Blood transfusions increase circulating plasma free hemoglobin levels and plasma nitric oxide consumption: a prospective observational pilot study

Abstract

Introduction: The increasing number of reports on the relation between transfusion of stored red blood cells (RBCs) and adverse patient outcome has sparked an intense debate on the benefits and risks of blood transfusions. Meanwhile, the pathophysiological mechanisms underlying this postulated relation remain unclear. The development of hemolysis during storage might contribute to this mechanism by release of free hemoglobin (fHb), a potent nitric oxide (NO) scavenger, which may impair vasodilation and microcirculatory perfusion after transfusion. The objective of this prospective observational pilot study was to establish whether RBC transfusion results in increased circulating fHb levels and plasma NO consumption. In addition, the relation between increased fHb values and circulating haptoglobin, its natural scavenger, was studied.

Methods: Thirty patients electively received 1 stored packed RBC unit (n = 8) or 2 stored packed RBC units (n = 22). Blood samples were drawn to analyze plasma levels of fHb, haptoglobin, and NO consumption prior to transfusion, and 15, 30, 60 and 120 minutes and 24 hours after transfusion. Differences were compared using Pearson's chi-square test or Fisher's exact test for dichotomous variables, or an independent-sample t test or Mann-Whitney U test for continuous data. Continuous, multiple-timepoint data were analyzed using repeated one-way analysis of variance or the Kruskall-Wallis test. Correlations were analyzed using Spearman or Pearson correlation.

Results: Storage duration correlated significantly with fHb concentrations and NO consumption within the storage medium (r = 0.51, P < 0.001 and r = 0.62, P = 0.002). fHb also significantly correlated with NO consumption directly (r = 0.61, P = 0.002). Transfusion of 2 RBC units significantly increased circulating fHb and NO consumption in the recipient (P < 0.001 and P < 0.05, respectively), in contrast to transfusion of 1 stored RBC unit. Storage duration of the blood products did not correlate with changes in fHb and NO consumption in the recipient. In contrast, pre-transfusion recipient plasma haptoglobin levels inversely influenced post-transfusion fHb concentrations.

Conclusion: These data suggest that RBC transfusion can significantly increase post-transfusion plasma fHb levels and plasma NO consumption in the recipient. This finding may contribute to the potential pathophysiological mechanism underlying the much-discussed adverse relation between blood transfusions and patient outcome. This observation may be of particular importance for patients with substantial transfusion requirements.

Figure 1

Packed red blood cell storage duration correlates with increased free hemoglobin and nitric oxide consumption. (A) Histogram of storage duration for all transfused packed red blood cell (pRBC) units (n = 52). Mean storage duration was 17.2 days (range 2 to 32 days). (B) Correlation between storage duration and the free hemoglobin (fHb) concentrations of the storage medium (SM) for each pRBC unit (n = 52). (C) Correlation between the fHb concentration and nitric oxide (NO) consumption within the SM in a random selection of 22 pRBC units. Black lines depict a nonlinear regression (exponential) curve.

Figure 2

Transfusion of stored red blood cells significantly increases plasma free hemoglobin and nitric oxide consumption. (A) Course of plasma free hemoglobin (fHb) in patients receiving 1 (white bars) or 2 (grey bars) packed red blood cell (pRBC) units from baseline levels before blood transfusion (BT) to 15, 30, 60 and 120 minutes and 24 hours after BT. ***Significant changes of fHb levels within groups compared with baseline fHb concentrations; P < 0.001. (B) Course of plasma nitric oxide (NO) consumption in a random selection of four patients transfused with 1 pRBC unit and nine patients transfused with 2 pRBC units. *Significant differences of fHb levels within groups compared with baseline fHb concentrations; P < 0.05.

Figure 3

Post-transfusion plasma haptoglobin, and pre-transfusion haptoglobin impact on post-transfusion free hemoglobin and nitric oxide consumption. (A) Change of plasma haptoglobin in patients receiving 1 (white bars) or 2 (grey bars) packed red blood cell (pRBC) units from levels before blood transfusion (BT) to 15, 30, 60 and 120 minutes and 24 hours after BT. (B) Change of plasma free hemoglobin (fHb) in patients with pre-transfusion haptoglobin plasma concentrations of 1.9 g/l or higher (white bars), between 0.75 and 1.9 g/l (light-grey bars), or 0.75 g/l or lower (dark-grey bars). Significant changes of fHb levels within groups compared with baseline (pre-transfusion) levels: *P < 0.05, **P < 0.01, ***P < 0.001. Statistical differences between groups are indicated with black bars and according P values. NS, nonsignificant.