National Patterns and Predictors of β-Blocker Use in Patients With Coronary Artery Disease FREE

THE USE of β-blockers by patients with coronary disease is well supported by data from randomized controlled trials. These medications reduce mortality in patients after myocardial infarction, reduce ischemic events in patients with mildly symptomatic disease, and improve survival compared with diuretics when used for primary prevention in men.^1- 5 They are also among the most cost-effective cardiac interventions.^6- 7

Despite these benefits, existing data suggest that β-blockers are underused by clinicians. Estimates for in-hospital use of these medications in patients with myocardial infarction range from 29% to 58%.^8- 12 Relatively few estimates of use in the outpatient setting exist. An analysis of insurance claims by Brand et al¹³ found that cardiologists prescribed β-blockers for fewer than 50% of their patients with no strong contraindication after myocardial infarction. Recently, Soumerai et al¹⁴ studied elderly survivors of myocardial infarction and noted that only 21% with no strong contraindication received a β-blocker. Notably, those who did receive these medications had a 43% lower mortality rate.

Because prior studies of β-blocker use have focused on selected populations of recently hospitalized patients, we undertook this study to investigate national patterns in β-blocker use over time in ambulatory patients with coronary disease. We also identified predictors of underuse of these medications that might form barriers to their appropriate outpatient use.

We used data from the National Ambulatory Medical Care Surveys (NAMCS) for 1980, 1981, 1985, and 1989 through 1996.^15- 25 These surveys were conducted by the National Center for Health Statistics to assess the office practices of US physicians. For each survey year, a random sample of physicians, stratified by specialty and geographic location, was selected from the master files of the American Medical Association and the American Osteopathic Association. Between 70% (1994) and 78% (1981) of selected physicians agreed to participate. The number of physicians participating ranged from 1345 to 1883 annually.

Twenty percent to 100% of patient visits to the participating physician in a randomly selected week were sampled. For each visit, the physician completed an encounter form describing the patient's demographics, medical diagnoses (both for the visit and preexisting conditions), services provided, and continuing and newly prescribed medications. Visit weights were calculated by the National Center for Health Statistics to reflect the physician and visit sampling algorithm and to adjust for nonresponse. These weights allow extrapolation of the survey data to national patterns of practice.

Although β-blockers may be beneficial for both primary and secondary prevention, we focused on their use in patients with diagnosed coronary disease.^1- 3,26 Visits by these patients were identified by the following International Classification of Diseases, Ninth Revision (ICD-9) codes: 410.0 through 410.9, 411.1, 411.81, 411.89, 412, 413.0, 413.1, 413.9, 414.00 through 414.09, 414.10, 414.11, 414.8, 414.9, 429.0, 429.1, 429.2, 429.5, 429.6, and 429.7. We also used the appropriate NAMCS-specific reason-for-visit codes. Some patients were coded as having myocardial infarction (ICD-9 code 410); we presumed them to have sustained recent myocardial infarctions. We recognized that many prior myocardial infarctions were probably not reported given the lack of direct relevance to the office visit.

Similarly, we identified patients with potential contraindications to β-blocker use by employing the ICD-9 codes for congestive heart failure (398.91, 402.01, 402.11, 402.91, 404.11, 404.13, 404.91, 404.93, and 428), diabetes (250), conduction blocks (426 and 427.8), peripheral vascular disease (440.2, 440.3, and 443), bronchospasm (490, 491, 492, and 493), and depression (311). Because conduction blocks and bronchospasm were relatively strong contraindications for β-blocker use, we excluded patients with these conditions from the statistical analyses. The final numbers of office visits sampled were 1448 in 1980, 1507 in 1981, 1899 in 1985, 832 in 1989, 1253 in 1990, 790 in 1991, 797 in 1992, 739 in 1993, 886 in 1994, 891 in 1995, and 710 in 1996.

