An arrhythmia is a heart rhythm disorder caused by a problem in the electrical system, or "wiring," of the heart muscle. The heart may beat too slowly (bradycardia), too quickly (tachycardia), or in a chaotic or irregular pattern (extra beats or fibrillation).
Medications to treat arrhythmias are chosen based on the nature, frequency and severity of the abnormal heart rhythm, and whether it arises in the upper chambers (atria) or lower chambers (ventricles) of the heart. These medications are divided into four types.
Type I Antiarrhythmics: Sodium Channel Blockers
Type I is divided into three classes (A, B, and C) with slightly different mechanisms of activity.
Each time the heart contracts, it sends out an electrical signal. The signal is generated by the flow of electrolytes through passageways in the heart called "ion channels." Electrolytes, or ions, include sodium, potassium, magnesium and calcium. Class I antiarrhythmic medications block sodium channels and decrease the conduction of electrical signals from cell to cell in the heart.
Class IA antiarrhythmics have been used for many years, typically for supraventricular tachycardia (SVT), abnormal heart rhythms that arise in parts of the heart above the ventricles (lower chambers) or less often in the ventricles themselves. These are called supraventricular arrhythmias. These drugs have only a moderate effect on sodium channels and usually prolong the duration of repolarization - the time it takes to "recharge" the heart after every beat. Some of these drugs, such as quinidine, also reduce the force of heart muscle contractions. They may not be suitable for patients with heart failure and other conditions that weaken the pumping ability of the heart. Class IA medications include:
- Disopyramide (Norpace)
- Procainimide (Procainimide HCI, Procan, Procanabid, Pronestyl)
- Quinidine (Quinidine sulfate, Quinaglute, Quinidex, Cardioquin)
Class IB antiarrhythmics are the least effective at blocking sodium channels. They work by slowing nerve impulses in the heart, but they can make abnormal heart tissue less sensitive. Usually, they shorten the time required for the heart to electrically “reset” or repolarize for a new heartbeat. They are used primarily to treat ventricular arrhythmias. They include:
- Tocainide (Tonocard®)
- Mexiletene (Mexitel®)
- Lidocaine is used in emergency situations, and must be administered intravenously (through a tube into a vein).
- Phenytoin (Dilantin®) belongs to a class of drugs called anticonvulsants and usually is used to treat seizures. It also is used in certain patients with cardiac arrhythmias caused by digitalis medicine.
Class IC antiarrhythmics are strong sodium channel blockers. They also slow nerve impulses in the heart, but have little effect on repolarization. They may be used for supraventricular and some ventricular arrhythmias. Class Ic antiarrhythmics cannot be used in patients who have had a prior heart attack or a weakened heart muscle due to heart failure (CHF).
- Flecainide (Tambocor®)
- Propafenone (Rhythmol®)
There is a chance that some antiarrhythmic drugs (and other types of drugs) may cause new heart rhythm problems, or make existing ones worse. If you have an arrhythmia, it is important to consult with an electrophysiologist or other heart rhythm expert. More than 80 marketed drugs, including some that are not used for heart problems, have been found to block potassium channels, prolong the QT interval (the time it takes the heart to recharge after each beat) and induce a fatal heart rhythm called torsades de pointes in some individuals.
Type II Antiarrhythmics: Beta-Adrenergic Antagonists, or Beta Blockers
Beta-blockers are used in the treatment of high blood pressure (hypertension), to relieve angina (chest pain) and in heart attack patients to help prevent additional heart attacks. Beta-blockers also are used to correct some irregular heartbeats. They affect the response to nerve impulses in certain parts of the body, decrease the heart's need for blood and oxygen and reduce its workload. They also help the heart beat more regularly. Beta-blockers must be used with care in patients with asthma or diabetes. Some common beta-blockers are:
- Acebutolol (Sectral®)
- Atenolol (Tenormin®)
- Betaxolol (Kerlone®)
- Bisoprolol (Zebeta®)
- Carvedilol (Coreg®)
- Esmolol
- Metoprolol(Toprol®, Lopressor®)
- Nadolol (Corgard®)
- Propranolol (Inderal®)
- Timolol (Blocadron®)
Type III Antiarrhythmics: Potassium Channel Blockers
They slow nerve impulses by acting directly on the heart tissues. Type III medications lengthen the duration of repolarization without affecting the heart's normal electrical conduction. Efforts to develop new antiarrhythmic drugs have focused on Type III medications because they are less likely to adversely affect the heart's pumping ability and they act on tissues in both the upper and lower chambers of the heart. Type III drugs used to treat heart rhythm disorders are:
- Amiodarone (Cordarone®)
- Azimilide (Stedicor®)
- Bepridil
- Dofetilide (Tikosyn®)
- Ibutilide (Corvert®)
- Sotalol (Betapace®)
- Tedisamil
At present, Type III antiarrhythmics are generally the most successful drugs for treating both supraventricular (SVT) and ventricular arrhythmias. They often are prescribed in addition to an ICD in patients at high risk for sudden cardiac arrest (SCA). Medications help to reduce the frequency and severity of abnormal rhythms so that patients receive fewer shocks from the ICD. Amiodarone and sotalol are the most frequenty used drugs of this class. Patients taking these and other antiarrhythmic drugs often must be monitored closely by a heart rhythm specialist.
Type IV Antiarrhythmics: Calcium Antagonists, or Calcium Channel Blockers
These drugs help to slow abnormally rapid heartbeats. They also widen the blood vessels and may decrease the heart's pumping strength. They are often used to treat high blood pressure, but usually are not prescribed for people with heart failure or other structural damage to the heart. (They may be used to treat heart faliure caused by a thickened heart muscle, hypertrophic obstructive cardiomyopathy. They also may be useful in treating coronary artery disease, or CAD (clogged blood vessels to the heart).
Some arrhythmias are treated with Type IV antiarrhythmics such as:
- Diltiazim (Cardizem®, Tiazac®)
- Verapamil (Dovera®, Isoptin®, Calan®)
