Effects of Fixed Low-Dose Warfarin, Aspirin-Warfarin Combination Therapy, and Dose-Adjusted Warfarin on Thrombogenesis in Chronic Atrial Fibrillation
Background and Purpose—Recent clinical trials have established that adjusted-dose warfarin (international normalized ratio [INR] 2.0 to 3.0) is highly effective in the reduction of ischemic stroke in patients with nonvalvular atrial fibrillation (AF). We hypothesized that the introduction of fixed low-dose warfarin alone or in combination with aspirin (300 mg) could normalize hemostatic markers, namely plasma fibrin D-dimer (an index of thrombogenesis), plasminogen activator inhibitor-1 (PAI-1, an index of fibrinolysis), fibrinogen, and von Willebrand factor (vWf, an index of endothelial dysfunction), in a manner comparable to adjusted-dose warfarin (target INR 2.0 to 3.0).
Methods—Sixty-one patients with AF (44 men, mean±SD age 64±19 years) who were not receiving any antithrombotic therapy were prospectively randomized into 1 of 3 treatment groups: warfarin (2 mg) (n=23; group 1), combination 1 mg warfarin plus 300 mg aspirin (n=21; group 2) or combination 2 mg warfarin plus 300 mg aspirin (n=17; group 3). Subjects from all 3 AF groups were matched for sex, age, and blood pressure. Blood samples were taken for sequential measurements for changes in plasma fibrin D-dimer, PAI-1, fibrinogen, and vWf before and at 2 and 8 weeks after randomization (phase 1). All patients were subsequently offered adjusted-dose warfarin therapy (phase 2), and an additional blood sample was taken 6 weeks later.
Results—When pretreatment results were compared with those from 60 age- and sex-matched healthy control subjects in sinus rhythm, there were significant elevations in levels of fibrinogen (P=0.025), vWf (P<0.0001), and fibrin D-dimer (P<0.0001) in patients with AF compared with control subjects. There were no significant changes in the levels of various indices measured after 2 and 8 weeks of therapy in all 3 groups, except for an increase in PAI-1 level (P=0.024) in group 3. After 6 weeks of therapy with dose-adjusted warfarin (INR 2.0 to 3.0), there was a significant decrease in plasma fibrinogen (P=0.023) and fibrin D-dimer (P=0.0067) levels. There were no significant changes in the levels of PAI-1 (P=0.198) or vWf (P=0.33).
Conclusions—The present results confirmed that high levels of vWf, fibrinogen, and fibrin D-dimer levels were present in patients with AF compared with control subjects. Moreover, the introduction of 300 mg aspirin plus low-dose warfarin (1 mg/d), low-dose warfarin alone (2 mg/d), or 300 mg aspirin plus low-dose warfarin (2 mg/d) did not significantly reduce any of the hemostatic markers studied (except PAI-1 levels), whereas conventional full-dose warfarin (INR 2.0 to 3.0) significantly reduced levels of fibrin D-dimer and fibrinogen. These results are in keeping with the disappointing ineffectiveness of low-intensity warfarin therapy, aspirin-warfarin combination, and ultralow-dose warfarin therapy in the recent prematurely terminated clinical trials and the established benefits of conventional adjusted-dose anticoagulation therapy.
Recent clinical trials have firmly established that adjusted-dose warfarin (target international normalized ratio [INR] 2.0 to 3.0), is highly effective in reducing ischemic stroke in patients with nonvalvular atrial fibrillation (AF).1 Because of bleeding and other contraindications, the interruption of anticoagulation treatment is frequent, even in clinical trials. In addition, patients require regular monitoring of anticoagulant intensity, which contributes to the expense and inconvenience of this treatment strategy. Therefore, there has been a necessity to develop new strategies to improve thromboprophylaxis in patients with AF.
