True systemic hypertension can be diagnosed when there is an increase in arterial pressure above accepted normal pressure for age, sex, and race. According to the seventh report on classification and characterization of BP by the Joint National Committee (JNC) on Evaluation, Detection, and Prevention of High Blood Pressure in 2003, a normal BP for adults 18 years or older is less than 120 mm Hg systolic with a diastolic of less than 80 mm Hg. Table 11.1 shows the classification of BP for adults by the JNC 7 report: normal, prehypertension, stage 1 hypertension, and stage 2 hypertension.

Hypertension in children and adolescents is defined as BP at the 95th percentile or greater. Prehypertension is defined as BP between the 90th and 95th percentile. The fourth report on high BP in children and adolescents by the National High Blood Pressure Education Program provides BP tables with percentiles based on gender, age, and height.

It is important to note that the classification of hypertension defines and specifies hypertension according to systolic and/or diastolic limits, each representing different pathophysiologic processes. Systolic hypertension is a marker of macrovascular disease and large arterial stiffening (atherosclerosis), and diastolic hypertension is a consequence of microvascular disease involving typically vessels less than 1 mm in size.

Uncontrolled hypertension can escalate to a hypertensive crisis defined as systolic BP greater than 180 mm Hg or diastolic BP greater than 120 mm Hg and categorized as urgent and emergent. According to the JNC 7 Report, hypertensive emergency is defined as elevated BP (usually ≥180/120 mm Hg) complicated by evidence of impending or progressive end-organ damage, whereas hypertensive urgency is severe elevation of BP without progressive end-organ damage.

The prevalence of hypertension depends on both the racial composition of the population and the criteria used to define hypertension. In a white suburban population like that in the Framingham study, nearly one-fifth has a BP greater than 160/95 mm Hg and almost one-half has a BP greater than 140/90 mm Hg. A higher prevalence of hypertension has been found in the non-white population. The frequency increases with the age of the population with nearly two-thirds of hypertensive patients being older than 50 years, and by the age of 75 years, almost 90% will have hypertension. The subtype of hypertension is also influenced by age. Younger individuals suffer from diastolic hypertension and combined systolic and diastolic hypertension, whereas older persons predominantly experience systolic hypertension. The number of hypertensive persons in the United States in 1983 was estimated to be 57.7 million—more than double the estimate made in 1960 to 1962. Currently, approximately 70 million Americans suffer from hypertension.

Hypertension is classified according to its subtypes: systolic, diastolic, and pulse pressure (Table 11.2). Historically, only diastolic hypertension—essential hypertension—was recognized, but more recently, there has been increasing emphasis and recognition of systolic hypertension, namely, isolated systolic hypertension (ISH). ISH is the most common subtype of hypertension affecting predominantly individuals older than 60 years of age. In fact, approximately 70% of hypertensive patients have ISH with approximately half of these individuals having very wide pulse pressure (>65 mm Hg) or pulse pressure hypertension (PPH). Isolated diastolic hypertension is most prevalent in individuals younger than 50 years and is an important marker of coronary heart disease and mortality in this age group. Some hypertensive
patients have both systolic and diastolic hypertension categorized as combined systolic and diastolic hypertension. The ranges of systolic, diastolic, and pulse pressure that define each subtype of hypertension are presented in Table 11.1.

The underlying mechanism of essential hypertension is unknown. A variety of abnormalities, including heredity, fetal undernutrition, abnormal sympathetic nervous system activity, cell membrane defects, renal retention of excess salt, microcirculatory alterations, endothelial
cell dysfunction, hyperinsulinemia secondary to insulin resistance, vascular hypertrophy, and altered renin-angiotensin system regulation are implicated. There is increasing evidence that “local renin-angiotensin” paracrine factors may be involved in the development of hypertension. However, characteristic hemodynamic changes are present as follows:

The higher the level of BP, the more likely that various cardiovascular diseases will develop prematurely through acceleration of arteriosclerosis. If untreated, approximately 50% of hypertensive patients die of coronary heart disease or congestive heart failure (CHF), approximately 33% of stroke, and 10% to 15% of renal failure.

