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Eur J Cardiothorac Surg 2005;28:512-513
© 2005 Elsevier Science NL

Letter to the Editor

The independent effects of cardiopulmonary bypass hemodilutional anemia and transfusions on CABG outcomes

Robert H. Habib a , b , * , Anoar Zacharias a , c , Thomas A. Schwann a , c , Christopher J. Riordan a , c

a Division of Cardiovascular Surgery, St Vincent Mercy Medical Center, 2213 Cherry Street, ACC Bldg, Suite 309, Toledo, OH 43608, USA
b Department of Medicine, Medical University of Ohio, Toledo, OH, USA
c Department of Surgery, Medical University of Ohio, Toledo, OH, USA

Received 11 April 2005; accepted 16 May 2005.

* Corresponding author. Address: Division of Cardiovascular Surgery, St Vincent Mercy Medical Center, 2213 Cherry Street, ACC Bldg, Suite 309, Toledo, OH 43608, USA. Tel.: +1 419 251 4998. (Email: robert_habib{at}mhsnr.org).

Key Words: Hematocrit • Perioperative outcomes • CABG • Survival • Intraoperative • Post-operative

We read with care and interest the recent article by Kuduvalli and colleagues [1] which investigated the role of perioperative transfusions on coronary artery bypass surgery (CABG) outcomes. We largely agree with the authors' conclusion that perioperative transfusions are associated with worse operative and 1-year mortality. Indeed, it conforms with the findings reported by our group in 2002 [2]. Yet, study design limitations (partly recognized by the authors) have lead them to underestimate the true adverse effects of excessive hemodilution on cardiopulmonary bypass (CPB). This was exacerbated by the authors' apparent unfamiliarity with findings of another report [3] that were not discussed in the article. There, in a systematic analysis of 3800 consecutive CABG patients with CPB and after adjusting for intraoperative and post-operative RBC transfusions, an independent association between increasing CPB hemodilutional anemia levels and worse operative outcomes and 6-year mortality were documented [3].

We wish to make (pose) the following comments (questions) to the authors that are primarily related to their handling of the two primary independent variables (hemoglobin levels and RBC transfusions):

(1) It is unclear why the authors chose the lowest laboratory hemoglobin (LL Hb; gdl–1) after arrival in the intensive care unit as opposed to the lowest Hb including during surgery. This is especially puzzling given their handling of the RBC transfusion data, which included units provided during surgery. Such inconsistency is a source of substantial confounding. The authors should elucidate why the intraoperative Hb was ignored while intraoperative transfusions were not. It is also noteworthy that their propensity model did not distinguish between transfusions during or after surgery. Why not?
(2) The pre- and intra-operative Hb values were not provided—nor were the CPB and ischemic times. Importantly, the reported median (interquartiles) LL Hb value of 14.1 (13.0–15.0)gdl–1 for the No RBC Transfusion cohort (Table 1) is surprisingly high and suggests phenomenally high preoperative Hb levels. Even the 30% off-pump patients cannot explain such values. In fact, such LL Hb values are higher than what is reported for preoperative Hb in many published CABG series. Please explain these data as they are critical to the interpretation of the study as a whole.
(3) Why are 28 patients transfused after post-operative day 3 included in the no transfusion cohort? Rationalizing this decision is particularly difficult to understand vis-a-vis the 1-year mortality effect. Why should the outcome effect at 1-year differ for patients transfused on day 3 differ from those on days 4 and 5? How this misallocation of patients affects the reported results is difficult to predict. At worst (although not ideal), these 28 patients could have been excluded altogether.
(4) The number of patients in each of the patient subgroups (A–D) in Figs. 4 and 5—which are based on the four possible combinations of RBC transfusion (Yes/No) and LL Hb >10gdl–1 (Yes/No)—and the corresponding P-values for these comparisons were not provided in the paper. Taking into account the above points, we contend that (a) the apparent lack of significance is a consequence of the number of patients in some of the groups, and (b) the systematically worse survival trends in the LL Hb <10gdl–1 (irrespective of transfusion status) is indicative of the importance of CPB hemodilutional anemia on outcomes.

In summary, and given the importance of the topic, we hope the authors would consider reporting to the readers their outcomes analysis results after incorporating the above outlined study design changes. We suggest that in doing so they might find significant independent adverse effects of CPB hemodilutional anemia and RBC transfusions on outcomes. Such findings would advance the position that changes to current CPB practice guidelines are needed such that both ‘RBC transfusions’ as well as their primary cause ‘CPB hemodilutional anemia’ are avoided.

References

  1. Kuduvalli M, Oo AY, Newall N, Grayson AD, Jackson M, Desmond MJ, Fabri BM, Rashid A. Effect of peri-operative red blood cell transfusion on 30-day and 1-year mortality following coronary artery bypass surgery. Eur J Cardiothorac Surg 2005;27(4):592-598.[Abstract/Free Full Text]
  2. Engoren M, Habib RH, Zacharias A, Schwann TA, Riordan CJ, Durham SJ. Effect of blood transfusion on long-term survival after cardiac surgery. Ann Thorac Surg 2002;74:1180-1186.[Abstract/Free Full Text]
  3. Habib RH, Zacharias A, Schwann TA, Riordan CJ, Durham SJ, Shah A. Adverse Effects of low hematocrit during adult cardiopulmonary bypass: should current practice be changed?. J Thorac Cardiovasc Surg 2003;125(6):1438-1450.[Abstract/Free Full Text]




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