Pathophysiology of Hypertension and Hypotension

Introduction

Hypertension and hypotension are common cardiovascular disorders that arise from disturbances in blood pressure regulation. Persistent sympathetic overactivity contributes significantly to primary hypertension, whereas impaired autonomic responses often underlie orthostatic hypotension. Understanding these mechanisms is essential for effective prevention, diagnosis, and management.

Hypertension

Physiological Aspects

Definition

• Hypertension is defined as a sustained elevation of systemic arterial blood pressure documented on repeated measurements.
• Both systolic and diastolic pressures are important in assessing cardiovascular risk.
• Although systolic pressure varies considerably with daily activities, a persistent increase is clinically significant and is associated with adverse cardiovascular outcomes.
• Diastolic pressure reflects peripheral vascular resistance and sustained elevation often indicates increased arteriolar tone.
• Chronic elevation of either systolic or diastolic pressure increases the risk of stroke, myocardial infarction, heart failure, and chronic kidney disease.
• Current guidelines generally define normal blood pressure in adults as less than 120/80 millimeters of mercury.
• Blood pressure values of 120–129 millimeters of mercury systolic with diastolic pressure less than 80 millimeters of mercury are considered elevated.
• Persistent blood pressure values of 130/80 millimeters of mercury or higher are classified as hypertension in many contemporary guidelines.
• Isolated systolic hypertension refers to elevation of systolic pressure with normal diastolic pressure and is particularly common in older adults because of reduced arterial compliance.
• Accurate diagnosis requires multiple blood pressure measurements obtained under standardized conditions on separate occasions.

Types of Hypertension

Hypertension is broadly classified into primary (essential) hypertension and secondary hypertension.

Essential Hypertension

• Primary hypertension accounts for approximately 90–95% of all cases and has no single identifiable cause.
• Multiple factors contribute to its development, including increased sympathetic activity, enhanced renin-angiotensin-aldosterone system activity, and increased blood volume.
• Excess dietary sodium intake, obesity, genetic predisposition, and reduced production of vasodilators such as nitric oxide also play important roles.
• Secondary hypertension results from identifiable causes, including renal, endocrine, vascular, or drug-induced disorders.

Table 100.1: JNC classification of hypertension.

Blood Pressure Category	Systolic Blood Pressure (mm Hg)	Diastolic Blood Pressure (mm Hg)
Normal	< 120	< 80
Prehypertension	120–139	80–89
Stage 1 Hypertension	140–159	90–99
Stage 2 Hypertension	≥ 160	≥ 100

Classification is based on the Joint National Committee (JNC 7) guidelines. The higher category should be assigned if systolic and diastolic values fall into different groups.

Risk Factors

Heredity

• Essential hypertension is a multifactorial disorder influenced by genetic, environmental, and lifestyle factors.
• A positive family history significantly increases the likelihood of developing hypertension, indicating a strong genetic predisposition.

Age

Blood pressure generally rises with advancing age because of progressive arterial stiffening and reduced vascular compliance.

Gender

• Before menopause, women have a lower prevalence of hypertension, partly because estrogen exerts favorable vascular effects.
• After menopause, the prevalence of hypertension in women approaches that observed in men.

Race

Certain ethnic groups, particularly individuals of African ancestry, exhibit a higher prevalence of hypertension and salt sensitivity.

Stress

Chronic psychological stress increases sympathetic nervous system activity, promoting vasoconstriction and sustained elevation of blood pressure.

Smoking

Cigarette smoking acutely raises blood pressure by stimulating catecholamine release and causing vascular dysfunction.

Obesity

• Obesity is a major modifiable risk factor and is associated with increased sympathetic activity, insulin resistance, and expanded blood volume.
• Reduction in body weight is often accompanied by a significant fall in blood pressure.

Hyperlipidemia

Hyperlipidemia accelerates atherosclerosis, thereby increasing vascular resistance and cardiovascular risk.

Sedentary Life

• A sedentary lifestyle predisposes to obesity, dyslipidemia, and hypertension.
• Regular physical activity lowers blood pressure and improves cardiovascular health.

Personality

Individuals exhibiting persistent behavioral stress patterns or heightened sympathetic activity may be more susceptible to hypertension.

Environmental Factors

Environmental influences, including socioeconomic factors, occupational stress, and urbanization, can affect blood pressure levels.

Salt Sensitivity

Salt sensitivity contributes to hypertension in susceptible individuals because excessive sodium intake promotes fluid retention and increases vascular reactivity.

Alcohol

Chronic excessive alcohol consumption is also associated with sustained elevation of blood pressure.

