Effects of Socioeconomic Status on Mortality after Acute Myocardial Infarction

Article Outline

• Abstract
• Methods
  • Data Source and Patient Selection
  • Baseline Characteristics
  • Socioeconomic Status
  • Hospital Characteristics
  • Outcomes
  • Statistical Analysis
• Results
  • Baseline Characteristics
  • Differences in Care Processes
  • Socioeconomic Status and 1-Year Revascularization
  • Socioeconomic Status and 1-Year Mortality
• Discussion
  • Effects of Socioeconomic Status on Acute Myocardial Infarction Incidence and Care
  • Effects of Socioeconomic Status on 1-Year Mortality
• Appendix 1. 
• References
• Copyright

Abstract 

Purpose

To assess the effects of socioeconomic status on mortality in patients with acute myocardial infarction.

Material and Methods

We studied a retrospective cohort of 5622 patients who presented to a hospital emergency department with an initial episode of acute myocardial infarction between April 1998 and March 2002 in the Province of Alberta, Canada. Our main outcome measure was 1-year all-cause mortality following the index emergency department visit; we used socioeconomic status (measured by neighborhood median household income) as our main predictor after controlling for patient and hospital characteristics and revascularization.

Results

Socioeconomic status profoundly affected the rate of emergency department presentation and the process and outcome of acute myocardial infarction care. In patients belonging to the lowest versus the highest socioeconomic status quartile, the risk of presenting to the emergency department was 72% higher (P <.001); at 1 year, revascularization was lower (36% vs 48%, P <.001), and mortality higher (19.1% vs 9.1%, P <.001). Socioeconomic status was independently associated with 1-year mortality after adjustment for baseline characteristics and 1-year revascularization, and socioeconomic status was especially influential in non-revascularized patients.

Conclusions

Given the influence of socioeconomic status on mortality after acute myocardial infarction and the key role of revascularization in modulating this relationship, our study has important implications for access to and process of cardiac care.

Keywords: Acute myocardial infarction, Socioeconomic status, Emergency department, Revascularization

In acute myocardial infarction, it is well established that a person’s socioeconomic status has a profound effect on the incidence of acute myocardial infarction, medical management such as the utilization of cardiovascular services including coronary angiography, percutaneous coronary intervention (PCI), and coronary artery bypass grafting (CABG), and clinical outcomes (eg, mortality). These relationships have been demonstrated in community and hospital inpatient settings.¹, ², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰, ¹¹, ¹² It remains unclear, however, how revascularization in particular modulates the effects of socioeconomic status on outcomes after controlling for baseline patient and hospital characteristics. Because of possible effects of health insurance coverage, we undertook the present study in the Province of Alberta, Canada where all residents are fully insured for medically necessary services. Our purpose was to elucidate the effects of socioeconomic status on 1-year mortality in patients with acute myocardial infarction and to evaluate whether revascularization modified this relationship.

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Methods 

Data Source and Patient Selection 

The Alberta Ministry of Health and Wellness maintains a comprehensive electronic data warehouse,¹³ which includes data on ambulatory care (Ambulatory Care Classification System [ACCS]), hospital discharges, physician billings, and health insurance registration status (Alberta Health Care Insurance Plan [AHCIP]), as well as inter-hospital transfer, exit (out-of-province migration), and mortality status.

We studied all residents of Alberta, Canada aged ≥18 years presenting to an acute care hospital emergency department with an “initial episode of care” for acute myocardial infarction (based on the fifth digit of ‘1’ for the International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9-CM] code for acute myocardial infarction, 410.x1) as the most responsible diagnosis between April 1, 1998 and March 31, 2001. We excluded from index emergency department presentations any “subsequent” or “other” episode of care <8 weeks following the initial episode for further observation, evaluation, or treatment.

Baseline Characteristics 

To generate a more complete patient risk profile we considered co-morbidities such as diabetes and hypertension to be present if they were coded within 1 year before the index emergency department visit in either the ACCS database or in a physician billing record, or within 5 years in a hospital discharge record.

