How Do Class IB Antidysrhythmics Work?

Antidysrhythmics, also known as antiarrhythmics, are drugs used to prevent abnormal cardiac rhythms such as atrial fibrillation, atrial flutter, ventricular tachycardia, and ventricular fibrillation. These drugs work by blocking sodium, potassium, and calcium channels in the heart muscles. Some drugs show autonomic effects.

Antiarrhythmic drugs are grouped into four main classes:

• Class I, sodium-channel blockers;
• Class II, beta-blockers;
• Class III, potassium-channel blockers;
• Class IV, calcium-channel blockers; and

miscellaneous antiarrhythmics or unclassified antiarrhythmics.

Class I antidysrhythmics block sodium channels (bind more tightly to cardiac tissue that is depolarizing a lot) and limit the electrical conductance of sodium over the cell membrane and reduce the rate of phase 4 (resting phase) depolarization in automatic cells. They are divided into subclasses a, b, and c.

Class IB antidysrhythmics are anesthetic drugs that show cardiac activity. They block inactivated sodium channels more than that of the open state and do not delay channel recovery time. This causes slow conduction of electrical impulse through cardiac tissue. These drugs decrease the duration of action potential during phase 0 depolarization in the Purkinje fibers (conducting fibers in the heart) and ventricular muscles. These drugs do not affect or slightly increase the effective refractory period (resting state in a depolarized cell and sodium channels are inactive in this phase) in ventricular action potentials.

Class IB antidysrhythmics are usually administered intravenously in a clinical setup to treat and prevent:

• Ventricular arrhythmias (abnormal heartbeats that originate in the lower heart chambers)
  • Ventricular fibrillation (life-threatening heart rhythm starting in the lower chambers of the heart)
  • Ventricular tachycardia (abnormal heartbeats that occur in the lower chambers)
  • Ventricular premature beats
• Digitalis-induced cardiac arrhythmias

Serious side effects may include:

• Atrioventricular block (arrhythmia caused by delayed conduction between ventricles)
• Ventricular extrasystoles (premature contraction of heart ventricles)
• Neurotoxicity (parathesis)
• Fits

Other common side effects include:

• Bradycardia (slower than normal heart rate)
• Hypotension (low blood pressure)
• Drowsiness
• Nausea and vomiting
• Blurred vision
• Disorientation
• Nystagmus
• Twitchings

The information contained herein is not intended to cover all possible side effects, precautions, warnings, drug interactions, allergic reactions, or adverse effects. Check with your doctor or pharmacist to make sure these drugs do not cause any harm when you take them along with other medicines. Never stop taking your medication and never change your dose or frequency without consulting your doctor.

Drug names include:

• Dilantin
• Dilantin 125
• Lidocaine
• Lidocaine CV
• Lidopen
• Mexiletine
• Phenytek
• Phenytoin