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From the University of Ottawa, Centre for Transfusion Research, Ottawa, Ontario, Canada.
Address correspondence to: Dr. Paul C. Hébert, Centre for Transfusion and Clinical Epidemiology Program, Ottawa Health Research Institute, General Campus, 501 Smyth Road, Box 201, Ottawa, Ontario K1H 8L6, Canada. Phone: 613-737-8197; Fax 613-739-6266; E-mail: phebert{at}ohri.ca
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Abstract |
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Source: We identified all relevant articles through the combined use of electronic searches of the MEDLINE and EMBASE databases, the Cochrane library as well as hand searching of all randomized clinical trials and review articles. The electronic search included articles published between 1966 and April 2001. The search included textword searches using "autotransfusion," "cell salvage," "device," or Medical Subject Headings "autologous blood transfusion" or a "randomized controlled trials" filter.
Principal findings: Five randomized controlled trials (RCT) were identified involving cell salvage and vascular surgeries. In infra renal abdominal aortic aneurysm surgery the risk ratio (the risk of receiving at least one unit of allogeneic red cells) was 0.37 [95% confidence intervals (CI) of 0.06 to 2.36]. In elective aorto-femoral bypass surgery the risk ratio was 0.97 (95% CI of 0.66 to 1.42). The pooled risk ratio for cell salvage in vascular surgery was 0.67 (95% CI of 0.35 to 1.28).
Conclusion: Cell salvage, a commonly used technique to recover red cells from the operative field, has been the subject of several studies in vascular surgery. There is insufficient evidence to recommend the routine use of cell salvage in elective abdominal aortic aneurysm and aorto-femoral bypass surgeries. A large RCT would elucidate whether cell salvage is effective as a blood conservation technique.
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Introduction |
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Cell salvage is a blood conservation technique developed and adopted in the past 20 years as a means of decreasing blood transfusions. The technique recovers blood lost in the operative field, purifies it and returns the recovered red blood cells (RBCs) to the patient thereby potentially avoiding exposure to blood products. Many hospitals have included cell salvage as part of the procedures made available to surgical teams that undertake interventions with large anticipated blood losses.7
The cell salvage technique is widely used in orthopedic surgery where evidence suggested a reduction in exposure to blood products, however a similar benefit was not observed in cardiac surgery.7 In vascular surgery, there are many published studies that support the use of cell salvage. However, a significant proportion of these studies make use of historical controls or are observational in nature. In addition, the results of a recent randomized controlled trial8 did not observe any decrease in red cell transfusions using cell salvage. The authors concluded that this technique should no longer be considered standard of care. Given genuine uncertainty regarding the use of cell salvage in vascular surgery, we conducted a systematic review of published studies to determine if cell salvage reduced the exposure of red cells.
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Methods |
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We selected studies that met the following criteria. The article was required to state that the study: 1) used a form of random allocation; 2) incorporated a control group that did not receive blood products recovered from a cell salvage device; 3) enrolled patients who underwent an elective abdominal vascular surgery (infra renal abdominal aortic aneurysm or a aorto-bifemoral bypass); and 4) reported the proportion of patients receiving at least one unit of allogeneic blood (the primary outcome). Studies were excluded from the meta-analysis if: 1) they were duplicate publications; 2) primarily involved patients under the age of 18 yr; and 3) allocated patients in the postoperative period.
One of the investigators (G.A.) examined all titles and abstracts obtained in these searches to determine eligibility of the randomized trial in this review. References of retrieved articles were also identified and examined to find additional studies. All randomized controlled trials identified were critically appraised.
Data extraction and synthesis
Once identified, the abstracted data included the proportion of patients receiving at least one unit of allogeneic packed RBCs as a primary outcome. We also gathered data on the mean number of RBC units transfused,9 the number of patients receiving autologous predonation, the quantity of red cells transfused in millilitres as well as clinical outcomes including complication from surgery and mortality rates. Information related to cell salvage technique included the model of the cell saver, the type of blood collection (washed vs unwashed) and the length of time of cell saver use. Other pertinent information related to the study itself included baseline demographics, exclusion criteria, as well as the number of study participants enrolled in the trial. Finally, we assessed indicators of the quality of the studies, specifically blinding, method of randomization and the completeness of follow-up once randomized.