Socioeconomic Status 

We followed a common practice of using the neighborhood median household income as a proxy for socioeconomic status.², ⁴, ⁶, ⁷, ⁸ Patients’ income was obtained by linking the above administrative databases with 2001 census data available from Statistics Canada.¹⁴ There were 4305 dissemination areas (DA) corresponding to the 6-digit postal code and 138 forward sortation areas (FSA) corresponding to the 3-digit postal code in our database. For ease of presentation, median household income was grouped into quartiles, Q1-Q4, as follows: ≤$38,796, $38,797-49,347, $49,348-62,839, and ≥$62,840, respectively. To calculate the rate of emergency department presentations by income quartile, we also estimated the total number of AHCIP registrants whose neighborhood household incomes were in each of these categories. We also classified patients who resided in the 2 health regions of Edmonton and Calgary (where tertiary health care facilities exist) as metropolitan and all the rest as nonmetropolitan.

Hospital Characteristics 

Of the 92 hospitals in our database, 3 were identified as interventional (cardiac catheterization-capable) and 89 as noninterventional. The average annual volume of cardiac-related emergency department visits (including acute myocardial infarction, unstable angina, stable angina, cardiac arrest, sudden death, and chest pain) during the study period was categorized into quartiles: very low, ≤14,887; low, 14,888 to 26,601; medium, 26,602 to 52,192; and high, >52,192. The 2 hospitals with cardiac surgical facilities were high-volume, and the PCI-only facility was medium-volume.

Outcomes 

The AHCIP registry contains termination of insurance information as a result of death and was used to track the 1-year all-cause mortality status. Eleven patients moved away from Alberta within 1 year of their emergency department presentation, and hence their 1-year mortality status was unknown. The registry information is routinely validated by Alberta Health and Wellness using data from vital statistics.

Statistical Analysis 

Descriptive data were summarized as percentages for categorical variables and medians with inter-quartile ranges for continuous variables. The chi-square and Kruskal-Wallis tests were used, as appropriate, to test group differences. In addition, a series of multiple logistic and Cox regression models were constructed to predict 1-year revascularization and mortality and to assess how the impact of socioeconomic status has been influenced by other independent baseline and process-of-care variables. Socioeconomic status and age were treated as continuous variables in these regression models with the impact of socioeconomic status assessed in $10,000 increments and that of age in 10-year increments. All tests were 2-sided and significance level was set at 5%. The SPSS statistical software package (version 13.0; SPSS Inc, Chicago, Ill) was used to perform all analyses.

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Results 

Baseline Characteristics 

Our study population consisted of 5622 patients aged 18 years and older who presented to any hospital emergency department in Alberta with an initial episode of acute myocardial infarction between April 1, 1998 and March 31, 2001. They were grouped, for descriptive analyses, into quartiles of socioeconomic status (Table 1) and subsequent statistical comparisons were conducted with the highest quartiles as the referent. Relative to the number of registrants in these income quartiles, the overall rate of initial acute myocardial infarction emergency department presentation was 182 per 100,000 registrants: 243 for the lowest and 139 for the highest socioeconomic status quartile (relative risk = 1.75; 95% confidence interval [CI], 1.42-2.16). Patients in the lowest income quartile were more likely to be older, female, and to have co-morbidities, such as diabetes mellitus, hyperlipidemia, and peripheral vascular disease. Conversely, they were less likely to have an anterior myocardial infarction on the index emergency department visit and to reside in a metropolitan region (Table 1).

Table 1. The Characteristics of Acute Myocardial Infarction Patients who Presented to the Emergency Department in Alberta by Neighborhood Income Quartile (Q1 = Lowest Income Quartile)

Variable	Q1	Q2	Q3	Q4	P
n	1435	1393	1339	1445
Female sex, %	37.4	33.5	30.0	25.8	<.01
Age
Median years	70	67	66	62	<.01
(IQR), years	(58-79)	(56-76)	(54-75)	(52-72)
≥65 years, %	61.0	53.8	52.1	42.7	<.01
Metro residency, %	47.0	46.5	56.5	67.6	<.01
Diabetes, %	24.7	21.1	22.2	18.6	.01
Hypertension, %	56.0	53.1	53.2	52.6	.27
Hyperlipidemia, %	36.5	33.7	35.5	29.6	<.01
Malignancy, %	9.4	9.3	10.1	7.8	.21
Previous myocardial infarction, %	26.3	27.0	27.2	29.4	.26
Anterior myocardial infarction %	16.0	17.2	16.1	19.8	.02
Peripheral vascular disease, %	21.0	18.8	18.3	14.0	<.01

IQR = inter-quartile range.

