Retrograde autologous priming of the cardiopulmonary bypass circuit as a blood conservation strategy: should we get pumped up?

Patient blood management (PBM) is an important component of quality cardiac surgical care. Through anaemia management, coagulation optimization, blood conservation modalities and patient-centred decision-making, PBM aims to improve blood conservation and patient outcomes [1]. In particular, retrograde autologous priming (RAP) of the cardiopulmonary bypass (CPB)\ circuit is becoming an increasingly common blood conservation strategy in adult cardiac surgery. Routine use of RAP varies across institutions, from <20% in European and Australian centres up to ∼62% in Canadian centres [2, 3]. While major societal guidelines generally promote the use of RAP, the recommendations were based on low- to moderate-quality evidence including systematic reviews of observational studies and small randomized controlled trials (RCTs) [1, 4] In their systematic review and meta-analysis, Gupta et al. [5] conducted a more comprehensive evaluation of the safety and efficacy of RAP in adult cardiac surgery patients to validate the guidelines’ recommendations based on the best available evidence to date. The authors conducted careful risk of bias evaluation of the included studies, evaluated the quality of evidence for each outcome and performed leave-one-out analysis. The authors conclude from low- to moderate-quality RCT and observational evidence comparing RAP to no RAP that RAP during adult cardiac surgery may be an effective blood conservation strategy. RAP demonstrated a significant reduction in the number of patients transfused with RAP [RCT data: Risk Ratio (RR) 0.58, 95% CI: 0.51–0.65, P < 0.001, I2 = 0%; observational data: RR 0.65, 95% Confidence Interval (CI): 0.53–0.80, P < 0.001, I2 = 75%]. In addition, with RAP, there was a reduction in the number of units transfused per patient (RCT data: mean difference −0.38 units, 95% CI: −0.72 to −0.04, P = 0.03, I2 = 77%; observational data: mean difference −1.0 units, 95% CI: −1.76 to −0.29, P = 0.006, I2 = 93%). However, RAP did not demonstrate a significant difference in morbidity and mortality with regard to bleeding, acute kidney injury, perioperative mortality and major adverse cardiovascular events, and the findings were inconsistent with respect to length of stay. The results of this review are consistent with findings of a recently published meta-analysis by Hensley et al. on RAP and its association with intraoperative red cell transfusion incidence, whole hospital red cell transfusion incidence, acute kidney injury outcomes and stroke outcomes [6]. While Hensley et al. included 21 RCTs and observational English, full-text studies, Gupta et al. also included an additional 8 studies due to their broader inclusion criteria with conference proceedings and lack of language restrictions. International guidelines generally promote the use of RAP. The 2019 European Association for Cardio-Thoracic Surgery/European Association for Cardio-Thoracic Anesthesia/European Board of Cardiovascular Perfusion guidelines on CPB in adult cardiac surgery recommend RAP as part of the blood conservation strategy to reduce transfusions (Class 1, Level of Evidence A) [4]. The 2021 Society of Thoracic Surgeons/Society of Cardiovascular Anesthesiologists/the American Society of ExtraCorporeal Technology/Society for the Advancement of Patient Blood Management Update to the Clinical Practice Guidelines on Patient Blood Management recommend the use of RAP of CPB circuits whenever possible (Class 1, Level of Evidence B-R) [1]. These guidelines are largely based on studies investigating RAP in conjunction with other methods, especially with use of minimized bypass systems, which may cause confounding of the blood conservation improvements observed with RAP. Despite the convincing findings by Gupta et al., there are a few limitations and evidence gaps worthy of discussion. The authors included unadjusted data for some observational studies, lower confidence in the estimates of effect, and studies with short follow-up times. It is also unclear whether the denominator of the number of transfusion units per patient reflects the entire patient cohort or only the patients who were transfused. In addition, it may be difficult to account for advancements in surgical techniques, PBM policies and CPB technology amongst the included studies, which span from 1998 to 2019. Furthermore, RAP was not the sole blood conservation strategy in all studies. This may have caused the findings to be confounded by the effects of other blood conservation interventions including acute normovolemic hemodilution, intraoperative autologous red cell salvage, antifibrinolytics, hemoconcentrator, shortened and low-prime circuit and more [6]. In addition, 15 of the included studies involved isolated coronary artery bypass grafting patients, including 5 of 12 RCTs and 10 of 17 observational studies while other studies gathered evidence from combined coronary artery bypass grafting patients, valve surgery patients, reoperative cardiac surgery patients, and other patients. Whether the findings are robust in the different cardiac surgical populations is unclear. Lastly, existing individual RCTs were small and, while having aggregate data is supportive of a benefit in terms of blood conservation, larger RCTs remain necessary to further elucidate the safety, efficacy and other clinical outcomes of RAP. Overall, Gupta et al. must be congratulated for conducting this important systematic review and meta-analysis comparing RAP to no RAP in CPB. They show that RAP is a safe and effective blood conservation strategy during adult cardiac surgery. RAP of CPB circuits is a simple technique that may justify the excitement that accompanies current guidelines; confirming this with larger RCTs may allow us to really get pumped up. Conflict of interest: none declared.

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