Diagnostic thresholds, guideline frameworks, and the distinction between primary and secondary hypertension
Hypertension is the most prevalent modifiable cardiovascular risk factor worldwide, affecting approximately 1.28 billion adults.3 Despite its ubiquity in clinical practice, its pathophysiology is multifactorial and only partially understood, involving genetic predisposition, neurohormonal activation, vascular biology, and end-organ adaptation in ways that vary substantially between individuals.4
This module establishes the mechanistic foundation for the entire HTN series. The pathophysiological mechanisms covered here, including RAAS activation, sympathetic overactivity, pressure-natriuresis resetting, and vascular remodeling, are the direct basis for the pharmacological rationale presented in each subsequent module.
Blood pressure classification has undergone significant revision over the past two decades. Clinicians working across different health systems should be familiar with both major frameworks currently in use.
The 2017 ACC/AHA Hypertension Guidelines substantially lowered the diagnostic threshold.1 The classification is as follows:
| Category | Systolic (mmHg) | Diastolic (mmHg) |
|---|---|---|
| Normal | < 120 | < 80 |
| Elevated | 120–129 | < 80 |
| Stage 1 Hypertension | 130–139 | 80–89 |
| Stage 2 Hypertension | ≥ 140 | ≥ 90 |
| Hypertensive Crisis | > 180 | and/or > 120 |
The rationale for the 2017 reclassification was outcome data demonstrating that cardiovascular risk increases continuously above 115/75 mmHg, and that individuals previously labeled "prehypertensive" carry meaningful excess risk.1 A practical consequence is that approximately 46% of U.S. adults now meet ACC/AHA criteria for hypertension, compared with 32% under the earlier JNC 7 thresholds.
The 2023 European Society of Hypertension (ESH) and 2018 European Society of Cardiology/ESH (ESC/ESH) Guidelines retained the 140/90 mmHg threshold but introduced a "high normal" category spanning 130–139/85–89 mmHg.2,8 Treatment decisions under either framework depend not only on blood pressure level but on total cardiovascular risk, comorbidities, and patient-specific targets, all of which are addressed in HTN-02.
Primary (essential) hypertension accounts for approximately 90–95% of all hypertension cases.4 It has no single identifiable cause and represents the interaction of genetic susceptibility with environmental and lifestyle factors over time.
Secondary hypertension accounts for 5–10% of cases but is clinically important because it is frequently underdiagnosed and, when treated etiologically, can be cured or substantially ameliorated.4 The major secondary causes are listed below.
A full discussion of the clinical clues, diagnostic workup, and pharmacological implications of each secondary cause is presented in HTN-02.
Isolated systolic hypertension (ISH), defined as systolic blood pressure at or above 140 mmHg with diastolic below 90 mmHg, is the dominant pattern in patients over 60 years of age.4 It reflects age-related loss of arterial compliance and increased pulse wave velocity rather than elevated peripheral resistance alone. As the aorta and large arteries stiffen with age, pulse wave reflection returns earlier in systole, augmenting systolic pressure while diastolic pressure falls or remains stable.
ISH carries substantial cardiovascular risk, particularly for stroke and heart failure with preserved ejection fraction (HFpEF). It should not be dismissed as a normal consequence of aging. Pharmacological management of ISH in the elderly, including treatment targets and preferred agents, is addressed in detail in HTN-10.
RAAS, sympathetic activation, renal mechanisms, vascular remodeling, and the interaction of genetic and environmental determinants
Blood pressure is the product of cardiac output (CO) and total peripheral resistance (TPR): BP = CO × TPR. Early in hypertension, particularly in younger individuals, elevated cardiac output driven by sympathetic overactivation may predominate. Over time, the predominant mechanism shifts to increased peripheral vascular resistance, driven by structural and functional changes in resistance arteries and arterioles.4 Understanding which mechanisms are operative in a given patient has direct therapeutic implications.
The renin-angiotensin-aldosterone system (RAAS) is the dominant neurohormonal regulator of blood pressure and volume homeostasis, and is the target of the most widely used antihypertensive drug classes.15 The classical pathway begins with renin secretion from juxtaglomerular cells in response to reduced renal perfusion pressure, decreased sodium delivery to the macula densa, and beta-1 adrenergic stimulation. Renin cleaves angiotensinogen (of hepatic origin) into angiotensin I, a biologically inactive decapeptide.
Angiotensin-converting enzyme (ACE), expressed on pulmonary vascular endothelium, then cleaves angiotensin I into angiotensin II (Ang II). Ang II acts on angiotensin II type 1 (AT1) receptors to produce vasoconstriction, aldosterone release, sodium retention, sympathetic facilitation, and vascular and myocardial remodeling. angiotensin II type 2 (AT2) receptor stimulation by Ang II exerts generally counterregulatory effects including vasodilation, antiproliferation, and natriuresis.15 Aldosterone acts on mineralocorticoid receptors in the renal collecting duct, promoting sodium reabsorption and potassium excretion.
The tissue RAAS, operating locally within the vasculature, heart, brain, and kidney, is now recognized as equally important to the circulating RAAS in chronic hypertension and end-organ damage.15 ACE inhibitors, ARBs, and mineralocorticoid receptor antagonists exert effects at both circulating and tissue levels, which is a key reason their organ-protective benefits extend beyond blood pressure reduction alone.
Sympathetic nervous system (SNS) overactivation contributes to hypertension through multiple converging mechanisms:4
SNS hyperactivity is particularly prominent in younger hypertensives, obesity-related hypertension, and hypertension associated with obstructive sleep apnea.11 Ang II facilitates norepinephrine release from sympathetic terminals and reduces neuronal reuptake, creating a positive feedback loop between RAAS and SNS activation. This interaction is the mechanistic basis for the synergistic benefit seen when RAAS inhibitors and sympatholytic agents are combined in clinical practice.