Differences in Care Processes 

Lower socioeconomic status patients were less likely to visit an emergency department at a high-volume, metropolitan, tertiary care hospital and to be treated aggressively, as indicated by the lower rates of invasive diagnostic procedures and revascularization at 1 year (Table 2, Table 3). Of note, patients with lower socioeconomic status had a significantly lower rate of immediate hospitalization following emergency department care. However, the rate remained constant at 93%-94% (P = .60) across the income quartiles among those who were discharged alive from the emergency department. Figures 1A-1C depict overall and sex-specific PCI and CABG rates at 1 year by socioeconomic status group, showing that lower socioeconomic status groups had lower PCI but similar CABG rates, and that both PCI and CABG rates were lower for women than for men.

Table 2. Hospital Characteristics (Q1 = Lowest Income Quartile)

Variable	Q1	Q2	Q3	Q4	P
n	1435	1393	1339	1445
Hospital type
Invasive, %	33.9	26.4	30.0	35.4	<.01
Emergency department volume (visits)*
V. low (≤14,887), %	38.5	37.8	28.4	15.2	<.01
Low (14,888-26,601), %	24.5	29.7	33.2	38.1	<.01
Medium (26,602-52,192), %	17.9	15.4	16.1	16.6	.32
High (>52,193), %	19.0	17.1	22.3	30.2	<.01
Immediate hospitalization, %	89.1	89.9	91.5	92.1	.02
Emergency department survivors only, %	93.4	93.5	94.3	93.1	.60

Table 3. Use of Invasive Procedures and Outcomes

Variable	Q1	Q2	Q3	Q4	P
n	1435	1393	1339	1445
Angiography
1st episode,⁎⁎ %	15.3	14.6	14.9	17.3	.19
At 1 year, %	48.9	52.2	59.1	64.6	<.01
PCI
1st episode,⁎⁎ %	12.4	13.8	17.5	21.8	<.01
At 1 year, %	25.9	27.9	34.1	36.8	<.01
CABG
1st episode,⁎⁎ %	1.7	1.4	2.2	3.1	<.01
At 1 year, %	9.4	9.3	9.9	10.8	.51
Revascularization
1st episode,⁎ %	13.9	15.1	19.5	24.7	<.01
At 1 year, %	34.0	36.5	42.8	46.0	<.01
Died after emergency department presentation
In emergency department, %	4.6	3.8	3.0	1.0	<.01
At 30 days, %	13.7	11.4	8.3	5.5	<.01
Hospitalized	10.0	8.5	5.6	4.7	<.01
Not hospitalized	2.2	0.0	2.7	2.0	.55
At 1 year, %	20.1	16.2	13.1	9.5	<.01
Hospitalized	17.0	13.4	10.4	8.7	<.01
Not hospitalized	5.5	4.7	11.0	7.0	.42

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• Figure 1. 

  (A) Overall rates of PCI and CABG at 1 year by quartile of neighborhood income where Q1 = lowest and Q5 = highest. The sample sizes are indicated in Table 1. (B) Rates of PCI and CABG at 1 year by quartile of neighborhood income for men. (C) Rates of PCI and CABG at 1 year by quartile of neighborhood income for women.

Socioeconomic Status and 1-Year Revascularization 

As confirmed by logistic regression models in Table 4, socioeconomic status by itself was a highly significant predictor of 1-year revascularization (Model 1) and remained so after adjusting for baseline patient factors (Model 2): an increase in the odds ratio (OR) and 95% CI of 6% (4%-9%) corresponded with each $10,000 increase in the area income. However, socioeconomic status was no longer significant (P = .10) after further adjustment for the emergency department volume (an institutional factor), which was a powerful predictor of 1-year revascularization and accounted for much of the effects of socioeconomic status (Model 3).

