Author + information
- Received August 11, 1998
- Revision received November 24, 1998
- Accepted January 14, 1999
- Published online May 1, 1999.
- Robert J Goldberg, PhDa,* (, )
- Jorge Yarzebski, MD, MPHa,
- Darleen Lessard, MSa and
- Joel M Gore, MD, FACCa
- ↵*Reprint requests and correspondence: Robert J. Goldberg, PhD, Professor of Medicine & Epidemiology, Division of Cardiovascular Medicine, Department of Medicine, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, Massachusetts 01655
The purpose of the present study is to describe changes over two decades (1975 to 1995) in the incidence, in-hospital and long-term case–fatality rates associated with acute myocardial infarction (AMI) from a multihospital community-wide perspective.
Despite the magnitude of, and mortality associated with acute myocardial infarction (AMI), relatively limited population-based data are available to describe recent and temporal trends in the attack and case–fatality rates associated with AMI from a representative population-based perspective.
The community-based study included 5,270 residents of the Worcester, Massachusetts, metropolitan area hospitalized with confirmed initial AMI in all metropolitan Worcester, Massachusetts, hospitals (1990 census population = 437,000) in 10 one-year periods between 1975 and 1995.
The age-adjusted incidence rates of initial AMI increased between 1975 (244 per 100,000) and 1981 (272 per 100,00), after which time these rates declined through 1995 (184 per 100,000). The crude and multivariable-adjusted in-hospital case–fatality rates exhibited a consistent decline between 1975/1978 (17.8%), 1986/1988 (17.0%) and 1993/1995 (11.7%). Although there were no statistically significant differences in the unadjusted long-term case–fatality rates of discharged hospital survivors over the periods under study, declines in the multivariable-adjusted risk of dying within the first year after hospital discharge were observed between the earliest and most recently discharged patients with AMI.
The results of this population-based study of patients with validated initial AMI provide encouragement for efforts directed at the primary and secondary prevention of AMI given declining incidence and case–fatality rates.
Mortality attributed to coronary heart disease (CHD) in the United States has exhibited a consistent decline since the late 1960s in men and women, in all age groups and in the three major race/ethnic groups, with the extent of this
decline varying according to these characteristics (1–4). As a result of this decline in CHD death rates, in 1992 mortality attributed to CHD was less than 60% of the death rate observed three decades earlier at the height of this epidemic. Limited population-based data exist, however, to determine whether these trends in CHD mortality are due to a decrease in the incidence rates of new coronary events, reflecting changes in the prevalence and/or levels of the major coronary risk factors, or to improved survival in patients with CHD (5–17). Despite this unexplained and encouraging decline in national CHD death rates, as well as suggestive declines in the incidence and mortality rates from acute myocardial infarction in population-based studies, CHD continues as a major cause of death and disability in the U.S., accounting for nearly one half million deaths in 1995 and two million hospitalizations (18). Acute myocardial infarction (AMI) is responsible for approximately one half of the annual deaths due to CHD and over one third of the acute hospitalizations, reinforcing the importance of further study of the descriptive epidemiology of AMI and of temporal trends therein. Moreover, although the age-adjusted death rates from CHD have declined each year since the peak years of the mid to late 1960s, declines in the crude death rate and absolute number of deaths due to CHD appear to have reversed and have shown patterns of increasing in 1992 and 1993 (19). Given projected national increases in the number of elderly persons, and likely increases in the number of new cases of CHD occurring in these individuals, these trends have clear health and financial implications that make it important to monitor trends over time in the attack and case–fatality rates of AMI from a well characterized and stable population perspective.
The purpose of this study is to report 20-year trends in the incidence, in-hospital and long-term case–fatality rates in residents from a geographically defined, representative metropolitan area hospitalized with AMI over the period 1975 to 1995.
Residents of the Worcester metropolitan area hospitalized with a primary or secondary discharge diagnosis of AMI (International Classification of Disease [ICD-9] code 410) from all 16 Worcester standard metropolitan statistical area (SMSA) hospitals during 1975, 1978, 1981, 1984, 1986, 1988, 1990, 1991, 1993 and 1995 made up the study population. In addition, a random sample of records from related diagnostic rubrics in which the diagnosis of AMI might have occurred (e.g., ICD codes 411 [other acute and subacute forms of ischemic heart disease] and 412 [old myocardial infarction]) was carried out during each of the years under study to identify potentially misclassified cases of AMI. All hospitals in the Worcester SMSA (1990 census population = 437,000) participated in this study. Fewer hospitals were included in recent years due to hospital closures, mergers or conversion to chronic care facilities. The medical records of Worcester SMSA residents with a discharge diagnosis of AMI from these hospitals were individually reviewed and validated according to preestablished diagnostic criteria that have been previously described (10,20,21). In brief, these criteria included a clinical history of prolonged chest pain not relieved by rest or use of nitrates, serum enzyme level elevations in excess of the upper limit of normal as specified by the laboratory at each hospital and serial electrocardiographic tracings during hospitalization showing changes in the ST segment and/or Q waves typical of AMI. At least two of these three criteria needed to be satisfied for study inclusion. All autopsy-proven cases of AMI were included irrespective of the other criteria. Cases of perioperative-associated AMI were not included.
