What these drugs do
Calcium channel blockers (CCBs) come in two flavors. Dihydropyridine CCBs like amlodipine act mostly on blood vessels and are used for blood pressure. Non-dihydropyridine CCBs — diltiazem and verapamil — work on the heart’s electrical system in addition to the vessels. In electrophysiology, the non-dihydropyridines are the ones we use.
Specifically, they slow conduction through the AV node, the electrical relay between the upper chambers (atria) and the lower chambers (ventricles). They also slow the heart rate from the sinus node and modestly reduce the force of heart muscle contraction. That combination makes them useful for slowing the ventricular response in AF, breaking SVT circuits that use the AV node, and preventing SVT recurrence.
Diltiazem and verapamil are siblings rather than identical. Verapamil tends to slow the heart and depress contraction more; diltiazem tends to be slightly gentler on contractility and often better tolerated.
Who we prescribe them for
The main uses:
- Rate control in atrial fibrillation and atrial flutter — slowing the ventricular response so the heart doesn’t race during episodes. We use diltiazem more often than verapamil for chronic AF.
- SVT termination in the emergency department — IV verapamil or diltiazem can break SVT when adenosine is not appropriate.
- SVT prevention — daily oral dosing reduces recurrence of AV nodal reentry tachycardia (AVNRT) and similar SVTs.
- Verapamil-sensitive idiopathic VT — a specific kind of ventricular tachycardia that responds particularly well to verapamil. Most VTs do not, so this is a narrow situation.
We do not use these drugs in:
- Heart failure with reduced ejection fraction (HFrEF) — they further depress contraction and can worsen heart failure.
- Atrial fibrillation in a patient with WPW (Wolff-Parkinson-White) — slowing the AV node in this setting can push more impulses down an accessory pathway and trigger a dangerous rhythm.
- Advanced AV block without a pacemaker.
- Severe bradycardia or sick sinus syndrome without a pacemaker.
If we are not sure whether a patient has WPW, we get an ECG first.
How to take them
Diltiazem:
- Extended-release: usually 120–360 mg once daily. We start low and titrate.
- Immediate-release: 30–90 mg four times daily — less common as a long-term option.
Verapamil:
- Extended-release: 120–360 mg once daily.
- Immediate-release: 40–120 mg three times daily.
Take with or without food. If you miss a dose, take it as soon as you remember unless it’s close to the next dose — never double up.
Side effects to watch for
Common:
- Constipation — particularly with verapamil. This is the most common reason patients ask to switch. Fluid intake, fiber, and sometimes a stool softener help.
- Leg or ankle swelling — relatively common, particularly with higher doses.
- Slow heart rate — sometimes too slow.
- Dizziness or light-headedness.
- Fatigue.
- Headache, flushing.
Less common but important:
- Worsening heart failure in patients with reduced ejection fraction.
- Heart block — particularly when combined with other AV-nodal slowing drugs.
- Liver enzyme elevation — uncommon, but we check at baseline and periodically.
- Gum overgrowth — a quirky side effect of long-term use.
Drug interactions
A few combinations need careful attention:
- Beta-blockers — combining with diltiazem or verapamil meaningfully slows the heart and AV node. We sometimes do this on purpose, but at lower doses and with monitoring.
- Digoxin — verapamil (and to a lesser extent diltiazem) raises digoxin levels. We reduce the digoxin dose.
- Statins — diltiazem and verapamil raise levels of simvastatin and lovastatin; we limit doses or switch to atorvastatin or rosuvastatin.
- Dofetilide — verapamil is contraindicated with dofetilide.
- Amiodarone — combination further slows AV conduction; we watch for excessive bradycardia.
- Carbamazepine, phenytoin — these can change CCB levels in either direction.
Grapefruit juice modestly raises CCB levels and is best avoided.
Monitoring
We check heart rate and blood pressure at follow-up, and an ECG to confirm the rhythm and rate. For long-term use, we periodically check liver enzymes and kidney function.
When to call us
Call about pulse under 50 with symptoms, fainting or near-fainting, new or worsening leg swelling, new shortness of breath, or significant constipation that is not improving with the usual measures.