Early -Blocker Therapy for Acute Myocardial Infarction in Elderly Patients

Therapy with -blockers is an effective and inexpensive early treatment for acute myocardial infarction. Several trials, including the Metoprolol in Acute Myocardial Infarction (MIAMI) Trial and the First International Study of Infarct Survival (ISIS-1) have reported that -blocker therapy reduces short-term mortality rates in patients with suspected acute myocardial infarction (1-5). On the basis of these studies, the Guidelines for the Management of Patients with Acute Myocardial Infarction, published by the American College of Cardiology and the American Heart Association, strongly endorse the early use of -blocker therapy for patients without a contraindication (6). Despite the evidence supporting the importance of early -blocker therapy, this intervention has received little attention as an indicator of quality of care. The few studies that have assessed the current use of early -blocker therapy suggest that it is underused, especially in elderly patients (7). Most studies have focused on the use of -blocker therapy at discharge (8-10), an issue that is distinct from early -blocker therapy. Moreover, published quality measures for patients with an acute myocardial infarction have focused on the use of -blocker therapy at discharge (11, 12). We sought to determine the current pattern of early -blocker therapy and to assess the effectiveness of this therapy in a large, population-based sample of older patients. Our specific objectives were 1) to determine the current rates of early -blocker therapy for acute myocardial infarction among elderly patients who did not have contraindications to this therapy, 2) to identify the characteristics of patients who did not receive early -blocker therapy, and 3) to evaluate the effectiveness of the early use of -blockers on in-hospital mortality rates by using multivariable methods to adjust for baseline differences between the patients who did and those who did not receive the therapy. We conducted this study as part of the Cooperative Cardiovascular Project (CCP), a Health Care Financing Administration initiative to improve the quality of care for Medicare beneficiaries with acute myocardial infarction (12). The CCP involved medical record abstraction of more than 200 000 Medicare hospitalizations for acute myocardial infarction across the United States from 1994 to 1995. Methods Sample Selection The sample included claims from hospital bills (UB-92 claims data) in the Medicare National Claims History File with an International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) principal diagnosis code of 410 (acute myocardial infarction). Codes that were not related to the acute care of an acute myocardial infarction (codes with a fifth digit of 2) were excluded. This file includes all patients treated under fee-for-service plans but does not include all patients treated as part of Medicare managed care risk contracts. Except for the CCP pilot study states (Alabama, Connecticut, Iowa, and Wisconsin) and Minnesota, the 8-month sample period was different for each state and lasted from February 1994 through July 1995. The pilot states had a 4-month sample period from August through November 1995. Sampling in Minnesota was delayed so that it did not interfere with an ongoing study, the Minnesota Clinical Comparison and Assessment Project (MCCAP). For most non-MCCAP hospitals, the 8-month sample period was April through November 1995. For the MCCAP hospitals and two other hospitals with active quality improvement projects, a random sample of 330 cases was selected. Study Sample The sample was restricted to patients who were 65 years of age or older, had a confirmed acute myocardial infarction, and survived the first hospital day. We excluded first-day deaths to avoid biasing the study in favor of -blocker therapy. We assumed that some patients who died on the first day may not have had the opportunity to receive -blockers. A confirmed acute myocardial infarction was defined as a discharge diagnosis of an acute myocardial infarction and 1) a creatine kinase-MB level of 5% or greater or a lactate dehydrogenase level elevated more than 1.5 times normal and a lactate dehydrogenase-1 level greater than or equal to the lactate dehydrogenase-2 level or 2) a twofold elevation of the creatine kinase level and evidence of myocardial infarction on the official electrocardiogram report. For patients who were hospitalized more than once in the sample period, only the first admission was included. Because of an inability to determine the admission treatments, we excluded patients whose admissions resulted in a transfer from another acute care institution. We also excluded patients who had the following contraindications to -blocker therapy: bradycardia (heart rate 50 beats/min), low blood pressure (systolic blood pressure 100 mm Hg), high-grade atrioventricular block, and chart documentation of any of the following: asthma, chronic