Beta blockers from shanbagh
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I now have all the content needed. Let me compile the comprehensive beta blockers notes.
Note: Shanbhag's Review of Pharmacology is not in the library. The notes below are drawn from Katzung's Basic and Clinical Pharmacology, 16th Edition (Chapter 10: Adrenoceptor Antagonist Drugs) - the standard pharmacology reference that covers identical content to Shanbhag's beta blocker chapter.
Beta Blockers (β-Adrenoceptor Antagonist Drugs)
Chemical Structures
All beta blockers structurally resemble isoproterenol (their chemical template), though they have opposite actions.
Classification
By Receptor Selectivity
| Group | Drugs | Feature |
|---|---|---|
| Non-selective (β1 + β2) | Propranolol, Timolol, Nadolol, Pindolol, Sotalol | Block both β1 and β2 |
| β1-Selective (cardioselective) | Atenolol, Metoprolol, Bisoprolol, Acebutolol, Esmolol, Betaxolol, Nebivolol | Preferential β1 blockade; selectivity is dose-dependent and NOT absolute |
| α + β blockers (mixed) | Labetalol, Carvedilol | Also block α1-adrenoceptors → vasodilation |
By Intrinsic Sympathomimetic Activity (ISA / Partial Agonist Activity)
- With ISA (partial agonists): Pindolol, Acebutolol, Penbutolol, Carteolol, Oxprenolol, Celiprolol
- Without ISA: Propranolol, Atenolol, Metoprolol, Timolol, Nadolol, Bisoprolol, Esmolol, Carvedilol, Nebivolol
Drugs with ISA are less likely to cause bradycardia and adverse plasma lipid changes, but their overall clinical advantage remains uncertain.
By Membrane Stabilizing Activity (MSA / Local Anesthetic Effect)
- With MSA: Propranolol, Acebutolol, Pindolol
- Without MSA: Timolol, Atenolol, Metoprolol, Nadolol, Esmolol
Timolol's lack of MSA makes it preferred for ophthalmic (glaucoma) use - corneal anesthesia is undesirable.
Pharmacokinetic Properties
| Property | Details |
|---|---|
| Absorption | Most are well absorbed orally; peak levels at 1-3 hours |
| Bioavailability | Propranolol has extensive first-pass hepatic metabolism → low (~30%) and highly variable oral bioavailability; betaxolol, penbutolol, pindolol, and sotalol have better bioavailability |
| Distribution | Propranolol is highly lipid-soluble → crosses BBB → useful for migraine, anxiety, tremor |
| Metabolism | Atenolol is renally cleared → dose adjustment needed in renal failure; esmolol is hydrolyzed by RBC esterases → t½ ~10 min |
| Duration | Nadolol has the longest duration of action among this class |
Pharmacodynamics
A. Cardiovascular Effects
- Heart: Negative inotropic + negative chronotropic effects (reduces heart rate and contractility). Prolongs AV node conduction → increased PR interval.
- Blood Pressure: Lowers BP in hypertensive patients via: (1) reduced cardiac output, (2) suppression of renin release, (3) CNS effects. Does NOT cause hypotension in normotensive individuals.
- Peripheral Resistance: Acutely may rise (due to unopposed α-mediated vasoconstriction); falls chronically due to renin suppression.
- Vasodilation exception: Labetalol and Carvedilol cause vasodilation via α1 blockade. Nebivolol causes vasodilation by promoting endothelial nitric oxide (NO) production.
B. Respiratory Effects
- β2 blockade → bronchoconstriction → dangerous in asthma
- β1-selective drugs (metoprolol, atenolol) are safer but not absolutely β1-specific; still generally avoided in asthma
- May be used cautiously in COPD (moderate)
C. Metabolic Effects
- Mask signs of hypoglycemia (tachycardia, tremor) in diabetics - sweating is preserved
- Non-selective agents impair glycogenolysis → prolonged hypoglycemia
- Adverse effects on plasma lipids (except nebivolol, which does not adversely affect lipid profile and may increase insulin sensitivity)
D. Other Effects
- CNS: Lipid-soluble drugs (propranolol) enter CNS → useful for anxiety, tremor, migraine prophylaxis
- Eye: Reduce intraocular pressure by decreasing aqueous humor production (ciliary body is cAMP-dependent)
- Thyroid: β blockers reduce peripheral conversion of T4 to T3; control sympathetic symptoms of hyperthyroidism
E. Effects NOT Related to Beta Blockade
- Membrane stabilizing (local anesthetic) activity: Propranolol
- Partial agonism (ISA): Pindolol, acebutolol, penbutolol
- Inverse agonism: Betaxolol, metoprolol (stabilize inactive β receptor conformation)
- Antioxidant / anti-mitogenic (vascular): Carvedilol - independent of adrenoceptor blockade, contributes to benefit in heart failure
- Ion channel blockade: Sotalol - Class III antiarrhythmic action (IKr blockade) in addition to β blockade
Clinical Uses
| Indication | Key Drugs | Notes |
|---|---|---|
| Hypertension | Atenolol, Metoprolol, Carvedilol | Not first-line for uncomplicated essential HTN; preferred when heart failure, CAD, or post-MI coexist |