Secondary Hypertension

• Secondary hypertension refers to elevated blood pressure resulting from an identifiable underlying disorder.
• Renal diseases are among the most common causes of secondary hypertension.
• Important renal causes include chronic glomerulonephritis, chronic pyelonephritis, polycystic kidney disease, renal artery stenosis, and renin-secreting tumors.
• Endocrine disorders frequently associated with hypertension include primary hyperaldosteronism, Cushing syndrome, pheochromocytoma, acromegaly, and hypothyroidism.
• Excess production of mineralocorticoids or catecholamines increases vascular resistance and promotes sodium retention.
• Certain neurological conditions, such as raised intracranial pressure and disorders affecting autonomic pathways, may produce neurogenic hypertension.
• Long-term use of oral contraceptive pills can elevate blood pressure by increasing sodium retention and activating the renin-angiotensin-aldosterone system.
• Pregnancy-induced hypertension occurs in complicated pregnancies and is associated with abnormal placental vascular factors and endothelial dysfunction.
• Structural cardiovascular abnormalities, particularly coarctation of the aorta, are important causes of secondary hypertension in younger individuals.
• Hypercalcemia, severe polycythemia, excessive intravenous fluid administration, and prolonged glucocorticoid therapy may also raise blood pressure.
• Identification and treatment of the underlying cause often improve or resolve secondary hypertension.

Experimental Hypertension

• Experimental hypertension refers to elevated blood pressure induced in animal models to investigate the mechanisms of hypertension.
• Renal hypertension can be produced by constricting the renal artery, leading to activation of the renin-angiotensin-aldosterone system.
• In the two-kidney, one-clip model, hypertension is primarily mediated by increased renin secretion.
• Neurogenic hypertension can be induced by denervation of arterial baroreceptors or by lesions involving central cardiovascular regulatory centers.
• Additional experimental models include steroid-induced hypertension, partial reduction of renal mass, and salt-induced hypertension in genetically susceptible animals.
• These models are widely used for evaluating antihypertensive therapies and studying cardiovascular pathophysiology.

Effects of Hypertension

• Chronic hypertension imposes a sustained increase in afterload, forcing the heart to pump against higher vascular resistance.

Effects on CVS

On Heart

• The initial adaptive response is concentric left ventricular hypertrophy, characterized by increased thickness of the ventricular wall.
• Persistent pressure overload eventually leads to ventricular dilatation and progressive decline in cardiac function.
• Long-standing hypertension may culminate in heart failure because of impaired ventricular performance.
• Increased myocardial mass elevates oxygen demand and predisposes individuals to angina pectoris and ischemic heart disease.

On Blood Vessels

• Hypertension also produces structural changes in blood vessels.
• Chronic vascular stress causes thickening and stiffening of arterial walls, thereby reducing vascular compliance.
• Decreased arterial compliance further increases systolic blood pressure and perpetuates hypertension.
• Elevated blood pressure accelerates the development of atherosclerosis, resulting in narrowing of arterial lumina.
• Atherosclerotic involvement of coronary arteries increases the risk of myocardial infarction.
• Cerebral vascular involvement predisposes to ischemic stroke, cerebral thrombosis, and intracerebral hemorrhage.
• Thus, prolonged uncontrolled hypertension causes significant cardiovascular morbidity and mortality.

Effects on CNS

• Chronic hypertension accelerates cerebral atherosclerosis, increasing the risk of ischemic stroke and cerebral infarction.
• Persistent elevated pressure may produce small intracerebral microaneurysms, predisposing to intracerebral hemorrhage.
• Severe uncontrolled hypertension can cause hypertensive encephalopathy and hypertensive retinopathy.

Effects on Kidney

• In the kidneys, vascular sclerosis involving renal arterioles and glomeruli reduces glomerular filtration rate and impairs tubular function.
• Progressive renal damage may manifest as proteinuria, hematuria, and chronic kidney disease.

Physiological Basis of Management

• Management of primary hypertension initially emphasizes lifestyle modification, whereas secondary or advanced hypertension often requires pharmacological therapy.
• Common antihypertensive drugs include diuretics, calcium channel blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, vasodilators, and anti-adrenergic agents.

Weight Reduction

• Excess body weight is strongly associated with elevated blood pressure.
• Reduction of body weight should be an essential component of therapy in overweight or obese individuals.
• Even modest weight loss can produce clinically significant reductions in blood pressure and cardiovascular risk.

Dietary Modification

• Restriction of dietary sodium decreases extracellular fluid volume and lowers arterial pressure.
• Because individual salt sensitivity is difficult to predict, moderate sodium restriction is recommended for most hypertensive patients.
• Reduced sodium intake may also delay or prevent the development of hypertension in susceptible individuals.
• Limiting intake of saturated and trans fats reduces the progression of atherosclerosis and lowers cardiovascular risk.
• A diet rich in fruits, vegetables, whole grains, and dietary fiber helps control body weight and improve lipid profiles.
• Increased fiber intake decreases intestinal absorption of cholesterol and contributes to cardiovascular protection.

Regular Physical Exercise

• Regular aerobic or isotonic exercise is highly effective in reducing blood pressure.
• Sedentary behavior is associated with hypertension, obesity, and other cardiovascular disorders.
• Activities such as brisk walking for 30–60 minutes on most days of the week improve cardiovascular fitness.
• Exercise lowers blood pressure by reducing sympathetic activity, enhancing parasympathetic tone, improving endothelial function, and facilitating weight reduction.

