Children undergoing cardiac surgery with cardiopulmonary bypass (CPB) are often exposed to blood products.1 In this group, recent data have shown that a liberal transfusion strategy [haemoglobin (Hb) transfusion threshold >9 g dl−1] is associated with an increased transfusion rate without improving outcome.2,3 Furthermore, several studies have reported an association between red blood cell (RBC) transfusion and increased postoperative morbidity, hospital length of stay and mortality.4–7 Many reasons have been evoked to explain the observed association between RBC transfusion and altered outcome. Among these, the storage-induced changes in RBC are thought to play a role. Indeed, some pathophysiological alterations described during the storage process could potentially support this hypothesis.8–11 These include a decrease in 2,3-diphosphoglycerate (2,3-DPG) levels (resulting in an increased affinity of Hb for oxygen), reduced RBC deformability, increased circulating free iron levels and pro-inflammatory effects.10,12–14
The impact of RBC storage duration has mainly been studied in adult surgical and trauma patients and has yielded contradictory results. However, some pitfalls in the methodology of these studies may explain these diverging results.15,16 In contrast to adult patients, there is a lack of data on this issue in children. The aim of this retrospective study was to assess the effects of RBC storage duration on postoperative morbidity and mortality in children undergoing elective repair of congenital heart disease under CPB.
Materials and methods
The study design was reviewed and approved by the local ethic committee (QFCUH ethics committee, Ref. CEH 03/12). The need for parental informed consent was waived given the retrospective nature of the study. We retrospectively reviewed our paediatric cardiac surgery database, in which children aged less than 18 years undergoing curative or palliative surgery under CPB between January 2006 and December 2010 were systematically included. Jehovah's Witnesses, American Society Anesthesiology (ASA) score more than 4, emergency procedures due to unstable conditions (moribund state) and children exposed to more than two RBC units were excluded from the analysis. Anaesthesia, CPB management and surgical techniques followed routine standardised protocols during the entire observation period.
Transfusion policy was defined through a multidisciplinary approach involving surgeons, anaesthesiologists and paediatricians working in the paediatric ICU (PICU). During CPB, RBCs were transfused to maintain an Hb concentration of more than 7 g dl−1. After CPB and in the PICU, RBCs were administered to maintain the Hb between 7 and 10 g dl−1, according to the clinical condition of the patient taking into account the presence of cyanotic disease, pulmonary hypertension and cardiac dysfunction. In case of haemorrhage, fresh frozen plasma (FFP) and platelets were transfused according to the clinical situation and standard laboratory tests.
Data regarding transfused blood products were retrieved from the institutional computerised blood bank database. The age of the RBC units was calculated using the following equation:
Storage duration = transfusion date – date of collection
Our patients were divided a priori into two groups according to storage duration: the group ‘Young’ included patients who received one or two RBC units with storage duration 7 days or less; the group ‘Old’ included children who received one or two RBC units with storage duration more than 7 days. The 7-day cut-off was chosen on the basis of laboratory investigations demonstrating a decrease to near zero of the intracellular concentration of 2 to 3 DPG. Children transfused with two RBC units from the two different storage duration groups were excluded. In our institution, we use SAGM (saline, adenine, glucose, mannitol) solution as the additive solution for RBC conservation. No reduction technology was applied during the study period.
For each child, personal data, intraoperative characteristics, type of surgery illustrated by the RACHS-1 (Risk Adjustment for Congenital Heart Surgery) score,17 PRISM II18 score (Paediatric Risk of Mortality) and ASA status were recorded. Our primary endpoint was a ‘composite’ including in-hospital mortality and/or the presence of at least one organ system failure in the postoperative period. The definition of each organ failure is described in Table 1.19,20 The duration of mechanical ventilation was defined as time from ICU admission to the time of extubation. Pulmonary failure was defined as the need for mechanical ventilation for more than 90 h.21 Prolonged inotropic support was defined as the need for more than 5 μg kg−1 min−1 of inotrope for more than 48 h postoperatively. Neurological dysfunction was considered as a transient or permanent neurological deficit. Secondary endpoints were postoperative mechanical ventilation duration expressed in hours, ICU and hospital length of stay (expressed in days), and ICU and hospital mortality.
