Heart Disease and Stroke in Canada 1997

6. Risk Factors for Cardiovascular Disease

6.1 Risk Factors for Ischemic Heart Disease

Considerable research has identified the major risk factors for ischemic heart disease³⁸. Some factors such as family history of premature ischemic heart disease, age, and male sex are "non-modifiable." Smoking, elevated blood cholesterol, high blood pressure, and physical inactivity are considered "modifiable" through individual behaviour change or treatment. Both diabetes and obesity are considered to be risk factors for cardiovascular disease. There are other risk factors, not yet well defined, which may contribute to ischemic heart disease ³⁹ (section 6.1.7).

As part of the Canadian Heart Health Initiative⁴⁰, the prevalence of risk factors in Canadian adults has been determined using a standardized methodology⁴¹. Survey data were obtained from 23,251 Canadians between the ages of 18 and 74 and the results from ten provinces have been reported^42,43.

For the major risk factors of smoking and high blood pressure, there is a definite east to west gradient, with eastern provinces demonstrating a higher prevalence than western provinces (Table 9). The exceptions are high blood pressure in Quebec and smoking in Ontario, which have the lowest respective rates for these risk factors. There is a similar but less striking trend for sedentary lifestyle and obesity, but relative uniformity of risk from elevated blood cholesterol.

6.1.1 Smoking

In the 1986-92 Canadian Heart Health Surveys ⁴³, the prevalence of smoking was highest in Atlantic Canada and Quebec , while Ontario and Saskatchewan had the lowest rates (Table 9). Percentages ranged from a high of 36% (of the 18-74 population) in Newfoundland to 23% in Ontario. Smoking rates also varied by age, with males ages 18-24 having the highest rates at 34%, while 31% of females ages 25-34 smoke regularly (Table 10); the national average of all age groups is 27%. There is little difference in the overall smoking rates of men and women ( 28% and 25% respectively). Approximately one-third of the population between the ages of 18-44 smoke regularly; the percentages in each age group then decline as the population ages (Table 10). As reported in the Survey on Smoking in Canada 1994, there have been no real changes in the overall prevalence of smoking since 1986 ⁴⁴.

The 1994 Youth Smoking Survey, Health Canada, confirms that younger people smoke more than older people, and more young people are starting to smoke. It is estimated that 29% of 15-19 year-olds are current smokers, and smoke an average of 13 cigarettes per day. As well, 14% of 10-14 year-olds are current smokers, smoking an average 10 cigarettes per day⁴⁵. Smoking among teens 15-19 years of age has increased by 25% since 1991 ⁴⁵. This increase has dire implications for the future health of Canadians, as it is estimated that 85% of smokers started smoking prior to the age 16; therefore, if this trend of more teens starting younger continues, we can then anticipate a greater number of adult smokers in the future. With theincrease in cigarette smoking among young girls and women, and the fact that smoking is one of the four major risk factors for heart disease, it can be anticipated that the mortality due to ischemic heart disease will increase and the difference between men's and women's mortality rates in the younger age-group will be reduced.

It is estimated that 41,408 deaths in Canada in 1991 were attributable to smoking⁴⁵, which is 21% of all deaths in Canada. Of these smoking-related deaths, acute myocardial infarction accounted for 25% of male deaths and 20% of female deaths. Smoking exacts both a personal and an economic toll. In 1991, the estimated economic costs of smoking to Canadian society totalled approximately $15 billion. This estimate included 4 million days in hospital, 3.3 million doctor visits, 1.4 million prescriptions, and 28 million days of absenteeism from work. As well, the 41,408 deaths translates into approximately $10.6 billion in lost future income⁴⁵.

6.1.2 Elevated Blood Cholesterol

The association between elevated blood cholesterol and ischemic heart disease has long been known. Results from long-term population studies such as the Framingham study ³⁸ have validated the association. Studies in other countries continue to illustrate this point. A study of 49,000 men and women in Sweden demonstrated both an increased mortality rate from coronary artery disease and cardiovascular disease, but also showed an increase in all-cause mortality at elevated cholesterol levels, even after adjustment for age, smoking, systolic blood pressure and body mass index⁴⁶ A 2% decrease in ischemic heart disease has been associated with a 1% lowering of low density lipoprotein cholesterol (LDL-C) in middle-aged men⁴⁷ Forty-five and forty-three percent (45% and 43%) of men and women respectively have elevated total plasma cholesterol (>5.2 mmol/L)^43,48 (Table 10). In men, there is a rapid increase in the prevalence of elevated total cholesterol after age 34 (Table 10), whereas in women, the dramatic increase comes a decade later after age 44, then exceeds that of men after the age of 55.

