Generic and Brand Names
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Verapamil — Isoptin, Calan, Covera-HS, Verelan
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Diltiazem — Cardizem, Dilacor XR, Tiazac
Class
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Vaughan Williams Class IV antiarrhythmics
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Non-dihydropyridine calcium channel blockers (CCBs)
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Primary effect on cardiac tissue (AV and SA nodes) rather than systemic vasculature
Mechanism of Action
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Verapamil: Potent L-type calcium channel blocker in nodal tissue and myocardium; marked AV node suppression, negative inotropy
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Diltiazem: L-type calcium channel blocker with intermediate effects between verapamil and dihydropyridines; balanced AV node suppression with moderate vasodilation
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Both slow conduction through the AV node, prolong AV nodal refractory period, and reduce SA node automaticity
Indications
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Supraventricular tachycardia (SVT) — acute and prophylactic therapy
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Rate control in atrial fibrillation and atrial flutter
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Prevention of paroxysmal supraventricular tachycardia (PSVT)
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Angina pectoris (both)
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Hypertension (both, but not first-line for arrhythmia-specific use)
Dosage and Administration
Verapamil
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Acute SVT: 5–10 mg IV over 2 minutes; may repeat after 15–30 minutes
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Oral: 120–360 mg/day in divided doses or SR/ER formulation once daily
Diltiazem
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Acute AF/AFlutter: IV bolus 0.25 mg/kg over 2 minutes; may repeat after 15 minutes with 0.35 mg/kg; follow with infusion 5–15 mg/hour
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Oral: 120–360 mg/day (immediate or sustained-release formulations)
Monitoring
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ECG: PR interval, heart rate, rhythm
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Blood pressure
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Signs of heart failure or worsening LV function
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Liver function with prolonged use
Contraindications (Both)
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Severe hypotension (SBP <90 mmHg)
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Sick sinus syndrome without pacemaker
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2nd- or 3rd-degree AV block without pacemaker
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Severe left ventricular dysfunction or decompensated heart failure
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WPW syndrome with AF/flutter (risk of VF due to accessory pathway conduction)
Precautions
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Use cautiously in bradycardia, heart failure, conduction disturbances
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Caution when combining with beta-blockers (additive AV block, bradycardia)
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Dose adjustments in hepatic impairment
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Avoid abrupt withdrawal in angina patients
Adverse Effects
Common (Both)
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Bradycardia, hypotension, dizziness, headache, flushing, ankle swelling
Verapamil-specific
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Constipation (notable)
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More potent negative inotropy — greater risk in low EF patients
Diltiazem-specific
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Less constipation than verapamil
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Slightly better tolerated in LV dysfunction (but still caution)
Drug Interactions
Both
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Beta-blockers: Increased risk of AV block, bradycardia
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CYP3A4 substrates: Levels affected by inducers/inhibitors
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Digoxin: Verapamil significantly increases digoxin levels; diltiazem less so
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Statins: Increased levels of simvastatin/lovastatin — risk of myopathy
Overdose (Both)
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Manifestations: Severe bradycardia, AV block, hypotension, possible cardiac arrest
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Management: IV calcium, atropine, vasopressors, temporary pacing if required
Patient Counselling
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Take with food or as prescribed
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Avoid grapefruit juice (increases drug levels)
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Monitor pulse regularly; report slow heartbeat, dizziness, fainting, swelling, SOB
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Do not crush or chew sustained-release forms
Comparison Table — Verapamil vs. Diltiazem
| Feature | Verapamil | Diltiazem |
|---|---|---|
| Node effect | Strong AV node suppression | Moderate–strong AV node suppression |
| Vasodilation | Mild | Moderate |
| Negative inotropy | Stronger | Less pronounced |
| Constipation risk | High | Low |
| Use in LV dysfunction | Avoid in moderate–severe | May be tolerated in mild, still caution |
| Digoxin interaction | Significant ↑ digoxin levels | Mild ↑ digoxin levels |
| IV onset (arrhythmia) | 2–3 min | 2–5 min |
| Preferred for | AV node re-entry, PSVT | AF/AFlutter rate control with mild vasodilation benefit |
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