| JOURNAL HOME | CME HOME | THIS MONTH | PAST ISSUES | ETOC | COLLECTIONS |
| AUTHORS | REVIEWERS | EDITORIAL BOARD | FEEDBACK | RSS | HELP |
| ||||||||||||||
| ||||||||||||||
|
|
|
|
||||||||||||
|
||||||||||||||
Departments of *Anesthesiology,
Cardiothoracic Surgery, and
Laboratory Medicine, University of Washington, Seattle, Washington; and
Department of Research Administration, Children’s Hospital and Regional Medical Center, Seattle, Washington
Address correspondence and reprint requests to Michael J. Eisses, MD, 4800 Sand Point Way N.E., Department of Anesthesia, Mail Stop 4D-1, Children’s Hospital and Regional Medical Center, University of Washington School of Medicine, Seattle, WA 98105. Address e-mail to michael.eisses{at}seattlechildrens.org
Abstract
Interventions such as heparin-coated circuits, 
-aminocaproic acid, and reduced shed blood reinfusion have shown mixed results when applied individually for limiting hemostatic activation during cardiopulmonary bypass (CPB). We compared coagulation and fibrinolytic activation during conventional CPB (control) (CTRL) using noncoated circuits, no antifibrinolytics, and open cardiotomy with a combined strategy (HAC) that used heparin-coated circuits, -aminocaproic acid, and closed cardiotomy. Blood samples were drawn before, during, and after CPB for primary coronary bypass grafting surgery from 9 CTRL patients and 10 HAC patients. Thrombin-antithrombin complex and fibrinopeptide A levels (markers of thrombin and fibrin generation) were reduced in the HAC versus CTRL group after 30 min of CPB (P < 0.05). Average tissue plasminogen activator (tPA) levels were significantly lower in the HAC group by 30 min on CPB (P < 0.05), resulting in preservation of plasminogen activator inhibitor (PAI)-1 during CPB (P < 0.05). D-Dimer, a measure of intravascular fibrin formation and removal, was reduced in the HAC group during and after CPB (P < 0.005). Overall, the combined strategy was associated with a reduction in CPB-induced increases in markers of thrombin generation, fibrin formation, tPA release, and fibrin degradation and better preservation of PAI-1.
IMPLICATIONS: A combined approach during cardiopulmonary bypass (CPB) that uses heparin-coated circuits, -aminocaproic acid, and limited reinfusion of shed pericardial blood is associated with reduced activation of the coagulation and fibrinolytic systems that typically occurs during conventional CPB.
Cardiopulmonary bypass (CPB) activates a variety of hemostatic processes, including coagulation and fibrinolysis, leading to an increased risk of postoperative bleeding and thrombosis. Given the interactions between systems, activation of one system may lead to activation of others, and, conversely, suppression of one system may lead to suppression of others. Interventions designed to reduce excessive activation of hemostatic processes during CPB include using "biocompatible circuits" (1–5) and antifibrinolytic drugs (6–8) and limiting the reinfusion of shed pericardial blood into the CPB circuit (9,10). In prior studies, Aldea et al. (1,9,11) developed an approach that combines heparin-coated CPB circuits, -aminocaproic acid (EACA), and washing of shed red blood cells before reinfusion to reduce multiple system activation. Their approach uses a collapsible venous reservoir that minimizes the blood-air interface, prevents direct reinfusion of mediastinal blood into the venous reservoir, and uses a cell salvage device (cell saver) to wash red cells, which are then reinfused into the venous side of the circulation. They demonstrated a decrease in postoperative levels of prothrombin activation peptide F1.2, polymorphonuclear elastase, terminal complement complex, ß-thromboglobulin, and platelet factor 4 (9).
CPB-induced activation of the coagulation and fibrinolytic systems occurs at different times during and after CPB. For example, maximum levels of active tissue plasminogen activator (tPA) occur near the beginning of CPB (12), and maximum levels of thrombin-antithrombin complex (TAT) occur at the end of CPB or after heparin neutralization (13–15), whereas maximum levels of plasminogen activator inhibitor (PAI)-1 occur several hours postoperatively (16). Most therapies designed to mitigate the effects of CPB, such as antifibrinolytics, coated circuits, and processing of shed blood, take place during CPB itself, yet some studies evaluated only baseline and post-CPB samples (8,17) or selected only one time point during CPB (7,9,18). Few studies have examined activation markers immediately after the initiation of CPB and after reperfusion. We hypothesized that the combined approach used in this study would reduce CPB-induced activation in several ways, including reductions in markers of thrombin generation, active tPA levels, fibrinolytic activity, and active PAI-1 levels. We sampled repeatedly before, during, and after CPB to see the time course and magnitude of effects that the combined approach had on suppression of coagulation and fibrinolytic system activation due to CPB.
