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First myocardial infarction in patients of Indian subcontinent and European origin: comparison of risk factors, management, and long term outcome

BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7081.639 (Published 01 March 1997) Cite this as: BMJ 1997;314:639
  1. N Shaukat, British Heart Foundation research fellowa,
  2. J Lear, senior house officer in medicinea,
  3. A Lowy, lecturer in epidemiologya,
  4. S Fletcher, research assistanta,
  5. D P de Bono, professor of cardiologya,
  6. K L Woods, professor of therapeuticsa
  1. aDepartment of Medicine and Therapeutics and Public Health, University of Leicester, Leicester
  1. Correspondence to: Dr N Shaukat Department of Cardiology, Kettering General Hospital NHS Trust, Rothwell Road, Kettering NN16 8UZ.
  • Accepted 29 November 1996

Abstract

Objectives: To compare long term outcome after first myocardial infarction among British patients originating from the Indian subcontinent and from Europe.

Design: Matched pairs study.

Setting: Coronary care unit in central Leicester.

Subjects: 238 pairs of patients admitted during 1987-93 matched for age (within 2 years), sex, date of admission (within 3 months), type of infarction (Q/non-Q), and site of infarction.

Main outcome measures: Incidence of angina, reinfarction, or death during follow up of 1-7 years.

Results: Patients of Indian subcontinent origin had a higher prevalence of diabetes (35%v 9% in patients of European origin, P<0.001), lower prevalence of smoking (39% v 63%, P<0.001), longer median delay from symptom onset to admission (5 hours v 3 hours, P<0.01), and lower use of thrombolysis (50% v 66%, P<0.001). During long term follow up (median 39 months), mortality was higher in patients of Indian subcontinent origin (unadjusted hazard ratio=2.1, 95% confidence interval 1.3 to 3.4, P=0.002). After adjustment for smoking, history of diabetes, and thrombolysis the estimated hazard ratio fell slightly to 2.0 (1.1 to 3.6, P=0.02). Patients of Indian subcontinent origin had almost twice the incidence of angina (54% v 29%; P<0.001) and almost three times the risk of reinfarction during follow up (34% v 12.5% at 3 years, P<0.001). The unadjusted hazard ratio for reinfarction in patients of Indian subcontinent origin was 2.8 (1.8 to 4.4, P<0.001). Adjustment for smoking, history of diabetes, and thrombolysis made little difference to the hazard ratio. Coronary angiography was performed with similar frequency in the two groups; triple vessel disease was the commonest finding in patients of Indian subcontinent origin and single vessel disease the commonest in Europeans (P<0.001).

Conclusions: Patients of Indian subcontinent origin are at substantially higher risk of mortality and of further coronary events than Europeans after first myocardial infarction. This is probably due to their higher prevalence of diffuse coronary atheroma. Their need for investigation with a view to coronary revascularisation is therefore greater. History of diabetes is an inadequate surrogate for ethnic origin as a prognostic indicator.

Key messages

  • The cumulative incidence of reinfarction after a first myocardial infarct is threefold higher in patients of Indian subcontinent origin than in European patients

  • Patients of Indian subcontinent origin have poorer survival after first myocardial infarction

  • The commonest coronary angiographic finding in patients of Indian subcontinent origin is triple vessel disease

  • A history of known diabetes is not in itself an adequate marker of adverse prognosis in this ethnic group

  • After myocardial infarction, patients of Indian subcontinent origin have a high risk of recurrent myocardial ischaemia and an increased likelihood of needing coronary revascularisation

Introduction

Migrants to the United Kingdom from the Indian subcontinent have a higher mortality from coronary heart disease than the indigenous population.1 In Britain there are 1.6 million people of Indian subcontinent origin, and the effect of demographic change is likely to amplify the public health impact of this increased risk of coronary heart disease during the next two decades.2 The greater incidence of acute myocardial infarction is associated with high prevalence of diabetes mellitus,3 4 and patients of Indian subcontinent origin with coronary heart disease are more likely than Europeans to have diffuse coronary atheroma at angiography.5 6 7

Relatively little is known about the clinical course of myocardial infarction in patients of Indian subcontinent origin, though a greater prevalence of triple vessel disease suggests that there might be a higher risk of recurrent coronary events. Two studies have reported that clinical outcome and long term survival were not significantly different in patients of Indian and European origin5 8; a third study has found increased early mortality after first myocardial infarction in patients of Indian origin.9 We compared clinical characteristics and outcome in a large consecutive series of patients of Indian subcontinent origin and matched patients of European origin who were followed up for one to seven years after first myocardial infarction.