Elevated low density lipoprotein cholesterol (LDL-C) and decreased high density lipoprotein cholesterol (HDL-C), particularly in the presence of elevated triglycerides, are more precise indicators of ischemic heart disease risk than total blood cholesterol⁴⁹. Measurement of these lipid fractions may identify additional people at risk and at the same time prevent the entry into treatment of individuals who have elevated total cholesterol but low LDL-C and high HDL-C. Forty percent (40%) of men and 32% of women respectively have elevated LDL-C (>3.4mmol/ L) and 13% of men and 4% of women have depressed HDL-C levels (⁴⁸. While elevated LDL-C and low HDL-C are risk factors for ischemic heart disease in both men and women, a consistent association between high triglyceride levels and ischemic heart disease has been found only in women. In general, when HDL-C is taken into account, triglyceride levels do not improve the prediction of cardiovascular disease. However, in women with low level of HDL-C, high triglyceride levels have been associated with increased risk of ischemic heart disease^50-52. Recent findings from the Quebec Cardiovascular Study showed that other lipid fractions or associations may identify high risk subjects. This study, done in men, demonstrated that Apoprotein-B was the most important determinant of risk, small dense LDL-C increased the risk independently of Apoprotein-B, and the higher concentration of both of these lipid fractions increased the risk by 6-fold from baseline^53,54.

6.1.3 High Blood Pressure

High blood pressure is an independent risk factor for cardiovascular disease ^36,55 and can increase the risk by 2-3 fold⁵⁶. Twenty-five percent (25%) of Canadian men and 18% of Canadian women have high blood pressure (Table 10), defined as a systolic blood pressure>= 140mmHg or a diastolic blood pressure of>= 90 mmHg and/or are undergoing treatment^43,57.

At ages 18-24, 6% of men and 1% of women have high blood pressure (Table 10). From that age on in both men and women, there is a steady increase in the prevalence of high blood pressure. After the age of 64, the prevalence is approximately equal between men and women.

Twenty-six (26%) of individuals with high blood pressure are unaware of their condition. Of those aware, only 57% are treated and controlled; that is, they have a diastolic blood pressure =^43,57.

It is well known that hypertension predisposes to all cardiovascular disease outcomes: coronary artery disease, stroke, congestive heart failure, and peripheral artery disease. Hypertension tends to cluster with other atherogenic risk factors, including dyslipidemia, glucose intolerance, insulin resistance, obesity, and increased uric acid. Treatment of high blood pressure should be based on the multivariate risk profile⁵⁸. Data from the ongoing Framingham study has shown no change in hypertension prevalence in four decades⁵⁶. In hypertensive males and females 35-45% of acute myocardial infarctions are silent and/or unrecognized⁵⁹.

6.1.4 Physical Inactivity

Physical inactivity is recognized as a major risk factor for ischemic heart disease⁶⁰. The 1988 Campbell's Survey of the Well-Being of Canadians is part of a longitudinal study examining physical activity and health patterns of Canadians⁶¹. With age, there is a general decline in activity except for Canadians over 65 who are more active than those in the 45-64 age group. During the past 20 years, however, the Canadian population has become generally more active^61,-63.

The Canadian Heart Health Surveys show that a sedentary lifestyle is widely prevalent (Table 9); 38% of adult Canadians are inactive in their leisure time. Almost half of individuals aged 18-74 in Newfoundland have a sedentary lifestyle; over 40% of those in the other Maritime provinces also exhibit this risk factor. British Columbia has the lowest percentage (29%) of sedentary individuals.

The Heart and Stroke Foundation of Canada, in its position statement on physical activity, recommends that regular physical activity, when properly undertaken, can be effective in preventing and limiting the disabling effects of heart disease and stroke⁶⁴. Specific recommendations suggest that individuals of all ages should be active on a daily basis, and that individuals should incorporate fitness-enhancing activity into their normal routine. It is suggested that schools ensure that quality, daily physical and health education is provided. As well, it is recognized that physical activity is a vital component in maintaining good health, and that physical activity is often a beneficial component in cardiac and stroke rehabilitation programs.