Guyton's pressure-natriuresis model provides a unifying framework: the kidney regulates long-term blood pressure by adjusting sodium excretion in response to renal perfusion pressure.5 In normotension, this relationship is steep: a small rise in blood pressure produces a marked increase in natriuresis. In hypertension, this curve is reset to a higher operating pressure, resulting from reduced filtering surface, increased tubular sodium reabsorption driven by RAAS and SNS activation, or structural nephron loss.5 Diuretic therapy lowers blood pressure by promoting natriuresis and resetting the curve toward a lower operating point.
Inward eutrophic remodeling thickens the arteriolar wall with a reduced lumen diameter, increasing peripheral resistance. This structural change may persist even when neurohormonal activation is pharmacologically suppressed, contributing to treatment resistance. Hypertrophic remodeling involves increased smooth muscle mass in response to chronic pressure loading and direct trophic effects of Ang II.4
Normally, the endothelium produces nitric oxide (NO) via endothelial nitric oxide synthase (eNOS), causing vasodilation and inhibiting platelet aggregation. In hypertension, oxidative stress, principally superoxide generated by Ang II acting on vascular nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, quenches NO, reducing its bioavailability.12 Increased endothelin-1 production further shifts vascular tone toward sustained vasoconstriction. Endothelial dysfunction precedes structural damage and contributes to the prothrombotic, proinflammatory milieu that amplifies cardiovascular risk.
Hypertension has a significant inflammatory component. T-lymphocytes infiltrate target organs, including the kidney, vasculature, and brain, producing cytokines that promote sodium retention, vascular stiffness, and fibrosis.4 Ang II is a potent proinflammatory stimulus, activating nuclear factor kappa B (NF-κB) and promoting monocyte adhesion to endothelium.12 This inflammatory dimension may partly explain the non-hemodynamic organ protection observed with RAAS inhibitors beyond their blood pressure-lowering effect.
Primary hypertension is polygenic. Genome-wide association studies (GWAS) have identified hundreds of loci with small individual effects, including variants in RAAS component genes, epithelial sodium channel (ENaC) variants such as those causing Liddle syndrome (a monogenic salt-sensitive form), and aldosterone synthase (CYP11B2) variants.13
Several environmental factors amplify hypertensive mechanisms. Dietary sodium excess is particularly important in older adults, those with chronic kidney disease, and Black patients, who exhibit higher rates of salt sensitivity.9 Obesity and insulin resistance contribute through multiple pathways: hyperinsulinemia activates the SNS and promotes renal sodium retention, increased RAAS activity raises angiotensin II levels, and leptin excess provides additional sympathoexcitatory drive.11 Physical inactivity, chronic psychosocial stress, excess alcohol consumption, and low dietary potassium each independently amplify hypertensive mechanisms.6
Cardiovascular, cerebrovascular, renal, ophthalmologic, and vascular consequences of sustained hypertension
Sustained hypertension produces structural and functional injury across multiple organ systems. Recognizing target organ damage is essential for risk stratification and for determining treatment urgency and targets.7
Treatment thresholds, ASCVD risk assessment, and the framework for pharmacological decision-making
Treatment urgency and target blood pressure goals are determined not only by the blood pressure level itself, but by the patient's total cardiovascular risk burden.1,6 The ACC/AHA framework stratifies 10-year atherosclerotic cardiovascular disease (ASCVD) risk using the Pooled Cohort Equations, with high risk defined as 10% or greater.
Established ASCVD (prior myocardial infarction, stroke, peripheral arterial disease, or coronary revascularization) represents the highest-risk category and generally mandates pharmacotherapy regardless of blood pressure stage. Diabetes mellitus, chronic kidney disease (eGFR below 60 mL/min/1.73 m² or albuminuria), left ventricular hypertrophy on ECG or echocardiogram, current smoking, dyslipidemia, age (men at or above 55 years, women at or above 65 years), and a family history of premature ASCVD each contribute meaningfully to overall risk.1
Lifestyle modifications that reduce blood pressure and cardiovascular risk include the DASH diet (which may reduce systolic BP by approximately 11 mmHg in hypertensive patients), sodium restriction below 2.3 g/day, weight reduction, aerobic exercise, alcohol moderation, and smoking cessation.6 These interventions should accompany pharmacotherapy at all stages rather than serve as an alternative to it in high-risk patients.
The BP = CO × TPR framework as the basis for rational drug class selection
Understanding blood pressure as the product of cardiac output and total peripheral resistance helps predict the hemodynamic profile most likely to respond to each drug class.4 This framework is revisited in each pharmacology module as clinical rationale for drug selection and combination therapy.
| Drug Category | Primary Hemodynamic Target | Best-Suited Clinical Context |
|---|---|---|
| Beta-blockers | Reduce cardiac output (HR, contractility) | Young patients; high-sympathetic-tone states; post-MI; HFrEF |
| CCBs, ACEi, ARBs, alpha-blockers | Reduce total peripheral resistance | Established hypertension; older patients; elevated TPR predominates |
| Diuretics | Reduce preload via natriuresis (decreased venous return and CO) | Volume-dependent hypertension; CKD; as adjunct to RAAS inhibition |
| Combined alpha/beta-blockers (labetalol, carvedilol) | Reduce both CO and TPR | Hypertensive emergencies; both elevated CO and TPR are operative |
Early hypertension in younger individuals more often reflects elevated cardiac output from sympathetic overactivation, making beta-blockers particularly effective in this context. Established hypertension in older patients and in those with longstanding disease more often reflects elevated peripheral vascular resistance, where CCBs, RAAS inhibitors, and diuretics have stronger mechanistic rationale and better outcome trial evidence.