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European Heart Journal Advance Access originally published online on July 17, 2006
European Heart Journal 2006 27(16):1954-1964; doi:10.1093/eurheartj/ehl146
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© The European Society of Cardiology 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Stroke prophylaxis in atrial fibrillation: who gets it and who does not?

Report from the Stockholm Cohort-study on Atrial Fibrillation (SCAF-study)

Leif Friberg1,*, Niklas Hammar2,3, Mattias Ringh1, Hans Pettersson4 and Mårten Rosenqvist5

1 Karolinska Institute at South Hospital, Stockholm, Sweden
2 Department of Epidemiology, Institute of Environmental Medicine, Karolinska Institute, Stockholm, Sweden
3 AstraZeneca, Mölndal, Sweden
4 Department of Biostatistics, South Hospital, Karolinska Institute, Stockholm, Sweden
5 Department of Cardiology, South Hospital, Karolinska Institute, Stockholm, Sweden

Received 18 January 2006; revised 10 May 2006; accepted 23 June 2006; online publish-ahead-of-print 17 July 2006.

* Corresponding author: Cardiology Clinic, Nacka Hospital Lasarettsvägen 4, S-13183 Nacka, Sweden. Tel: +46 8 601 53 10; fax: +46 8 601 53 16. E-mail address: leif.friberg{at}stockholm.bonet.se


   Abstract
Top
 Abstract
Introduction
 Aim
 Method
 Results
 Discussion
 Summary
 References
 
Aims Underuse of warfarin for stroke prophylaxis in atrial fibrillation (AF) is extensive and represents a major problem in clinical practice. To identify factors associated with warfarin treatment in eligible AF patients.

Methods and results The study population consisted of all Swedish resident AF patients at the Stockholm South General Hospital during 2002 (n=2796). Medical records were examined and complemented by data from the Swedish National Hospital Discharge Register. Sixty-eight percent of the patients (1898/2796) had indications, and no apparent contraindications for warfarin treatment. Of these 54% (1029/1898) got warfarin. Factors favouring warfarin treatment after adjustment for other factors were history of ischaemic stroke, an implanted pacemaker, treatment in a cardiology rather than internal medicine ward and valvular defect. Factors associated with a reduced likelihood of warfarin treatment were paroxysmal type of AF and age >80 years. Important risk factors for stroke in AF like heart failure, hypertension, and diabetes did not increase the chances of warfarin treatment.

Conclusion Risk stratification using known risk factors of stroke seems to affect warfarin treatment only to a minor degree in clinical practice. Undertreatment was particularly common in patients with paroxysmal AF and in patients aged >80 years and calls for improved clinical routines in accordance with international guidelines.

Key Words: Atrial fibrillation • Thromboembolism • Anticoagulation • Warfarin • Aspirin • Guidelines • Adherence


   Introduction
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Aim
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The annual risk of ischaemic stroke in atrial fibrillation (AF) is estimated to be ~5–7%1–4 but varies greatly depending on age, sex, and concomitant disease. In certain groups, the risk approaches 20%.5,6

Warfarin and other oral anticoagulants can reduce the risk of embolic stroke but it increases the risk of bleedings.7–12 Aspirin (acetylacylic acid) offers less efficient protection, but is easier to manage and is associated with fewer bleeding complications.9,10,13–16

The European and American Societies of Cardiology have issued guidelines17 for the management of stroke prophylaxis in AF patients based on risk assessment and evidence from large trials. Previous studies have shown that these recommendations are poorly adhered to in clinical practice.18–24 In a recent study from the United States, it was concluded that risk stratification had little effect on warfarin use, but age >80 years and AF classification did have an influence.25


   Aim
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Method
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The aim of this study was to assess the extent of warfarin treatment in relation to ACC/AHA/ESC Guidelines, and to identify factors associated with the chance of warfarin treatment in eligible patients.