The importance of physical inactivity as a cardiovascular risk factor has been addressed formally at several levels. The first ever U.S. Surgeon General's Report on Physical Activity was released in July, 1996, to coincide with the Olympic Games in Atlanta, Georgia⁶⁵. The report is an exhaustive review of the benefits of exercise, thorough reviews of studies assessing the risks and benefits of exercise, and numerous recommendations for both the individual and the community. A new position is that even moderate activity on a regular basis can increase health and well-being; physical activity need not be strenuous to achieve health benefits. Sedentary individuals have a 1.2-2 fold increased risk of more premature death than active individuals. Five large cohort studies demonstrated that low levels of physical activity increased the risk of ischemic heart disease mortality. They also demonstrated a dose-response relationship; benefit occurs at moderate levels, but an increased benefit occurs with increasing levels of physical activity. The report reviewed 36 studies since 1953, looking for a specific coronary heart disease benefit, which was found to be derived from the beneficial effect on serum cholesterol, body mass index, blood pressure, and diabetes.

Twenty studies on hypertension and exercise were reviewed. The most active individuals had a 30% decreased risk of developing hypertension. One randomized trial on primary prevention of hypertension, focusing on nutrition and physical activity, found that the incidence of hypertension in the study group was one-half that of the control group, even after adjustment for gender, age, and body mass index. The report examined 22 studies on the effect on exercise on known hypertensives: aerobic exercise (30-60 minutes 3-4 times/week) decreased both the systolic and diastolic blood pressure by 6-7 mmHg.

The Surgeon General's Report places a new emphasis on amount rather than intensity of physical activity, and demonstrates that health benefits from exercise are achievable for most. It recommends cardiorespiratory fitness supplemented with strength-developing exercise at least twice per week. It has also shown that the level of decreased risk of coronary heart disease attributable to regular physical activity is similar to other lifestyle factors.

6.1.5 Diabetes Mellitus

Numerous studies have shown the clustering of cardiovascular disease risk factors in diabetics, including obesity, hypertension, elevated blood cholesterol, and smoking ⁶⁶. Overall, 4% of Canadian men and 5% of women report having diabetes mellitus ⁴³. This prevalence ranges from 1% in the youngest (15-34 years) to 12% in the oldest (55-74 years) age groups among men and from 3% to 9% among women respectively⁶⁷. The true prevalence of diabetes in Canada may be double that of self-reported diabetes, based on evidence from North American studies that approximately 50% of adult with diabetes have not been diagnosed with the condition⁶⁸. The prevalence of diabetes varies among ethnic groups. The prevalence of self-reported diabetes among the aboriginal population age 15 and over is 6.5% with a prevalence rate as high as 22% among those over 65 years of age. About two-thirds of the aboriginal population with diagnosed diabetes are women⁶⁹.

6.1.6 Obesity

Obesity, especially abdominal obesity, is associated with an increased risk of ischemic heart disease^70,71. Thirty-one percent (31%) of Canadian adults are obese (body mass index (BMI)>= 27) (Table 9) with the prevalence being greater among men than women (Table 10). With age, the prevalence of obesity increases in both men and women. Yet, for men a decrease is observed following age 65 so that in the 65-74 age group the prevalence of obesity in men and women is equal.

With increasing obesity there is a rise in the prevalence of an abdominal distribution of fat as measured by waist-hip ratio. A waist-hip ratio of> 0.9 for men and> 0.8 for women is an indication of abdominal obesity. More men (50%) than women (34%) have abdominal obesity with the prevalence also increasing with age^43,67.

Obesity and abdominal fat distribution are both associated with an increased prevalence of diabetes, high blood pressure and elevated plasma cholesterol. The prevalence of high blood pressure, for example, is more than doubled among individuals with abdominal obesity⁶⁷. Therefore, it is desirable to maintain a healthy BMI in the range of 20-25 for both men and women⁷² . Data from the long-term Framingham study shows that the degree of overweight is proportional to the rate of development of cardiovascular disease. Over a follow up of 26 years, data showed that each standard deviation increase in weight was associated with a 15% and 22% increase in cardiovascular disease events in men and women respectively⁷³. Weight control is important for the control of other cardiovascular disease risk factors. The percent of hypertension that is attributable to obesity is estimated to be 78% in men and 65% in women⁵⁹.