Many migrants of South Asian origin, a large proportion having come from East Africa, settled in Leicester from the early 1970s onwards. They now form 23% of the population of the city. The largest ethnic subgroup is Gujarati speaking. The migrant group is substantially younger than the indigenous population,2 and its distinctive epidemiological characteristics (notably a higher prevalence of non-insulin dependent diabetes mellitus and ischaemic heart disease) have been reported elsewhere.10

Methods

Patients–For all patients admitted to the coronary care unit of Leicester Royal Infirmary, a standard dataset is collected and entered on a computer database. From this we identified all patients of Indian subcontinent origin admitted with confirmed acute myocardial infarction between January 1987 and August 1993, by surname analysis.11 The diagnosis required at least two of three factors: (i) a typical history of cardiac pain not relieved by rest or nitrates; (ii) evolving electrocardiographic changes compatible with the diagnosis; (iii) a rise in serum cardiac enzymes to greater than twice the upper limit of the laboratory reference range. For each patient identified, a patient of European origin meeting the same diagnostic criteria was matched by age (within two years), date of admission (within three months), sex, and type (Q/non Q) and site of infarction. It was not possible to match six patients, and a further four were lost to follow up. The analysis includes 241 matched pairs (200 men and 41 women).

Data–The standard clinical data set was collected prospectively, validated by a single investigator before computer entry, and not subsequently modified. It comprised details of clinical presentation, medical history and coronary risk factors, treatment, complications, and acute outcome. To this was subsequently added information on events after discharge including reinfarction and cardiac investigations (exercise stress testing, coronary angiography, coronary angioplasty, and coronary artery bypass grafting). Data sources included general practitioner records, hospital notes, stress test registers, and catheter laboratory logs at the adjacent cardiothoracic centre which provides all invasive cardiology services for the district. Angiograms were coded (blind to ethnic origin) as one, two, or three vessel coronary disease. Reinfarction was accepted to have occurred only on the evidence of a diagnosis at discharge from hospital. Deaths were ascertained from the NHS central register.

Statistical analysis–Categorical variables were analysed with McNemar's test for matched data or by Embedded Image2 tests where the matched design was broken. Distribution of survival times was compared by using Kaplan-Meier plots and tested by the generalised Savage (Mantel-Cox) method. A Cox proportional hazards model was fitted in which potential confounding variables were fitted as covariates. Matching was taken account of by fitting a stratified model.12

Results

Clinical characteristics and inpatient course

Route of admission was similar in both groups: 77 (32%) of north European patients and 68 (28%) of patients of Indian subcontinent origin were admitted directly from a general practitioner referral; the remaining patients in both groups were admitted from the casualty department. Patients of Indian subcontinent origin had the expected lower prevalence of smoking and higher prevalence of diabetes (table 1).4 9 As noted in an earlier study, their delay from onset of symptoms was significantly longer, the diagnosis of myocardial infarction was less commonly made at presentation, and consequently the use of thrombolysis was lower in this group.13 In addition to delay the other main reason for the disparity in thrombolysis rates was a greater rate of missed diagnosis in patients of Indian subcontinent origin (34 % v 19%, P=0.001). Contraindication to thrombolysis was higher in the north European group (21% v 7%, P=0.001). Highest recorded serum creatine kinase concentration was slightly higher in patients of Indian subcontinent origin (median 1789 v 1523 IU/l, P=0.09). However, the incidence of early complications (left ventricular failure, hypotension, or tachyarrhythmia) did not differ significantly.

Table 1

Clinical characteristics, management, and coronary angiography in patients of Indian subcontinent and of northern European origin. Values are numbers (percentages) unless specified otherwise

View this table:

Mortality and morbidity during long term follow up

Diagnosis of angina during follow up was more common in patients of Indian subcontinent origin than in Europeans (54% v 29%, P<0.001), as was rate of reinfarction (34% v 12.5% at 3 years, P<0.0001) (fig 1). The increased tendency to reinfarction in patients of Indian subcontinent origin may be partly or fully attributable to ethnic differences in the risk factor profile. To take account of this, a Cox proportional hazards model was fitted in which variables that were different in the two groups (family history of coronary heart disease under the age of 50 years, use of thrombolytic drugs, maximum blood glucose concentration in hospital, smoking, and known diabetes) were fitted as covariates. Matching was taken account of by fitting a stratified model. The unadjusted hazard ratio for reinfarction in patients of Indian subcontinent origin was 2.76 (95% confidence interval 1.75 to 4.36, P<0.001). Addition of these variables made little difference to the estimated hazard ratio (adjusted hazard ratio=3.0, 1.7 to 5.3, P<0.001). The observed ethnic difference in risk could therefore not be explained by these risk factors. The proportional hazards assumption was checked and found to be valid.

Fig 1
Fig 1

Reinfarction after first myocardial infarction in patients of Indian subcontinent and of European origin

Survival after first myocardial infarction was also poorer in patients of Indian subcontinent origin (fig 2). The unadjusted hazard ratio was 2.12 (1.33 to 3.37, P=0.002); after adjustment for the same variables used in the reinfarction model, the estimated hazard ratio fell slightly to 2.0 (1.1 to 3.6, P=0.025).

