Yes, Cardizem (diltiazem) is an antiarrhythmic medication. It belongs to Class IV of the Vaughan Williams antiarrhythmic classification, which includes non-dihydropyridine calcium channel blockers. It is FDA-approved to treat atrial fibrillation with a rapid heart rate and paroxysmal supraventricular tachycardia, two of the most common abnormal heart rhythms seen in clinical practice.
How Cardizem Works as an Antiarrhythmic
Cardizem targets L-type calcium channels in the heart. These channels are responsible for carrying calcium into cardiac cells, and calcium flow is what drives the electrical signals through certain parts of the heart, particularly the atrioventricular (AV) node. The AV node acts as a gatekeeper between the upper and lower chambers of the heart, controlling how fast electrical signals pass through.
By blocking calcium from entering these cells, Cardizem slows conduction through the AV node and extends the time the node needs to “reset” before it can fire again (its refractory period). A 20 mg intravenous dose prolongs AV conduction time and the node’s refractory period by roughly 20%, according to the FDA label. This slowing effect is what brings a racing heart rate under control during arrhythmias like atrial fibrillation, where the upper chambers fire chaotically and bombard the AV node with electrical signals.
Which Arrhythmias It Treats
Cardizem has two FDA-approved uses for arrhythmias:
- Atrial fibrillation with rapid ventricular rate (RVR): In atrial fibrillation, the upper chambers quiver instead of contracting normally, sending hundreds of disorganized signals per minute toward the lower chambers. Cardizem slows how many of those signals get through the AV node, bringing the heart rate down to a safer range. It does not convert the rhythm back to normal. It controls the speed, not the pattern.
- Paroxysmal supraventricular tachycardia (SVT): These are episodes of sudden, rapid heartbeat originating above the ventricles. Many types of SVT involve a short-circuit loop that passes through the AV node. By slowing conduction in the node, Cardizem can interrupt the loop and terminate the episode.
An important distinction: Cardizem is a rate-control drug, not a rhythm-control drug. It lowers how fast the ventricles beat during an arrhythmia, but it does not restore normal sinus rhythm on its own. Other antiarrhythmics from different classes are used when the goal is to convert the heart back to a normal rhythm.
Acute and Long-Term Use
In emergency or hospital settings, Cardizem is given intravenously to rapidly slow a dangerously fast heart rate during atrial fibrillation or SVT. The IV form works quickly, typically within minutes.
For ongoing rate control, oral diltiazem is used at doses ranging from 120 to 480 mg daily, according to the American College of Cardiology. Extended-release formulations (sold under names like Cardizem CD, Cartia XT, and Tiazac) allow once-daily dosing. Many people with chronic atrial fibrillation take oral diltiazem indefinitely to keep their resting heart rate in a healthy range.
How It Compares to Verapamil
Verapamil is the other Class IV antiarrhythmic, and it works through the same basic mechanism: blocking L-type calcium channels to slow AV node conduction. In practice, though, diltiazem is the preferred choice for acute rate control in most hospitals. The reason comes down to side effects. Verapamil causes more significant drops in blood pressure and has a stronger tendency to weaken the heart’s pumping force. It also has more drug interactions. Diltiazem is generally better tolerated, which is why it has become the default calcium channel blocker for managing fast heart rhythms.
When Cardizem Should Not Be Used
Despite its effectiveness for certain arrhythmias, Cardizem is dangerous or contraindicated in several cardiac conditions:
- Wolff-Parkinson-White syndrome (WPW): In patients who have an extra electrical pathway between the upper and lower heart chambers, Cardizem can actually speed conduction through that abnormal pathway. During atrial fibrillation, this can trigger ventricular fibrillation, a life-threatening rhythm. IV diltiazem is specifically contraindicated in atrial fibrillation or flutter with a known accessory pathway.
- Ventricular tachycardia: IV diltiazem can cause cardiac arrest in patients with ventricular tachycardia. This is one reason correctly identifying the type of arrhythmia matters before treatment begins.
- Severe heart block or sick sinus syndrome: Because Cardizem slows AV node conduction, it can worsen existing conduction problems unless a pacemaker is in place.
- Significant heart failure: Diltiazem should be used cautiously, if at all, in patients with severe left ventricular dysfunction (an ejection fraction below 30%) or moderate to severe heart failure symptoms. It should not be given after a heart attack if there is evidence of fluid in the lungs, as it can acutely worsen heart failure.
- Low blood pressure: Cardizem is contraindicated when systolic blood pressure is below 90 mmHg or in cardiogenic shock.
These restrictions highlight why the specific type of arrhythmia and the patient’s overall cardiac health determine whether Cardizem is the right choice. It is a highly effective antiarrhythmic for the conditions it targets, but using it in the wrong setting can make a dangerous rhythm worse.