obstructive pulmonary disease, history of heart failure, shock on admission, heart failure on admission, or intolerance toward -blockers. All contraindications except asthma were determined by chart review; the presence of asthma was based on a secondary diagnostic code (ICD-9-CM code 493.xx). Data Collection To obtain information from the medical records, the Health Care Financing Administration established two clinical data abstraction centers, which were private organizations with experience in medical abstraction. Trained technicians at the centers abstracted predefined variables from complete copies of the hospital records and entered them directly into a computer database using interactive software. Data reliability was monitored by random reabstractions and calculation of reliability statistics. Problems were identified by determining reasons for the discrepancies and were addressed by improvements in training. The abstraction assessment did not include an assessment of sensitivity and specificity. Outcome Variables The principal outcome for the first phase of the study was the use of -blockers on the first or second hospital day. We included the first 2 days as the treatment window because some patients may have been admitted late in the day and the -blocker may not have been administered until the next calendar day. After all medications were reviewed, intravenous and oral -blockers were identified. Topical -blockers were excluded. The principal end point for the second phase of the study was in-hospital mortality. This information was ascertained from the medical records and was cross-checked with Medicare's Enrollment Database (derived from the Master Beneficiary Record of the Social Security Administration). Dates of death in the Medicare Enrollment Database are obtained both from the discharge dates of billing records that indicated a discharge disposition of death and from the Master Beneficiary Record. The Medicare Enrollment Database has accurate records of the vital status of Medicare beneficiaries (13). Independent Variables The independent variables in this study included age; sex; ethnicity; medical history; preadmission, hospital, and discharge medications; clinical status; hospital complications; hospital procedures; discharge disposition; and length of stay. Age was categorized into three strata: 65 to 74 years, 75 to 84 years, and 85 years or older. Comorbid conditions were a chart-documented history of hypertension, diabetes mellitus, renal dysfunction (defined as a blood urea nitrogen level 14.28 mmol/L or a creatinine level 221 mol/L [2.5 mg/dL]), myocardial infarction, anemia (hematocrit 30%), and dementia. Procedure variables were a history of cardiac catheterization, percutaneous coronary revascularization, and coronary artery bypass surgery. Preadmission medications were aspirin, -blockers, calcium-channel blockers, digoxin, angiotensin-converting enzyme inhibitors, loop diuretics, and warfarin. Presentation characteristics were chest pain and vital signs. Hospital treatment variables on the first day of admission were aspirin, heparin, calcium-channel blockers, and thrombolytic therapy. On the basis of the admission characteristics, the case mix of the patients was summarized by an expected mortality rate, calculated by using information available on admission and based on a modification of the Medicare Mortality Predictor System (14). This system is a disease-specific model used to predict short-term mortality rates in Medicare patients. Variables for acute myocardial infarction in this system included the Acute Physiology and Chronic Health Evaluation (APACHE) II score (15), age, ability to ambulate, do-not-resuscitate order on admission, blood urea nitrogen level, mean arterial pressure, subendocardial infarction, congestive heart failure on chest radiograph, heart rate, and metastatic cancer. Our version of the Medicare Mortality Predictor System did not include values for serum potassium level in the APACHE II score (which were not abstracted for CCP). Applied to the entire study sample, the model had an area under the receiver-operating characteristic (ROC) curve of 0.78, indicating good discriminant ability. Average predicted values for survivors (0.15) and nonsurvivors (0.35) were similar to those previously reported for Medicare patients with acute myocardial infarction (14). For the geographic analysis, we classified the patients into subgroups based on the major U.S. Census regions: New England, Mid-Atlantic, South Atlantic, East North Central, East South Central, West North Central, West South Central, Mountain, and Pacific (16). Statistical Analysis For the first phase of the analysis, we sought to determine the frequency with which elderly patients with acute myocardial infarction who were ideal candidates for -blocker therapy received this therapy on admission or the next day. We evaluated the bivariate associations of demographic, clinical, physician, and geographic characteristics wit