Methods
Studies on human subjects were performed according to the principles of the Declaration of Helsinki. The University of Washington (UW) Human Subjects Review Committee approved the study, and all participants signed informed consent. A total of 19 patients undergoing primary coronary artery bypass grafting surgery (CABG) with CPB were recruited consecutively and studied according to the clinical standards in place at that time. Only first-time CABG patients were included. Patients taking preoperative aspirin were included, but patients taking any other anticoagulant therapy (e.g., platelet inhibitors or heparin) were excluded.
The first 9 patients (all male; mean age, 59 ± 7 yr), labeled as controls (CTRL), underwent CPB with standard nonheparin-coated circuits with a hard-shell reservoir and oxygenator (Affinity; Medtronic, Minneapolis, MN) and no antifibrinolytic drugs. Cardiotomy and vented blood were routed to the noncoated reservoir as an open system. The next 10 patients (7 men and 3 women; mean age, 64 ± 13 yr), labeled as the combined therapy group (HAC), underwent CPB with heparin-coated circuits (Carmeda; Medtronic) with a collapsible soft-shelled reservoir and EACA in 3 doses: 10 g after anesthesia and before the start of surgery, 10 g in the priming fluid of the CPB circuit, and 10 g at the time of heparin reversal with protamine sulfate (7,19,20). Cardiotomy blood was routed to a cell saver (COBE Brat II), and vented blood was routed to a heparin-coated collapsible soft-shelled reservoir. Seven of 9 CTRL patients and all 10 in the HAC group took preoperative aspirin.
Anesthesia techniques were consistent for all patients. Patients received midazolam (1–3 mg), and anesthesia was induced with fentanyl, etomidate, and pancuronium. Anesthesia was maintained with isoflurane (0.6%–1.2%) and fentanyl (total dose, 1500–2500 µg).
Bypass circuits were all primed with Plasmalyte (Baxter). IV porcine heparin was administered to maintain a target activating clotting time of 450 s in the CTRL group and 300 s in the HAC group by using a heparin dose-response assay (Hepcon Instrument; Medtronic Perfusion Systems) (1,2,11). Pump flow rates were targeted to a cardiac index of 2.4 L · min–1 · m–2 across both groups, with maintenance of near-normal patient temperatures. Mean arterial blood pressures were maintained at or more than 60 mm Hg by using phenylephrine after adequate pump flow was obtained. Protamine administration was standardized across both groups by using a standard heparin assay (Hepcon HMS). In the CTRL group, the cell saver was used to collect and process blood only after protamine administration, whereas in the HAC group, it was used throughout the procedure. Processed cell saver blood was given IV after protamine administration in both groups.
Blood was collected from an arterial catheter at 10 time points: 1) after the induction of anesthesia, 2) after sternotomy, 3) after heparin administration, 4) after 5 min of CPB, 5) after 15 min of CPB, 6) after 30 min of CPB, 7) 5 min before reperfusion of the heart (release of cross-clamp), 8) 5 min after reperfusion, 9) 3 min after protamine sulfate administration, and 10) 2 h after surgery ended. At each time point, 4.5 mL of arterial blood was anticoagulated with 11 mM citrate and 25 µM D-Phe-Pro-Arg-chloromethylketone for assays of prothrombin activation peptide F1.2, TAT, plasmin-antiplasmin complex (PAP), tPA antigen, D-dimer, and fibrinopeptide A (FPA). Next, 4.5 mL of arterial blood was added to 0.5 mL of 0.105 M citrate for assays of PAI-1 antigen and PAI-1 activity. Finally, 4.5 mL of blood was added to acid citrate (Biopool Int.) for measuring tPA activity (21).