For the periods under study, the complications of AMI were assessed on the basis of information available from the clinical charts (22,23). Congestive heart failure was regarded as present when there was evidence of pulmonary edema or bilateral basilar rales with an S3gallop. Cardiogenic shock was defined as a systolic blood pressure of less than 80 mm Hg unresponsive to fluids, cyanosis, cold extremities, congestive heart failure and persistent oliguria occurring at any time during the acute hospital phase. Since one of the major objectives of this study was to identify new cases of AMI occurring in Worcester SMSA residents to calculate incidence rates, when the review of the hospital chart indicated that the present hospitalization was not the first for CHD, records of previous hospitalizations for CHD were reviewed where available.
The present report is based on the 5,270 residents of the Worcester metropolitan area who satisfied the diagnostic criteria for initial AMI in the 10 1-year periods examined. Of these, 519 patients were hospitalized in 1975; 558 in 1978; 639 in 1981; 459 in 1984; 527 in 1986; 446 in 1988; 472 in 1990; 530 during each of the years 1991 and 1993, and 590 in 1995.
The hospital records of patients with an initial validated AMI were abstracted for demographic and clinical data, including age, gender, complications during hospitalization (e.g., congestive heart failure, cardiogenic shock), myocardial infarction type (Q wave or non-Q wave) and location (anterior or inferior/posterior). The approaches used to ascertain survival status after hospital discharge included a review of records for additional hospitalizations and a statewide and national search of death certificates for residents of the Worcester SMSA. For patients discharged from the hospital, follow-up was continued through the end of calendar year 1996. The maximum follow-up varied from 1 to 21 years depending on the index period of hospitalization. Of 4,487 patients discharged from area-wide hospitals, some form of additional follow-up was obtained for 4,438 (99%).
Differences in the distribution of demographic and clinical characteristics between patients hospitalized for AMI in the respective periods were examined by using chi-square tests of statistical significance. The short-term prognosis in each period was examined by calculating in-hospital case–fatality rates. The simultaneous effect of several potentially confounding variables that might influence in-hospital death rates was accounted for by means of a logistic multiple regression technique. A life table approach was used to examine trends in long-term survivorship to include patients followed up for various lengths of time after hospital discharge. The log-rank test was used to calculate the overall significance of the difference between long-term survival curves of patients discharged from the hospital between 1975 and 1995. The Cox proportional hazards regression approach was used to examine trends in long-term survival while adjusting for the effect of several potentially confounding variables that might influence survival after hospital discharge. Annual incidence rates of AMI were calculated in the usual manner. Since differences over time were noted according to age, age-adjusted rates were computed by means of the direct method of standardization, using the Worcester SMSA population census estimates in 1980.
Temporal trends in the incidence rates of initial AMI
Trends in the age-adjusted incidence rates of initial AMI over the 20-year period under study are shown in Figure 1. The age-adjusted incidence rates increased between 1975 (244 per 100,000) and 1981 (272 per 100,000), with a decline thereafter until 1990; the incidence rates of AMI slightly increased after this period, reaching rates in 1995 (184 per 100,000) similar to those seen approximately 10 years earlier but considerably lower than those observed during the initial study years.
Characteristics of patients with initial AMI
The demographic, medical history and clinical characteristics of patients with initial AMI, stratified according to five aggregated periods to make trends over time more interpretable, are shown in Table 1. Over the periods under study, patients hospitalized during the most recent years were significantly older, and included an increasingly greater proportion of women, persons with a history of diabetes, hypertension, or stroke and those with a non-Q wave and inferior/posterior AMI. Temporal, albeit inconsistent, trends in the occurrence of congestive heart failure and cardiogenic shock during the acute hospitalization were noted. Use of various cardiac medications during the acute hospitalization and in hospital survivors at the time of discharge from area-wide hospitals is provided. Increases in the use of each of these beneficial cardiac medications were observed over the 20-year period under study.