ISH is a manifestation of atherosclerotic disease involving predominantly the aorta and its major branches. Both ISH and PPH reflect arterial stiffening and represent the pulsatile component of BP, whereas the mean arterial pressure (MAP) represent the static state. Systole, then, is directly influenced by the ventricular contractile state, the ejected volume, heart rate, and, importantly, compliance of the aorta and pulse wave velocity. With increasing age and factors such as diabetes, smoking, increased cholesterol as well as some genetic factors, compliance of the aorta diminishes markedly (Fig. 11.1). The net result is stiffening of the arterial
tree and inability to absorb the pulsatile load, causing the systolic pressure to rise sharply. Pulse wave velocity has gained much attention lately because it may be a richer substrate for characterizing the physiology of blood flow and pressure. Under normal physiologic conditions, the ejected blood volume is transmitted peripherally as a propagated wave. After reaching the periphery, in particular, at the bifurcation sites, a reflected wave or a retrograde wave is generated from the encountered impedance. Under normal conditions, wave reflection arrival coincides with the beginning of diastole, thereby augmenting the diastolic pressure. However, with stiffening of the aorta as occurs with aging, both the propagated and the reflected waves travel much more rapidly, and early return of the reflected arterial wave during late systole augments the systolic component, effectively increasing afterload. The ensuing loss of diastolic augmentation results in a disproportionate increase in the systolic BP and lower diastolic BP—a hallmark of ISH and of PPH.

Mechanistically, pulse pressure is also an index of the stiffness of conduit vessels and the rate of pressure wave propagation and reflection within the arterial tree. Approximately half of patients with ISH also have wide pulse pressure; however, both individuals who are normotensive (systolic, <140 mm Hg) with a low diastolic pressure or persons with combined systolic and diastolic hypertension (systolic >140 mm Hg and diastolic >90 mm Hg) can have wide pulse pressure (normal values being ≤40 mm Hg).

The end-organ damages caused by long-standing hypertension are as follows:

Hypertensive patients are at increased risk for coronary artery disease, silent myocardial ischemia, CHF, and stroke. However, whether preoperative hypertension is predictive of perioperative major cardiac morbidity remains controversial. Some investigators have shown that patients with untreated, poorly controlled, or labile preoperative hypertension are at increased risk for perioperative BP lability, dysrhythmias, myocardial ischemia, and transient neurologic complications. Some suggested that preoperative hypertension predicted perioperative MI. However, Goldman and Caldera demonstrated that mild-to-moderate hypertension did not increase the risk of major morbid events. Instead, preoperative hypertension may predict several intermediates of outcome, such as BP lability and myocardial ischemia. The controversy may be due to the wide variability in the hypertensive population. Hypertension may affect perioperative morbidity through the extent of end-organ damage and not the manifestation of the disease itself. LVH, which signifies long-standing poorly controlled hypertension, can increase the risk of myocardial ischemia from imbalances of myocardial oxygen supply and demand regardless of the presence or absence of coronary artery disease.

ISH (systolic BP >140 mm Hg and diastolic BP <90 mm Hg) has been identified as a risk factor for cardiovascular complications in the general population and treatment reduces the future risk of stroke. Aronson et al. and Fontes et al. found that among the various components of BP, preoperative pulse pressure hypertension was independently and significantly associated with postoperative stroke, renal failure, and mortality in patients undergoing coronary artery bypass. Neither ISH nor isolated diastolic hypertension were predictive of vascular complications. This finding is consistent with results of longitudinal studies finding that increase in pulse pressure is a better predictor of major vascular complications than increase in either systolic or diastolic BP. Interestingly, rises in pulse pressure as few as 10 mm Hg in both normotensive and hypertensive individuals is associated with 20% or more increase risk of renal, coronary, and cerebral events.

Both systolic hypertension and high pulse pressure are strongly associated with risk for perioperative cerebrovascular accidents and acute renal failure. The brain and the kidneys as well as the heart differ from most organs in that they receive large amount of blood flow and have very low resistance. As such, the pulsatile load (pulse pressure) tends to be very high. Over a period of many years of poorly or untreated hypertension, endothelial injury and vascular remodeling changes can promote both arteriosclerosis and atherosclerosis, which are known risk factors for cerebral and renal vascular complications. The pulsatile stress also can cause dislodgment of plaques from the aorta and its major branches, resulting in embolic stroke.

In hypertensive patients, autoregulation of cerebral blood flow is reset to a higher range than normal, and although it protects the brain against sudden increases in pressure, it makes it more vulnerable to hypotension. Therefore, when BP is lowered acutely, hypertensive patients will show signs of cerebral ischemia at a higher level of BP than normotensive patients.

Hypertension may accelerate cognitive decline with age. Hypertension, particularly systolic, is a major risk factor for initial and recurrent stroke and for transient ischemia attacks caused by extracranial atherosclerosis.

Chronic renal insufficiency is a common sequela of hypertension. Patients with hypertension should have a measurement of baseline serum creatinine. In the original Cardiac Risk Index, an elevated serum creatinine level (gt;3.0 mg per dL [>265.2 mmol per L]) was one of the independent risk factors for perioperative cardiovascular morbidity and mortality. This has been confirmed in the revised Cardiac Risk Index in which a preoperative serum creatinine level greater than 2.0 mg per dL (176.8 mmol per L) was one of six independent factors that predicted increased cardiovascular risk.