated with further renal injury. To understand this paradox, we evaluated the effects of reversal of anemia during CPB with allogenic RBC transfusion in a novel large-animal model of post-cardiac surgery acute kidney injury with significant homology to that observed in cardiac surgery patients. Adult pigs undergoing general anesthesia were allocated to a Sham procedure, CPB alone, Sham+RBC transfusion, or CPB+RBC transfusion, with recovery and reassessment at 24 h. CPB was associated with dilutional anemia and caused acute kidney injury in swine characterized by renal endothelial dysfunction, loss of nitric oxide (NO) bioavailability, vasoconstriction, medullary hypoxia, cortical ATP depletion, glomerular sequestration of activated platelets and inflammatory cells, and proximal tubule epithelial cell stress. RBC transfusion in the absence of CPB also resulted in renal injury. This was characterized by endothelial injury, microvascular endothelial dysfunction, platelet activation, and equivalent cortical tubular epithelial phenotypic changes to those observed in CPB pigs, but occurred in the absence of severe intrarenal vasoconstriction, ATP depletion, or reductions in creatinine clearance. In contrast, reversal of anemia during CPB with RBC transfusion prevented the reductions in creatinine clearance, loss of NO bioavailability, platelet activation, inflammation, and epithelial cell injury attributable to CPB although it did not prevent the development of significant intrarenal vasoconstriction and endothelial dysfunction. In conclusion, contrary to the findings of observational studies in cardiac surgery, RBC transfusion during CPB protects pigs against acute kidney injury. Our study underlines the need for translational research into indications for transfusion and prevention strategies for acute kidney injury.

Fig. 1.

Hemodynamics. A: total volume of crystalloid administered over 24 h. B: serial hematocrit values over 24 h. C: mean arterial blood pressure (MABP). D: global oxygen delivery (Do₂) at end of intervention period estimated using different methods in cardiopulmonary bypass (CPB) and Sham pigs, as described in text. E and F: mixed venous oxygen saturation (Sv_O2; E) and arterial lactate concentrations (F) in experimental subjects over time. Bars and whiskers represents means ± SE. Hatched line indicates intervention/CPB period. For A and D, P values were derived from ANOVA with intergroup comparisons using a t-test. For B, C, E, and F, P values were determined using ANOVA for repeated measures adjusted for baseline differences. *P < 0.05 vs. Sham. †P < 0.05 vs. CPB. ‡P < 0.05 vs. Sham+transfusion (Tx).

Fig. 2.

Measures of creatinine clearance, endothelial function, and regional oxygenation. Graphs show measured creatinine clearance at baseline, 1.5 h postintervention, and at 24 h (A), renal artery blood flow at 24 h (B), cortical microvascular flow at 24 h before and in response to acetylcholine (ACh) infusion (C), nitric oxide (NO) bioavailability at 24 h (D), measured Po₂ in the outer medulla at 24 h (E), and renal cortical nucleotide levels at 24 h (F). ATP is expressed as a ratio to ADP (both measured in ng/mg protein), and adenosine is expressed as ng/mg protein. Values are means ± SE. For creatinine clearance, P values were determined using ANOVA for repeated measures adjusted for baseline estimated at 114.2 ml/min. For ACh studies, P values were derived from ANOVA with adjustment for baseline values estimated as renal blood flow = 0.24 l/min and cortical microvascular flow =302 CPU. *P < 0.05 vs. Sham. †P < 0.05 vs. CPB. ‡P < 0.05 vs. Sham+Tx.

Fig. 3.

Histological analysis. A: representative hematoxylin and eosin (H&E)-stained photomicrographs of cortical tubules and graphs demonstrating mean proximal tubular diameter. B: differences in protein expression [normalized densitometry ratio of ET-1 (Western blot as shown) to β-actin (not shown)] and localization of ET-1 to dilated injured tubules. Scale bars = 140 μm (A) and 72 μm (B). C: differences in protein expression [normalized densitometry ratio of Western blots as shown to β-actin (not shown)] and immunofluorescence staining for endothelial nitric oxide synthase (eNOS). D: percent immunostaining for Dolichos biflorus agglutinin (DBA) lectin (brown), which stains a component of the endothelial glycocalyx. Scale bars = 47 μm (C) and 70 μm (D). E and F: cell counts/mm² and representative photomicrographs of immunofluorescence for platelet-activating complex (PAC), a specific marker of platelet activation (E), and MAC387, a monocyte marker (F). Scale bars = 31 μm (E) and 50 μm (F). Values are means ± SE (at least n = 4/group). P values were derived from ANOVA with intergroup comparisons using a t-test. *P < 0.05 vs. Sham. †P < 0.05 vs. CPB. ‡P < 0.05 vs. Sham+Tx.