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1.1 Introduction The global prevalence of hypertension was estimated to be 1.13 billion in 2015, with the overall prevalence of around 26% of the world’s population and around 30–34% of those aged 18 years and above. Hypertension is a risk factor for many cardiovascular, cerebrovascular, and cardiovascular-related conditions, and substantially contributes to premature mortality. In 2017, it was estimated that hypertension is the leading modifiable risk factor for all-cause mortality world- wide, accounting for over 10 million deaths and 218 million disability-adjusted life years. It is believed that there is a global epidemic of yet unknown proportions as abnormally elevated blood pressures are often asymptomatic. Indeed, the first diagnosis of hypertension is often made when the individual presents with a myocardial infarction or a stroke. Hypertension is thus rightly often referred to as the ‘silent killer’. Modifiable factors impacting hypertension include lifestyle changes such as smoking, diet, and physical activity levels. Public health initiatives for hypertension, such as population and opportunistic screening and patient edu- cation to promote ‘heart-healthy’ lifestyles, are critical for primary prevention, detection, and management of hypertension. 1.2 Historical perspective The theories regarding blood circulation and blood pressure go far back in history. In ancient Greece, Hippocrates and Galen knew about arteries and veins. Galen chapter 1 4 • epidemiology of hypertension was convinced veins and arteries were not connected and blood flowed both backward and forward from the heart. His teachings were unchallenged for more than 1000 years. It was only during the Middle Ages that these teachings were challenged, and new experiments were conducted and these laid the basis for our modern under- standing of the heart and circulation. It was William Harvey in 1616 who first described a one-way circulation of blood and correctly suggested the existence of capillaries. In 1733, Stephen Hales, an English clergyman, was the first person to measure blood pressure, albeit in a horse. Almost 150 years later, Ritter von Basch invented a machine that could measure the blood pressure of a human being in a non-invasive manner. This was the forerunner of the device introduced by Riva-Rocci in 1896, which proved to be a prototype of today’s refined instru- ments. This, along with the invention of the stethoscope by René Laennec, helped Russian scientist N. S. Korotkoff in 1905 to monitor the pulse whilst the blood pressure cuff was inflated, giving birth to the term ‘Korotkoff sounds’. Before long, patients were having their blood pressure checked, and high blood pressure came to be known as ‘hypertension’. At that time, it was felt that in certain conditions, raised blood pressure helped to maintain the perfu- sion of various organs and was not particularly harmful, hence the term ‘benign essential hypertension’, as opposed to ‘malignant hypertension’, which was thought to be very high blood pressures that could result in brain haemorrhage and heart failure. It was only towards the middle of the twentieth century that various surgical and pharmacological interventions were considered for the treatment of hyper- tension, although its exact risk was uncertain. Some of the initial surgical proced- ures that were considered included thoracic sympathectomy, while drug therapy consisted mainly of sedatives. Diuretics were introduced for hypertension in the 1950s and 1960s, but scepticism still persisted in the medical community about the benefit of treating this ‘benign essential’ condition. 1.3 Hypertension and cardiovascular risk It was not until the late 1970s and 1980s that large-scale epidemiological studies, including the Framingham study, clearly demonstrated the association between high blood pressure and cardiovascular risk. At the same time, a Veterans Administration study demonstrated that treating hypertensive patients with di- uretics appeared to protect them from future events. These studies, and espe- cially the more recent prospective studies collaboration by Lewington et al., have demonstrated that there is an almost linear relationship between blood pressure and cardiovascular and cerebrovascular risk. From the ages of 40–69, there is a twofold increase in mortality rates from ischaemic heart disease, and more than a twofold increase in stroke mortality for each 20 mmHg increase in systolic blood pressure and 10 mmHg increase in diastolic blood pressure. They also found that there is no evidence of a threshold wherein blood pressure is not directly related
to risk, even at levels as low as 115/75 mmHg. They also concluded that a 10 mmHg higher systolic or 5 mmHg higher diastolic blood pressure would be asso- ciated, in the long term, with a higher risk of about 30% of death from ischaemic heart disease. It is well established that hypertension is associated with a higher incidence of cardiovascular events, including myocardial infarction, heart failure and periph- eral artery disease, cerebrovascular events, including haemorrhagic or ischaemic stroke, and cardiovascular-related conditions, including end-stage renal disease. Many of these associations have been demonstrated across multiple age and ethnic groups. Emerging evidence has also associated hypertension with an in- creased risk of atrial fibrillation, cognitive decline, and dementia. It has also been shown that in people with low blood pressure who are not receiving any antihypertensive therapy, the incidence of cardiovascular disease is less. However, this cannot be used to support the benefits of therapy, as naturally occurring low blood pressure may offer a degree of protection that is not pro- vided by a similarly low blood pressure resulting from antihypertensive therapy. However, many randomized controlled trials have demonstrated a significant benefit for treating individuals with high blood pressure, especially when they are at high risk of future cardiovascular events due to coexisting factors. For an individual patient, the greatest risk is from a higher level of blood pres- sure. However, for the population at large, the greatest burden from hyperten- sion occurs among people with only minimally elevated blood pressure, as they are more likely to develop overt hypertension in the following years and often the majority of them are picked up once they have had an event.