Results from the Framingham Heart Study indicate that the lifetime risk of atrial fibrillation is about 25%.¹ An increased risk of embolic stroke is a major risk factor associated with the diagnosis of atrial fibrillation.² Although the risk of ischemic stroke related to atrial fibrillation is significantly reduced with warfarin (Coumadin) treatment, anticoagulation treatment can possibly lead to an increased risk of complications from bleeding.^3-5 To help guide the clinician in determining the degree of risk of thromboembolic events posed by atrial fibrillation in individual patients, 2 approaches have been employed. The first is the use of readily available clinical criteria, such as the CHADS² (congestive heart failure, hypertension, age > 75 years, diabetes, and previous stroke or transient ischemic attack) score, which delineates higher-risk patient populations for whom anticoagulation is recommended and lower risk populations for whom it may not be necessary.^4,6,7 The other strategy, which is usually applied more selectively, is the use of transthoracic and transesophageal echocardiography to help assess thromboembolic risk in patients with atrial fibrillation.⁴

Despite clinical criteria guidelines useful for predicting thromboembolic risk, previous studies have shown that anticoagulation therapy is underused in many high-risk populations with atrial fibrillation.^8-12 To determine current use of anticoagulation for patients with atrial fibrillation, we recently analyzed the National Ambulatory Medical Care Survey (NAMCS), a large ambulatory patient database, to determine the time trends in the use of anticoagulation therapy for the prevention of thromboembolism in patients with atrial fibrillation.¹³

Patients and methods

The NAMCS documents the responses of a group of ambulatory physicians to a standardized survey instrument. The physicians are not employed by the federal government and are nationally representative. We examined the 1994 through 2003 NAMCS database to establish the utilization patterns of anticoagulant therapy among patients diagnosed with atrial fibrillation.¹³ Demographic, medication, and comorbidity data were obtained. Univariate and multivariate analyses were performed by standard techniques.¹³

Results

Over the course of the study period, approximately 40.5 million atrial fibrillation patient visits occurred, and 18.5 million (45.6%) of these patients received anticoagulant therapy. The frequency of anticoagulation therapy increased with increasing age (34%, 45%, and 49% frequency in those aged 18-59, 60-75, and >75 years of age, respectively; P < .01). The percentage of patients receiving anticoagulation therapy also increased with the presence of 1 or more comorbid factors believed to be associated with an increased frequency of thromboembolic events (previous cerebrovascular event, congestive heart failure, diabetes mellitus, or hypertension). Anticoagulation therapy was prescribed in 51% of patients with any comorbid factor and in 42% of those without any comorbid factor (P < .03). Interestingly, the presence of one comorbid factor, diabetes mellitus, which has been shown to be associated with thromboembolism in patients with atrial fibrillation, was associated with a lower frequency of anticoagulation therapy (37% of patients with and 46% of patients without diabetes received anticoagulation therapy). Among all patients with atrial fibrillation, the use of rate control agents was associated with receiving anticoagulant therapy (56% of those treated with a rate control agent vs 33% of those not treated with a rate control agent; P < .001).

We analyzed the rate that anticoagulation therapy was prescribed between 1994 and 1997 and between 2001 and 2003. An increased frequency was noted in patients aged 18 to 59 years (24% to 43%; P < .04) and in those aged 75 years and older (40% to 49%; P < .02), but not in those aged 60 to 75 years (49% to 45%; P = .92). Logistic regression analyses showed a yearly increase of 7% in the likelihood of being prescribed anticoagulant therapy for all patients with atrial fibrillation during this period.

When we evaluated the frequency of anticoagulation therapy in patients with atrial fibrillation who were estimated to be at relatively low risk for a thromboembolic event (individuals under 65 years of age without known congestive heart failure, previous cerebrovascular event, hypertension, or diabetes mellitus), 30% received anticoagulant therapy. The chance of receiving anticoagulant therapy in these low-risk patients was increased 2-fold for patients who had a cardiologist as a provider (45% with vs 17% without a cardiologist provider.)

Discussion

These and other recent results suggest that although compliance with guidelines for anticoagulation therapy for patients at relatively high risk for thromboembolic phenomena appears to be improving, many patients who qualify for anticoagulant therapy are not receiving it.^8-13 However, the target rate for anticoagulation of patients with atrial fibrillation who are at relatively high risk for thromboembolic events has not been determined, and some studies suggest that as many as 20% of such patients may have either an absolute or strong relative contraindication to anticoagulation.¹⁴ Nonetheless, despite improvements in recent years, there appears to be additional opportunity to reduce thromboembolic risk in many patients with atrial fibrillation.

Although many who are at risk for the thromboembolic complications of atrial fibrillation are undertreated, it appears that anticoagulation therapy may be overused in many low-risk patients who may be more safely treated with an antiplatelet agent. The guidelines vary somewhat in age cutoffs and risk factors, but it is clear that many of the patients under the age of 65 with no clear risk factors whom we identified could have been treated with aspirin alone.⁴ This finding is of note because the highest relative increase in the use of anticoagulation over the past decade has been in this group. It is also of interest that having a cardiologist as a provider was associated with an increased frequency of warfarin therapy in this low-risk group. There may be several factors, such as referrals of more complex patients and a higher use of echocardiography to better risk-stratify patients, that underlie the association of cardiology care with a higher frequency of anticoagulation therapy in the lower-risk population with atrial fibrillation. More studies will be needed to determine the reasons anticoagulant therapy is prescribed as well as the outcomes in these low-risk patients.

Conclusions

Results of our study showed that there has been an increase in the use of anticoagulant agents for patients with atrial fibrillation over the past 10 years, especially among those at highest risk for thromboembolic stroke. However, a significant number of patients at risk are not receiving anticoagulant treatment. Furthermore, it appears that anticoagulation therapy may be overprescribed in many low-risk patients who may be more safely treated with an antiplatelet agent.

References