What You Need to Know About Anti-hypertensive Agents

Anti-hypertensive agents; know which class to use. If you are hypertensive or know anybody that has hypertension, it’s best to know the anti-hypertensive agents, their mechanism of action, and when to use any of the classes.

 

What are Anti-hypertensive Agents?

This refers to those agents or classes of drugs that are used for the management of hypertension. The major categories of anti-hypertensive drugs are:

  1. Sympatholytic
  2. Diuretics
  3. Renin-inhibitors
  4. Vasodilators
  5. Angiotensin-converting enzymes inhibitors
  6. Angiotensin 2 receptor antagonists
  7. Calcium channel blockers

 

Read also: What is Pain and Analgesics

 

Sympatholytic

These are classes of anti-hypertensive that decrease the activities of the sympathetic nervous system.

Examples include:

  1. Beta-adrenergic blocking agents (i.e. beta-blockers)
  2. Alpha-adrenergic blocking agents or blockers
  • Centrally acting alpha agonists
  1. Postganglionic adrenergic neuron blockers (neuronal blockers)
  2. Ganglion blockers

 

Beta-adrenergic blockers

Examples are

  • Propranolol
  • metoprolol
  • Atenolol
  • Labetalol
  • Timolol
  • Carvedilol
  • Nebivolol
  • Oxprenolol
  • Penbutolol
  • Bisoprolol
  • Nadolol
  • Carteolol
  • Pindolol
  • Betaxolol acebutolol

 

Mechanism of Action

  1. This class of anti-hypertensive agents selectively block the beta one (1) adrenergic receptors in the heart and other tissues, thus reducing cardiac output by decreasing the heart rate and contractility.
  2. They also reduce sympathetic outflow from the CNS.
  3. They inhibit renin secretion

Note: Beta-blockers have a greater tendency of occupying the beta one (1) receptor in the heart rather than the beta two (2) receptor in the lungs, this is called relative cardio selectivity.

Relative cardio selectivity is dose-dependent and is lost as dosages are increased. Therefore, no beta-blocker is totally safe in patients with bronchospastic disease e.g. asthma and Chronic Obstructive Pulmonary Disease (COPD)

 

Indications 

It is used in mild hypertensive patients. Other uses include; angina, congestive heart failure, and secondary prophylaxis in myocardial infarction and cardiac arrhythmia

 

Adverse effects 

This includes:

  • Rebound hypertension on sudden withdrawal
  • Broncho constriction
  • Bradycardia
  • GIT upset
  • Nightmares
  • Forgetfulness

 

Read also: What is Pneumonia? All You Need To Know

 

Alpha-adrenergic blockers

examples are

  • Terazosin
  • Doxazosin
  • Prazosin
  • Indoramin
  • Tolazoline
  • Phentolamine
  • Phenoxybenzamine

 

Mechanism of Action

They selectively block alpha-adrenergic receptors causing vasodilation of both arteries and veins.

 

Indications

  • It is used in mild to moderate hypertensive patients
  • It is also used in diabetic patients and also increases High-Density Lipoprotein (HDL) and decreases low-Density Lipoprotein (LDL) and Triglycerides (TGs)
  • Other uses include; benign hypertrophy of the prostrates

 

Adverse effects

  • Postural hypotension and syncope (first dose effect)
  • Impotence (inhibits ejaculation)
  • Nasal stuffiness
  • Dry mouth
  • Headache
  • Drowsiness
  • Fluid retention
  • Blurred vision

 

Centrally Acting Alpha Agonists

Examples include

  • Methyldopa (Aldomet)
  • Clonidine
  • Guanabenz
  • Moxonidine
  • Guanfacine

 

Mechanism of Action

They act primarily within the CNS on alpha two (2) receptors to decrease sympathetic outflow to the cardiovascular systems.

 

Indications

  • They are used as alternatives to initial hypertensive in mild to moderate hypertension
  • Methyl dopa is often preferred for the treatment of hypertension in pregnant women since long experience has shown that it does not harm the fetus.

 

Side Effects

  • Sedation
  • Dry mouth
  • Methyl dopa is known to cause coomb’s positive hemolytic anemia.