F1.2 (Dade-Behring; coefficient of variation [CV] = 6%), TAT complex (Dade-Behring; CV = 5%), PAP (Dade-Behring; CV = 6%), D-dimer (Diagnostica Stago; CV = 5%), tPA antigen (Diagnostica Stago; CV = 7%), PAI-1 antigen (Biopool Int.; CV = 6%), and FPA (in-house; CV = 10%) were measured by using enzyme immunoassay methods. PAI-1 activity (CV = 5%) and tPA activity (CV = 5%) were measured by using bioimmunoassays (Biopool Int.). All assays were performed by the Department of Laboratory Medicine at the UW. Assay controls run with each batch of samples were within the expected ranges before the results were reported. The laboratory is accredited by, and participates in, the external proficiency testing program of the College of American Pathologists.
Statistical analysis was performed through biostatistical consultation with the Department of Research Administration at Children’s Hospital and Regional Medical Center. The estimates of the effects of patient group and time point were calculated by using a generalized estimating equation model (22,23). The model family was Gaussian. The mean link function was the identity link. The working correlation structure used was the identity matrix. The scale parameter option was "scale (x2)." Stata software (Stata Corp., College Station, TX) was used for all statistical calculations.
Because the sampling time points in this study span highly distinct phases of the surgical experience (pre-CPB, during CPB, and postsurgery), time point was treated as a set of indicator variables rather than as a continuous covariate. The generalized estimating equation model included the time point indicators and the group x time point interaction terms, which allows for a separate mean to be fitted for each group at each time point. The primary comparisons of interest were the between-group differences at each specific time point, which are marked with asterisks when significantly different. Analyses of within-group changes relative to the group’s baseline mean (sample point 1) were also performed via Wald testing on the appropriate linear combinations of model coefficients and are depicted by different plotting characters: squares, P = not significant; circles, P < 0.05; triangles, P < 0.005.
Results
There were no significant differences in preoperative demographic data (Table 1); no significant differences in CPB prime volumes, hematocrit, or platelet counts on CPB; and no significant differences in CPB times between groups (Table 2). Although the total fluid volume processed by the cell saver was larger in the HAC group, the amount of processed red blood cells transfused was not different (Table 2).
|
|
|
Both groups showed a modest increase in FPA during surgery before CPB (Fig. 2). During CPB, FPA levels in CTRL decreased to baseline levels and then increased again after heparin neutralization. In the HAC group, FPA levels decreased to less than baseline after CPB started and remained low until after heparin neutralization. FPA levels in the HAC group were 40% and 60% less than CTRL at 30 min on CPB and after reperfusion, respectively. Fibrinogen levels decreased in both groups immediately after CPB started because of hemodilution, and they remained low throughout CPB. Overall, there was less fibrin formation in the HAC group during CPB compared with the CTRL group.
|
|
In the CTRL group, average active PAI-1 levels (Fig. 3) were unchanged during surgery before CPB, decreased more than 5-fold by 5 min on CPB, and then increased toward the end of CPB, reaching 15-fold more than baseline by 2 h postoperatively. In contrast, average active PAI-1 levels in the HAC group were not significantly different from baseline at any time during or after CPB. Average active PAI-1 levels were twofold higher in the HAC group compared with the CTRL group by 5 min on CPB and remained higher at 30 min on CPB.
In the CTRL group, average total PAI-1 levels (Fig. 3) showed little change during early surgery but decreased 30% by 5 min on CPB and remained less than baseline until late CPB, when total PAI-1 levels began to increase. In the HAC group, average total PAI-1 levels showed little change from baseline until 2 h after surgery, when the levels increased significantly. Total PAI-1 levels were twofold less near the end of CPB in the HAC group compared with the CTRL group and remained significantly less after neutralization of heparin.
The combined strategy reduced the increase in tPA during CPB, better preserved PAI-1 activity during early CPB, and reduced the increase in PAI-1 at the end of surgery; this was associated with normal rather than low postoperative tPA activity. Overall, the combined strategy reduced the typical CPB-induced swings in fibrinolytic activators and activator inhibitors and brought the patients back toward baseline status.
In the CTRL group, average PAP levels (Fig. 4) were unchanged from baseline through early CPB. PAP levels increased near the end of CPB and peaked after neutralization of heparin. In the HAC group, average PAP levels showed a 60% increase during early surgery after the administration of EACA. They declined toward baseline by 5 min on CPB before increasing mid-CPB and peaking postoperatively. PAP levels were 60% higher in the HAC group compared with the CTRL group during early surgery but were no different during and after CPB.
|
Discussion
Studies looking at strategies individually for reducing CPB-induced systemic activation by using heparin coating, antifibrinolytics, and limited shed blood reinfusion have had mixed results (3,5,7,24). In contrast, an approach that combines these interventions has been shown to be safe with regard to thromboembolic complications (1) and resulted in reduced activation of a number of systems assessed during CPB, fewer intensive care unit (ICU) days, fewer hospital days, and fewer postoperative complications (1,2,9,11,25). In this study, we evaluated in more detail the effect of this combined approached on the coagulation and fibrinolytic systems during CPB.
