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Ann Thorac Surg 1996;62:417
© 1996 The Society of Thoracic Surgeons
DR L. HENRY EDMUNDS (Philadelphia, PA): This is a comprehensive, well-designed, well-executed study that squarely addresses two issues: First, what is the amount of thrombin that we can safely circulate during open heart operations? And second, does surface-bound heparin reduce the morbidity from cardiopulmonary bypass? To my mind, Aldea and associates have answered the second question persuasively, but I think the jury is still out regarding the first.
The first point is that the Duraflo II heparin-coated surface leaks heparin. In Boonstra's work with coated circuits, for the same systemic dose, the amount of heparin in plasma is clearly higher at the end of bypass.
Øvrum's work shows that when the amount of systemic heparin is reduced and a heparin-coated circuit is used, the amount of circulating thrombin is increased. There was a significant increase in F1.2 in the patients who received half-dose systemic heparin and Duraflo-coated circuits at the end of bypass as compared with patients who received a full dose of heparin and an uncoated circuit.
Finally, from our own work, when the same dose of systemic heparin was used with coated or uncoated circuits, there was no difference in the amount of F1.2 that circulated.
This study and many studies from Europe show that it is clinically safe to reduce the amount of circulating heparin and therefore increase the amount of circulating thrombin. However, at what concentration of thrombin do gross clots appear? Is this concentration the same for all patients?
The second question seems to have a clearer answer. Many studies have shown that surface-bound heparin reduces the amount of complement activation and also the amount of neutrophil activation, and may in fact spare platelets. The benefits shown in Aldea's study are partly due to a reduction in the inflammatory response to cardiopulmonary bypass as well as to an increase in circulating thrombin. This translates into less fluid retention, less temporary organ dysfunction, less bleeding, fewer thrombotic complications, and probably fewer emboli. The question is whether there are cheaper and safer ways to do this.
In vitro enoxaparin, which is a low-molecular-weight heparin, completely prevents complement activation during simulated cardiopulmonary bypass and also prevents the release of elastase. Animal studies and clinical trials have not been done with this drug, but the surprising antiinflammatory effect is interesting. "Blood anesthesia" produced by selective, temporary inhilution of specific plasma proteins and cells may be just as effective and much safer. Nevertheless, Aldea and associates have clearly shown that control of blood activation during open heart operations can produce great benefits. Their blood management program, which includes heparin-coated circuits, reduced systemic heparin and discarding plasma from the pericardial aspirate reduces the morbidity associated with first-time revascularization operations and does so conclusively.
DR ALDEA: Doctor Edmunds has pointed out two very important issues. Clearly, there is some thrombin formed during cardiopulmonary bypass regardless of the degree of anticoagulation therapy. The question is, is the small amount of thrombin that is being formed of clinical significance? I think that Dr Edmund's work has demonstrated that heparin-bonded circuits may not necessarily reduce thrombin formation during bypass. Other work by other investigators has shown some decrease in thrombin formation. Moreover, I think the work from Dr Øvrum's groups has documented a 5- to 10-fold increase in thrombin formation, which occurs after protamine reversal and 2 hours after bypass. That degree of elevation overshadows the amount of thrombin that is actually being formed during bypass.
We have not specifically looked at the amount of thrombin formed during cardiopulmonary bypass with our methods and are currently doing so. However, we did elect to look at the amount of blood activation and debris that exist during circulation by analyzing the arterial blood filters.
Scanning electron microscopic evaluations were done on two arterial filters at 200x magnification: a heparin-coated filter with the lowest ACT during bypass of 223 seconds, and a conventional circuit with full anticoagulation therapy. The amount of debris on the arterial surface, which is proof of blood activation and debris during bypass, is greater than with conventional circuits. This finding was not unique to these particular two filters. We have analyzed 40 filters with similar results.
So in answer to your question, we have not tried in this study to find the lower limits of safe anticoagulation, but rather tried to find a common ground where the benefits of heparin-bonded circuits could be achieved with a lower anticoagulation protocol.
DR LUDWIG K. von SEGESSER (Zürich, Switzerland): This is an important presentation that provides further evidence for the efficacy and safety of cardiopulmonary bypass with low systemic heparin treatment using heparin-coated equipment. I also agree with Dr Edmunds that there is no such thing as a no-risk procedure. Most surgeons would agree with that.
However, we overlook now more than 300 procedures with even tighter ACT criteria than those shown here. And like Aldea and associates, we have found less bleeding, lower transfusion requirements, and no device failures. This is major progress, as transfusion-related infectious diseases remain a major threat and add substantial costs to perfusion with unnecessarily high ACT values.
I would like to ask Dr Aldea up to what extent he will apply his findings in his future practice.
DR ALDEA: Before beginning this study, we initially used heparin-bonded circuits for high-risk procedures, which included reoperations in emergency catheterization laboratory crashes. Based on these findings, we will now use heparin-bonded circuits routinely for all cardiopulmonary bypass procedures. We believe that the results are so overwhelmingly positive that we can no longer withhold this treatment from patients undergoing coronary bypass operations.
DR LEO CUELLO (San Antonio, TX): This is a well-presented and well-planned study. However, based on my experience and protocol in saving blood, I think these patients may have bled too much in both studies. I think that the criteria for blood transfusion and blood products should be lowered. I think blood should not be replaced with blood, but can be replaced with albumin. We need to study the concept of normovolemic anemia, in which blood is replaced only if patients show signs of hypovolemia, and not replace blood, as many surgeons do, based on a laboratory value.
I have two questions. First, why did you replace blood with products postoperatively and not use albumin? Second, what was the average preoperative and postoperative hemoglobin? If patients have a preoperative hemoglobin value of more than 13 g, over 80% of the patients will need practically no allogeneic blood. If the hemoglobin is less than 11 g preoperatively, they might need blood. Any patient who leaves the hospital with hemoglobin of 13 g and who received 1 or 2 U of packed red blood cells probably did not need any allogeneic blood transfusion.
DR ALDEA: We agree that patients should not have transfusions based on thresholds alone. We did use thresholds in a context of this study to evaluate the relative efficacy of this newer therapy. The average hemoglobin in our patients before bypass was 11.3%, and the average hematocrit was 35%. The discharge values were very similar. So we did not overtransfuse the patients.
To emphasize this point, when patients are not taking intravenous nitroglycerin and heparin or aspirin, even though they presented to the operating room on an urgent basis (two thirds of our patients require urgent revascularization procedures), only 8% of them required transfusion in the heparin-bonded treatment group. These patients received only 0.18 U per patient, mostly packed red blood cells.
I think that whatever transfusion threshold one uses, given the patient population, they will do better with these particular methods.
Related Article
Ann. Thorac. Surg. 1996 62: 410-418.
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