 

Ganglion blockers

Examples are

  • Trimethaphan
  • Hexamethonium
  • Pentolium
  • Mecamylamine
  • Tubocurarine
  • Pempidine
  • Chlorisondamine
  • Benzohexonium

 

Mechanism of action

They inhibit autonomic activities by interfering with neurotransmission with autonomic ganglion.

 

Indications

  • Trimethaphan is used in hypertensive emergencies

 

Adverse effects

  • Impotence
  • Constipation
  • Urinary retention
  • Dry mouth due to parasympathetic effect.

 

 

Neuronal blockers

Examples are

  • Reserpine
  • Guanethidine
  • Guanadrel
  • Bethanidine
  • Betylium
  • Debrisoquine

 

Mechanism of action

  • Reserpine acts centrally as well as peripherally by depleting catecholamine stores in the brain and in the peripheral adrenergic system.

 

Indications

  • The use of reserpine and guanethidine and other neuronal blockers are steadily declined with the advent of agents that are safer and more effective.
  • Reserpine is still in use in the treatment of mild hypertension in combination with diuretics.

 

Adverse effects

  • Sedation at high dose
  • Because reserpine also depletes norepinephrine in the CNS and may depress the mood, it should be avoided in patients with a history of depression.

 

Vasodilators

Vasodilators are used to treat hypertension, heart failure, and angina; however, some vasodilators are better suited than others for these indications.

Some vasodilators that act primarily on resistance vessels (arterial dilators) are used for hypertension, heart failure, and angina; however, reflex cardiac stimulation makes some arterial dilators unsuitable for angina.

Venous vasodilators are very effective for angina, and sometimes used for heart failure, but are not used as primary therapy for hypertension.

 

Examples of Vasodilators

  • Hydralazine
  • Nitroprusside
  • Minoxidil
  • Diazoxide
  • Benazepril
  • Captopril
  • Fosinopril
  • Moexipril
  • Perindopril
  • Quinapril
  • Enalapril

 

Mechanism of action

  • They act by directly relaxing peripheral vascular smooth muscles – arterial, venous or both

 

Indications

  • They are used primarily in combination with other antihypertensive agents for the treatment of moderate and severe hypertension.
  • They are used as second-line treatment in patients that are refractory to initial therapy of other drugs.
  • Hydralazine is used intravenously or intramuscularly in managing hypertensive crises.

 

Adverse effects

  • When used alone, they cause reflex tachycardia, fluid retention, and angina hence they need to combine them with other antihypertensive agents such as diuretics plus either a beta-adrenergic blocker or other sympatholytic agents.
  • Hydralazine has been associated with lupus-like syndrome whereas minoxidil may cause hypertrichosis (increase in hair growth) particularly in women. In fact, minoxidil has been marketed as a topical formulation for the treatment of several forms of baldness in men and women.

 

Calcium Channel Blockers

Calcium channel blockers are sometimes classified as vasodilators while others classify them among the sympatholytic. Examples are; nifedipine, amlodipine, verapamil, cilnidipine, clevidipine, isradipine, nisoldipine, nimodipine, felodipine, nitredipine, lercanidipine, diltiazem etc.

 

Mechanism of Action

  • Currently approved calcium-channel blockers (CCBs) bind to L-type calcium channels located on the vascular smooth muscle, cardiac myocytes, and cardiac nodal tissue (sinoatrial and atrioventricular nodes). These channels are responsible for regulating the influx of calcium into the muscle cells, which in turn stimulate the smooth muscle contraction and cardiac myocyte contraction.

 

Indications

They are used as alternatives for the initial treatment of hypertension in a select patient populations that are unable to take beta-adrenergic blockers, such as patients with high coronary disease risk or diabetes mellitus, who also have a bronchospastic diseases such as asthma or Reynold’s disease. However, blacks and elderly patients respond well to this agent.

 

Adverse Effects

  • Nifedipine use has been associated with flushing, headache, and peripheral edema. Using the sustained release product daily has shown to effectively reduce this effect.
  • Verapamil is associated with constipation.
  • Other adverse effects are; dizziness, redness in the face, gingival overgrowth, rapid heart rate, slow heart rate, etc.

 

Note: Beta-adrenergic blockers when used with calcium channel blockers may have an additive effect on inducing heart failure and bradycardia. Hence, concomitant use of both should be avoided.