6.1.7 Other Factors

Recent research suggests that a number of other factors may play a role in the development of cardiovascular disease. These factors include altered thrombogenic, inflammatory and immunologic responses, dietary iron, and psychosocial factors, including anger. Anger is associated with myocardial ischemia and arrhythmias, theoretically through activation of the sympathetic nervous system which leads to sinus tachycardia, hypertension, impaired myocardial perfusion and a high degree of cardiac electrical instability⁷⁴. Biological antioxidants such as beta carotene, ascorbic acid, vitamin E, and selenium may have a protective effect; Canada is presently directing a large international trial, called HOPE, involving 9,000 high risk men and women to determine whether or not vitamin E reduces cardiovascular mortality, myocardial infarction and stroke⁷⁵. A recent study suggests that smokeless tobacco use is related to cardiovascular disease, as well as increasing the risk of oral cancer ⁷⁶. Recently, hemostatic variables have been included in studies of cardiovascular risk. Most myocardial infarctions and cardiac deaths are precipitated by acute occluding coronary thrombi; as well, thrombosis participates in atherogenesis. In prospective studies, fibrinogen was found to be an independent predictor of myocardial infarction in both males and females and of stroke in males; it appears that the fibrinogen level provides information on risk profile over and above that supplied by established risk factors ⁷⁷. Ongoing studies are evaluating the role of elevated homocysteine and folic acid deficiency as risk factors for cardiovascular disease. Several studies have shown that total serum homocysteine is a strong and independent risk factor for stroke ⁷⁸. and that elevated homocysteine contributes to premature arteriosclerosis, arterial thrombotic events and venous thromboembolism⁷⁸ . Further work is required before recommendations can be made regarding these factors³⁹.

6.1.8 Multiple Risk Factors

The hallmark of cardiovascular disease risk is the synergistic effect of more than one risk factor on overall cardiovascular disease risk. Even moderate elevations in more than one risk factor increases cardiovascular disease risk⁷⁹.

Seventy-five percent (75%) of Canadian adults have at least one of the major cardiovascular disease risk factors (high blood pressure, elevated blood cholesterol, regular smoking and sedentary lifestyle)^43,80 (Table 9). The prevalence of at least one risk factor increases substantially with age until it reaches a plateau at age 55 in both men and women (Table 10). Over ninety percent (>90%) of Canadian adults age 65-74 have at least one major risk factor. Men, aged 55-64, and women, aged 65-74 years of age have the highest occurrence of two or more risk factors (63% and 64% respectively) (Table 10).

6.1.9 Public Knowledge on Causes of Cardiovascular Disease

More than 90% of Canadians recognize that cardiovascular disease is preventable. However, the majority do not recognize the specific risk factors ⁸⁰ (Table 11), although the higher the level of education, the more likely individuals are to identify specific risk factors. Smoking was one of the most commonly reported risk factors associated with cardiovascular disease and was reported more often by smokers than by non-smokers⁸⁰. The next most commonly reported risk factors were obesity, stress, and a lack of exercise (Table 11).

6.1.10 Socioeconomic Status and Risk Factors

Prevalence and awareness of risk factors varies with socioeconomic status. Canadians with a lower level of education (a proxy measure of socioeconomic status) are more likely to have risk factors for cardiovascular disease⁸⁰ (Table 12) yet are less likely to identify these as risk factors (Table 11). Lower socioeconomic groups are less aware than upper socioeconomic groups of high blood pressure and elevated blood cholesterol as risk factors.

6.2 Risk Factors for Stroke

Risk factors for stroke can be divided broadly into two categories: non-modifiable and modifiable. The non-modifiable risk factors include the patient's age and a family history of stroke. The presence of diabetes is associated with an increased risk of stroke, however it is not clear whether optimal diabetic control can result in a decrease in stroke incidence.

High blood pressure is the most important modifiable risk factor for stroke. Other modifiable risk factors for stroke are smoking, physical inactivity, and diabetes. Pre-existing conditions such as the presence of atrial fibrillation and a previous transient ischemic attack (TIA) are also important risk factors for stroke⁸¹.

6.2.1 High Blood Pressure

High blood pressure remains the number one preventable risk factor for stroke with an estimated 56.4% (males) and 66.1% (females) of strokes attributable to this factor⁸². The overall age-adjusted risk of stroke among hypertensives compared to normotensives is 3.1 for men and 2.9 for women. The risk of stroke increases with both increasing systolic and diastolic pressures.

Recent evidence indicates that isolated systolic hypertension is a major risk factor for stroke in the elderly⁸³ and is a stronger predictor of stroke than elevated diastolic blood pressure⁸⁴. Isolated systolic hypertension is defined as a systolic blood pressure of 160 mmHg or over and a diastolic pressure of less than 90 mmHg. Isolated systolic hypertension may be present in approximately 30% of women and 10% of men over the age of 80. There is good clinical evidence that treating such isolated systolic hypertension in those 60 years of age and over will decrease the incidence of stroke. Therapy also slows progression of carotid artery narrowing secondary to atherosclerosis⁸³.