Fig 2
Fig 2

Survival after first myocardial infarction in patients of Indian subcontinent and of European origin

Diagnostic procedures and revascularisation

The two ethnic groups did not differ significantly in the use of exercise stress testing, coronary angioplasty, or coronary artery bypass grafting (table 1). Rates of positive results on stress testing were similar in the two ethnic groups (62% in patients of Indian subcontinent origin and 51% in patients of European origin). The distribution of coronary angiography over time after first infarction showed no difference in the use of this procedure in the first six months, but subsequently it tended to be used less in patients of Indian subcontinent origin despite their higher reinfarction rate. At angiography (table 1), triple vessel disease was strikingly more common in patients of Indian subcontinent origin, whereas single vessel disease was the most likely finding among Europeans (51% v 41%; P<0.0001).

Discussion

These data provide the largest and longest age controlled comparative analysis of the outcome of myocardial infarction in patients of Indian subcontinent origin so far published. Patients of Indian subcontinent origin presented later and gave rise to more diagnostic difficulty than European patients. In consequence, thrombolytic treatment was less likely to be used.

Ethnic differences in mortality and reinfarction

Although the most recent study found a significant increase in early mortality in patients of Indian subcontinent origin after myocardial infarction,9 two previous studies found no such ethnic difference in mortality or in reinfarction. The first study included 77 white patients and 54 patients of Indian subcontinent origin, the latter predominantly Gujaratis (as in the present study).5 No age adjustment was made in the analysis, although the patients of Indian subcontinent origin were considerably younger. Coronary angiography was performed within three months of infarction in 62 European patients and 41 patients of Indian subcontinent origin. European patients had a significantly lower prevalence of triple vessel disease and significantly lower atheroma scores in vessels not associated with the infarct territory. Of the 113 patients followed up at one year, 71% of European patients and 58% of patients of Indian subcontinent origin were alive and had had no further cardiac events (difference not significant).

The second study analysed 102 matched pairs of patients of Indian subcontinent and European origin admitted to Birmingham hospitals with first myocardial infarction.8 Matching factors were age, sex, time, and season of infarction. The proportions in each ethnic group surviving to one, two, three, and four years were not significantly different (though there was a trend to lower survival in patients of Indian subcontinent origin). The smaller sample size and less efficient statistical analysis of survival (contrasting simple proportions rather than distributions of survival times), both reducing statistical power, may account for the different conclusions drawn in this study and in ours. The fact that this longer term study found a substantial ethnic effect raises the possibility that the risk associated with Indian subcontinent origin is a late effect. However, a late effect was not apparent from the survival curves, which start to diverge immediately after infarction. This, and the fact that the hazard ratio associated with ethnic origin seems to be constant, suggests an immediate and sustained effect on risk of reinfarction.

Diabetes and coronary angiography

It is well recognised that patients of Indian subcontinent origin with myocardial infarction have a greater prevalence of diabetes.4 5 14 15 Diabetic patients in general tend to have more diffuse coronary artery disease than non-diabetic patients and a higher mortality after infarction. Wilkinson et al found that controlling for a history of diabetes removed much of the excess early mortality risk in patients of Indian subcontinent origin.9 This was not our finding: controlling for diabetes had little effect on the hazard ratio for both mortality and reinfarction. Hughes et al found no difference in atheroma score in the coronary arteriograms of patients of Indian subcontinent origin who were normoglycaemic and of those who had impaired glucose tolerance or diabetes (6.2 (SD 4.9) v 5.7 (3.9), P>0.8).5 The practical implication is that patients of Indian subcontinent origin with first myocardial infarction, regardless of a history of diabetes, are more likely than European patients to have triple vessel disease. This should be considered when deciding on the need for coronary angiography.

The use of coronary angiography was slightly (though not significantly) lower among patients of Indian subcontinent origin than European patients in our study, whereas the angiographic findings and prognostic difference indicate that the reverse should have been the case. The reasons are unclear. It has previously been reported that referral and investigation for angina are relatively delayed in patients of Indian subcontinent origin despite more extensive coronary atheroma at presentation.6 13

Implications

The age distribution of the population with Indian subcontinent origin in the United Kingdom is younger than that of the indigenous population. Demographic shift alone will increase the public health impact of the increased incidence of coronary heart disease in the former group in the coming decades.2. In the absence of any proved primary preventive strategy to mitigate the influence of ethnic origin on risk, it is particularly important that optimum secondary preventive care is achieved. Revascularisation by coronary artery bypass grafting improves survival among patients with triple vessel disease,16 and therefore accurate identification of such patients should be given a high priority.

Acknowledgements

We thank the staff of the Office of Population, Censuses, and Surveys (now the Office for National Statistics) for providing mortality data.

Footnotes

  • Funding NS and D de B are supported by the British Heart Foundation.

  • Conflict of interest None.

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