Definitions

Normative BP ranges in adults have been based on long-term, end-organ risk as determined by large-cohort epidemiologic data. In children and adolescents, normal BP ranges have been defined based on data from relatively large cohort studies in presumed-healthy subjects and are stratified by gender, age, and height. The seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC7) and the Fourth Report on the Diagnosis, Evaluation, and Treatment of High Blood Pressure in Children and Adolescents (Fourth Report) (NHBPEP) define hypertension in children and adolescents as an average SBP or DBP at the 95th percentile or above for gender, age, and height on three or more occasions. BPs (SBP or DBP) between the 90th and 95th percentiles have historically been referred to as “high normal” but in these most recent guidelines have been redefined as prehypertensive. This change reflects the addition of prehypertension to the adult diagnostic criteria as defined by JNC7; there is increased risk of developing hypertension in those with prehypertension. Stage 1 hypertension is defined as a BP above the 95th and up to 5 mm Hg over the 99th percentile for gender, age, and height. Stage 2 hypertension is more than 5 mm Hg higher than the 99^th percentile. More timely antihypertensive therapy is recommended for stage 2 hypertension.

White coat hypertension is defined as having a BP consistently above the 95th percentile for age in a physician’s office or clinic but being normotensive outside of the clinical setting. This diagnosis usually requires ambulatory BP monitoring for confirmation.

There are many causes of hypertension (Figure 47-1). Whereas BP for which a clear cause can be determined is described as secondary hypertension, hypertension without a clear correctable cause is referred to as primary or essential hypertension. This is necessarily a diagnosis of exclusion.

Figure 47-1 Causes of hypertension.

Physiology

BP is determined by both cardiac output (CO) and systemic vascular resistance (SVR). Factors increasing output or resistance can result in hypertension. The relationship between BP and CO is summarized by the following equations.

Multiple causes of secondary hypertension are shown in Figure 47-1. Each condition may increase BP via increases in heart rate, SV, or SVR. Considering these three fundamental factors in BP elevation aids in the appropriate therapeutic approach to a given patient.

Measurement of Blood Pressure

Accurate and consistent measurement of BP is a precondition to appropriate medical management. BP measurements, even when performed correctly, demonstrate significant variability. As a result, the arithmetic mean of three or more BP measurements on separate occasions is used to diagnose hypertension. The mean BP of multiple measurements most closely approximates those obtained by ambulatory BP monitoring.

Accepted epidemiologic statistics for BP are all based on measurements made by auscultation. Despite this, oscillometric measurements of BP are increasingly used because of their ease of use and the (false) perception that they are more precise. Oscillometric “measurements” of SBP and DBP are based on proprietary algorithms that extrapolate these from measured mean BPs. These estimates have been shown to be consistently at least 5 to 10 mm Hg higher than those measured by auscultation. Because of this, high BPs derived from oscillometric measurements should be repeated by auscultation as a matter of course.