Recent studies have focused on the use of low-intensity anticoagulation or aspirin-anticoagulation regimens. For example, ultralow-dose warfarin (1 mg/d) has been successfully used to prevent thrombosis of central venous catheters2 and to prevent venous thrombosis in malignancy.3 In the MRC thrombosis prevention trial, 5499 men aged 45 to 69 years at high risk of ischemic heart disease were treated with low-intensity anticoagulation (mean INR 1.47), 75 mg/d aspirin, or placebo.4 The trial investigators found that aspirin reduced nonfatal ischemic heart disease (by 32%) and low-intensity warfarin mainly reduced fatal events (by 39%), whereas the combination of warfarin-aspirin therapy was more effective than either agent alone.4 The possibility that low-intensity antithrombotic strategies would be suitable as thromboprophylaxis for AF seemed attractive and plausible.
Recent evidence has established that AF confers a hypercoagulable state that is independent of underlying structural heart disease or etiology.5 The usefulness of the measurement of various markers of hypercoagulability to indicate the effectiveness of antithrombotic therapy has been demonstrated in a randomized trial in which fixed ultralow-dose warfarin (1 mg) or aspirin (300 mg) did not significantly reduce elevated β-thromboglobulin (a marker of platelet activation) and fibrin D-dimer (an index of thrombogenesis) levels in patients with nonvalvular AF, although conventional adjusted-dose warfarin therapy (INR 2.0 to 3.0) did normalize levels of these markers.6 This trial was consistent with the beneficial effect of adjusted-dose warfarin in the prevention of stroke and thromboembolism in patients with AF and suggested that fixed ultralow-dose warfarin or aspirin may not exert similar beneficial effects in the reduction of thrombogenesis. This was in keeping with the subsequent publication of the results from the second Copenhagen Atrial Fibrillation Aspirin and Anticoagulant Therapy Study (AFASAK 2) clinical trial.7
We had hypothesized that the introduction of fixed low-dose warfarin regimens (2 mg) alone or in combination with aspirin (300 mg) could normalize hemostatic markers, namely plasma fibrin D-dimer (an index of thrombogenesis), plasminogen activator inhibitor-1 (PAI-1, an index of fibrinolysis), fibrinogen, and von Willebrand factor (vWf, an index of endothelial dysfunction), in a manner comparable to adjusted-dose warfarin (target INR 2.0 to 3.0). The present study was initiated before the premature termination of the third Stroke Prevention in Atrial Fibrillation Trial (SPAF III), the Minidose Warfarin (MIWAF) trial, the Primary Prevention of Arterial Thrombo-embolism in Non-rheumatic Atrial Fibrillation (PATAF) trial, and the Copenhagen AFASAK 2 trial of the effect of fixed aspirin-warfarin combination, conventional-dose warfarin, and aspirin regimens in patients with AF.7 8 9 10
Subjects and Methods
We studied outpatients with chronic nonvalvular AF who were not receiving antithrombotic therapy and had been referred to a specialist outpatient clinic for the consideration of anticoagulation. Chronic AF was confirmed on ECG on ≥2 separate occasions (≥6 weeks apart). All subjects underwent a 2-dimensional, targeted M-mode, Doppler ultrasound examination with a Hewlett-Packard Sonos 100 echocardiograph to exclude any significant valvular disease or moderate-to-severe left ventricular systolic dysfunction.
After informed consent was obtained, patients were prospectively randomized into 1 of 3 groups: low-dose warfarin (2 mg) (n=23; group 1), combination ultralow-dose warfarin (1 mg) plus aspirin (300 mg) (n=21; group 2), or combination low-dose warfarin (2 mg) plus aspirin (300 mg) (n=17; group 3). Patients were seen at baseline (visit 1) and then at 2 and 8 weeks after randomization (phase 1); subsequent to this, all patients were offered conventional adjusted-dose warfarin therapy (achieving INR 2.0 to 3.0) (phase 2), but due to patient withdrawals, complete samples for hemostatic markers were taken at the 6-week follow-up visit in only 20 fully anticoagulated patients. Nevertheless, patients were seen at intervals during that period to ensure therapeutic anticoagulation was rapidly achieved and maintained at INR 2.0 to 3.0.