   Method
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All patients with a diagnosis of AF or atrial flutter treated at the Departments of Cardiology or Internal Medicine at the South Hospital in Stockholm, Sweden, during 2002 were identified by ICD-10 diagnosis codes I48.9 and I49.5 from the local patient register. Stroke patients were taken care of by the neurology section within the internal medicine department in the hospital. Thus, all patients presenting with stroke at the hospital were included. Both inpatients and outpatients were included. Approval for the study was obtained from the Regional Ethical Committee.

The initial number of patients identified was 3405. All medical records were examined by two cardiologists according to a predefined protocol. Only patients with a documented episode of AF or flutter during 2002 were included. The diagnosis could not be verified in 605 patients who were, therefore, excluded.

For the remaining 2800 patients, we obtained information on previous hospitalizations and diagnostic codes from the National Hospital Discharge Register (HDR) going back to 1987 nationally and for Stockholm County to 1972. In this register we found information for all but four patients who where temporary visitors to Sweden. These four patients were excluded and the results presented below concern 2796 patients.

Definitions
To avoid ambiguity in the classification of persistent AF and paroxysmal AF, we considered all patients who underwent cardioversion as having persistent AF. Information about possible contraindications to warfarin treatment was obtained from medical records and from the HDR. As contraindications we considered presumed poor compliance (dementia, psychiatric illness, alcohol, or drug abuse), frequent falls, previous severe bleeds, malignant hypertension, severe haemostatic defects, severe liver disease, cancer with an increased risk of bleeds, recent trauma, or surgical procedures. Age >90 years was considered a contraindication unless the patient had daily professional help with medication.

The prophylactic regime prescribed at the time of the patient's last contact with the hospital during the year was documented.

Thrombo-embolic stroke, bleedings, or myocardial infarction before the index date of each patient was identified by the HDR. For conditions which are often cared for in an outpatient setting, e.g. hypertension, diabetes, and heart failure, we combined information from the HDR and from medical records to obtain a history.

Echocardiography had been performed in more than two-third of the patients, which complemented the information from the HDR about valvular disease. Insufficiencies grades 2–4/4 were considered as valvular defects. Stenosing defects of at least moderate severity were also considered as valvular defects.

Statistical methods
We used logistic regression to identify variables associated with warfarin treatment among eligible patients. Our model strategy was as follows. First, crude associations for each variable with the odds of receiving correct treatment were studied in univariable models. Secondly, we used multivariable models to study the adjusted associations. We added variables to the model and kept those that showed a significant univariate association. All variables were tested with Wald {chi}2 statistic and considered significant if P<0.05. Finally, in order to compare the crude and adjusted associations, each variable that was not included in the model were added one at a time.

The associations are presented as odds ratios (OR) with 95% confidence intervals (CI). Hosmer–Lemeshow goodness-of-fit test was used to examine if the final model adequately fitted the data. All analyses were performed in SPSS 13.0.

We also used classification trees to identify variables associated with treatment of warfarin and to describe the patients that did and did not receive it. We used the CHAID algorithm to build the tree.26 A CHAID analysis starts with all data in one group. Each possible split on each explanatory variable is considered, to find the split that leads to the strongest association with the dependent variable. Resulting groups were split until the following criteria were reached; tree depth was limited to three levels, no group with less than 50 patients were formed, and no split with Bonferroni adjustment less than 0.05 was executed.


   Results
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Patient characteristics
The patients had a mean age of 74 years and 54% were men (Table 1). Intermittent AF, either paroxysmal or persistent, was predominant in younger patients, whereas permanent AF was more common in the elderly. In one-third of the patients AF was paroxysmal. Nearly half the patients had hypertension, ischaemic heart disease, or heart failure. Valvular defects were found in 791 patients and consisted mostly of mitral (41%) or tricuspid insufficiencies (33%), mostly of grade 2 (79 and 74%, respectively). Mitral stenosis was found in 20 patients. Contraindication against warfarin treatment was present in 21% (590/2796) of the patients. In most cases contraindications were relative; presumed poor compliance and elderly patients who previously had suffered frequent falls being the most frequent by far. Of the 2796 patients 41% used warfarin, 35% used aspirin in various dosages and 18% had no prophylaxis at all. Further information on given thromboembolic prophylaxis is presented in Table 2 and Figures 1 and 2.