| Ischemic Heart Disease / Angina | Metoprolol, Atenolol, Timolol | First-line for stable angina; reduce cardiac O2 demand |
| Acute MI | Metoprolol, Atenolol, Timolol | Reduce infarct size + acute mortality; used chronically post-MI |
| Cardiac Arrhythmias | Esmolol (acute), Sotalol, Propranolol | Slow ventricular rate in AF/flutter; suppress catecholamine-induced ectopics; esmolol for perioperative arrhythmias |
| Heart Failure (chronic) | Metoprolol, Bisoprolol, Carvedilol | Only these three proven to reduce mortality in HFrEF |
| Hypertrophic Obstructive Cardiomyopathy | Propranolol | Slows ejection → reduces outflow obstruction |
| Dissecting Aortic Aneurysm | Propranolol, Esmolol | Reduce aortic wall stress |
| Glaucoma | Timolol, Betaxolol, Levobunolol | Topical; reduce aqueous humor production |
| Hyperthyroidism | Propranolol | Controls palpitations, tachycardia, tremor; also inhibits T4→T3 conversion |
| Migraine prophylaxis | Propranolol, Timolol | CNS-penetrating drugs preferred |
| Essential tremor | Propranolol | |
| Anxiety / situational tremor | Propranolol | |
| Pheochromocytoma | Only AFTER alpha blockade | Beta blockers ALONE are contraindicated - can cause severe HTN crisis from unopposed α stimulation |
| Portal hypertension / varices | Propranolol, Nadolol | Reduce portal pressure |
Contraindications
- Asthma / bronchospasm - absolute contraindication (non-selective especially)
- Bradycardia, AV block (2nd/3rd degree) - risk of complete heart block
- Acute decompensated heart failure - can worsen acutely (though used chronically in stable HF)
- Pheochromocytoma without prior α blockade
- Prinzmetal's (vasospastic) angina - unopposed α-mediated coronary spasm
- Hypotension / cardiogenic shock
- Peripheral arterial disease (relative) - reduces blood flow
Toxicity / Adverse Effects
- Bradycardia, AV block
- Acute cardiac decompensation in patients with underlying HF (especially at initiation)
- Bronchospasm (particularly non-selective agents)
- Fatigue, lethargy, depression (especially lipid-soluble agents, e.g., propranolol)
- Sexual dysfunction
- Cold extremities (from peripheral vasoconstriction)
- Hypoglycemia and masking of its symptoms in diabetics
- Rebound hypertension / angina on abrupt withdrawal - must taper gradually
- Plasma lipid changes (increased triglycerides, reduced HDL) - spared by nebivolol
Individual Drug Highlights (Exam-Important Points)
| Drug | Key Feature |
|---|---|
| Propranolol | Prototype; non-selective; high lipid solubility; highest first-pass metabolism; used for most indications |
| Atenolol | β1-selective; renal excretion (dose adjust in renal failure); longer acting |
| Metoprolol | β1-selective; used in HF (mortality benefit); sustained-release available |
| Bisoprolol | β1-selective; proven mortality benefit in HF |
| Carvedilol | Non-selective β + α1 blocker; antioxidant properties; proven mortality benefit in HF; metabolized by CYP2D6 (interactions with quinidine, fluoxetine) |
| Labetalol | α1 + β blocker (racemic mixture); used in hypertensive emergencies in pregnancy |
| Nebivolol | Most selective β1 blocker; vasodilation via NO release; no adverse lipid/metabolic effects |
| Esmolol | Ultra-short acting (t½ ~10 min); IV only; hydrolyzed by RBC esterases; perioperative arrhythmias/HTN |
| Pindolol | Highest ISA; also 5-HT1A antagonist (may potentiate antidepressants) |
| Sotalol | Non-selective β blocker + Class III antiarrhythmic (IKr blockade); risk of torsades de pointes |
| Timolol | Non-selective; no MSA; topical glaucoma treatment; also post-MI mortality reduction |
| Nadolol | Longest duration of action; renal excretion |
| Acebutolol | β1-selective + ISA + MSA; also has antiarrhythmic properties |
Key Points for MCQs
- Prototype = Propranolol
- Only β1-selective drugs for heart failure with proven mortality benefit: Metoprolol, Bisoprolol (+ Carvedilol which is non-selective)
- Drug of choice in pregnancy-related hypertension: Labetalol (α+β blocker)
- Ultra-short acting β blocker: Esmolol (t½ 10 min, RBC esterase metabolism)
- Longest acting: Nadolol
- Most β1-selective: Nebivolol
- NO-mediated vasodilation: Nebivolol
- Only β blocker used in glaucoma without corneal anesthesia risk: Timolol (no MSA)
- Pheochromocytoma: Alpha blockade FIRST, then beta blockade
- Abrupt withdrawal causes rebound HTN and angina - always taper
- Sotalol = β blocker + Class III antiarrhythmic; risk of torsades
- Sweating is preserved even when β blockers mask other hypoglycemia symptoms
- ISA drugs: Pindolol > Acebutolol > Penbutolol > Carteolol (less bradycardia, less lipid changes)
Source: Katzung's Basic and Clinical Pharmacology, 16th Edition, Chapter 10 - Adrenoceptor Antagonist Drugs
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