Baseline results in patients with AF were compared with those from age- and sex-matched normotensive healthy control subjects drawn from healthy hospital staff and from those who were attendees at hospital for hernia repair, varicose veins, or minor surgery. None had diabetes, and all were without any signs or symptoms of cardiovascular or connective tissue disease. The study protocol was approved by the West Midlands District Ethics Committee.
Blood Samples and Assay Procedures
Blood samples were taken from the antecubital fossa vein and placed into sodium citrate; they were centrifuged within 2 hours of collection at 3000g and 4°C for 20 minutes to obtain plasma, which was then separated and stored at −70°C before assay.
The plasma markers were measured in batches: vWF with an established ELISA (DAKO), fibrin D-dimer with an ELISA from Agen, fibrinogen with a modified Clauss technique on a Pacific hemostasis coagulometer and reagents from Alpha Laboratories, and PAI-1 with an ELISA from Immuno GmbH. Intra-assay and interassay coefficients of variation for all ELISA assays were <5% and <10%, respectively.
Data and Statistical Analyses
Results are expressed as mean±SD, except for PAI-1 and fibrin D-dimer, for which results are expressed as median and interquartile range (IQR). Comparisons between cases and controls were performed with the t test or Mann-Whitney U test. With 3 sets of data (pretreatment and 2 and 8 weeks after treatment in phase 1), results were analyzed with Friedman’s repeated-measures ANOVA (RMANOVA). Paired data, which were comparisons of data for baseline with those for 6-week treatment with dose-adjusted conventional adjusted-dose warfarin therapy (phase 2), were measured with paired t test or paired Wilcoxon test, as appropriate. Correlations were performed with Spearman’s rank correlation, and categorical data were compared with use of the χ2 test. All statistical calculations were performed on a microcomputer with a commercially available statistical package (Minitab release 11; Minitab Inc). A value of P<0.05 was considered statistically significant.
We studied 61 patients with chronic AF (44 men and 17 women, mean age 64±19 years). Baseline indices in these patients were compared with those for 60 age- and sex-matched healthy subjects in sinus rhythm (mean age 66±6 years). There were no significant differences in mean age, sex ratio, and blood pressures between patients and control subjects (Table 1⇓). Similarly, there were no significant differences among the 3 treatment groups (groups 1, 2, and 3) in mean age, sex ratio, and blood pressures (data not shown).
Baseline for Patients Versus Control Subjects
There were significant elevations in levels of plasma fibrinogen (P=0.025), vWf (P<0.0001), and fibrin D-dimer (P<0.0001) in patients with AF compared with control subjects. There was a nonsignificant trend toward higher plasma PAI-1 levels in the patients with AF (P=0.0706) (Table 2⇓).
There were 16 patients with lone AF, which was defined for the purposes of the present study as AF in the absence of any known predisposing factors. PAI-1 levels were significantly lower in those with lone AF compared with those without lone AF (n=45) (P=0.02). There were no statistically significant differences in age, blood pressures, or levels of plasma fibrinogen (P=0.24), vWf (P=0.34), and fibrin D-dimer (P=0.777) in the patients with one AF compared with those without one AF (Table 3⇓).
There were significant correlations between fibrin D-dimer and diastolic blood pressure (r=−0.274), PAI-1 (r=−0.257), and fibrinogen (r=0.322) levels (Spearman, all P<0.05). There were no significant correlations between INR and measured parameters at baseline or between other measured parameters (data not shown).
In group 1 (2 mg warfarin), there were no significant changes in the levels of various measured indices and INR after 2 and 8 weeks of therapy with 2 mg/d warfarin (Table 4⇓). There was a nonsignificant trend toward a reduction in fibrin D-dimer levels (P=0.055).
In group 2 (1 mg warfarin and 300 mg aspirin), there were no significant changes in the levels of any of the various measured indices and INR after 8 weeks of therapy with 1 mg warfarin and 300 mg/d aspirin (Table 4⇑).