Temporal trends in hospital death rates
The crude, age-adjusted and multivariable-adjusted risks of dying after hospitalization for AMI all showed a consistent reduction during the most recent time periods (Table 2). Adjusting only for age revealed a statistically significant approximate halving of the risk of dying after AMI among patients hospitalized in the 1990s as compared with those hospitalized 20 years earlier. Adjustment for age plus several additional demographic and clinical confounders revealed relatively similar declines in the risk of dying during the acute hospitalization during the most recent study years; the lowest adjusted risk of dying was consistently observed during the most recent period of hospitalization (1993/1995).
Long-term survival after hospital discharge
When the crude long-term survival rates in hospital survivors were examined, there were no discernible trends apparent in long-term survival. The 1, 2, and 5-year survival rates were 88%, 83% and 69% among patients discharged in 1975/1978; 87%, 81% and 68% among those discharged in 1981/1984; 90%, 84% and 71% in those discharged in 1986/1988, and 87%, 81% and 69% in those discharged in 1990/1991. In patients discharged in the most recent cohort (1993/1995), 1- and 2-year survival rates were 89% and 83%, respectively (Fig. 2).
Since patients discharged from the hospital over the periods under study differed with regard to a number of factors that may affect long-term prognosis, an age- and multivariable-adjusted analysis was carried out to control for various potentially confounding factors. For this analysis, follow-up was truncated at 1 year given the high mortality risk during this period and the more limited duration of follow-up available for the most recently hospitalized patient cohort. The 10 study years were again aggregated into five periods to make data analyses more interpretable. The results of the age-adjusted analysis showed trends toward improved 1-year survival among patients discharged from the hospital beginning in the mid to late 1980s and beyond. The likelihood of surviving to the 1st year after hospital discharge was greatest, and statically significant, among patients discharged during the early to mid 1990s. After adjusting for several factors affecting long-term survival in addition to age, similar trends of improving survival over time were observed, with these differences again being statistically significant among patients discharged in 1986/1988 and 1993/1995 compared with patients hospitalized during the earliest study years (Table 3).
The results of this population-based study carried out in a large representative metropolitan area in the Northeast suggest declines in the incidence and hospital case–fatality rates of AMI over a two-decade long period as well as improving trends over time in the first year after hospital discharge among hospital survivors of AMI.
Incidence rates of AMI
National surveillance efforts for monitoring trends in the attack and death rates of AMI remain sorely lacking as a limited number of population-based studies in different geographic settings have examined these end points. These studies include the Minnesota Heart Survey (4,17,24), the Corpus Christi Heart Project (12,25), the Rochester (Minnesota) Epidemiology Project (5,6)and the Charleston Heart Study (15). Despite the varying time periods involved in each of these studies, and differing clinical and demographic characteristics of the study samples, each of these studies has observed declines in the attack rates of AMI. Declines in the attack rates of AMI have also been observed in the more select studies of male employees of the DuPont Company (11). Population-based studies in Finland, Australia and New Zealand have also shown declines in the incidence rates of acute coronary events over the different periods examined (7–9,13,14). Whereas an earlier publication from the Worcester Heart Attack Study showed increases in the age-adjusted incidence rates of initial AMI in greater Worcester residents hospitalized between 1975 and 1981 (10), more recent data from this study showed declines in these incidence rates through the mid (20)and late 1980s (21). On the other hand, modest increases in the annual discharge rate of persons with a nonvalidated diagnosis of AMI have been observed in the National Hospital Discharge Survey between the mid 1960s and early 1990s (26). Surveillance data from the community-based Pawtucket Heart Health risk factor intervention program failed to detect changes in the incidence rate of definite AMIs during the 1980s (27).
The reasons for the declining incidence rates of AMI observed in the present study remain unknown, since concomitant changes in the major and less well established risk factors for acute coronary disease among residents of the metropolitan Worcester population are not monitored as part of our surveillance activities. Although appropriate caveats must be kept in mind in extrapolating from the results of studies carried out in other settings, national (28,29)as well as population-based estimates (30,31)of changes in the commonly accepted risk factors for CHD have, in general, demonstrated consistent improvements over the past several decades. Despite encouraging trends in the population awareness and control of high blood pressure, declining prevalence rates of cigarette smoking and declines in average population serum cholesterol levels, some inconsistencies in coronary risk factor changes over time remain. For example, the prevalence of obesity in the U.S. has increased over the period of declining CHD death rates (18), and only a small proportion of the population is engaged in recommended levels of physical activity (18,32).
