Effects on Coagulation and Fibrinolysis With Reduced Versus Full Systemic Heparinization and Heparin-Coated Cardiopulmonary Bypass
Background Extracorporeal circulation with circuits coated with surface-bound heparin has allowed reduced levels of systemic heparinization. Clinical benefits have included reduced postoperative bleeding and less homologous blood usage. However, the effects on the hemostatic and fibrinolytic systems have remained in part unknown.
Methods and Results Indications of thrombin generation, platelet activation, and fibrinolytic activity were investigated in patients undergoing coronary artery bypass surgery. Two groups were perfused with cardiopulmonary bypass (CPB) circuits completely coated with surface-bound heparin: one group with low systemic heparin dose (activated clotting time [ACT] >250 seconds; n=17) and a second group having a full heparin dose (ACT >480 seconds; n=18). A third control group was perfused with ordinary uncoated circuits and full heparin dose (n=17). The plasma level of thrombin-antithrombin complex and prothrombin fragment 1.2 increased in all groups during bypass, and somewhat more in both the heparin-coated groups toward the end of CPB, compared with the control group (P<.01). However, the increase during CPB was minimal compared with the major elevation observed 2 hours after surgery in all groups. Platelet release of β-thromboglobulin increased in all groups (P<.01) during CPB and significantly more in the high-dose group compared with the other two groups (P=.03). Fibrinolytic activities were similar in all groups, and there were no indications of major consumption of coagulation factors.
Conclusions Reduced systemic heparinization (ACT >250 seconds) in patients having extracorporeal circulation with completely heparin-coated circuits did not lead to increased thrombogenicity. Thrombin formation remained within low ranges during CPB compared with patients receiving a full heparin dose and with the major elevations observed after surgery.
Surface-bound heparin in circuits used for extracorporeal circulation increases thromboresistance, allowing reduced systemic heparinization during CPB.1 2 3 Reduced intravenous heparin in combination with heparinized CPB circuits has been documented to decrease postoperative bleeding and requirements for homologous blood transfusions and to reduce the need for protamine.1 2 3
Previous clinical studies with heparin-coated CPB equipment have been limited to situations in which only the oxygenator and tubings could be coated with heparin.1 3 4 5 When the intravenous heparin amounts were reduced, the cardiotomy reservoir for shed blood return had to be excluded or substituted with various cell-saver devices. These circumstances made investigations of reduced systemic anticoagulation during heparinized CPB surgery rather complex. Hence, the effects on the coagulation cascade and the fibrinolytic system have remained partly unknown.
Because all components for routine open-heart surgery have become available with heparin-coated surfaces, the present study was initiated to evaluate the influence on thrombin generation, platelet activation, and fibrinolytic activity in patients perfused with a complete heparin-coated extracorporeal circuit (Duraflo II, Baxter Healthcare Corp). The effects of reduced systemic heparinization, with a target ACT of >250 seconds, were compared with full heparin dose and ACT of >480 seconds and with an uncoated control group.
Low-operative-risk patients admitted for first-time elective coronary artery bypass surgery with two- or three-vessel disease were eligible for the study. Exclusion criteria were urgent operation, redo surgery, age >75 years, and impaired left ventricular function. Also, neurological disorders and liver and renal failures were reasons for exclusion. Anticoagulation or antiplatelet therapy was discontinued 7 days before surgery.
Low-Dose Group, Coated (n=17)
All surfaces in contact with blood were coated with a water-insoluble heparin complex. The circuit included silicone and polyvinyl chloride tubings connected to a hard-shell cardiotomy reservoir (DII BCR 3500); a soft-shell venous reservoir (DII BMR-1900); a woven, hollow polypropylene fiber membrane oxygenator (Univox Gold, Baxter-Bentley); and a 25-μm arterial filter (DII AF-1025). Systemic heparin (Nycomed Pharma) was given in a bolus dose of 100 IU/kg. The lower level of ACT (HemoTec Inc) was 250 seconds before institution of bypass. The dose of protamine sulfate (Novo Nordisk) for neutralization of heparin was 1.3 mg/100 IU heparin.
High-Dose Group, Coated (n=18)
The circuit was identical to that of the low-dose group in all aspects. Systemic heparin was given as a bolus dose of 400 IU/kg. ACT was 480 seconds before institution of bypass. The dose of protamine sulfate for reversal of the anticoagulation was 1.3 mg/100 IU heparin.
Control Group, Uncoated (n=17)
The extracorporeal circuit was uncoated but otherwise identical to the heparin-coated equipment. Heparin 400 IU/kg was administered, with a target ACT of 480 seconds before CPB was started. The protamine/heparin ratio was similar to that in the coated groups.