In group 3 (2 mg warfarin and 300 mg aspirin), there was no significant changes in the levels of fibrinogen, vWf, or fibrin D-dimer and INR values after 8 weeks of therapy with 2 mg warfarin and 300 mg/d aspirin (Table 4⇑). However, there was a significant increase in PAI-1 levels (from 6.0 IU/dL at week 0 to 7.8 IU/dL at week 8, P=0.024).
In phase 2 (subsequent treatment with conventional dose-adjusted warfarin, INR 2.0 to 3.0), there were significant decreases in the levels of plasma fibrinogen (P=0.023) and fibrin D-dimer (P=0.0067) and in the median INR value (P<0.00001) after 6 weeks of therapy with dose-adjusted warfarin (mean±SD INR 2.4±0.7). There were no significant changes in PAI-1 and vWf levels (Table 5⇓). There were no significant correlations between the INR and fibrin D-dimer or PAI-1 levels in any of the treatment groups.
The present results have confirmed the presence of high levels of vWf, fibrin D-dimer, and fibrinogen and a trend toward an increased PAI- 1 level in patients with AF compared with healthy control subjects in sinus rhythm, consistent with a hypercoagulable state associated with this arrhythmia.5 6 11 The presence of high fibrin D-dimer levels suggests ongoing fibrin turnover and thrombogenesis in patients with chronic AF at baseline compared with control subjects in sinus rhythm.6 11 12 High vWf levels are suggestive of endothelial dysfunction, whereas increased plasma fibrinogen levels indicate abnormal hemorheology and clotting, which are significantly correlated to thrombogenesis (as indicated by fibrin D-dimer levels).11 The high PAI-1 levels suggest a tendency toward a hypofibrinolytic state in chronic AF, which also appears to be correlated with fibrin D-dimer levels.13 The presence of all of these abnormalities in hemostatic markers fulfills 2 components of the Virchow’s triad for thrombogenesis: abnormal blood constituents and vessel wall. The presence of abnormal hemodynamic flow in chronic AF, with atrial stasis, fulfills the third component of Virchow’s triad and may contribute to the increased risk of stroke and thromboembolism in patients with AF.5
Anticoagulation with warfarin is effective thromboprophylaxis in patients with AF.1 However, there are problems with conventional full-dose warfarin: inconvenience and safety. This may account for the reluctance in clinical practice to prescribe anticoagulants for all patients with AF. The possibility that low-intensity antithrombotic strategies would be suitable as thromboprophylaxis for AF seemed attractive and plausible.
The introduction of conventional adjusted-dose warfarin (achieving INR 2.0 to 3.0) appears to normalize abnormal levels of thrombogenic markers,6 11 suggesting that conventional full-dose warfarin treatment was effective in preventing excessive fibrin turnover and consistent with the beneficial antithrombotic effects of conventional adjusted-dose warfarin in clinical trials. For example, Lip et al11 found that conventional adjusted-dose warfarin significantly reduces circulating fibrin D-dimer levels by two thirds (P<0.001), consistent with the 68% reduction in the risk of stroke and thromboembolism in clinical trials with adjusted-dose warfarin, as reported by the Atrial Fibrillation Investigators.1 Similar observations of the marked reduction in hemostasis with conventional anticoagulation have been reported in other studies.6 14 By contrast, aspirin therapy results in a nonsignificant 25% reduction in markers of thrombogenesis,6 11 broadly consistent with the 21% risk reduction in stroke from the pooled analysis of the Atrial Fibrillation Investigators.15 Similarly, Yamamoto et al16 reported that 330 mg/d aspirin suppressed platelet function in patients with AF but did not significantly affect the increased coagulation activity in these patients.
