Systemic and microcirculatory effects of blood transfusion in experimental hemorrhagic shock

The microvascular reperfusion injury after retransfusion has not been completely characterized. Specifically, the question of heterogeneity among different microvascular beds needs to be addressed. In addition, the identification of anaerobic metabolism is elusive. The venoarterial PCO₂ to arteriovenous oxygen content difference ratio (P_v-aCO₂/C_a-vO₂) might be a surrogate for respiratory quotient, but this has not been validated. Therefore, our goal was to characterize sublingual and intestinal (mucosal and serosal) microvascular injury after blood resuscitation in hemorrhagic shock and its relation with O₂ and CO₂ metabolism.

Anesthetized and mechanically ventilated sheep were assigned to stepwise bleeding and blood retransfusion (n = 10) and sham (n = 7) groups. We performed analysis of expired gases, arterial and mixed venous blood gases, and intestinal and sublingual videomicroscopy.

In the bleeding group during the last step of hemorrhage, and compared to the sham group, there were decreases in oxygen consumption (3.7 [2.8-4.6] vs. 6.8 [5.8-8.0] mL min^-1 kg^-1, P < 0.001) and increases in respiratory quotient (0.96 [0.91-1.06] vs. 0.72 [0.69-0.77], P < 0.001). Retransfusion normalized these variables. The P_v-aCO₂/C_a-vO₂ increased in the last step of bleeding (2.4 [2.0-2.8] vs. 1.1 [1.0-1.3], P < 0.001) and remained elevated after retransfusion, compared to the sham group (1.8 [1.5-2.0] vs. 1.1 [0.9-1.3], P < 0.001). P_v-aCO₂/C_a-vO₂ had a weak correlation with respiratory quotient (Spearman R = 0.42, P < 0.001). All the intestinal and sublingual microcirculatory variables were affected during hemorrhage and improved after retransfusion. The recovery was only complete for intestinal red blood cell velocity and sublingual total and perfused vascular densities.

Although there were some minor differences, intestinal and sublingual microcirculation behaved similarly. Therefore, sublingual mucosa might be an adequate window to track intestinal microvascular reperfusion injury. Additionally, P_v-aCO₂/C_a-vO₂ was poorly correlated with respiratory quotient, and its physiologic behavior was different. Thus, it might be a misleading surrogate for anaerobic metabolism.