Table 4. Logistic Regression for 1-Year Revascularization

Variable	χ2	OR	95% CI	P
Model 1. Socioeconomic status only
Socioeconomic status (per $10,000)	47	1.09	1.06-1.12	<.01
Model 2. Socioeconomic status + baseline patient factors*
Socioeconomic status (per $10,000)	24	1.06	1.04-1.09	<.01
Model 3. Socioeconomic status + baseline patient and hospital factors
Socioeconomic status (per $10,000)	3	1.02	1.00-1.05	.10

Socioeconomic Status and 1-Year Mortality 

Mortality rates by income quartile are presented in Table 3, showing an inverse relationship between socioeconomic status and mortality in the emergency department, at 30 days, and at 1 year. This relationship was maintained among patients who were immediately hospitalized (P <.01). Compared with those who were immediately hospitalized upon discharge from the emergency department, those who were not immediately hospitalized had substantially lower mortality rates. These rates did not vary significantly by socioeconomic status (P = .55 and .42 at 30 days and 1 year, respectively).

The impact of socioeconomic status on 1-year mortality was further assessed by Cox regression models (Table 5). Socioeconomic status alone was highly significant, as shown by a relative reduction in 1-year mortality per $10,000 increase in area income of 13% (Model 1), which was attenuated to 6% but remained highly significant (P <.01) after adjustment for age and the presence of comorbidities such as hyperlipidemia and diabetes mellitus (Model 2), as well as further adjustment for hospital emergency department volume and 1-year revascularization (Models 3). Furthermore, there was a significant interaction between socioeconomic status and revascularization (P = .03), thereby confirming that the effect of socioeconomic status was largely confined to nonrevascularized patients (Figure 2).

Table 5. Cox Regression for 1-Year Mortality

Variable	Wald	HR	95% CI	P
Model 1. Socioeconomic status only
Socioeconomic status (per $10,000)	59	0.87	0.83-0.90	<.01
Model 2. Socioeconomic status + baseline patient factors⁎
Socioeconomic status (per $10,000)	11	0.94	0.91-0.98	<.01
Model 3. Socioeconomic status + baseline patient factors + revascularization⁎⁎
Socioeconomic status (per $10,000)	9	0.94	0.91-0.98	<.01

OR = odds ratio; CI = confidence interval; HR = hazards ratio.

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• Figure 2. 

  1-year mortality rate by socioeconomic status and revascularization status at 1 year. The sample sizes from Q1 to Q4 are: 1176, 1137, 967, and 991, respectively, for the not revascularized; and 659, 650, 752, and 597, respectively, for the revascularized.

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Discussion 

In this population-based study of acute myocardial infarction patients presenting to the acute care emergency department in Alberta, Canada, we found that, despite universal health insurance coverage, socioeconomic status as measured by median neighborhood household income had profound effects on the rate of emergency department presentation, access to invasive coronary procedures, and mortality outcomes. By further examining the interactions with revascularization, we provide novel insights that the effects of socioeconomic status on mortality were modulated by revascularization.

Effects of Socioeconomic Status on Acute Myocardial Infarction Incidence and Care 

Our finding that low socioeconomic status is associated with a greater rate of emergency department visits following the onset of acute myocardial infarction is not surprising and may be inferred from the well-known inverse relationship between socioeconomic status and the risk of acute myocardial infarction.¹, ¹⁵, ¹⁶ However, there is a paucity of such population-based data, presumably because system-wide ambulatory care databases such as those in Alberta and the Veterans Administration Healthcare System in the US¹⁷ have not been well developed. As a result, most population-based studies relied on hospital discharge data, and yet, 7%-10% of patients (or 5%-7% of the emergency department survivors) in the high to low income quartiles who presented to the emergency department in our study were not immediately hospitalized. By choosing a more proximal point of care, our emergency department-based study is more comprehensive and representative of the acute myocardial infarction population than most other inpatient-based studies. This study, therefore, extends the results of the conventional, inpatient-based studies to examine all patients regardless of whether they were admitted or not from the emergency department.