In all groups, additional heparin was given if the ACT level was below target. The extracorporeal bypass circuit was disconnected before administration of protamine. An ACT of >130 seconds in the postoperative period represented a level of consideration for supplemental doses of protamine.
The patients in the low-dose group and the uncoated control group were prospectively randomized. Because of indications of more thrombin formation in the low-dose group, the high-dose group entered the study immediately after inclusion of the two other groups was terminated. These patients were operated on consecutively, following identical selection criteria. Informed consent was obtained from all patients, and the study protocol was approved by the regional Ethical Committee (March 3, 1993). All operations were performed by one of two surgeons (E.Ø., G.T.).
Anesthesia and Operation
The anesthesia protocol included mainly a combination of diazepam (0 to 0.2 mg/kg), midazolam hydrochloride (0 to 0.2 mg/kg), fentanyl (6 to 8 μg/kg), and pancuronium bromide supplemented with isoflurane and nitrous oxide.
The extracorporeal circulation was established with a Stöckert roller pump with the pulsatile flow control PFC III (Stöckert Instrumente GmbH). Mild hypothermia (blood temperature, 32°C) was instituted immediately after bypass was started. The heart-lung machine was primed with 2000 mL of Ringer’s acetate. Hemodilution was further accentuated (and standardized) by autologous blood removal for blood conservation,6 which aimed at an intraoperative hematocrit of >22%. Ventilation of the lungs was terminated when the patient was on CPB and was reinstituted a few minutes before conclusion of bypass.
Myocardial protection consisted of antegrade administration of crystalloid cardioplegia (St Thomas’s Hospital No. 2) and topical cooling with ice slush. Cardiotomy suction was used deliberately during the entire period of heparinization. The mediastinal shed blood was autotransfused hourly in a closed system up to 18 hours after the operation.
ACT was tested before surgery, after heparin administration, before cardiopulmonary bypass, 10 minutes after start of bypass and thereafter every 20 minutes on bypass, after protamine administration, and 2 hours after surgery.
Test Blood Sampling
Blood samples were drawn with a syringe from the central venous cannula at the following intervals: after induction of anesthesia, immediately after institution of cardiopulmonary bypass, after release of the aortic cross-clamping, at the end of CPB, and 2 hours after surgery. The first 10 mL of the sample was discarded. All samples were immediately cooled on ice and centrifuged, and the plasma was stored at a temperature of −70°C until assayed.
Evidence of thrombin generation was evaluated by the plasma concentration of TAT complex and PF 1.2, both analyzed by ELISA (Enzygnost TAT micro and Enzygnost F 1.2 micro, Behringwerke AG).
Platelet counts were determined with an automatic cell counter (Cobas Minos ST, Roche). Activation of platelets was assessed by release of β-TG, quantified by ELISA with specific rabbit antibodies (Asserachrom β-TG, Diagnostica Stago). For this assay, samples were collected in Diatube (Diagnostica Stago) as described by the manufacturer.
PAP complex, providing information on fibrinolytic activity, was studied by an ELISA technique (Diagnostica & Analys, Senice AB).
d-Dimer concentrations, indicating fibrin degradation, were determined by an immunoassay technique with monoclonal antibodies specific to a neoantigen on the d-dimer structure (Nycocard D-dimer, Nycomed Pharma).
Fibrinogen concentration was determined according to Clauss.7
Total plasma heparin was quantified in the low-heparin-dose group to increase the validity of ACT. Analysis was done in citrated plasma and determined as a complex with the antithrombin present in plasma (Coatest Heparin, Chromogenix).
Comparison of continuous variables for all groups was done with the Kruskal-Wallis ANOVA. When indicated, a Mann-Whitney test was applied for comparing two groups. Discrete variables were treated by means of contingency tables, with Yates’ correction and Fisher’s test when one of the expected cell values was <5. Longitudinal changes between two time points only were analyzed by paired Student’s t test and Wilcoxon paired test. Correlation analyses were done between the levels of thrombin generation markers, ACT, and CPB times. The values are presented as medians with quartiles (or range, if indicated). A value of P<.05 was considered significant.
The clinical data are listed in Table 1⇓. There were no significant differences in demographic or operative parameters, except for somewhat longer extracorporeal time in the high-dose group compared with the low-dose group (due to one single patient with extended bypass time). The amounts of heparin given (Table 2⇓) and the levels of ACT during bypass were similar in the two groups with a full systemic dose of heparin (Fig 1⇓) and significantly lower (P<.001) in the low-dose group, which received ≈30% of the doses in the other two groups. The total plasma concentrations of heparin in the low-dose group were 1.3 (0.9 to 1.5) U/mL at start of CPB and 1.5 (1.3 to 1.8) U/mL at the end.