 

Renin-inhibitors

Renin comes one level higher than the angiotensin-converting enzyme (ACE) in the renin-angiotensin system. Inhibitors of renin can therefore effectively reduce hypertension.

Aliskiren (developed by Novartis) is a renin inhibitor that has been approved by the U.S. FDA for the treatment of hypertension. Others are; valturna, tekturna HCT, amturnide, hydrochlothiazide/aliskerin, aliskerin/amlodipine etc.

 

Diuretics

Most people with hypertension, of which 90-95% have hypertension of unknown origin (primary or essential hypertension), are effectively treated with diuretics. Anti-hypertensive therapy with diuretics is particularly effective when coupled with reduced dietary sodium intake.

The efficacy of these drugs is derived from their ability to reduce blood volume, cardiac output, and with long-term therapy, systemic vascular resistance.

Some examples of diuretics are:

  1. Loop diuretics
  • Bumetanide
  • Ethacrynic acid
  • Torsemide
  • Furosemide
  1. Thiazide diuretics
  • Epitizide
  • Hydrochlothiazide
  • Bendroflumethiazide
  • Methyclothiazide
  • Polythiazide
  1. Thiazide-like diuretics
  • Indapamide
  • Chlorthalidone
  • Metolazone
  1. Potassium-sparing diuretics
  • Amiloride
  • Triamterene
  • Spironolactone
  • Eplerenone
  1. Carbon anhydrase inhibitors
  • Acetazolamide
  • Methazolamide
  • Dorzolamide

 

 

Mechanism of Action

Diuretic drugs increase urine output by the kidney (i.e. promotes diuresis). This is accomplished by altering how the kidney handles sodium. If the kidney excretes more sodium, then water excretion will also increase. Most diuretics produce diuresis by inhibiting the reabsorption of sodium at different segments of the renal tubular system.

 

Adverse Effects

The most important and frequent problem with thiazide and loop diuretics is hypokalaemia. This sometimes requires treatment with a potassium-sparing diuretic. A potentially serious side effect of potassium-sparing diuretics is hyperkalemia.

 

 

Angiotensin-converting enzyme (ACE) inhibitors

ACE inhibitors are considered ‘’first-line’’ therapy in the treatment of stage one hypertension. They may also be used in hypertension caused by renal artery stenosis, which causes renin-dependent hypertension owing to the increased released of renin by the kidneys.

 

Mechanism of Action

ACE inhibitors produce vasodilation by inhibiting the formation of angiotensin II. This vasodilation is form by the proteolytic action of renin (released by the kidney) action on circulating angiotensinogen to form angiotensin I. Angiotensin I is then converted to angiotensin II by ACE.

 

Adverse Effects

  • Hypotension
  • Cough
  • Headache
  • Dizziness
  • Fatigue
  • Renal impairment
  • Hyperkalaemia etc.

 

 

Angiotensin II receptor blockers (ARB’S)

ARB is used in the treatment of hypertension and heart failure in a similar manner as ACE inhibitors. Some examples are Teveten, Diovan, Cozaar, Micardis, Atacand, etc.

 

Mechanism of Action

ARB are receptor antagonists that block type I angiotensin II receptors of blood vessels and other tissues. These receptors are coupled to Gq-proteins and IP3 signal transduction pathway that stimulates vascular smooth muscle contraction.

 

Adverse Effects

  • Dizziness
  • Headache
  • Hyperkalaemia
  • Hypotension
  • Rash
  • Diarrhea
  • Dyspepsia
  • Abnormal liver function etc.

 

 

 

Conclusion

Hypertension may be primary, which may develop as a result of environmental or genetic causes, or secondary, which has multiple aetiologies including renal, vascular, and endocrine causes. Knowing the anti-hypertensive agent to use and using them on time helps to prevent complications of high blood pressure such as stroke and myocardial infarction.

Evidence suggests that reduction of hypertension by 5mmHg can decrease the risk of stroke by 34%, ischemic heart disease by 21%, and reduces the likelihood of dementia, heart failure, and mortality from cardiovascular disease.

Leave a Comment

Your email address will not be published. Required fields are marked *