Environmental Basis of Cardiovascular Disease

1.1 Introduction

The term cardiovascular disease (CVD) refers to a group of illnesses caused by the disorders of the heart, blood vessels and blood flow. The most common cause of cardiovascular diseases is atherosclerosis, which is the hardening of arteries due to the formation of an atheromatous plaque. Abrupt changes in blood flow in atherosclerotic vessels result in acute myocardial infarction and stroke, which are the major clinical manifestations of chronic changes in the vessel wall. In the heart, ischemic injury due to atherosclerotic disease often leads to arrhythmia, hypertrophy, cardiomyopathy and heart failure. Heart disease is accompanied by chronic metabolic and physiological changes that precede and contribute to its clinical manifestations. These include metabolic changes such as high cholesterol (hypercholesterolemia) and insulin resistance and physiological changes such as an increase in blood pressure (hypertension) and changes in cardiac contractility. Although the causes of diabetes are not well understood, diabetes primarily affects the heart and blood vessels and is, therefore, considered to be a major CVD risk factor. Therefore diabetes and obesity are included in this discussion of heart disease.

Significant CVD is also associated with rheumatic disease, which is due to myocardial damage caused by streptococcal bacteria and congenital malformation of the structures of the heart or blood vessels. Several other types of congenital CVD are also common. These defects could be overt, resulting from gross malformation of major blood vessels or myocardial tissue in the fetus, or they may be more subtle, leading to an increase in susceptibility to stress or exercise. Congenital defects or prolonged hypertension and infectious diseases could also result in the dilation and rupture of the aorta leading to aortic aneurysm and dissection. Additionally, cardiovascular disorder associated with deep vein thrombosis and pulmonary embolism could result from blood clots in the leg veins that can dislodge and move to the heart and the lungs.

As a group, CVD is the leading cause of death world-wide (Figure 1.1). According to the WHO in 2004, CVD accounted for nearly 30% of all deaths worldwide. It killed twice as many people as infectious and parasitic disease and 3 times as many people as all forms of cancer. Globally, most (43–45%) cardiovascular deaths are due to coronary heart disease (CHD) or ischemic heart disease (IHD), whereas stroke accounts for 33% of CVD. A similar distribution of CVD deaths has been reported for countries such as the US (Figure 1.2).

These statistics suggest that heart disease is the major cause of mortality world-wide. Although the prevalence of heart disease varies considerably (vide infra) it still remains a major cause of death in all human populations regardless of their geographic location or ethnicity. It shows no preference for gender or economic status. Both men and women appear to be equally susceptible. World-wide, more women (31.5%) than men (26.8%) die of heart disease. Even in low-income countries (per capita ≤$825) IHD is the number two leading cause of death (9.4%), second only to deaths caused by lower respiratory infections (11.2%), whereas in middle and high income countries ($10, 066 or more) IHD and cerebrovascular disease account for 25 to 28% of all deaths (WHO, 2005). What is more alarming is that the prevalence of heart disease in increasing. The WHO estimates that 80% of all current CVD deaths are in developing, low- and middle-income countries and it is estimated that by 2010, CVD will be the leading cause of death in developing, low-income countries as well. In developed countries, the emergent epidemics of diabetes and obesity are threatening to erode the pattern of recent gains in health. In the US, the increase in obesity alone has been forecasted to slow down the increase in life expectancy that has been steadily increasing since the early 20th century. Thus, CVD is the most frequent cause of death throughout the world, independent of economic status, gender, or ethnic differences.

The universally high burden of CVD and the extraordinarily high rates of CVD mortality across all communities, suggests that humans as a species are particularly prone to heart disease. It may be argued that CVD is an inevitable consequence of aging, that blood pressure and cholesterol levels inexorably increase with age and that if an individual survives middle age without succumbing to infectious disease, sporadic cancers, accidents or violence, their most likely fate is cardiovascular death. This view is consistent with data showing that the risk of dying from CVD increases with age. In the US, the percentage of population with CVD increases from 14.9 and 8.7% for men and women of 20–39 years of age to 78.8 and 84.7% for men and women more than 80 years of age. It has been suggested that because heart disease develops more often in old individuals, it is not subject to direct selective pressure, i.e. that natural selection during evolution is unable to weed out these diseases as they do not affect reproductive success. Natural selection, it is believed, tends to maintain the frequency of genes that increase reproductive success even if the genes have other effects that increase disease susceptibility in older age. However, as we shall see, these arguments do not take into account the important role of the environment, which affects not only the long-range evolutionary susceptibility to disease, but also the proximate causes that lead to the disease development in a specific individual. Moreover, changes in the environment can modify (slow down or accelerate) age-dependent changes in the heart and blood vessels. In addition, a changing environment could continuously alter the context within which the effects of a gene manifest. Thus, a gene could be beneficial in one environment but not the other. As a result, changes in the environment could impart maladaptive predilection to a previously well-adapted genetic...