The observed declines in incidence rates of AMI seen in the present study provide encouragement for primary prevention efforts. However, despite adjusting for the advancing age of patients with acute coronary disease, the upward trends in AMI incidence during the most recent study years provide continued reinforcement for the modification of risk factors associated with CHD, search for additional modifiable risk factors and population-wide health promotional efforts. The effective primary as well as secondary prevention of AMI includes careful attention to efforts of smoking cessation, weight control and lipid and blood pressure management. These data also provide additional rationale for community surveillance efforts to identify possible increases in the magnitude and/or mortality from AMI and application of targeted intervention strategies to forestall or prevent subsequent cases of AMI from occurring.
In-hospital case–fatality rates after AMI
Although appropriate reservation also needs to be exercised in interpreting studies reporting data about changes over time in in-hospital case–fatality rates after AMI, studies carried out in male employees of the DuPont Company (11)and in hospitalized patients in Rochester (6)and Minneapolis–St. Paul, Minnesota (4,17), Baltimore, Maryland (33)and the Pee Dee area of South Carolina (15)all show improvements over time in in-hospital survival after AMI.
These hospital death rates, which are typically higher than those observed in randomized clinical trials of AMI patients given the more restrictive inclusion criteria and upper age limits typically imposed in these trials, provide encouragement for either changes in the natural history of AMI and/or the more widespread application of treatment strategies used in the management of AMI. We did not control for differences in the treatment of patients hospitalized with AMI over time given difficulties in the interpretation of these data and the lack of detailed information about indications for the use of these agents. We have previously shown, however, marked increases over time in the use of therapies shown to be effective in the management of AMI, including aspirin and beta-adrenergic blocking agents (34,35), and increases in the use of thrombolytic therapy and angiotensin-converting enzyme inhibitors since the more recent introduction of these medications in the mid to late 1980s (36–38). The increased use of these medications, as well as coronary revascularization approaches that have been shown previously in this study (39), likely contributed to improving trends in hospital survival after AMI.
Long-term survival after AMI
In contrast to the crude unadjusted findings in which the long-term prognosis of discharged hospital patients was unchanged, improving trends in 1-year survival rates were observed when age and other factors that could impact on postdischarge survival were controlled for. Although the majority of published reports examining trends in the prognosis of hospital survivors have failed to show improvements in long-term survival (40), including the present study when patients in the earliest hospitalized cohorts were followed through the mid to late 1980s (10,20), relatively few population-based studies have examined trends in postdischarge survival during more recent periods including the present era of increasing use of thrombolytics. An improvement in long-term survival has been observed in AMI patients discharged from greater Minneapolis–St. Paul hospitals between 1970 and 1980 (24)and in those discharged between 1985 and 1990 (17).
Although the results of our study show encouraging improvements in long-term survival of AMI patients discharged during more recent study years, we did not collect data on changes in lifestyle practices or treatment approaches used by patients upon leaving the hospital and are, therefore, unable to examine the impact of these interventions on postdischarge survival. It is likely, however, that the improving trends in 1-year survival rates likely reflect these practices as well as therapeutic efforts applied during the acute hospitalization to limit the extent of acute myocardial damage. Although we did not adjust the postdischarge survival rates for medications that patients may have been prescribed at the time of hospital discharge given problems in the interpretation of these data, there have been marked increases over time in the prescribing of medications shown to be of benefit in the secondary prevention of AMI that may have contributed to these improving survival trends (38). For example, there was an approximate 16-fold increase in the use of antiplatelet agents at the time of discharge in hospital survivors in the most recent as compared with the earliest study periods; there was more than a three fold increase in the use of beta-adrenergic blocking agents. It is likely that adjustment for these and additional treatment approaches would have reinforced the improving trends in long-term survival observed. These prescribing trends, as well as increasing use of coronary revascularization approaches, may also have contributed to the positive long-term outlook observed in patients discharged in the mid 1990s as compared with earlier years given the favorable impact of these treatment approaches on extent of acute myocardial damage.
The results of this community-wide study carried out in a metropolitan area whose sociodemographic characteristics reflect those of national averages suggest that declining CHD mortality rates in the U.S. may reflect decreases in both the incidence and case–fatality rates of AMI. These results provide continued encouragement for the application of primary and secondary preventive strategies that have been shown to be effective in the prevention and treatment of AMI.
☆ This project was supported by funding provided by the National Heart, Lung, and Blood Institute (R01 HL35434).
- acute myocardial infarction
- coronary heart disease
- International Classification of Disease
- standard metropolitan statistical area
- Received August 11, 1998.
- Revision received November 24, 1998.
- Accepted January 14, 1999.
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