In interpreting thrombin generation, we used both TAT with a short half-life (10 minutes) and F1.2 with a longer half-life (90 minutes). Using F1.2 alone can be misleading given that increases may be related to accumulation. Prior studies have generally reported peak levels of TAT at the end of CPB (7,14,15), after heparin neutralization with protamine (3,13,16), or occasionally several hours postoperatively (26). We found that during conventional CPB, increased thrombin generation occurs soon after CPB is started, continues throughout CPB with peak TAT levels after reperfusion or heparin neutralization, and returns toward baseline by two hours after surgery. Some studies have reported that heparin-coated circuits in combination with aprotinin (27) or with leukocyte filtration (28) reduced thrombin generation during CPB, whereas other studies (3,5,29) have reported no effect of heparin coating on thrombin generation. Antifibrinolytic drugs have had variable results in reducing thrombin generation, with some studies showing no change (8) and others showing a decrease with EACA (7). Limiting reinfusion of shed blood alone leads to a reduction in thrombin activation markers by the end of CPB but not during CPB (9). The HAC group in our study had a significant reduction in thrombin generation markers within 15 minutes of starting CPB that continued into the postoperative period. This reduction in thrombin generation markers was associated with lower levels of the fibrin formation marker FPA by 30 minutes on CPB as well.
The combined approach potentially reduces excess thrombin and fibrin generation in three ways: First, limiting the reinfusion of shed blood reduces the reinfusion of tissue factor, the initial precursor to in vivo thrombin generation (30,31). Second, reduced fibrinolytic activation may limit plasmin-induced platelet activation. Finally, heparin coating reduces kallikrein/kinin (32) and complement system (32) activation, which further reduces the overall hemostatic and inflammatory activation.
Conventional CPB results in dramatic swings in the fibrinolytic system, including increased secretion of tPA during CPB, increased plasmin generation, and fibrin degradation, all of which are associated with an increased risk of bleeding (32). A dramatic increase in PAI-1 activity postoperatively may shift the balance in fibrinolytic activity to a potentially prothrombotic state. Heparin coating of the circuit has been associated with suppression of tPA release (5) and reduced PAP levels (33), but not with a reduction in fibrin degradation (34), preservation of PAI-1 during CPB, or suppression of high postoperative PAI-1 levels (5). EACA reduces D-dimer levels but does not decrease plasminogen activation (35), preserve PAI-1 during CPB, or reduce postoperative PAI-1. Restricting shed blood reinfusion alone may have helped in suppressing tPA secretion but did not preserve PAI-1 (9).
In the CTRL group, peak levels of active tPA occurred during the first 30 minutes of CPB. Inhibition of fibrinolytic activity when active tPA was at its maximum was achieved with an EACA dosing scheme reported to be safe by other groups (7,19,20); it included both a bolus of EACA before CPB started and EACA in the pump prime. This resulted in the highest EACA levels immediately after beginning CPB. In our study, there were no new postoperative myocardial infarctions or strokes in either group. We have also studied smaller EACA doses, and both may be similarly effective (9).
In the HAC group, both tPA secretion and D-dimer formation were suppressed compared with conventional CPB. Reduced release of tPA was associated with preservation of PAI-1 during CPB. The typical post-CPB acute-phase increase in PAI-1 was reduced in the HAC group, and this resulted in preservation of postoperative active tPA at normal baseline levels. This may be a result of an overall reduction in the inflammatory response compared with conventional CPB.
Notably, plasmin formation as measured by the marker PAP was significantly increased in the HAC group just before CPB. The increase in PAP in the HAC group was associated with the administration of EACA. Lysine-binding site blockers increase the rate of plasminogen activation by tPA (35) while simultaneously reducing the ability of plasmin to lyse fibrin. Overall, the combined approach was associated with reduced tPA release, reduced plasmin lysis of fibrin, and reductions in the typical swings in fibrinolytic activation, inhibition, and increased fibrin degradation seen during conventional CPB. This resulted in a quicker return to normal fibrinolytic regulation postoperatively.