6.2.2 Smoking

Cigarette smoking remains a well-known risk factor for stroke^85,86. The Honolulu Heart Program showed that cigarette smoking, as an independent risk factor, significantly increased the risk of stroke, intracerebral and subarachnoid hemorrhage. The risk is dose related with smoking more than 25 cigarettes per day conferring the highest risk. A recent analysis of 32 separate studies showed smoking to be a significant independent contributor to stroke incidence in both sexes and at all ages⁸⁷. The risk of stroke was approximately 50% higher in smokers than in non-smokers and rose substantially with the number of cigarettes smoked per day in both men and women. Based on the data from the Nurses Health Study and the Framingham study, cessation of smoking is followed by a reduction in risk of stroke to baseline over a remarkably short time. The risk of stroke falls to approximately 50% within one year and reaches the levels of those who have never smoked within five years⁸⁸. There appears to be no age related effects as there is a decrease in risk with smoking cessation in both young and older individuals.

6.2.3 Atrial Fibrillation

Atrial fibrillation is a well-known risk factor for stroke and is a common problem, occurring in up to 10% of patients over the age of 75. Recently, several studies have shown conclusively that the risk of stroke in patients with untreated atrial fibrillation is high^89-91. This may reach 7% per year in patients who, in addition to atrial fibrillation, also have a previous history of embolic disease, hypertensive heart disease, a previous history of congestive heart failure or echocardiographic evidence of a left atrial abnormality. Proper assessment and careful use of anticoagulants or antiplatelet agents can significantly decrease the risk of embolic stroke. Studies have demonstrated that the annual incidence of a stroke in those with chronic non-valvular atrial fibrillation is 4.5%; oral warfarin decreases the incidence to 1.4%⁹². As well, after TIA or a stroke, warfarin decreases the risk of a second stroke from 12% to 4% ⁹².

6.2.4 Transient Ischemic Attack

Patients who have had a recent TIA or a completed stroke are at a very high risk of a subsequent stroke⁹³. This is dependent on the degree of carotid stenosis and the presence of associated risk factors. In patients with bilateral carotid stenosis, the risk may be as high as 39% in the first 18 months⁹⁴. This risk is highest immediately after the event and then slowly decreases thereafter. In most patients, the stroke is secondary to thrombus formation in the carotid artery with subsequent distal embolization. Infrequently, an episode of hypotension in the presence of critical carotid stenosis may result in an infarction in the same arterial territory. Recognition of TIA is important because treatment can significantly decrease the risk of recurrence of stroke^94-96.

6.2.5 Physical Inactivity

Leisure time activity and work-related vigorous physical activity have been shown to lower the incidence of stroke. Although most initial evidence was related to heart disease, there are now some data which show that increasing activity also leads to a decrease in the incidence of stroke⁹⁷. Underlying mechanisms, though not fully understood, are likely multifactorial. Exercise has a beneficial effect on risk factors for cardiovascular disease; thus physical activity may help reduce elevated blood pressure and improve weight loss and the LDL-C to HDL-C ratio. These changes would then improve the risk factor profile for stroke. Exercise is also associated with positive lifestyle choices that include non-smoking, improved eating habits and maintenance of a healthy body weight⁹⁸.

6.2.6 Diabetes Mellitus

Individuals with diabetes have an increased risk of stroke. Men with diabetes have a 6-fold increased risk of death from stroke, while women with diabetes have an 8.2-fold increased risk of death⁹⁹. Of stroke deaths, 16% of male deaths and 33% of female deaths are attributable to diabetes⁹⁹.

6.2.7 Other Risk Factors

Other risk factors for stroke include an increased hematocrit, elevated fibrinogen levels, obesity, alcohol consumption and lipid abnormalities. Prospective studies have shown that fibrinogen levels are an independent predictor of acute myocardial infarctions in both men and women, but of stroke in men only¹⁰⁰. Several ongoing investigations are looking at the relationship between a patent foramen ovale (PFO) and subsequent embolic events. Results from a recently conducted study consisting of 140 subjects with a PFO who presented with a stroke, demonstrated that the stroke was not commonly due to a coexisting cause of stroke¹⁰¹. Several studies have shown that total serum homocysteine is a strong and independent risk factor for stroke⁷⁸. The relationship between the risk of stroke and the above risk factors remains controversial⁸¹.

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