In the NAMCS, β-blocker use was evidenced by the reporting of the generic or proprietary names for acebutolol, atenolol, betaxolol, bisoprolol, esmolol, nadolol, labetalol, metoprolol, penbutolol, pindolol, propranolol, timolol, or sotolol among the medication codes for each visit. β-Blockers used either as single agents or as a component of combination medications were included. No patients were identified who were taking carteolol or carvedilol. We assessed the use of calcium channel blockers (CCBs) in a similar manner.

Patient and physician characteristics reported in the NAMCS were also included in the analyses. Geographic area was based on the 4 US census regions: Northeast, Midwest, South, and West. The primary specialty of the physician was defined according to American Medical Association categories. Insurance coverage was said to be private if private insurance coverage, primarily or as a supplement to Medicare, was reported.

We determined the rate of β-blocker use using patient visit data pooled from 1993, 1994, 1995, and 1996. These visit data were weighted using National Center for Health Statistics visit sampling weights to derive national estimates of use. We modified these weights using the method described by Potthoff et al²⁷ to derive effective sample sizes for use in statistical testing.

We carried out univariate and multivariate analyses to identify predictors of β-blocker use. The univariate analyses were performed by cross tabulation of potential predictors, including patient age, sex, race, location, and comorbidities, with use of β-blockers. The results were statistically evaluated using χ² tests of association, with P values of .05 or less considered statistically significant. Multivariate logistic regressions were performed to evaluate the independent effect of each of the potential predictors on β-blocker use. Demographic and clinical variables were chosen for inclusion in the model if they were significant in the univariate analyses or of a priori relevance to the decision to use β-blockers. Adjusted odds ratios and 95% confidence intervals were calculated for each potential predictor after the other covariates in the model were adjusted for.

Also, longitudinal analyses were performed using data from 1980, 1981, 1985, and 1989 through 1996. We developed a multivariate logistic model similiar to that described above, but based on the entire panel of years.

Nationally, an estimated 70.8 million office visits were made by patients with coronary artery disease between 1993 and 1996. Nine percent of these (6.7 million) were made by patients with atrioventricular conduction block or bronchospasm secondary to asthma or chronic obstructive pulmonary disease. We excluded these visits from our analyses. Forty-four percent of visits were by women, and the average patient age was 69 years. β-blocker use was reported in only 20.9% of visits by patients with coronary artery disease. As shown in Table 1, use was significantly lower among patients older than 75 years and residents of the Northeast. Patients visiting cardiologists (26.9%) and internists (21.0%) had β-blocker use reported more often than patients seeing family or general practitioners (16.5%). Coverage with private insurance was also found to be correlated with β-blocker use.

Hypertension and reported myocardial infarction were positively correlated with β-blocker use, whereas the diagnosis of congestive heart failure was negatively correlated. No significant association between β-blocker use and diabetes, depression, or peripheral vascular disease was noted.

Univariate analyses were also performed to identify whether similar characteristics predicted CCB use (data not shown). Like β-blockers, calcium antagonists were reported as being used more often in visits by patients with hypertension and less often in visits by patients with congestive heart failure. However, use of CCBs was similiar in patients with and without prior myocardial infarction, and did not vary significantly across demographic categories.

When predictors of β-blocker use were assessed using multivariate logistic regression, younger age, residence in the Northeast, visits to cardiologists and internists, the presence of hypertension, and prior myocardial infarction continued to be significant predictors of β-blocker use (Table 2). Congestive heart failure remained negatively correlated with β-blocker use. White race and private insurance coverage were 2 predictors that approached statistical significance, with odds ratios of 1.42 (95% confidence interval, 0.98-2.06) and 1.22 (95% confidence interval, 0.96-1.56), respectively. Sex, diabetes, and the use of CCBs were not independently associated with β-blocker use.

We repeated the above analyses after excluding patients with the diagnosis of congestive heart failure. β-Blocker use was reported in 22% of the 93% of visits by patients without congestive heart failure. The results of the multivariate regressions were unchanged.

We examined the use of β-blockers over time, using NAMCS data for 1980, 1981, 1985, and 1989 through 1996. Estimated annual office visits by patients with coronary artery disease and without strong contraindications ranged from 13.6 to 19.7 million. β-Blocker use was reported in 29% of visits in 1981. Use subsequently decreased to 14% of visits by 1990, before increasing again to 25% of visits in 1996. These data are shown in Figure 1, along with comparable data for CCBs.