Cardiac surgery and CPB can reduce coagulation factors and platelets through hemodilution by the pump priming fluid, blood loss as part of surgery, losses associated with cell salvage and washing, and consumption through a disseminated intravascular coagulation-like process. Consumption of coagulation factors is usually associated with excessive hemostatic activation and a continued decline in levels of coagulation factors and platelets after the initial reduction due to hemodilution. In this study, there were lower levels of FPA, F1.2, and TAT during CPB in the HAC group; this suggests less coagulation activation. There was a decrease in both antithrombin and fibrinogen levels due to hemodilution at the start of CPB, but the levels were statistically unchanged for the remainder of CPB and were not different between groups. A larger study would be required to determine whether there were differences between groups in the consumption of coagulation factors. Average blood volumes processed by the cell saver were larger in the HAC group, but the actual volume transfused was similar (Table 2).
In our study, the HAC group received less systemic anticoagulation. The use of less anticoagulation has been controversial with regard to thrombotic complications. In this study, reduced anticoagulation was used only in conjunction with the combined approach of heparin-coated circuits, EACA, and elimination of shed blood reinfusion. Aldea et al. (1,2,9,11) have reported such an approach as safe with regard to thrombotic complications, such as graft occlusion, stroke, and myocardial infarction. Although some studies (36,37) have reported an increase in hemostatic activation markers in patients receiving less anticoagulation, our study and an earlier study by Aldea et al. (11) did not. One potential explanation is that the use of such a combined approach not only includes heparin bonding of the circuit but also limits reinfused shed blood. One recent study (38) reports a correlation between the increase in thrombin generation markers during CPB and the amount of reinfused shed blood. Despite these discrepancies and possible explanations, care must be taken with smaller heparin doses to avoid potential thrombotic complications, especially rarer complications such as CPB circuit and intracardiac thrombosis. Although Aldea et al. have shown a reduction in postoperative complications, a recent article by Mangano (39) suggests that antifibrinolytic drugs may increase mortality in CABG. Therefore, the routine use of antifibrinolytic drugs for lower-risk surgery with CPB remains controversial and will require further adequately powered, randomized studies.
There are limitations to this study. First, the patient groups were studied consecutively on the basis of a change in the conduct of CPB at our institution. Although the groups were not randomized, they were demographically similar. The focus of this study was the time course of coagulation and fibrinolytic activation during CPB and its suppression by the combined approach. Clinical outcome was not evaluated because of the small group size required by the high intensity of testing and because clinical studies of the combined approach had already shown less blood loss, less blood use, fewer days in the ICU, fewer days in the hospital, and fewer postoperative complications (1,2,9,11,25). We evaluated first-time CABG or low-risk patients, in which the routine use of individual aspects of this combined approach is still debated. Some aspects of this approach may not be feasible when cardiotomy volumes are large, such as during valve surgery. In particular, surgeries that result in larger cell saver volumes may lead to a loss of circulating platelets and coagulation factors through the washing process and may ultimately lead to more bleeding and larger transfusion requirements. The cost-effectiveness of these alternative approaches has yet to be determined. At our institution, full-dose aprotinin therapy costs approximately US$1200 (three vials). The cost of an uncoated circuit with a noncoated cardiotomy reservoir is approximately US$725, compared with US$755 for a heparin-bonded circuit with no reservoir and 30 g of EACA. Cell salvage with identical costs was used for both groups.
In conclusion, a combined approach using heparin-coated circuits, EACA, and a closed cardiotomy system that limits the reinfusion of shed blood is associated with decreased activation of the coagulation and fibrinolytic systems starting immediately after initiating CPB and extending into the early postoperative period.
References
This article has been cited by other articles:
|
|
 |
|
  M. Pretorius, B. S. Donahue, C. Yu, J. P. Greelish, D. M. Roden, and N. J. Brown Plasminogen Activator Inhibitor-1 as a Predictor of Postoperative Atrial Fibrillation After Cardiopulmonary Bypass Circulation, September 11, 2007; 116(11_suppl): I-1 - I-7. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. M. van den Goor, R. Nieuwland, P. M. Rutten, J. G. Tijssen, C. Hau, A. Sturk, L. Eijsman, and B. A. de Mol Retransfusion of pericardial blood does not trigger systemic coagulation during cardiopulmonary bypass Eur. J. Cardiothorac. Surg., June 1, 2007; 31(6): 1029 - 1036. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
