How do antiarrhythmics work?
There are many drugs with very different mechanisms that act as antiarrhythmics. Some affect the heart's ability to take up and use calcium, sodium, or potassium. Charged particles (ions) of these substances play crucial roles in the heart's electrical system. These particles move in and out of heart cells through what are called channels. You can think of a channel as being like a pore in a cell's outer covering or membrane. Some antiarrhythmics act by blocking particular types of channels. Other antiarrhythmics affect the way the nervous system interacts with the heart. These are called beta blockers. Some drugs act as antiarrhythmics and also affect heart rates.
So the bottom line is that this is a fairly complicated topic! There are a lot of different mechanisms that can be involved. A number of useful antiarrhythmic drugs are contraindicated for HCM patients because they tend to increase the force of each heartbeat.
In the rest of this page, we will focus on the major antiarrhythmic drugs used for HCM patients. And we will concentrate on drugs that don't already have their own page here, such as beta blockers, calcium channel blockers, and sodium channel blockers.
Some important antiarrhythmic drugs used in HCM
Amiodarone (Cordarone) is a commonly used antiarrhythmic drug in hypertrophic cardiomyopathy. It is a powerful and effective drug, used to treat both atrial and ventricular arrhythmias. But amiodarone has several potentially serious side effects. These side effects include lung toxicity (2%-7% in some studies, but as high as 10%-17% in others), liver function test abnormalities (4%-9%), hyperthyroidism (about 2%), hypothyroidism (2%-4% in some studies, but as high as 8%-19% in others), proarrhythmia (2%-5%), and optic neuropathy, which can lead to blindness. A minor side effect is skin sensitivity to sunlight, which can be avoided with use of appropriate sunblocks.
It is very important for those on amiodarone to follow up with strict monitoring of lung, liver, and thyroid functions. These tests should be performed every 3-6 months. See your doctor to schedule these tests.
Although amiodarone can have serious side effects, it is a very useful drug when given in the proper doses and monitored carefully. Its negative effects take some time to occur, so it is common for doctors to prescribe amiodarone for relatively short periods of weeks to months. Long-term use can be appropriate if there is sufficient monitoring. If it is intended for long-term use, many doctors suggest avoiding long-term use in younger patients, because over the years they have a greater chance of developing toxicity.
Dronedarone (Multaq) was developed in recent years to find a drug that had many of the advantages of amiodarone with few of its disadvantages. By chemically modifying amiodarone to remove parts that are particularly toxic, drug researchers hoped to build a drug that would be widely usable, highly effective, and with little toxicity. Unfortunately, it has proved to be much less effective than was hoped. While it has little toxicity, a 2018 FDA safety review found that it doubled the rates of death, stroke, and heart failure in patients with permanent atrial fibrillation. Sadly, it is not the near-miracle drug that had been hoped for.
Sotalol (BetaPace) is used to treat several kinds of arrhythmias. It acts by blocking potassium channels. In HCM patients it is used widely to treat atrial fibrillation (afib). Sotalol is effective at controlling afib, but because of its side effects it must be used with caution. Sotalol can prolong the QT interval in the cardiac cycle. When the QT interval becomes too long, there is a danger of a serious ventricular tachycardia which can be fatal. For this reason, Sotalol is always started in a hospital setting so the patient's heart can be monitored.
Dofetilide (Tikosyn) is also used to treat atrial fibrillation and atrial flutter in HCM. Dofetilide acts by blocking potassium channels. Like both versions of Sotalol, Dofetilide can prolong the QT interval in the cardiac cycle, making a dangerous ventricular arrhythmia possible. Dofetilide is also always initiated with a hospital stay. Its effectiveness at controlling atrial fibrillation is comparable to, and possibly better than, Sotalol AF.
Disopyramide (Norpace) is an antiarrhythmic drug but is mainly used in HCM because it can help relieve obstruction. It is discussed further here.
Other tools for managing arrhythmias
Beyond antiarrhythmic drugs there are other tools for treating arrhythmias. One set of tools is implantable cardioverters-defibrillators (ICDs), and pacemakers, which provide electrical stimulus to regulate heart rhythms. Another set of tools is surgical, including catheter ablations and maze surgery. Managing arrhythmias is challenging, and none of these tools is useful for everyone.