A multivariate logistic regression model using the entire panel of years produced results consistent with the 1993 through 1996 model, with similar predictors achieving statistical significance (Table 2). As shown, white race and private insurance were significantly associated with use of β-blockers when data from 1980 through 1996 were used.

Despite strong evidence that β-blockers improve survival, our data suggest that only one fifth of ambulatory patients with coronary disease and without strong contraindications were taking these medications. Most prior studies that have examined the use of β-blockers have been based on selected health plans or on inpatients in academic medical centers.^{8- 9,11- 14,28- 30} To our knowledge, this is the first study to use a nationally representative sample in the ambulatory setting and to examine patterns of β-blocker use in coronary artery disease.

In contrast to prior studies, we studied all patients with coronary artery disease, rather than just survivors of myocardial infarction. This may account for the lower estimates of β-blocker use in our study compared with prior studies; our focus on outpatients may also explain the lower rate of use. When we restricted our sample only to patients who were survivors of myocardial infarction, the rate of use was 37.3%, compared with the 29% to 58% found in other studies.^{8- 9,11- 14,28- 30} Unfortunately, sample size limitations prevent us from making more detailed conclusions about this group.

While the largest clinical trials of β-blocker therapy have been based on patients who have had a myocardial infarction, smaller randomized trials have shown that β-blockers reduce adverse outcomes in patients with stable angina and decrease the risk of myocardial infarction in patients with unstable angina.^4,31- 32 Experimental data indicate that β-blockers may retard the progression of atherosclerosis.^29,33 There are some randomized trial data supporting the use of β-blockers in patients in whom coronary artery disease has not even been diagnosed.^1,34- 35 Thus, β-blocker therapy should probably not be limited to those patients who are hospitalized for myocardial infarction. Our study population is a readily identifiable group of patients in the outpatient setting for whom therapy with β-blockers would be expected to yield clinical benefits.

A number of reasons may contribute to the low rate of β-blocker use. Physicians may hesitate to use these agents in patients with congestive heart failure, diabetes, or peripheral vascular disease, although this reluctance is not supported by the literature.^36- 40 Indeed, in the case of congestive heart failure, a growing body of evidence suggests that there may be morbidity and even mortality benefits for patients who take these medications.^41- 43 Also, physicians may be wary of potential adverse effects, such as fatigue, impotence, and depression, although, again, the data to support these concerns are lacking.^38,44

Our data did suggest that β-blockers were prescribed less often for patients with congestive heart failure. While the NAMCS cannot distinguish acute from chronic congestive heart failure, relatively few outpatients would be presenting with acute pulmonary edema. There was no correlation in our data between β-blocker use and the diagnoses of diabetes and peripheral vascular disease, although the sample sizes were small. We excluded patients with conduction blocks and bronchospasm, because we believe that these conditions constitute stronger contraindications to the use of β-blockers. One of the strongest negative predictors of β-blocker use after other factors were controlled for was advanced age, with patients older than 75 years receiving the medication 35% less often than their younger counterparts. This has been a consistent finding in previous studies.^{10- 11,14,45} In patients hospitalized for myocardial infarction, Gurwitz et al¹¹ found adjusted odds ratios for β-blocker use of 0.36 for patients aged 75 to 84 years and 0.26 for patients aged 85 years and older, compared with patients younger than 55 years. While elderly patients have traditionally been excluded from large randomized studies, they are at greatest risk of poor outcomes when they are hospitalized for myocardial infarction and may derive more absolute benefit from β-blockers than younger patients.^14,45

Nonclinical factors may affect the use of β-blockers. For instance, our data suggest that physicians in the Northeast prescribe β-blockers at a significantly higher rate than physicians in other parts of the country. Pilote et al⁴⁶ found a higher rate of β-blocker use in New England than in other regions, a finding also observed in studies of other physician practices.^47- 48

β-Blocker use was also higher among patients visiting cardiologists and internists than among those visiting family and general practitioners, a difference that persisted after patient characteristics were controlled for. Prior studies have not explicitly evaluated the effect of physician specialty on β-blocker prescribing practices. There is evidence, however, that cardiologists may be more aware than generalists about important clinical trial data. Ayanian et al⁴⁹ found that more cardiologists (75.3%) than family practitioners (51.6%) and internists (54.6%) recognized that β-blockers definitely improved survival after myocardial infarction.

