Calcium Channel Blockers* - MOA, adverse effects*, therapeutic uses mechanism of action in Angina Pectoris, adverse effects precautions 2nd mbbs pharmacology refer kd tripathi

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Calcium Channel Blockers (CCBs)

(Based on Katzung's Basic & Clinical Pharmacology 16e and Lippincott Illustrated Reviews: Pharmacology - closest equivalents to KD Tripathi content for 2nd MBBS)

I. MECHANISM OF ACTION (MOA)

Calcium is required for smooth muscle and cardiac muscle contraction. During membrane depolarization, voltage-gated L-type (long-acting) calcium channels open and allow Ca²⁺ to enter cells down its concentration gradient.

CCBs block voltage-sensitive L-type calcium channels in cardiac and vascular smooth muscle, preventing Ca²⁺ influx. This results in:

Vascular smooth muscle relaxation - arteriolar vasodilation, reduced peripheral vascular resistance (PVR), reduced afterload
Negative inotropy - reduced force of myocardial contraction (especially verapamil)
Negative chronotropy & dromotropy - slowed SA node automaticity and AV nodal conduction (especially verapamil and diltiazem)

Calcium influx is also increased in ischemia (due to membrane depolarization from hypoxia), which activates ATP-consuming enzymes and worsens injury. CCBs protect ischemic tissue by blocking this entry.

II. CLASSIFICATION

Class	Drug(s)	Vascular Selectivity	Cardiac Effect
Phenylalkylamine	Verapamil	Low (more cardiac)	Strong -ve inotrope & chronotrope
Benzothiazepine	Diltiazem	Intermediate	Moderate -ve inotrope & chronotrope
Dihydropyridines (DHPs)	Nifedipine, Amlodipine, Felodipine, Nicardipine	High (peripheral vascular)	Minimal direct cardiac effect

Memory aid: "Verapamil = Vessel + heart; Nifedipine = predominantly peripheral."

Amlodipine (DHP) is most peripherally active → Diltiazem is intermediate → Verapamil has the strongest cardiac (myocardial) effects.

III. THERAPEUTIC USES

1. Hypertension

All CCBs are effective antihypertensives - reduce PVR via arteriolar vasodilation.
Long-acting DHPs (amlodipine, felodipine) or extended-release nifedipine are preferred.
Avoid short-acting nifedipine (causes reflex tachycardia and fluctuating BP).

2. Angina Pectoris (detailed below)

3. Cardiac Arrhythmias

Verapamil and diltiazem are used for SVT (supraventricular tachycardias), rate control in atrial flutter/fibrillation.
They slow AV node conduction (negative dromotropic effect).

4. Hypertrophic Cardiomyopathy

Verapamil improves diastolic relaxation and reduces outflow obstruction.

5. Raynaud's Phenomenon

DHPs (nifedipine) reduce vasospasm in peripheral vessels.

6. Migraine Prophylaxis

Verapamil is used off-label for migraine prevention.

7. Esophageal Spasm

Nifedipine relaxes esophageal smooth muscle.

IV. MECHANISM OF ACTION IN ANGINA PECTORIS

Angina results from an imbalance between myocardial oxygen supply and demand. CCBs address both sides:

A. Stable (Effort) Angina

CCBs reduce myocardial oxygen demand by:

Decreasing afterload - arteriolar vasodilation lowers PVR, reducing cardiac work
Decreasing contractility (verapamil/diltiazem) - reduces O₂ consumption
Decreasing heart rate (verapamil/diltiazem) - prolongs diastole, improves coronary perfusion time

They also increase oxygen supply by dilating coronary arteries.

From Katzung: "In the treatment of effort-induced angina, calcium channel blockers reduce myocardial oxygen consumption by decreasing vascular resistance, thereby decreasing afterload."

B. Vasospastic (Variant/Prinzmetal) Angina

CCBs are the drugs of choice - they directly relax coronary artery spasm.
Beta-blockers are contraindicated in vasospastic angina (may worsen spasm by leaving alpha-adrenergic vasoconstriction unopposed).
Nitrates + CCBs together are very effective.

C. Unstable Angina

CCBs are used as adjunct therapy but beta-blockers and antiplatelet agents take priority.