The D’Agostino and Pearson test was used to assess the normal distribution of the data. Because of the non-Gaussian distribution, continuous variables are presented as median [interquartile range] and the Mann–Whitney U test was used for comparisons. Categorical variables are presented as numbers (%) and the Fisher exact test or Chi-square test was used to compare groups. Univariate and multivariate logistic regression analyses were used to identify the factors independently associated with the composite endpoint. In these analyses, storage duration was entered as a continuous variable. When children received two RBC units, the age of the older unit was used for regression analyses. Results for the logistic regression analyses are expressed as odds ratio (OR) and 95% confidence interval (95% CI). A P value of less than 0.05 was considered significant. A P value of less than 0.1 was defined to test variables in the multivariate analysis. Statistical analyses were performed with Prism 6 for Mac OS (version 6.0a; GraphPad software inc., San Diego, California, USA) and Statistix software for Windows (version 9; Analytical Software, Tallahassee, Florida, USA).
From the 1014 children recorded in our database, 570 children were included in the final analysis, 118 in the group ‘Young’ and 452 in the group ‘Old’ (Fig. 1).
Table 2 summarises the clinical data of the study population. No significant differences were observed between both groups except for aortic cross-clamping and CPB times, which were longer in the group ‘Old’. Furthermore, the proportion of children exposed to ultrafiltration was higher in the group ‘Old’. However, the volume of ultrafiltration did not differ between groups.
In the entire study enrollment, the median RBC storage time was 12 [8 to 17] days, 6 [5 to 7] days in the group ‘Young’ and 14 [11 to 19] days in the group ‘Old’. Figure 2 shows the distribution of RBC length of storage. No difference was observed between groups regarding transfusion rates (Table 3). Primary composite endpoint as well as secondary endpoints did not differ between groups (Table 4). Univariate logistic regression analysis (Table 5) identified several risk factors for the occurrence of the primary composite endpoint, which were subsequently included in multivariate logistic analysis. Preoperative body weight, PRISM score, duration of mechanical ventilation, ICU stay and the number of platelet units transfused were identified as the independent risk factors for occurrence of the primary composite endpoint (Table 6). Of note, storage duration did not appear as a risk factor. Indeed, univariate logistic regression analysis showed that storage duration used as a continuous variable was not a risk factor for the occurrence of our composite endpoint (OR 0.98, 95% CI 0.96 to 1.01).
This retrospective analysis aimed to assess the effect of RBC storage duration on postoperative outcome in children undergoing cardiac surgery with CPB, transfused with one or two RBC units. Five hundred and seventy transfused children were reviewed. RBCs storage duration had no impact on postoperative morbidity and mortality in our population.
Only a few studies have evaluated the effect of RBC storage duration on postoperative outcome in children. In a first study, Ranucci et al.22 observed an increased incidence of pulmonary complications in children transfused on CPB with RBC stored for more than 4 days. Interestingly, the duration of RBC storage did not affect outcome in children transfused outside CPB. In a second study, Karam et al.23 prospectively evaluated the association between duration of RBC storage and outcome in critically ill children. The incidence of multiple organ dysfunction and the ICU length of stay were significantly higher in children exposed to RBC units stored for more than 14 days.
These results are in contrast with those in our study. This can be explained by differences in the methodology used. First, the definition used to categorise blood transfusion between ‘old’ and ‘fresh’ blood substantially differs between the studies. In the studies by Ranucci et al.22 and Karam et al.,23 the authors used the median storage time they observed in their patients to set up the cut-off between groups. Ranucci et al.22 used a 4-day cut-off, while Karam et al.23 used a 14-day cut-off. Obviously, there is no clear definition of what are ‘fresh’ or ‘old’ RBCs, and we used a 7-day cut-off on the basis of laboratory investigations.24 In stored RBC, 2,3-DPG decreases progressively, resulting in a shift of the oxygen dissociation curve to the left, corresponding to an increased affinity of Hb for oxygen and, therefore, a decreased off-loading of oxygen to the tissues.25 This change appears to be significant after 7 days of storage.26 Second, both Ranucci et al.22 and Karam et al.23 used storage duration as a dichotomised variable. As emphasised by van de Watering,16 dichotomisation of storage duration could introduce a bias when evaluating the effect of this variable on postoperative morbidity. Storage time should preferentially be collected and analysed as a continuous variable, as this retains the most information. Using this approach, we did not observe an effect of storage duration on children's outcome. Our results are in line with those of Manlhiot et al.27 who reviewed a cohort of 1225 children exposed to high-risk cardiac surgery. When children were exposed to less than four RBC units, duration of storage, expressed as a continuous variable, did not influence postoperative outcome. However, a negative impact was observed in children having received four or more RBC units.