Coverage with private insurance also appeared to be positively correlated with β-blocker use. Sial et al³⁰ found that uninsured patients received β-blockers less often than insured patients after hospitalization for myocardial infarction; they did not compare privately insured with non–privately insured patients.

Nonwhite race was found in one of our multivariate analyses to predict a lower rate of β-blocker use, but not in the univariate analysis. Race-related variations have been described for other cardiac interventions, such as angioplasty and bypass surgery.^50- 53 Two past studies assessing the impact of race on β-blocker use yielded conflicting results, although in different patient populations.^8,14

Some of the findings of our longitudinal analyses were unexpected. The 2 major randomized trials demonstrating that β-blockers benefited survivors of myocardial infarction were published in 1981 and 1982.^2- 3 Our data did not show an increase in β-blocker use after these years; if anything, there was a reduction in use from 1980 to 1990. The availability of substitute classes of medications, in particular CCBs, may be one explanation for this trend. Aggressive marketing of CCBs may have reinforced clinicians' negative perceptions of β-blockers. Indeed, the decline in β-blocker use in our sample during the 1980s was paralleled by an increase in the use of CCBs, which were unavailable at the beginning of that decade.

The subsequent rise in β-blocker use from 1993 through 1996 may be the result of a variety of factors, including national practice guidelines,⁵⁴ the controversy surrounding CCB use in patients with coronary artery disease,⁵⁵ and the promising data on therapy with β-blockers in patients with congestive heart failure.^41- 43 Scientific, professional, and commercial influences on physician prescribing practices are poorly understood and warrant further investigation.

Several limitations of this study deserve comment. The NAMCS data are based on visits, not on patients. Thus, patients who make more frequent visits will be overcounted. The direction of the bias depends on whether patients for whom β-blockers are prescribed make fewer or more frequent office visits. To evaluate this further, we performed additional analyses restricted to first visits and general medical examinations, which are unlikely to be repeated in a given year. The rate of β-blocker use from 1993 through 1996 estimated from first visits and general examinations was 20.4%, which is comparable to the rate found in the full sample.

Our results are also subject to potential reporting bias, as the data are based on encounter forms filled out by physicians and not externally validated. Some visits by patients with coronary disease may have been omitted if physicians did not include coronary disease as 1 of the diagnoses coded for that visit. Similarly, the low rates of prior myocardial infarction and congestive heart failure in the study may reflect underreporting or the use of nonspecific ICD-9 codes.

Underreporting of β-blockers also could occur, but we believe that this is mitigated by the fact that our analysis was limited to visits with a reported diagnosis of coronary artery disease. Physicians who identify coronary artery disease as a diagnosis or as a reason for a visit may be less likely to underreport the use of β-blockers.

Retrospective assessment of physician practices requires caution. The database does not provide enough clinical detail to comment definitively on the appropriateness of therapy in particular instances. However, the overall rate of β-blocker use in this population was sufficiently low that it would be difficult to explain on clinical grounds alone.

In summary, we found potential underuse of β-blockers in a national, population-based sample of office visits by patients with coronary artery disease. Some of this underuse may be attributable to exaggerated concerns regarding adverse effects of β-blocker therapy, particularly in older patients. However, our analyses indicate that nonclinical factors, including region, insurance type, and physician specialty, may also play a role and may form barriers to the effective use of β-blockers. These findings highlight a substantial opportunity for better outpatient management of coronary artery disease.

Accepted for publication February 5, 1998.

Reprints: Randall S. Stafford, MD, Institute for Health Policy, Massachusetts General Hospital, 50 Staniford St, Ninth Floor, Boston, MA 02114.