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  		Table 1 Characteristics of the study population

 

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  		Table 2 Fulfilment of guideline recommendations on warfarin treatment

 

Figure 1
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  		Figure 1 Thrombo-embolic prophylaxis actually given to AF patients who ought to have warfarin (n=1898) or aspirin <300 mg od (n=791) according to the ACC/AHA/ESC guidelines. Other/comb refers to dipyridamol and clopidogrel, either given alone or in combination with aspirin. If a combination-included warfarin, it is presented in the warfarin sector.

 

Figure 2
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  		Figure 2 Less than half of the patients with AF (n=2796) got thrombo-embolic prophylaxis according to the recommendations issued jointly by AHA, ACC, and ESC. If aspirin >300 mg is considered as an acceptable alternative to warfarin, although warfarin was recommended, the fraction receiving correct treatment is just a little more than half the patients.

 
Anticoagulation and adherence to guidelines
Approximately, two-thirds of all patients had indications and no contraindications for warfarin treatment. Of these about half received it. Aspirin >300 mg was the recommended treatment for 791 patients with a low risk of thrombo-embolic stroke, but only 186 of them got it (24%). Most patients receiving aspirin got it in a low dose without proven benefit against AF-related stroke.9,10 Contrary to recommendations one of the four patients with lone AF got warfarin. Overall, only 46% received treatment in accordance with the recommendations, whether this meant warfarin, aspirin, or no treatment at all.

Predictors for receiving anticoagulation treatment
The chances of receiving warfarin treatment was in the multivariate analysis seen to be increased in patients with permanent AF, in patients aged 60–80 years, in patients with previous thrombo-embolic stroke or TIA, valvular defects, pacemakers, and in patients who were discharged from a cardiology ward rather than an internal medicine ward (Table 3).


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  		Table 3 Factors associated with warfarin treatment in 1898 AF-patients with indication for, and no contraindication against, warfarin prophylaxis

 
We remade the analysis of which patients gets warfarin with all 2796 patients in the cohort, including those who should not have warfarin either because of contraindications or lack of indication for warfarin, e.g. patients with lone AF (Table 4). The main observations did not change much by this. Factors associated with less frequent use of warfarin in the whole group of patients were, not surprisingly, some of the factors indicating contraindication to warfarin treatment, e.g. history of previous cerebral bleeding and alcohol abuse.


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  		Table 4 Factors associated with chances of receiving warfarin-prophylaxis in 2796 patients with AF (including patients with contraindications against, and patients lacking indication for, warfarin prophylaxis)

 
With both methods it was clear that age did not affect the chances of warfarin treatment in a linear way; both the oldest and the youngest had lower chances to get the recommended treatment.

Heart failure, hypertension, and diabetes did apparently not influence warfarin treatment either way.

Using the CHADS-2 scoring system,6,27 we identified 679 patients with high risk of embolic stroke who were without contraindications to warfarin. These patients had a score of at least 3p based on the following risk factors: previous thrombo-embolic stroke (2p); heart failure (1p); hypertension (1p); diabetes (1p); and age >75 years (1p). In this high-risk group, 56% received warfarin. After adjustment for age and type of AF, these high-risk patients appeared to have a non-significant tendency for better chances of receiving warfarin (OR 1.3, 95%CI 1.0–1.6, not in table).

The classification tree analysis indicated that the most important factor influencing the probability of warfarin treatment was whether AF was permanent/persistent or paroxysmal (Figure 3). In patients with permanent/persistent AF, old age was the most important predictor of warfarin treatment and in the oldest age group (>80 years) the type of clinic. In patients with paroxysmal AF, on the other hand, a history of stroke was of primary importance and in those with no previous stroke the presence of a pacemaker.


