Beta Adrenergic Receptor Antagonist / Beta-Blockers

BETA ADRENERGIC RECEPTOR ANTAGONIST / BETA-BLOCKERS


CLASSIFICATION:

EFFECTS:

  • Beta-blockers have common effects and particular effects.

Common effects 

  • The effects of beta-blockers are primarily cardiovascular.
    • Slow heart rate by ↓slope of slow diastolic depolarization and constitute group II of antiarrhythmic drugs. 
    • ↓ heart oxygen requirement,  preventive treatment of angina pectoris.
    • ↓ pathological arterial hypertension,
    • ↓ cardiac output
    • ↓ renin secretion, 
    • ↓ sympathetic tone by a central effect.
  • Beta-blockers ↓intraocular pressure by decreasing aqueous humor secretion.

Particular effects

  • Beta-mimetic activity or intrinsic sympathomimetic activity or ISA.
  • True beta-blockers – Prevent endogenous catecholamines from inducing beta effects.

Beta-1 receptors blockerCardioselective/selective β1/II generation beta blockers:

  • Preferred in diabetes mellitus , bronchial asthma, peripheral vascular disease & hyperlipidemia.

Nebivolol – 

  • NO-mimetic vasodilatator effect.
  • Most cardioselective.

Beta blockers with membrane-stabilizing effect:

  • Exhibits “Local anesthetic or antiarrhythmic effect” by reducing transmembrane ion exchanges. 
  • Due to decreased depolarization rate by sodium entry.
  • Should be avoided in Glaucoma – Risk of corneal anaesthesia.
  • Pindolol, Acebutolol, Carvedilol, Betaxolol, Propranolol, Metoprolol & Labetalol.
    • Propranolol inhibits transport of iodine in thyroid follicle.
    • Carvedilol would have, at least in vitro, antioxydant properties. 

PHARMACOKINETICS:

Acts by being liposoluble & water-soluble drugs.

  • Liposoluble beta-blockers (propranolol & oxyprenolol):
    • Quickly and completely absorbed by the digestive tract.
    • Metabolized in liver and can undergo an inactivation known as first metabolism. 
    • Bound at 90% to plasma proteins.
    • Great volume of distribution, i.e. they diffuse readily in tissues.
    • Short duration of action (Esmolol)
    • Short plasma half-life.

Water-soluble beta-blockers like atenolol, nadolol and sotalol:

  • Less absorbed by digestive tract & more irregular way.
  • Little metabolized by liver.
  • Do not cross blood brain barrier.
  • Not very bound to plasma proteins.
  • Eliminated primarily by kidney in unchanged form – Hence, contraindicated in renal failure.
  • Restricted volume of distribution.
  • Long plasma half-life(Atenolol).
  • Nadolol has longest half-life.
  • Intermediate products such as pindolol & celiprolol.
  • Labetalol has both alpha and & beta blocking action.
  • Elimination in milk – Contraindicated during lactation.

USES:

Cardiovascular uses:

  • Angina pectoris(unstable angina)

Tachycardia:

  • Includes hyperthyroidic patients (Grave’s disease) taking propranolol.
    • Arterial hypertension
    • Myocardial infarction 

Congestive heart failure

  • Three beta-blockers having shown efficacy in this therapeutic use are 
    • Bisoprolol, 
    • Metoprolol 
    • Carvedilol.

Other therapeutic uses:

  • Prevention of primary and secondary digestive bleeding in portal hypertension by rupture of esophageal varices(propranolol is usually used)
  • Migraine, tremor, transitory somatic symptoms of anxiety, alcohol addiction in which there appears a beta overstimulation (propranolol is usually prescribed).
  • Glaucoma – Primary open angle glaucoma.
    • By ophthalmic solutions.
    • Diffuses into general circulation causing adverse effects.
  • Atherosclerosis

ADVERSE EFFECTS:

  • Aggravation of congestive heart failure (with calcium channel blockers).
  • Non-selective (Ist generation beta blockers) mask all effects of hypoglycemia Sweating, Palpitations, Dizziness.
  • Congestive heart failure is both an indication & contraindication to beta-blockers use.
    • Aggravation of bradyarrythmia; bradycardia, atrioventricular blocks. 
    • Beta-blockers like intrinsic sympathomimmetic activity like celiprolol, Oxprenolol, Pindolol, Pendutolol, Acebutolol, Alprenolol are useful.
    • Due to  partial agonistic activity apart from bloacking activity.

Masked hypoglycemic symptoms in diabetics:

  • Tachycardia, sweating & tremors – Due to sympathetic stimulation acts as warning sings.
    • Beta-blockers mask these symptoms (Except sweating) – Mediated via cholingeric system.
    • Hence, patient directly fall into coma.
  • Delayed recover from hypoglyβ1cemia – Due to βmediated hyperglycemia.
  • Raised risk of developing type II diabetes after antihypertensive treatment.

Aggravation of asthmatic disease:

  • Bronchoconstriction can occur due to βblockade.
  • Causes hypotension, bradycardia, heart rhythm disorders & shock.
  • Coldness of extremities.
  • Worsened risk of anaphylactic shock.

Metabolic disorders:

  • Increases triglycerides, cholesterol and VLDL (very low density lipoproteins).

Rebound of symptoms: 

  • Tachycardia, arterial hypertension, fainting, sweats, nervousness , especially risk of angina, myocardial infarction and sudden death.
  • Diffusion in body after topical use of ophthalmic solutions elicit bradycardia, asthma, in elderly.

Various, rare disorders: 

  • Immunological disorders, lupus, fibroses.

CONTRAINDICATIONS :

  • Decompensated CCF
  • Bradycardia , HR< 60/min
  • COPD
  • Asthma
  • Variant angina
  • Carbohydrate intolerance
  • Hyperlipidemia
  • During lactation – Elimination in milk.
  • Atherosclerosis
  • Sick sinus syndrome
  • Partial and complete heart block
  • Raynaud’s disease – 
    • Due to vasoconstriction.
    • Worsens peripheral vascular disease.
  • Sotalol is contraindicated in renal failure.

Exam Important

  • Beta blockers are contraindicated in Decompensated CCF, Asthma , Atherosclerosis
  • Beta blockers are antiarrhythmic agents TYPE II
  • Acebutol, Atenolol & Metoprolol are cardioselective beta blocker
  • Beta blockers are contraindicated in Asthma
  • Atenolol is longer acting than metoprolol 
  • Labetalol has both alpha and & beta blocking action
  • Nadolol has longest half life
  • Sotalol  is contraindicated in renal failure
  • Beta blocker without local anaesthetic effect is Atenolol
  • Beta blockers are contraindicated in Sick sinus syndrome
  • Combination use of beta blockers and calcium channel blockers cause  Heart block
  • Contraindication of topical beta blockers asthma
  • First line drug choice for management of hypertension in patients with angina Beta Blockers
  • Effect of beta blocker’s on heart are  Decrease in heart rate, Decreases cardiac output & Precipitates heart failure
  • First line drug for primary open angle glaucoma is Beta blockers
  • Shorest acting beta blocker Esmolol
  • Mechanism of action of timolol is  Nonselective beta blocker
  • Beta blockers mask all effects of hypoglycemia Sweating, Palpitations, Dizziness
  • Lipid insoluble beta blockers Are long acting
  • Lipid insoluble beta blockers are Incompletely absorbed orally
  • Lipid insoluble beta blockers Do not cross blood brain barrier

 

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