Combination in Angina

CCB + beta-blocker: additive reduction in heart rate and BP; however, combining verapamil or diltiazem with beta-blockers risks severe bradycardia and AV block (use DHPs like amlodipine instead).
CCB + nitrate: combined vasodilation, counteracts reflex tachycardia from nitrates.

Angina Treatment Algorithm:

V. ADVERSE EFFECTS

A. Dihydropyridines (Nifedipine, Amlodipine, Felodipine)

Adverse Effect	Mechanism
Peripheral edema (most common)	Arteriolar dilation - fluid shifts to interstitium
Flushing / headache	Vasodilation
Reflex tachycardia	Especially with short-acting nifedipine - compensatory sympathetic activation from rapid BP drop
Hypotension	Excess vasodilation
Worsening angina	Short-acting nifedipine can precipitate angina (avoid immediate-release formulations)

"Use extended-release formulations (short-acting agents can worsen angina)" - Lippincott

B. Non-Dihydropyridines (Verapamil, Diltiazem)

Adverse Effect	Mechanism
Bradycardia	SA node depression
AV block	AV nodal depression
Constipation	Very common with verapamil (smooth muscle relaxation in gut); less with diltiazem
Gingival hyperplasia	Verapamil (also seen with nifedipine and phenytoin)
Heart failure exacerbation	Negative inotropy - avoid in systolic HF
Edema	Particularly diltiazem

C. Class-wide adverse effects

Dizziness, nausea
Interaction with CYP3A4 (verapamil, diltiazem, nicardipine inhibit CYP3A4 and P-glycoprotein - increased levels of co-administered drugs)
Amiodarone + verapamil/diltiazem: additive AV conduction depression

VI. PRECAUTIONS AND CONTRAINDICATIONS

Situation	Drug to Avoid	Reason
Systolic heart failure (HFrEF)	Verapamil, diltiazem	Negative inotropy worsens pump function
Bradycardia / Sick sinus syndrome	Verapamil, diltiazem	Further rate reduction
AV block (2nd/3rd degree)	Verapamil, diltiazem	Further conduction suppression
Vasospastic angina	Beta-blockers preferred - not CCBs (actually CCBs ARE preferred here; avoid beta-blockers)	-
Concurrent beta-blocker therapy	Verapamil or diltiazem	Risk of severe bradycardia, AV block, and cardiac failure; DHPs (amlodipine) are safe to combine
Post-MI (recent)	Short-acting nifedipine	Associated with increased mortality
Pregnancy	Use with caution; nifedipine is sometimes used for preterm labour (tocolysis)	-
Wolff-Parkinson-White + AF	Verapamil	Can accelerate accessory pathway conduction - risk of ventricular fibrillation

VII. QUICK DRUG PROFILES (2nd MBBS High-Yield)

Drug	Class	Key Use	Key SE
Nifedipine	DHP	HTN, vasospastic angina, Raynaud's	Reflex tachy, flushing, edema
Amlodipine	DHP (long-acting)	HTN, stable angina	Edema, less tachycardia
Verapamil	Phenylalkylamine	SVT, HTN, HCM, migraine	Constipation, bradycardia, AV block, HF
Diltiazem	Benzothiazepine	SVT, HTN, angina	Bradycardia, AV block, edema (intermediate profile)

Key Exam Points (KD Tripathi style mnemonics)

"VAN" rule for vasospastic angina: Verapamil, Amlodipine, Nitrates - YES; Beta-blockers - NO
Verapamil's 3 C's: Cardiac conduction depression, Constipation, Contractility reduction
Amlodipine advantage: Long half-life (~35-50 hours), once-daily dosing, no reflex tachycardia, safe to combine with beta-blockers
Nifedipine IR risk: Short-acting nifedipine causes reflex tachycardia and may precipitate MI - use only extended-release
CCBs are preferred in: asthma + angina (beta-blockers are contraindicated), elderly hypertensives, angina + peripheral vascular disease

Sources: Lippincott Illustrated Reviews: Pharmacology (p. 416-423); Katzung's Basic & Clinical Pharmacology, 16th Edition (Chapters 12, 13)

Note: KD Tripathi's "Essentials of Medical Pharmacology" is not available in this library, but Katzung and Lippincott cover identical content for 2nd MBBS syllabus and are widely accepted as equivalent references.

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