The reason that lower socioeconomic status patients were less likely to be immediately hospitalized, however, is that they were more likely to die in the emergency department, because the rate of hospitalization was similar (93%-94%, P = .60) across the income quartiles among those who were discharged alive from the emergency department and not immediately hospitalized. The higher emergency department mortality rate among patients with lower socioeconomic status, on the other hand, is consistent with our data that low socioeconomic status was associated with advanced age, female sex, nonmetropolitan residency, and more frequent co-morbidities at emergency department presentation. Despite their higher risk profiles, we showed that low socioeconomic status patients were less likely to be treated aggressively (eg, catheterization and PCI procedures), as shown in numerous other studies.², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰ We further showed that socioeconomic status ceased to be a significant predictor of revascularization after adjustment for hospital emergency department volume (Table 4). This implies that less aggressive treatment of patients with lower socioeconomic status was related to their nonmetropolitan residency, because they were less likely to be treated in metropolitan, tertiary care hospitals with on-site interventional facilities.³, ⁴, ⁵, ⁷, ¹⁸

Effects of Socioeconomic Status on 1-Year Mortality 

Consistent with other studies that examined the relationship between socioeconomic status and acute myocardial infarction mortality in the community and after hospital discharge,², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰, ¹¹, ¹², ¹⁵, ¹⁶, ¹⁹, ²⁰ our study of acute myocardial infarction patients who presented to the emergency department also showed that low socioeconomic status is associated with higher mortality within 30 days and 1 year before and after adjustment for baseline factors and revascularization. In addition, we further demonstrated that this inverse relationship persisted only among patients who were immediately hospitalized. Among patients who survived the emergency department phase but were not immediately hospitalized, the 1-year mortality rates, which did not vary by socioeconomic status, tended to be lower than those who were hospitalized. Although failure to properly diagnose and treat acute myocardial infarction patients in the emergency department remain a serious health care issue, our data substantiate the contention that such occurrence may be relatively infrequent.²¹, ²²

We found, moreover, that not only were socioeconomic status and revascularization independent predictors of 1-year mortality, but the impact of socioeconomic status on mortality was also largely confined to nonrevascularized patients. This heretofore unreported finding that revascularization leveled the socioeconomic status playing field likely relates to the effectiveness of revascularization irrespective of socioeconomic status. Because low socioeconomic status patients are more likely to be female and older, less likely to be revascularized, and tend to have worse outcomes, it follows that the unrealized benefits of revascularization may be greater for low socioeconomic status patients. Hence, a targeted approach to improve quality of care of these patients seems warranted.¹⁸, ²³ Such an approach may include greater adherence to evidence-based medicine²⁴ and speedier transfer of patients from community to tertiary care centers for early PCI.²⁵, ²⁶, ²⁷

Some limitations of our study exist. We based our socioeconomic status indicator on the median household income by dissemination area (DA) (previously called enumeration area), supplemented by the data for the forward sortation area (FSA) for missing DA-level income data. Because each FSA consisted of about 20,000 households as compared with about 300 households for each DA, our DA-derived socioeconomic status indicator is less prone to misclassification of personal income than commonly used, FSA-derived indicators in most area-based socioeconomic status studies.²⁸ Based on previous research, it is likely that Canadian hospital discharge abstracts in general, and acute myocardial infarction coding in particular, are reasonably accurate and of high quality.³, ⁶, ¹⁹ Also, some key factors are unavailable in our administrative database, including admission electrocardiogram, angiogram results, education level, functional status, quality of life, and physician characteristics. Finally, we limited our study only to 1-year mortality outcomes without considering other important time points and indicators, such as complications and re-hospitalization.

In summary, socioeconomic status markedly affects the rate of emergency department presentation, access to invasive procedures, and clinical outcomes after acute myocardial infarction. However, there are complex relationships among socioeconomic status, procedure use, and other patient-related and non-patient-related factors, some of which (eg, the use of revascularization) are modifiable. Given these complex, interrelated, and under-researched relationships, additional studies are required to assess and improve equity and quality of cardiac care.

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Appendix 1. 

ICD-9-CM Codes for Key Diagnoses and Procedures

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