Hypertension is arbitrarily defined as a diastolic blood pressure (DBP) of 90 mm Hg or higher, a systolic blood pressure (SBP) equal to or higher than 140 mm Hg, or both, on 3 separate occasions. It affects 24% of the population of the United States and is common among black (28%), white (24%), and Hispanic (14%) Americans. The prevalence of hypertension increases with age and is more than 70% among people 65 years and older. Among principal diagnoses given by family physicians for outpatient visits, only acute respiratory tract infection (7%) is more common than hypertension (6%). The annual direct medical cost of caring for hypertension exceeds $10 billion.
This article will discuss the pathophysiology and diagnosis of hypertension from an evidence-based perspective. An upcoming Applied Evidence article will cover treatment of hypertension and prognosis.
Idiopathic, or essential, hypertension accounts for more than 95% of cases and appears to be caused by a complex interaction between genetic predisposition and environmental factors. The predisposition to essential hypertension is polygenic in origin and may find full expression when combined with environmental factors, such as obesity, low physical activity levels, high stress levels, high alcohol consumption, high dietary sodium, and low dietary potassium, calcium, and magnesium. The complex interaction of genetics and environment may affect sodium, catecholamines, the renin-angiotensin system, insulin, and cell membrane function, causing elevation of the blood pressure.
The more common identifiable causes of hypertension include chronic renal disease (2%-5%), renovascular disease—including renal artery atherosclerosis and fibromuscular dysplasia—(0.2%-0.7%), Cushing syndrome (0.1%-0.6%), pheochromocytoma (0.04%-0.1%), and primary hyperaldosteronism (0.01%-0.3%). Although obesity, excessive alcohol consumption, oral contraceptive therapy, and sleep apnea may cause hypertension, they are not typically included as identifiable causes of hypertension. The prevalence of the latter conditions as identifiable causes of hypertension remains to be defined.
The presence of hypertension must be confirmed by blood pressure measurements obtained with proper technique. The blood pressure of all patients 18 years and older should be measured at each health care visit because of the high prevalence of hypertension. Patients should be encouraged to abstain from nicotine and caffeine for at least 30 minutes before the measurement of the blood pressure. Measurement should be made with a mercury sphygmomanometer or a recently calibrated aneroid device. The bladder of the blood pressure cuff should encircle 80% of the arm. The pressure should be taken after at least 5 minutes of rest with the patient sitting, back supported, and arm bared and supported at heart level. The first sound heard (phase 1) is the SBP, and the last sound heard (phase 5) is the DBP. Two readings separated by 2 minutes should be averaged. Hypertension is present when an accurately measured blood pressure is high on 3 separate occasions.
A major consensus report, the Sixth Report of the Joint National Committee on Prevention Detection, Evaluation, and Treatment of High Blood Pressure,11 designates 6 categories of blood pressure:
- Optimal—SBP less than 120 mm; DBP less than 80 mm
- Normal—SBP less than 130 mm; DBP less than 85 mm
- High normal—SBP is 130 to 139 mm; DBP is 85 to 89 mm
- Stage 1 hypertension—SBP is 140 to 159 mm; DBP is 90 to 99 mm
- Stage 2 hypertension—SBP is 160 to 179 mm; DBP is 100 to 109 mm
- Stage 3 hypertension—SBP is 180 mm or higher; DBP is 110 mm or higher
It is important to note that the recommended diagnostic evaluation is based on a consensus and should not be considered evidence-based. A complete history, physical examination, and limited diagnostic testing (urinalysis, complete blood count, potassium, sodium, fasting glucose, creatinine, total cholesterol, high-density cholesterol, and electrocardiogram) are recommended once the presence of hypertension has been confirmed. This evaluation has 3 purposes:
- Identify other cardiovascular risk factors. Most patients with hypertension have multiple cardiovascular risk factors at the time of initial evaluation. Risk factors include smoking, hyperlipidemia, diabetes, age older than 60 years, sex (men or postmenopausal women), and family history of cardiovascular disease in a female relative before age 65 years or a male relative before 55 years.
- Identify end-organ damage. Evidence of end-organ damage includes left ventricular hypertrophy, angina, previous myocardial infarction, previous angioplasty or coronary revascularization, heart failure, stroke or transient ischemic attack, nephropathy, peripheral arterial disease, and retinopathy.
- Identify secondary causes of hypertension. Estimating the pretest probability of a secondary (identifiable) cause of hypertension is problematic, because referral bias is a major problem in hypertension prevalence studies; patients are typically included in these studies only after being referred to a study center by their primary care physician for resistant or difficult to control hypertension. On the basis of the best available estimates, it would be reasonable to assume that patients presenting to primary care physicians have a 5% probability of an identifiable cause of hypertension.