Interestingly, most of the previous studies did not consider the number of RBC units transfused. Multiple transfusions are correlated with the severity of illness and worse outcomes. Patients who received more transfusions were probably more severely ill than those who did not, and patients who are more severely ill have more adverse clinical outcomes. Moreover, patients who received more RBC units are at an increased risk to receive older units.28,29 For these reasons, we only included children exposed to one or two RBC units.
We could hypothesise that, more than the storage duration, the number of transfused units will have an impact on morbidity and mortality. This point has been emphasised by Roberson et al.26 who assessed the effect of 7 vs. 42-day-old stored RBC transfusion on tissue oxygenation and microcirculation in healthy volunteers. They reported that RBC transfusion of one unit of RBC stored for 42 days does not have any overt detrimental effects on oxygenation or microcirculation compared with one unit stored for 7 days. Thus, not only the volume of blood transfused but also the underlying patient's clinical conditions probably play an important role in the reported effects of RBC transfusion on outcome.
Our results are in accordance with those recently published in the ‘Age of Red Blood Cells in Premature Infants Study’ (ARIPI).30,31 This prospective, double-blind, randomised, controlled trial was performed in 377 premature infants with birth weights less than 1250 g, who were admitted to six Canadian tertiary neonatal ICUs. The primary goal of the study was to determine whether transfusion of RBC stored for 7 days or less decreased morbidity in this high-risk group. In this trial, the use of fresh RBC (mean ± SD age: 5.1 ± 2.0 days) was not associated with an improved outcome when compared with standard transfusion practices (14.1 ± 8.3 days).
Our results have to be interpreted taking into account some limitations. First, it is a retrospective, observational study; therefore, it is subject to uncontrolled confounding and potential misclassification biases. As an example, more re-do surgical procedures were included in the group ‘Old’ and this could have an impact on severe morbidity and mortality. However, after adjustment using the multivariate analysis, these factors were not independently associated with our composite endpoint. However, some other variables influencing severe postoperative morbidity and mortality may not have been collected for adjustment in the multivariate model. We made strenuous efforts to reduce the number of confounding variables. We chose a 7-day cut-off to define young and old RBC units based on laboratory findings instead of the median storage value. Nevertheless, storage duration was used as a continuous variable in the logistic regression analyses. The study protocol was strictly defined and followed. Furthermore, we only analysed nonemergency surgery including children who received a small amount of RBC, which allowed us to exclude more severely ill children and children with major bleeding, conditions known to be associated with poorer prognosis. Interestingly, our univariate or multivariate analysis highlighted the relation between some factors (such as low preoperative body weight, high PRISM score, increased duration of mechanical ventilation, prolonged ICU stay and larger volumes of platelet transfusion) and hospital mortality. These results are in accordance with a previous study that evaluated risks and predictors of blood transfusion in the paediatric population.32,33
Finally, even if the total studied population is large, the group ‘Young’ only included 118 children. We performed a power analysis to define the number of children required in each group to find a significant difference in term of our primary outcome. We computed the results of our univariate logistic regression analysis for storage duration and the incidence of the composite endpoint. If we accepted a 5% α-error and a 20% β-error, the power analysis showed that 17 874 patients would have been necessary to highlight a possible significant difference in the composite endpoint.
In conclusion, the results of our retrospective analysis showed that duration of RBC storage did not influence postoperative morbidity and mortality in children undergoing cardiac surgery and transfused with one or two units of RBC.
Acknowledgements relating to this article
Assistance with the study: none
Financial support and funding: solely supported by departmental sources
Conflicts of interests: none
Presentation: presented, in part, during the Best Abstract Runner-up session at the ESA Anaesthesiology Congress, Barcelona, June 2013.
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