The MI Incidence Cohort
From 1987 to 2006, an estimated total of 2816 incident infarctions occurred in Olmsted County. Medical history available prior to the event covered a mean (SD) time of 39 (20) years. The mean (SD) age at index was 68 (15) years old. Of the 2816 incident events, 1222 (43%) occurred among women () and 1107 (39%) among persons age 75 years and older. The distribution of age at index did not change over time whereas the percent of MIs experienced by women decreased over time ().
Trends in Characteristics of MI from 1987 to 2006 in Olmsted County, Minnesota
Of all incident events, 1689 occurred prior to the introduction of troponin in August 2000 and 1127 thereafter. Among these, 278 (25%) did not meet infarction criteria as defined by CKMB and met only troponin-based criteria. Compared to cases identified by CKMB-based criteria, those meeting only troponin-based criteria were older, more likely to be women, and had more comorbidities (). They were less likely to experience pain, present with Q waves, and were in a lower Killip class. Most infarctions meeting only troponin-based criteria were non-ST elevation infarctions. These differences were independent of age and sex. With regard to treatment, MIs meeting only troponin-based criteria were less likely to receive evidence-based therapies. Among all MIs, the use of reperfusion/revascularization during hospitalization increased over time, without reaching statistical significance. The use of aspirin, beta blockers and statins at dismissal increased over time ().
Incidence of Hospitalized MI
Temporal trends differed according to the type of biomarker used for diagnosis (). When all infarctions were included irrespective of the biomarker used for diagnosis, the incidence rates did not change between 1987 and 2006. The overall age and sex adjusted incidence rate (95% CI) of hospitalized infarctions was 186 per 100,000 (150-221) in 1987 and 180 per 100,000 (151-209) in 2006 (P=0.171 for the year effect). When only cases meeting CK/CKMB criteria were considered, a significant temporal decline in the incidence of MI was detected (p=0.020) as the age and sex-adjusted incidence rate (95% CI) of hospitalized infarctions declined to 141 per 100,000 (115-167) in 2006. This represents a 1.1% per year decline in the incidence of infarctions meeting CK/CKMB criteria. Thus, assuming a linear decline from 1987 to 2006, the age and sex adjusted relative risk of experiencing an MI as defined by CK/CKMB in 2006 compared to 1987 was 0.80 (95% CI, 0.67 to 0.98) indicating a 20% decline in incidence rates over the last 2 decades.
Figure 1 Trends in incidence of hospitalized MI from 1987 to 2006 in Olmsted County, Minnesota. Incidence rates are shown on a linear scale and are adjusted by the direct method to the age distribution of the total US population in 2000. They are reported per (more ...)
While the incidence of MI was higher in men () and in older persons (data not shown), none of the aforementioned trends differed by age or sex. The incidence trends diverged markedly according to the presence or absence of ST elevation (). The incidence rates of ST elevation MI (STEMI) declined by 41% over the time period irrespective of troponin (relative risk [RR], 0.59, 95% CI, 0.47 to 0.76 for STEMI including troponin-only cases, versus 0.56, 95% CI, 0.44 to 0.71 for STEMI excluding troponin-only cases). The incidence rates of non ST elevation MI (NSTEMI) increased by 49% over time when troponin-only cases were included (RR 1.49, 95% CI, 1.23 to 1.81). Temporal trends in NSTEMI did not change when troponin-only cases were excluded.
Figure 2 Trends in incidence of hospitalized MI from 1987 to 2006 in Olmsted County, Minnesota by ST segment elevation status. Incidence rates are shown on a linear scale and are adjusted by the direct method to the age distribution of the total US population (more ...)
The incidence rate analyses were repeated using a minimum difference of 0.03 ng/ml between any two troponin measurements to define a change in values. Doing so increased the estimated number of incident infarctions by 1.2% without impacting temporal trends, which attests to their robustness.
Myocardial Infarction Severity
When analyzing all infarctions irrespective of the biomarker used for diagnosis, most patients were in Killip class 1, but the proportion of those in Killip class 2, 3, or 4 declined over time as did the proportion of patients with ST-segment elevation (). Electrocardiographic Q waves were observed in 54% of cases and its frequency increased during the first half of the period followed by a decrease thereafter. The CKMB ratio showed a similar pattern.
The trends in hemodynamic presentation, ECG findings, and CKMB ratio were similar when cases meeting only troponin criteria were excluded. The median time between symptom onset and first ECG (ascertained in 97% of cases) was 1.7 (25th-75th percentile 0.8-4.4) hours and did not change over time.
Among all incident infarctions, the 30-day case fatality rate () was higher in women and in older persons, and decreased markedly over time. Indeed, after adjusting for age and sex, the overall 30-day case fatality rate declined by 4.3% per year (P=0.001). Thus, compared to the reference year of 1987, for an incident infarction occurring in 2006, the age and sex adjusted hazard ratio of death within 30 days of the event was 0.44 (95% CI, 0.30 to 0.64; P<0.001) indicating a 56% decline in 30-day case fatality rate over the last 2 decades. The temporal trends in 30-day case fatality did not differ by age or sex (year*age interaction P=0.630, year*sex interaction P=0.884) and were similar when cases meeting only troponin-based criteria were excluded.
Thirty-day case fatality rates for incident hospitalized MI overall and by age, sex and time period.
Among all incident infarctions, the mean (SD) follow-up was 6.0 (5.3) years. Among persons who survived for 30 days after the incident infarction, survival did not improve further over time. Indeed, compared to 30-day survivors of an infarction occurring in 1987, the age and sex adjusted hazard ratio of death among 30-day survivors of an infarction occurring in 2006 was 1.04 (95% CI, 0.81 to 1.35; P=0.717). Further adjustment for cardiovascular risk factors, comorbidity and Killip class yielded similar results. The temporal trends in long term survival among 30-day survivors did not differ by age or sex (year*age interaction P=0.168, year*sex interaction P=0.798) and were similar when cases meeting only troponin-based criteria were excluded.
The distribution of the causes of deaths after hospitalized MI changed over time (P=0.001). During the first year quartile (1987-91), 62% of deaths were ascribed to cardiovascular causes compared to 50% during the most recent year quartile (2002-2006).
When cardiovascular death was examined, among persons who survived at least 30 days, the survival free of cardiovascular death improved over time. Indeed, compared to 30-day survivors of an infarction occurring in 1987, the age and sex adjusted hazard ratio of cardiovascular death among 30-day survivors of an infarction occurring in 2006 was 0.54 (95% CI, 0.38 to 0.75; P=0.001). These findings were similar when cases meeting only troponin criteria were excluded.
The 5-year Kaplan-Meier survival estimate (95% CI) after incident infarction was 67% (65%-69%), lower than that expected among the Minnesota population of 79% (P<0.001 for comparison between observed and expected survival). When the analysis was stratified by year groups, similar estimates of observed and expected survival were obtained over time.