Epidemiology of CVD
At the beginning of the 20th century, CVD was responsible for less than 10 percent of all deaths worldwide, but by 2001 that figure was 30 percent. About 80 percent of the global burden of CVD death occurs in low- and middle-income countries. Murray and Lopez (1996) predicted that CVD will be the leading cause of death and disability worldwide by 2020 mainly because it will increase in low- and middle-income countries. By 2001, CVD had become the leading cause of death in the developing world, as it has been in the developed world since the mid 1900s (Mathers and others 2006; WHO 2002a). Nearly 50 percent of all deaths in high-income countries and about 28 percent of deaths in low- and middle-income countries are the result of CVD (Mathers and others 2006). Other causes of death, such as injuries, respiratory infections, nutritional deficiencies, and HIV/AIDS, collectively still play a predominant role in certain regions, but even in those areas CVD is now a significant cause of mortality.
Predominant Cardiovascular Diseases
This chapter focuses on the most common causes of CVD morbidity and mortality:
ischemic heart disease (IHD)
congestive heart failure (CHF).
These diseases account for at least 80 percent of the burden of CVD in all income regions, which share many of the same common risk factors; accordingly, similar interventions are appropriate. A fourth manifestation, rheumatic heart disease (RHD), which accounts for 3 percent of all disability-adjusted life years (DALYs) lost as a result of CVD, does not contribute significantly to the overall global burden of CVD. The burden of RHD will likely continue to diminish, but it is still an important inflammatory cause of heart disease in developing countries and accordingly is addressed in this chapter. We do not address many other forms of CVD because of the scope of this volume; the regional rather than global nature of some inflammatory diseases, such as Chagas disease; or the congenital abnormalities or genetically based cardiomyopathies for which prevention and treatment options remain limited.
Ischemic Heart Disease
IHD is the single largest cause of death in the developed countries and is one of the main contributors to the disease burden in developing countries. The two leading manifestations of IHD are angina and acute myocardial infarction. In 2001, IHD was responsible for 7.3 million deaths and 58 million DALYs lost worldwide (WHO 2002b). Seventy-five percent of global deaths and 82 percent of the total DALYs resulting from IHD occurred in the low- and middle-income countries.
Angina is the characteristic pain of IHD. It is caused by atherosclerosis leading to stenosis (partial occlusion) of one or more coronary arteries. Patients with chronic stable angina have an average annual mortality of 2 percent or less. Acute myocardial infarction (AMI) is the total occlusion of a major coronary artery with a complete lack of oxygen and nutrients leading to cardiac muscle necrosis. AMI is usually diagnosed by changes in the electrocardiogram; by elevated serum enzymes, such as creatine phosphokinase and troponin T or I; and by pain similar to that of angina. Thirty-day mortality after an AMI is high: even with best medical therapy it remains at about 33 percent, with half the deaths occurring before the individual reaches the hospital. Even in a hospital with a coronary care unit where advanced care options are available, mortality is still 7 percent. In a hospital without such facilities or therapies, the mortality rate is closer to 30 percent. Even though mortality among patients who have recovered from an AMI has declined in recent decades, approximately 4 percent of patients who survive initial hospitalization die in the first year following the event (Antman and others 2004).
Stroke is caused by a disruption in the flow of blood to part of the brain either because of the occlusion of a blood vessel (ischemic stroke) or the rupture of a blood vessel (hemorrhagic stroke). Many of the same risk factors for IHD apply to stroke; in addition, atrial fibrillation is an important risk factor for stroke. The annual risk of stroke in patients with non-valvular atrial fibrillation is 3 to 5 percent, with 50 percent of thromboembolic stroke being attributable to atrial fibrillation (Wolf, Abbott, and Kannel 1991). Chapter 32 discusses the diagnosis and management of the clinical syndromes in greater detail.
Congestive Heart Failure
CHF is the end stage of many heart diseases. It is characterized by abnormalities in myocardial function and neurohormonal regulation resulting in fatigue, fluid retention, and reduced longevity. CHF is caused by pathological processes that affect the heart; IHD and hypertension-related heart disease are the most common etiologies. The risk of developing CHF is two times more in hypertensive men and three times more in hypertensive women compared with those who are normotensive. CHF is five times more common in those who have had an AMI than in those who have not. The prognosis for those with established CHF is generally poor and worse than for those with most malignancies (McMurray and Stewart 2000) or AIDS, with a one-year mortality rate as high as 40 percent and a five-year mortality between 26 and 75 percent.