Figure 3
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  		Figure 3 Classification tree showing the factors that at each step had the strongest association with the chances of getting the recommended treatment. Previous stroke/TIA refers to thrombo-embolic events, not haemorrhages. P-values were <0.00001 at all nodes except for pacemakers where it was 0.001.

 

   Discussion
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 Abstract
 Introduction
 Aim
 Method
 Results
 Discussion
Summary
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Many previous studies have shown that underutilization of anticoagulation is frequent in AF.18–24 Our study confirms earlier observations and adds information about circumstances that affect clinical decision-making on anticoagulation. This study indicates that important risk factors for embolic stroke do not substantially increase the chance of warfarin treatment. Paroxysmal AF and old age are associated with a reduced chance of warfarin treatment contrary to guideline recommendations. These findings from a Swedish hospital are consistent with recent observations in the United States.25

Study population
The age- and gender distribution among the patients in our study is similar to what has been reported in other studies.28–30 The prevalence of coexisting disease was relatively high in our study when compared with other studies.23,31 This might be an effect of the fact that our study benefited from access to the Swedish nationwide register about hospital discharges which supplied us with extensive diagnostic information that is not readily available in most other countries.

Adherence to guidelines
The majority of the AF patients did not receive thrombo-embolic prophylaxis as recommended by the guidelines. Nearly half the patients who ought to have warfarin were left without it. In comparison with earlier studies showing even lower adherence, our results indicate that there might have been an improvement in clinical practice in recent years.20–24,37 Frykman et al.19 in another study performed in Sweden in the late 90s found that only 40% of eligible patients aged <80 years with permanent AF and at least one risk factor had warfarin when compared with 78% of patients with the same characteristics in our study (Table 3).

An exaggerated concern of bleeding risk, in combination with underestimation of stroke risk, may partly explain the apparent reluctance to provide AF patients with warfarin treatment.21,32 Another reason may be that the guidelines still were relatively unknown as they were presented only in the preceding year. In essence, most of the facts underlying the recommendations were common knowledge among cardiologists at the time. Lack of knowledge about the recommendations might partly explain why patients treated in internal medicine wards had less chance to get the recommended treatment when compared with patients in cardiology wards. It could, perhaps, also explain why pacemaker patients, who were treated by cardiologists, received the recommended treatment more often than others. Patients in the internal medicine wards were older than in the cardiology wards and had more often contraindications to warfarin treatment (39 vs. 15%). However, in the analysis of differences in treatment in internal medicine and cardiology wards, patients with contraindications were not included. Still, there is a possibility of some underestimation of the actual number of patients with contraindications against warfarin.

Patients are often reluctant to take warfarin because of practical difficulties or fear of bleedings. Documentation of such refusals were seldom documented in the records and was found only in 2.6% of the records. Doctors too, might at times be reluctant to commence warfarin treatment, because of the time it takes to make the patient understand why and how this drug should be taken. Some professionals also have doubts about the applicability of the advice in guidelines in their day-to-day clinical practice.33

Who is likely to get the right treatment?
Among those with the best chances of getting warfarin treatment were patients with pacemakers and patients treated in cardiology wards (rather than internal medicine wards). These circumstances are, to our knowledge, not associated with increased stroke risk.

Major risk factors for embolic stroke in AF patients are prior ischaemic stroke or transient ischaemic attack (TIA), heart failure, hypertension, age >75 years (especially in women), and diabetes mellitus.18,29,34

We found no association between four of these five major risk factors, and the chances of getting warfarin. Patients who had experienced a prior stroke or TIA did have a clearly increased chance of getting warfarin. Patients with heart failure, diabetes, and hypertension did not appear to have better chances of getting warfarin than others.