Mental Relaxation

• Chronic psychological stress contributes to sustained elevation of blood pressure through increased sympathetic activation.
• Stress-management strategies, including meditation, relaxation techniques, yoga, and mindfulness practices, may help reduce blood pressure.
• Adequate sleep and maintenance of emotional well-being further support optimal cardiovascular health.

Stoppage of Alcohol and Smoking

• Complete cessation of smoking is essential because tobacco use promotes vasoconstriction, endothelial dysfunction, and atherosclerosis.
• Excessive alcohol intake elevates blood pressure and increases cardiovascular risk.
• Therefore, alcohol consumption should be minimized or avoided in individuals with hypertension.
• Long-term adherence to lifestyle modification significantly improves blood pressure control and reduces the risk of cardiovascular complications.

Antihypertensive Drugs

• Pharmacological therapy is indicated when lifestyle modification alone fails to adequately control hypertension or when blood pressure is markedly elevated at presentation.
• Lifestyle measures should be continued even after initiation of antihypertensive medication.

Vasodilators

Direct vasodilators, such as hydralazine, lower blood pressure by relaxing vascular smooth muscle and reducing peripheral vascular resistance.

Anti-adrenergic Drugs

• Alpha-adrenergic blockers, such as phentolamine and prazosin, reduce vascular tone by inhibiting the vasoconstrictor effects of norepinephrine.
• Beta-adrenergic blockers, including propranolol and atenolol, decrease heart rate and cardiac output.
• Beta blockers also suppress renin release from juxtaglomerular cells, thereby reducing activity of the renin-angiotensin-aldosterone system.

Calcium Channel Blockers

• Calcium channel blockers, such as verapamil, diltiazem, and amlodipine, inhibit calcium entry through L-type calcium channels in vascular smooth muscle.
• This action decreases vascular resistance and lowers blood pressure.
• Verapamil and diltiazem additionally slow atrioventricular conduction and reduce heart rate.

Diuretics

• Thiazide diuretics are commonly used as first-line agents for uncomplicated hypertension.
• They promote sodium and water excretion, decrease plasma volume, and reduce peripheral vascular resistance.
• Loop diuretics, such as furosemide, are particularly useful in patients with renal impairment or heart failure.
• Spironolactone, an aldosterone antagonist, is especially effective in hypertension associated with hyperaldosteronism or resistant hypertension.

ACE Inhibitors

• Agents such as captopril and enalapril inhibit the formation of angiotensin II and increase bradykinin levels, producing vasodilation.
• These drugs are particularly beneficial in patients with diabetes mellitus, chronic kidney disease, or renovascular hypertension.

Angiotensin Receptor Antagonist

Angiotensin receptor blockers, including losartan and valsartan, selectively block angiotensin II type 1 receptors, thereby reducing vasoconstriction and aldosterone secretion.

Hypotension

• Hypotension refers to abnormally low arterial blood pressure, generally defined in adults as a systolic pressure below 90 millimeters of mercury.
• Borderline hypotension is often considered when systolic pressure ranges between 90 and 99 millimeters of mercury.
• Clinically, hypotension may be classified as chronic, acute, or postural (orthostatic) hypotension.

Chronic Hypotension

• Chronic hypotension is characterized by persistently low blood pressure over a prolonged period.
• Many individuals remain asymptomatic, whereas others may experience fatigue, weakness, dizziness, or lethargy.
• Important causes include primary adrenal insufficiency, hypopituitarism, malnutrition, chronic diarrhea, and prolonged bed rest.

Acute Hypotension

• Acute hypotension is a sudden fall in blood pressure that may compromise tissue perfusion.
• Severe reductions in blood pressure can produce syncope, shock, or organ dysfunction.
• Common causes include acute hemorrhage, severe vomiting, diarrhea, excessive diuresis, acute myocardial infarction, and generalized vasodilation, such as in septic or anaphylactic shock.

Postural Hypotension

• Postural (orthostatic) hypotension is defined as a fall in systolic blood pressure of at least 20 millimeters of mercury within a few minutes of standing from a supine position.
• It usually results from impaired autonomic regulation and inadequate compensatory vasoconstriction.
• Patients commonly experience dizziness, lightheadedness, blurred vision, or syncope on standing.

Acute Postural Hypotension

• Acute orthostatic hypotension is usually transient and may cause brief fainting episodes.
• It commonly occurs after prolonged standing, sudden standing following prolonged bed rest, or immediately after strenuous exercise.
• Reduced venous return and temporary pooling of blood in the lower limbs contribute to the fall in blood pressure.

Chronic Postural Hypotension

• Chronic orthostatic hypotension may be primary or secondary.
• Primary orthostatic hypotension is often idiopathic and is commonly seen in older adults because of age-related autonomic dysfunction.
• Secondary causes include diabetic neuropathy, amyloidosis, vitamin B1 deficiency, spinal cord disorders affecting autonomic pathways, use of sympatholytic medications, and surgical sympathectomy.