Abstract

Introduction Coronary heart disease is a leading cause of death in New Zealand and around the world. The causes of coronary artery disease are complex and may vary amongst different populations.

Methods A case-control study was conducted to determine the effects of smoking, hypertension, body size and age on risk of myocardial infarction. 621 New Zealand men aged between 25 and 65 were included in the study; 136 cases and 484 controls. Crude and adjusted odds rations were calculated.

Results Smoking (odds ratio 3.41, 95% CI 2.25 – 5.16), obesity (2.30, 95% CI 1.17-4.50) and hypertension (1.92, 95% CI 1.07 – 3.45) were significantly and independently associated with an increased risk of myocardial infarction.

Discussion The major risk factors for myocardial infarction are modifiable. It is important that public health initiatives focus in improving these factors in order at improved outcomes for myocardial infarction.

Introduction

Coronary artery disease is one of the leading causes of death worldwide, accounting for 16.3% of all deaths in high income countries (World Health Organization, 2008). In New Zealand, cardiovascular disease is the main cause of preventable deaths (Ministry of Health, 2008). Death usually occurs as the result of a heart attack, or myocardial infarction. 

The causes of coronary heart disease are complex. Known risk factors include older age; cigarette smoking; a history of hypertension; diabetes; cholesterol ratios; obesity; dietary patterns; psychosocial factors such as stress; and lack of physical activity (Wilson et al., 1998; Yusuf et al., 2004). Most of these risk factors can be modified and coronary heart disease is largely preventable (Yusuf et al., 2004). Men are more likely than women to experience myocardial infarction (Wilson et al., 1998). The purpose of this study was to investigate the effects of age, smoking, hypertension and body size on the risk of developing coronary artery disease in a population of New Zealand men using a case control study design.

Methods

Study participants were New Zealand men aged 25 to 65. There were 621 participants; 136 cases and 485 controls. Controls were selected from the electoral roll while cases were men who had a myocardial infarction. Variables measured were age, smoking status, body size and hypertension. Age was recorded as a continuous variable. Body size was indicated using body mass index (BMI) and recorded as a continuous variable. Smoking was defined as whether the participant was a current smoker. Hypertension was defined as having a past history of hypertension. Cases and controls were compared to ensure broad similarity between groups.

Statistical analysis was undertaken using the Epiinfo statistical program (version 3.5.3). Age was re-coded into four categories; 25 to 39, 40 to 49, 50 to 59 and 60 to 65. BMI was re-coded into three categories; normal (BMI<25); overweight (BMI of 25-30) and obese (BMI>30). These cut off points are commonly used to indicate whether someone is overweight or obese.

Crude odds ratios were calculated for each variable. Logistic regression was used to control for confounding. All variables were included in the analysis as all are risk factors for myocardial infarction and may be confounders. Dummy variables were created for age group and body size. Effect modification of smoking and hypertension was investigated as a history of smoking could plausibly result in higher blood pressure.  An interaction variable was created for smoking and hypertension using logistic regression.

Results

The characteristics of the study participants are shown in Table 1. Cases and controls were broadly similar in terms of age. 46% of cases were smokers compared to 21% of controls. 13% of cases were obese compared to 6% of controls. 17% of cases had a history of hypertension compared to 9% of controls. The groups were similar on all other variables.

Crude and adjusted odds ratios are shown in table 2. None of the variables include in the logistic regression model were found to be confounders as none of the variables showed a difference of greater than 10% between the crude and adjusted odds ratios. After adjusting for confounding, there were no statistically significant differences between age groups in the risk of developing myocardial infarction. Adjusted odds ratios (OR) and 95% confidence intervals for each age group compared to the 25-39 age group are as follows: 40-49 (OR=1.78, 95% CI 0.82-3.88), 50-59 (OR=1.30, 95% CI 0.60-2.84), 60-65 (OR=2.04, 95% CI 0.90-4.60).  Smoking was the strongest predictor of risk of myocardial infarction, followed by obesity, followed by hypertension. The independent effect of smoking showed smokers were almost 3.5 times more likely to develop myocardial infarction than non-smokers (OR=3.41 95% CI 2.25 – 5.16). The independent effect of obesity showed that those who were obese were more than twice as likely as those of normal body size to develop myocardial infarction (OR=2.30, 95% CI 1.17-4.50). Being overweight did not increase the risk of myocardial infarction compared to those of normal body size (Adjusted OR=1.44, 95% CI 0.96-2.16).  The independent effect of hypertension showed that those with a history of hypertension were almost twice as likely as those without a history of hypertension to develop myocardial infarction (OR=1.92, 95% CI 1.07 – 3.45). The test for interaction between smoking and hypertension showed no effect modification (p=0.846).

Discussion

This study shows that smoking increases the risk of myocardial infarction more than any other factor we measured. Obesity and a history hypertension also increased the risk of myocardial infarction significantly. These results support previous findings that smoking is the strongest predictor of myocardial infarction (Yusuf et al., 2004). The magnitude of the effects of obesity and hypertension is similar to effects found in other studies (Yusuf et al., 2004).  

An interesting result in this study was the finding that age was not associated with an increased risk of myocardial infarction. This could be because our study did not include men older than 65. It may be that age becomes an important risk factor for myocardial infarction later in life.

Some limitations of this study were that blood pressure was not measured directly meaning that those with undiagnosed hypertension would not have been detected and medication for hypertension was not taken into account. Smoking status only indicated whether a participant was a current smoker and did not take into account length of time as a smoker, past history of smoking, or number of cigarettes smoked. Differences in these factors may have resulted in differences in risk of myocardial infarction. Body size was measured using BMI, which, although it is a commonly used, is not as accurate at predicting risk of myocardial infarction as some other measures such as weight-hip ratio (Yusuf et al., 2004).

Some known risk factors were not included in the study, such as diabetes or cholesterol ratios, and this limits the ability of this model to predict risk of myocardial infarction. Cholesterol ratios are of particular importance as these have been shown to strongly predict the risk of myocardial infarction (Yusuf et al., 2004).

The results of this study are important because they reinforce the idea that the leading risk factors for myocardial infarction are potentially modifiable. Lifestyle changes such as quitting smoking, improving diet and physical activity and losing weight are all likely to result in lower risk of myocardial infarction. The implications for public health are that a focus on reducing smoking, improving diet, promoting physical activity and preventing and treating obesity is likely to be the most effective way to reduce the burden of coronary heart disease in New Zealand.

Any future studies should include diabetes and cholesterol ratios in their investigations. Further study is also required to investigate these risk factors in women. However, as many risk factors for myocardial infarction are now know, it is important to now consider the most effective ways to improve lifestyle factors that lead to coronary artery disease.

References

Ministry of Health. (2008). A Portrait of Health: Key results of the 2006/07 New Zealand Health Survey. Wellington: Ministry of Health.

Wilson, P. W. F., D'Agostino, R. B., Levy, D., Belanger, A. M., Silbershatz, H., & Kannel, W. B. (1998). Prediction of coronary heart disease using risk factor categories. Circulation, 97(18), 1837-1847.

World Health Organization. (2008). Fact sheet No 310: Top ten causes of death. Geneva: World Health Organization.

Yusuf, P. S., Hawken, S., Ôunpuu, S., Dans, T., Avezum, A., Lanas, F., et al. (2004). Effect of potentially modifiable risk factors associated with myocardial infarction in 52 countries (the INTERHEART study): Case-control study. Lancet, 364(9438), 937-952.