Class I are sodium-channel blockers. Reduce phase 0 slope and peak of action potential. Class I drugs are broken down into 3 classes based on degree of sodium channel blockade.

Class IA have moderate Na+ channel blocking effects. Moderate reduction in phase 0 slope; increase APD; increase ERP.

Class IB are weak sodium-channel blockers. Small reduction in phase 0 slope; reduce APD; decrease ERP.

Class IC are strong sodium-channel blockers. Large reduction in phase 0 slope; no effect on APD or ERP.

Class II are beta-blockers. Block sympathetic activity; reduce heart rate and conduction.

Class III are potassium-channel blockers. Delay phase 3 repolarization and consequently increase APD and ERP.

Class IV are calcium-channel blockers. Most effective at SA and AV nodes; reduce heart rate and conduction.

Study note: Know that Class 1C agents may be very dangerous in patients with CAD or diminished LV function. These agents are now only used in patients with Afib who have otherwise very healthy hearts. Be able to describe action potential duration (APD) and effective refractory period (ERP).

Class 1a:
1. disopyramide
2. procainamide
3. quinidine

Class 1b:
1. lidocaine (most important class 1b drug)
2. phenytoin (sometimes considered class 1b drug)
3. mexiletine
4. tocainide (no longer sold in the US)

Class 1c:
1. flecainide
2. propafenone

These three drugs are class III antiarrhythmics. Their mechanism of action is blocking potassium channels during repolarization of cardiac cells. These agents prolong the duration of the action potential WITHOUT altering Phase 0 of depolarization or the resting membrane potential. Instead they prolong the REFRACTORY PERIOD.

Amiodarone (Cordarone/Pacerone)
This drug’s dominant effect is prolonging the action potential duration and the refractory period. It has antianginal as well as antiarrhythmic properties. The effects are complex showing class I, II, III, and IV actions. It contains iodine and is structurally related to thyroxine.

It is effective in the treatment of severe refractory supraventricular and ventricular tachyarrhythmias. Amiodarone has a prolonged half-life of several weeks because it distributes extensively in adipose tissue (high volume of distribution). Full effects may not be seen for up to six weeks.

Sotalol (Betapace)
This medication is a class III antiarrhythmic drug as well as a potent Beta blocker. It blocks a rapid outward potassium current. This blockade prolongs both repolarization and the duration of the action potential, therefore lengthening the effective refractory period.  It does have a strong antifibrillatory effect.Sotalol has the lowest rate of adverse side effects. Like any medication that prolongs the QT interval, there is a serious risk of torsade de pointes (3-4% of patients).

Dofetilide (Tikosyn)
This medication is used often as a first-line antiarrhythmic for patients with Afib, CHF, or CAD with impaired LV function.Note that these drugs increase APD and ERP.

Calcium channel blockers are Class 4 antiarrhythmic drugs. They decrease the inward current carried by calcium, resulting in decreased rate of spontaneous depolarization and they bind to open, depolarized channels, thereby preventing repolariztion until the drug wears off. They also slow the conduction in tissues that are dependent on calcium currents, such as the AV node. They block most effectively when the heart is beating fast. The major effect is seen on vascular smooth muscle and the heart.

They are indicated for the treatment of atrial arrhythmias, re-entrant SVT, and reduction of ventricular rate in atrial flutter and atrial fibrillation. They are also used to treat hypertensive patients who also have asthma, angina, diabetes, and/or peripheral vascular disease

They are contraindicated in patients with pre-existing depressed cardiac function (due to their negative inotropic properties). In patients with hepatic dysfunction, verapamil should be avoided because it is extensively metabolized in the liver.

Side effects include: constipation, dizziness, headache, and feeling of fatigue

verapamil- it is the least selective; shows greater action in the heart (negative inotrope) than on vascular smooth muscle, but it has significant effects on both; used for the treatment of angina, supraventricular tachycardias, and migraine headache; high incidence of side effects

diltiazem (Cardizem)- has both cardiac and vascular smooth muscle effects, but has less pronounced negative inotropic effects on heart; very few side effects; used for treatment of SVT, angina, and HTN; use with caution with beta blockers

nicardipine (Cardene)-shows greater action on vascular smooth muscle than on the heart; used for the treatment of hypertension, angina; safe to use with other cardiovascular drugs (warfarin, digoxin, beta blockers)

nifedipine (Procardia)- shows greater action on vascular smooth muscle (peripheral vasodilation) than on the heart; used for the treatment of hypertension and angina; safe to use with beta blockers; highest incidence of side effects