Postoperative parameters such as mediastinal drainage and hemoglobin concentration at discharge were similar in all three groups, with no significant intergroup differences (Table 3⇓). None of the patients were transfused with homologous blood products. One patient in the control group sustained a minor perioperative myocardial infarction, and there were no incidents of stroke. All patients survived.
The mean hematocrits during CPB and body weights were similar in all groups, and all figures are given uncorrected for hemodilution.
Indications of thrombin generation, as assessed by TAT complex (Fig 2⇓), increased constantly during CPB in all groups. The levels at the end of CPB were significantly more elevated in the two heparin-coated groups compared with the control group (P<.001). In the full-heparin-dose group, the levels of TAT complex increased from a baseline of median 5.6 (4.5 to 7.3) μg/L to 81.2 (57.3 to 123.6) μg/L at the end of CPB, and in the low-dose group from 3.5 (3.0 to 6.5) μg/L to 49.1 (27.8 to 65.4) μg/L. In the control group, the intraoperative levels increased from 7.1 (3.5 to 21.6) μg/L to 20.6 (12.4 to 29.6) μg/L. Correction for the somewhat more extended CPB times in the high-dose group did not change the statistical significance of the differences. The intraoperative elevation of TAT complex was modest compared with the concentrations seen 2 hours after surgery. The maximal levels reached a median of 249.6 (48.3 to 402.2) μg/L in the full-dose group, 246.0 (50.7 to 597.6) μg/L in the low-dose group, and 190.1 (52.6 to 541.9) μg/L in the control group. The intergroup differences were not statistically significant.
Prothrombin Fragment 1.2
As for the TAT complex, the levels of PF 1.2 were significantly more elevated in the two heparin-coated groups than in the control group at the end of CPB (P<.01) (Fig 3⇓). In the full-heparin-dose group, an elevation from a baseline of median 1.4 (1.2 to 1.7) nmol/L to 4.1 (2.6 to 5.3) nmol/L was recognized, compared with an increase from 1.2 (0.9 to 1.8) nmol/L to 4.2 (3.4 to 4.8) nmol/L in the low-dose group, and from 1.6 (1.3 to 2.5) nmol/L to 2.4 (1.7 to 3.1) nmol/L in the control group. Similar to the TAT complex, correction for the somewhat longer CPB times in the high-dose group did not change the intergroup significant differences. Also, for PF 1.2, a major elevation was observed 2 hours after surgery, reaching 11.0 (3.9 to 17.1) nmol/L in the full-dose group, 18.3 (6.4 to 25.2) nmol/L in the low-dose group, and 18.4 (7.1 to 23.0) nmol/L in the control group. There were no intergroup statistical differences.
In all three groups, there were no statistical correlations between the thrombin generation markers and the levels of ACT (r<.18, P>.1 for all). Similarly, in the low-heparin-dose group, the levels of TAT complex and PF 1.2 were not correlated to plasma heparin concentrations (r<.19, P>.1 for all).
β-TG concentration, an indicator of platelet activation and release, increased significantly during CPB in all groups and persisted at a level four to five times higher after surgery compared with baseline (P<.01) (Fig 4⇓). At the end of bypass, the plasma concentration was a median of 216.8 (145.6 to 281.6) IU/mL in the full-heparin-dose group and was more elevated (P=.03) than the 159.4 (115.3 to 195.9) IU/mL in the control group. The corresponding level was 154.9 (106.3 to 182.2) IU/mL in the low-dose group (not statistically significant compared with the two other groups).
PAP complex assessment detects situations of hyperfibrinolysis, and the plasma concentrations increased significantly (P<.01) in all groups after the operation (Fig 5⇓), to a level of two to three times that of the preoperative values. There were no significant intergroup differences at any time.
Plasma concentration of d-dimer increased significantly in all groups (P<.01). From a similar preoperative level of 0.5 mg/L, the concentrations reached 3.0 (1.5 to 6.0) mg/L in the low-dose group, 2.0 (0.7 to 4.0) mg/L in the high-dose group, and 3.0 (1.0 to 4.0) mg/L in the control group 2 hours after the operation. There were no significant intergroup differences.
Platelet counts revealed a small reduction during CPB but were at preoperative levels 2 hours after surgery (Fig 6⇓). No differences were observed between the groups.
Plasma fibrinogen concentrations decreased during CPB, as expected because of hemodilution, and increased to preoperative levels after surgery, with no intergroup differences (Fig 6⇑).