As for age we found that elderly patients, especially those >80 years of age, had clearly reduced chances of getting the recommended warfarin treatment even in the absence of any identified contraindications. In fact, we saw a marked drop in the frequency of warfarin use in patients above the age of 80 years. Possibly this was influenced by the lingering of a nowadays abandoned clinical rule-of-thumb saying that lifelong warfarin-treatment end by the age of 80.

Patients with paroxysmal AF had a reduced chance of getting warfarin, compared with patients with permanent AF, although the same treatment is recommended by the guidelines. The realization that the stroke-risk in paroxysmal AF is comparable with the risk in permanent AF is quite recent.35

Overtreatment with warfarin
Our study showed that one-quarter of patients with lone AF were on warfarin although their stroke risk is not thought to exceed the risk of bleeds associated with warfarin therapy.36 For this reason, the guidelines consider warfarin contraindicated in lone AF.

Methodological considerations
An important strength of the present study was the comparatively large study population consisting of all patients diagnosed with AF during 1 year at a single comparatively large hospital serving ~600 000 inhabitants or one-fifteenth of the Swedish population. This assured representativity of the study population and provided possibilities to assess the association between different patient characteristics and warfarin treatment with a reasonable statistical precision. Another strength of the study was that the information about each patient was extracted from several sources according to a predefined protocol in order to achieve high reliability of data.

A limitation to our study is that it is retrospective and based on standard clinical information. Some doctors wrote elaborate records, others very brief. We tried to minimize the impact of inhomogeneous information in the medical records by applying a strict protocol in extracting information from the medical records, and by extending the reading to include records from contacts made by different doctors during preceding years.

We believe that omission of information played a more important role than incorrect information. If a medical record stated that a patient had diabetes mellitus, he or she probably was a diabetic. If nothing about diabetes was found in the record, the patient still might be a diabetic. It is likely that we have somewhat underestimated the presence of some risk factors affecting the indications for warfarin.

We know from clinical experience that omissions of contraindications against warfarin treatment sometimes are made deliberately. Dementia might be suspected but may not be entered into the record out of respect for the patient. If alcohol or drug abuse is suspected but denied by the patient, the doctor may be reluctant to risk warfarin treatment, without making an entry in the record about it. Thus, it is likely that we also have underestimated the number of patients with contraindications against warfarin.

The consequence of this is that we most likely underestimated both the number of patients with indications for warfarin treatment, and the number of patients with contraindications against it. Our estimate of the proportion of patients suitable for warfarin treatment who did not get it was 46% (Figure 1) and even if we overestimated this figure it would still leave a large proportion of eligible patients without warfarin treatment.


   Summary
Top
 Abstract
 Introduction
 Aim
 Method
 Results
 Discussion
 Summary
References
 
Our results indicate that a large proportion of AF patients do not receive adequate stroke prevention. Of the five major risk factors for embolic stroke, only a prior stroke was associated with clearly better odds to receive oral anticoagulants. Heart failure, hypertension, diabetes, and age >75 years did not seem to favourably influence the chance of treatment.

Paroxysmal AF and advanced age was strongly associated with poor prospects of receiving the recommended treatment. Patients treated by cardiologists appeared to have better chances to get correct treatment.

Thus, risk-stratification using known risk factors of stroke seems to affect the choice of treatment only to a minor degree in clinical practice. It is urgent to increase the awareness of the need for adequate stroke prophylaxis in AF patients.

Conflict of interest: M.R. is national coordinator in a phase II trial for an antiarrhythmic compound manufcatured by Astra Zeneca Inc. He has also participated in a phase II trial and in a phase III trial investigating a thrombin inhibitor manufactured by Boehringer-Ingelheim. He has received honoraria from Astra Zeneca Inc. for giving lectures in the field of anticoagulation. N.H. does epidemiological research for Astra Zeneca Inc. The other authors have no affiliations or conflicting economic interests to declare.


   References
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