The effect of cell salvage on the proportion of patients who received allogeneic blood was summarized with an overall estimate of the relative risk (RR) and 95% confidence intervals (CI) by using a Mantel and Haenszel’s fixed-effects model.10 In this report, a RR with 95% CI incorporating 1.0 suggests that there is no difference between groups or insufficient data to conclude that cell salvage decreased overall exposure to red cells. A RR with a point estimate and 95% CI less than 1.0 suggests that fewer patients in the cell salvage group received at least one unit of allogeneic red cells while a RR greater than 1.0 suggests that more patients in the cell salvage group received at least one red cell transfusion.
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Results |
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We identified two randomized controlled trials in patients undergoing elective aorto-femoral bypass surgery.8,12 Neither study demonstrated a clinically important decrease in the number of patients exposed to red cells following the use of cell salvage. The Clagett study reported a RR of 0.94 (95% CI from 0.63 to 1.40) and the Kelley-Patteson study reported a RR of 1.50 (95% CI from 0.28 to 7.93). The pooled RR was 0.97 (95% CI from 0.66 to 1.42). There were insufficient data from both studies to compare and analyze the differences between mean units transfused.
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Discussion |
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In the subgroup of patients who underwent infra renal abdominal aortic aneurysm surgery, the pooled RR for cell salvage was 0.37 with 95% CI ranging from 0.06 to 2.36. In this instance, the very large CIs were primarily a function of divergent study results. In addition to the limited information available for review, there were a number of clinical and methodological differences between the trials. These differences may also offer plausible explanations for our inability to observe benefits from this blood conservation technique. Key differences between studies included referral patterns that may have caused differences in surgical complexity and severity of illness, surgical technique, cell salvage techniques, the type and frequency of complications, approach to randomization as well as other factors.
In the Clagett study, the greater use of re-transfused salvaged red cells was associated with an increased use of allogeneic blood required to maintain comparable hemoglobin concentrations. In an accompanying editorial, Ouriel noted that despite two additional red cell units recovered from the cell salvage procedure, postoperative hematocrit levels were comparable between groups immediately following surgery.8 One would have expected a measurable increase in red cell mass given that patients received extra salvaged red cells. A plausible explanation for the inability to detect a rise in postoperative hemoglobin concentrations elaborated in the editorial was excess hemolysis of salvaged red cells in study participants. Unfortunately, markers of hemolysis were not measured by Clagett and colleagues. Other potential explanations may include differences in fluid administration resulting in dilutional anemia in the postoperative period.
Hemolysis from cell salvage procedures may occur because of suction pressures in the collection of shed red cells that increase shear stress in the plastic tubing and filters in the device, as well as the centrifugation process.15 In their trial Spark and colleagues standardized the suction pressures not to exceed 150 mmHg and filter sizes used during surgery. As a consequence, they did not observe significant differences in biochemical and morphological markers of hemolysis. Differences in the type of device may also have caused different rates of clinically important hemolysis observed in the two trials.
The marked difference in reported cross clamp time and the length of time required for the operative intervention may indicate differences in the selection of study participants or surgical techniques. Indeed, reported results from the Clagett trial averaged almost twice the amount of time patients were anesthetized, doubled the length of time of cross clamp and operation time when compared to Spark et al. (Table II). The longer cross-clamp times16 or the re-infusion of hemolyzed red cells may have resulted in an increased rate of complications such as nosocomial infections, lengths of stay and perioperative mortality. Unfortunately, both studies were not large enough to detect changes in most clinically important outcomes.