Unfortunately, the results of clinical trials that test the strategy of the use of low-intensity anticoagulation and aspirin-anticoagulation combinations as adequate thromboprophylaxis in AF have been disappointing.7 8 9 10 This is not surprising because various studies (including the present study) of similar regimens of low-dose warfarin, aspirin, or combination aspirin/low-dose warfarin have not shown any significant effect on the elevated indices of thrombogenesis in patients with AF. For example, fixed ultralow-dose warfarin (1 mg) did not significantly reduce thrombogenesis or platelet activation in AF,6 which is in keeping with subsequent results from the AFASAK 2 study.7 The latter group also reported that after 3 months of therapy with fixed-dose warfarin (1.25 mg), the level of INR increased significantly from baseline in patients receiving warfarin in any dose but only dose-adjusted warfarin (INR 2.0 to 3.0) had a marked effect on F1+2 levels, another index of thrombogenesis.17 However, therapy with fixed minidose warfarin, combined minidose warfarin-aspirin, and aspirin alone did not significantly alter F1+2 levels. In the SPAF III study,8 the use of low-intensity warfarin plus 300 mg aspirin did not demonstrate any reduction in stroke risk compared with conventional anticoagulation. These findings lent weight to the premise that conventional adjusted-dose anticoagulant therapy is more appropriate than aspirin for the prevention of stroke and systemic embolism in patients with AF. Thus, the ineffectiveness of regimens with either aspirin, fixed-dose or low-intensity warfarin, or combination low-dose warfarin/aspirin regimens in the reduction in thromboembolic events in clinical trials of AF7 8 9 10 parallels the findings of various studies of hemostatic markers with similar regimens. Indeed, this suggests that the measurement of hemostatic markers might be used as surrogate end points to test the clinical effectiveness of an antithrombotic regimen.
The present study was started before the published results of ASAFAK 2, SPAF III, and other similar trials, and our aim was to study the effect of higher low-dose warfarin (ie, the use of 2 mg warfarin with or without 300 mg aspirin and 1 mg warfarin with 300 mg aspirin) in patients with chronic AF. This would follow on from our previous work6 in which we investigated the effect of ultralow-dose warfarin (1 mg) on hemostatic markers in AF; ideally, the present study would have been completed before the scheduled termination of AFASAK 2, SPAF III, and other similar trials.
The present study is limited by the relatively short follow-up period of phase 1. With the trend (nonsignificant) toward a decrease in fibrin D-dimer levels in patients administered low-dose warfarin (2 mg), it is conceivable that given a longer period of time, statistically significant decreases in this marker may be demonstrated. We did not relate these hemostatic abnormalities to underlying clinical risk factors or detailed assessments of left atrial or ventricular size or function, because previous studies have indicated that there was no significant relationship between these markers of thrombogenesis and underlying medical history or other cardiac abnormalities in patients with AF.5 11 12 Furthermore, the main aim of this study was to demonstrate the effects of the introduction of treatment with 300 mg aspirin or low-dose (2 mg) warfarin on these markers in patients with AF rather than to correlate the abnormalities with the underlying medical history or with detailed measurements of cardiac size or ventricular function.
The results of the present study show that patients with AF have not only an increased fibrin turnover but also a reduced fibrinolytic state and abnormal endothelial function. The introduction of fixed low-dose warfarin (2 mg) or aspirin-warfarin combination therapy did not significantly reduce markers of thrombogenesis, whereas conventional adjusted-dose warfarin (INR 2.0 to 3.0) significantly reduced the increased levels of fibrin D-dimer in patients with AF. These results are in keeping with the disappointing ineffectiveness of low-intensity warfarin therapy, aspirin-warfarin combination, and ultralow-dose warfarin therapy in the recent prematurely terminated clinical trials and the established benefits of conventional adjusted-dose warfarin therapy (INR 2.0 to 3.0).
We acknowledge the support of the City Hospital NHS Trust Research & Development Program for the Haemostasis Thrombosis and Vascular Biology Unit.
- Received August 16, 1999.
- Revision received January 13, 1999.
- Accepted January 13, 2000.
- Copyright © 2000 by American Heart Association
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