The worldwide burden of CHF is substantial and continues to rise. Throughout the developed world the prevalence is about 2 to 3 percent, with an annual incidence rate of 0.1 to 0.2 percent (McMurray and Stewart 2000). However, the incidence and prevalence of CHF rise dramatically with age. Prevalence is 27 per 1,000 population for those older than 65, compared with 0.7 per 1,000 for those younger than 50 (McKelvie 2003). CHF occurs more frequently in men, and incidence and mortality differ substantially according to gender and socioeconomic status. CHF causes 53,000 deaths in the United States each year and contributes to another 213,000, and the death rate attributed to CHF rose by 155 percent from 1979 to 2001 in the United States (American Heart Association 2002). CHF is the first-listed diagnosis in 1 million hospitalizations.
Rheumatic Heart Disease
RHD is the consequence of an acute rheumatic fever (ARF)—that is, a poorly adapted autoimmune response to group A Beta-hemolytic streptococci. It affects the connective tissue, mainly the joints and the heart valves. The most serious complications are valvular stenosis, regurgitation following the valvulitis, or both (Ephrem, Abegaz, and Muhe 1990). RHD is also a predisposing factor for infective endocarditis, a disease of younger adults, predominantly males (Koegelenberg and others 2003).
According to 2001 estimates, RHD accounts for 338,000 deaths per year worldwide, two-thirds of them in Southeast Asia and the Western Pacific (WHO 2002b). About 12 million people in developing countries, most of them children, suffer from RHD (WHO 1995). Steer and others' (2002) review of developing countries suggests that RHD prevalence in children is between 0.7 and 14 per 1,000, with the highest rates in Asia. RHD and ARF are the most common causes of cardiac disease among children in developing countries (Ephrem, Abegaz, and Muhe 1990; Schneider and Bezabih 2001; Steer and others 2002) and account for almost 10 percent of sudden cardiac deaths (Kaplan 1985).
Until the 1950s, ARF accounted for a substantial portion of cardiovascular problems among schoolchildren in developed countries, and even though it is now far less common, outbreaks still occur (Carapetis, Currie, and Kaplan 1999), suggesting that neither antibiotics nor other public health measures have been totally effective in controlling ARF.
The Epidemiological Transition
Over the past two centuries, the industrial and technological revolutions have resulted in a dramatic shift in the causes of illness and death. Before 1900, infectious diseases and malnutrition were the most common causes of death; however, primarily because of improved nutrition and public health measures, they have gradually been supplanted in most high-income countries by CVD and cancer. As improvements continue to spread to developing countries, CVD mortality rates are increasing.
Known as the epidemiological transition, this shift is highly correlated with changes in personal and collective wealth (the economic transition), social structure (the social transition), and demographics (the demographic transition). Omran (1971) provides an excellent model of the epidemiological transition that divides it into three basic ages: pestilence and famine, receding pandemics, and degenerative and human-created diseases (table 33.1). Olshansky and Ault (1986) add a fourth stage: delayed degenerative diseases.
The consistent pattern for most high-income countries going through the epidemiological transition has been initially high rates of stroke, mostly hemorrhagic. Only in the third phase, with the presence of increased resources, but coupled with increased diabetes and smoking rates and adverse lipid profiles, do rates of IHD climb. This phase is also accompanied by better control of severe hypertension, reducing the rates of hemorrhagic stroke, which is then replaced by ischemic stroke. Most regions appear to be following this pattern and have a predominance of IHD. The two exceptions are East Asia and the Pacific and Sub-Saharan Africa. The pattern in East Asia and the Pacific is dominated by China and appears to be a result of China's stage in the transition but may also be following a pattern similar to Japan's—that is, dominated by more strokes and fewer IHD deaths—whereas Sub-Saharan Africa is in an earlier phase of the epidemiological transition.
Even though countries tend to enter these stages at different times, the progression from one stage to the next tends to proceed in a predictable manner. The six World Bank regions are at various phases of the epidemiological transition (table 33.1), and where development has occurred, it has often been at a more compressed rate than in the high-income countries. Although rates of IHD and stroke fell 2 to 3 percent per year in the high-income countries during the 1970s and 1980s, the rate of decline has since slowed. Overweight and obesity are escalating at an alarming pace, while rates of type 2 diabetes, hypertension, and lipid abnormalities associated with obesity are on the rise. This trend is not unique to the developed countries, however. According to the World Health Organization, worldwide more than 1 billion adults are overweight and 300 million are clinically obese. Even more disturbing are increases in childhood obesity that have led to large increases in diabetes and hypertension. If these trends continue, age-adjusted CVD mortality rates could increase in the high-income countries in the coming years. These trends are discussed in greater detail in chapter 45.
The risk of developing CVD depends to a large extent on the presence of several risk factors. The major risk factors for CVD include tobacco use, high blood pressure, high blood glucose, lipid abnormalities, obesity, and physical inactivity. The global variations in CVD rates are related to temporal and regional variations in these known risk factors. Discussions of the strength of the associations of the various factors with CVD are found elsewhere (chapters 30, 44, and 45). Although some risk factors, such as age, ethnicity, and gender, obviously cannot be modified, most of the risk is attributable to lifestyle and behavioral patterns, which can be changed.