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The randomized trials also included patients undergoing aorto-femoral bypass surgery.8,12 This surgical intervention requires significantly less RBC transfusions as compared to aortic aneurysm surgery. Both major studies8,12 evaluating cell salvage during aorto-femoral bypass were not able to document any savings in allogeneic RBC use with this technique. Inferences from these studies and the resultant meta-analysis are weak given the small sample size of both individual studies and the pooled results. The use of cell salvage for surgical procedures with low blood loss such as aorto-femoral bypass will likely not reduce or eliminate exposure to allogeneic RBCs.
Although only three randomized controlled trials met inclusion criteria, each study adequately described the study population, the cell salvage techniques and their complications, the transfusion protocols as well as important surgical and anesthetic benchmarks. In addition, the reports adequately described the use of standard concealment of randomization technique, reported on all major objective outcomes and major co-interventions. The reporting of outcomes and co-interventions is all the more important in studies where blinding is not possible.
In conclusion, the results of this meta-analysis did not find sufficient evidence that cell salvage decreases exposure to allogeneic red cells in abdominal vascular surgeries. Unfortunately, there are too few studies involving too few patients to draw inferences regarding the benefit of therapy. Indeed, the fact that three randomized controlled trials documented divergent outcomes underscores the need for further research. In addition, we identified even fewer data examining the consequences of this blood sparing technique on clinically important outcomes such as infections, organ failure and mortality. Given the substantial cost associated with this technique, we believe that it is all the more important to conduct a large randomized controlled trial in this patient population.
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Footnotes |
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Accepted for publication August 26, 2003. Revision accepted February 13, 2004.
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2 Coffin CM. Current issues in transfusion therapy. 1. Risks of infection. Postgrad Med 1986; 80: 219–24.
3 Crosby ET. Perioperative haemotherapy: II. Risks and complications of blood transfusion. Can J Anaesth 1992; 39: 822–37.
4 Faust RJ, Warner MA. Transfusion risks. Int Anesthesiol Clin 1990; 28: 184–9.[Medline]
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7 Huet C, Salmi LR, Fergusson D, et al. A meta-analysis of the effectiveness of cell salvage to minimize perioperative allogeneic blood transfusion in cardiac and orthopedic surgery. International Study of Perioperative Transfusion (ISPOT) Investigators. Anesth Analg 1999; 89: 861–9.
8 Clagett GP, Valentine RJ, Jackson MR, Mathison C, Kakish HB, Bengtson TD. A randomized trial of intraoperative autotransfusion during aortic surgery. J Vasc Surg 1999; 29: 22–31.[Medline]
9 Lemmer JH Jr. Reporting the results of blood conservation studies: the need for uniform and comprehensive methods. Ann Thorac Surg 1994; 58: 1305–6.[Medline]
10 Normand SL. Meta-analysis: formulating, evaluating, combining, and reporting. Stat Med 1999; 18: 321–59.[Medline]
11 Varga ZA, Thompson JF, Locke-Edmunds JC, Baird RN, Farndon JR. Clinical and experimental studies of intraoperative autotransfusion using a new filtration device. Br J Surg 1995; 82: 765–9.[Medline]
12 Kelley-Patteson C, Ammar AD, Kelley H. Should the cell saver autotransfusion device be used routinely in all infrarenal abdominal aortic bypass operations? J Vasc Surg 1993; 18: 261–5.[Medline]
13 Spark JI, Chetter IC, Kester RC, Scott DJ. Allogeneic versus autologous blood during abdominal aortic aneurysm surgery. Eur J Vasc Endovasc Surg 1997; 14: 482–6.[Medline]
14 Thompson JF, Webster JH, Chant AD. Prospective randomised evaluation of a new cell saving device in elective aortic reconstruction. Eur J Vasc Surg 1990; 4: 507–12.[Medline]
15 Adan A, Brutel de la Riviere A, Haas F, van Zalk A, de Nooij E. Autotransfusion of drained mediastinal blood after cardiac surgery: a reappraisal. Thorac Cardiovasc Surg 1988; 36: 10–4.[Medline]
16 Gelman S. The pathophysiology of aortic cross-clamping and unclamping. Anesthesiology 1995; 82: 1026–57.[Medline]
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