Several reports have demonstrated the clinical safety with heparin-coated extracorporeal circuits and reduced systemic heparin.1 2 3 5 Significant reduction of postoperative bleeding and lower requirements for homologous blood transfusion have been achieved. In addition, when intravenous heparin amounts were reduced, the subsequent reduced need for protamine may have diminished the well-known side effects of protamine, such as hemodynamic instability and hypersensitivity.8 9 In the present study, in contrast to a previous report on a larger patient series,2 no decreased postoperative bleeding was seen when the heparin dosages were lowered. However, the limited number of patients in each group precluded any significant conclusions with regard to clinical effects.
Regardless of the clinical benefits reported, no prior studies have fully investigated the effects on coagulation and fibrinolysis when the intravenous amounts of heparin and protamine are reduced in combination with heparinized extracorporeal circuits. In particular, the potential hazards of increased thrombogenicity have been of concern upon reduction of the level of systemic anticoagulation.10 Since thrombin is the key factor in the coagulation amplification system, contributing to generation of fibrin as well as to activation of platelets and the fibrinolytic system, markers of thrombin generation were studied. PF 1.2 is formed by proteolytic cleavage of prothrombin when it is transformed to thrombin and is thus a direct indicator of thrombin formation. Thrombin is inactivated through complex formation with antithrombin, forming TAT complexes, and assay of TAT complex formation is accordingly another indicator of generated thrombin. Platelet activation will take place in response to vessel wall injury and thrombin formation and can be estimated by the amount of β-TG released to plasma from the α-granules.
In the present study, the levels of PF 1.2 and TAT complex insidiously increased during CPB, indicating that thrombin is generated during bypass in all groups, even with standard heparinization and ACT >480 seconds. This is in agreement with other investigations of thrombin formation using ordinary uncoated circuits and demonstrates that heparin is only partially effective as an anticoagulant during CPB surgery.11 12 The present data did not support another study13 demonstrating less thrombin formation with heparinized circuits and full heparin dosage.
The similar concentrations of TAT complex and PF 1.2 observed in both heparin-coated groups with different heparin doses is an indication that the present reduction of intravenous heparin is within acceptable limits in regard to thrombin formation. Although the thrombin formation was even lower in the control group, this difference is presumably not of biological significance, particularly when the extensive elevation of the thrombin markers recognized 2 hours after completed surgery is considered. The high postoperative levels must be regarded as part of normalization of the hemostatic mechanisms that were suppressed by heparin during the surgical trauma. A similar pattern of increased postoperative thrombin generation has been presented by others, who used CBP with ordinary uncoated circuits12 or partly heparin-coated CPB and full systemic heparinization.14 As for activation of platelets, the levels of β-TG were even lower in the low-dose group compared with the other two groups. In fact, the presence of full heparin dose did not give any evidence of advantages in regard to the parameters for the coagulation system. Consequently, completely heparin-coated circuits for extracorporeal circulation with reduced ACT to 250 seconds can safely be performed with respect to thrombogenicity, at least with CPB times of about 1 hour. The lack of correlation between intraoperative ACT and levels of thrombin markers supports this suggestion. Furthermore, in eight patients, the ACT dropped below target during CPB, the lowest being 207 seconds, with no increased elevation of the thrombin markers. Another indication of adequate anticoagulation in the low-heparin-dose group was the absence of increased fibrinolytic activity. Since thrombin activates endothelial cells to produce tissue plasminogen activator,15 hyperfibrinolysis would be likely in the case of undesired high thrombin levels. In fact, no significant differences were recognized for the plasma concentrations of d-dimer and PAP complex. The increased levels observed 2 hours after the operation in all groups confirm the normal occurrence of fibrinolysis initiated by the surgical trauma and extracorporeal circulation. Perioperative and postoperative platelet counts and fibrinogen levels were equivalent to preoperative values, thus indicating no excessive consumption of coagulation factors in either group.
Clinically, when intravenous heparin was reduced, there was no evidence of harmful thrombogenicity. There was no incidence of perioperative myocardial infarctions or stroke. Visually, no clot formation was seen in the surgical field or in any part of the extracorporeal circuit. This compares well with other reports,1 2 3 5 even in cases with more extended reduction of systemic heparinization than in the present series.5
In conclusion, a completely heparin-coated circuit for CPB combined with low systemic heparinization can safely be performed in elective coronary artery bypass surgery. No clinical, technical, or laboratory evidence of undesired thrombogenicity was observed, and there were no signs of increased fibrinolysis or consumption of coagulation factors. The indications of thrombin formation during CPB in all groups represent a warning not to reduce systemic anticoagulation too far. On the other hand, no advantages could be demonstrated with full heparin dose when heparin-coated extracorporeal circuits were used.
Selected Abbreviations and Acronyms
|ACT||=||activated clotting time|
|PF 1.2||=||prothrombin fragment 1.2|
- Received March 20, 1995.
- Revision received June 7, 1995.
- Accepted June 7, 1995.
- Copyright © 1995 by American Heart Association
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