Class IB antiarrhythmics

Class IB antiarrhythmics

Class IB antiarrhythmics are used for treating acute ventricular arrhythmias. They include:

• lidocaine
• mexiletine.

Pharmacokinetics

Mexiletine is absorbed from the GI tract after oral administration. Lidocaine is administered I.V. to prevent rapid metabolism by the liver after it enters the hepatic portal circulation.

All bound up?

 

Lidocaine is distributed widely throughout the body, including the brain. Lidocaine and mexiletine are moderately bound to plasma proteins. (Remember, only that portion of a drug that’s unbound can produce a response.)

Class IB antiarrhythmics are metabolized in the liver and excreted in urine. Mexiletine also appears in breast milk.

 

Pharmacodynamics

Class IB drugs work by blocking the rapid influx of sodium ions during the depolarization phase of the heart’s depolarization-repolarization cycle. This decreases the refractory period, which reduces the risk of arrhythmia. 

 

Make a IB-line for the ventricle

Because class IB antiarrhythmics especially affect the Purkinje fibers (fibers in the conducting system of the heart) and myocardial cells in the ventricles, they’re used to treat only ventricular arrhythmias.

Warning!

Adverse reactions to class IB antiarrhythmics

Adverse reactions to class IB antiarrhythmics include drowsiness, light-headedness, paresthesia, sensory disturbances, hypotension, and bradycardia.

 

Lidocaine toxicity can cause seizures and respiratory arrest.

Adverse reactions to mexiletine include hypotension, atrioventricular block, bradycardia, confusion, ataxia, and double vision. Mexiletine may also produce nausea and vomiting.

 

Pharmacotherapeutics

Class IB antiarrhythmics are used to treat ventricular ectopic beats, ventricular tachycardia, and ventricular fibrillation.

 

Class IB antiarrhythmics are usually the drug of choice in acute care because they don’t produce immediate serious adverse reactions.

 

Drug interactions

Class IB antiarrhythmics may exhibit additive or antagonistic effects when administered with other antiarrhythmics, such as
phenytoin, propranolol, procainamide, and quinidine. Other drug interactions include the following:

Now I get it!

 

How lidocaine works

Lidocaine works in injured or ischemic myocardial cells to retard sodium influx and restore cardiac rhythm. Normally, the ventricles contract in response to impulses from the sinoatrial (SA) node. But when tissue damage occurs in the ventricles, ischemic cells can create an ectopic pacemaker, which can trigger ventricular arrhythmias. The illustrations below show how these arrhythmias develop at the cellular level’and how lidocaine suppresses them.

 

Ischemic myocardial cell

Normal myocardial cells permit a limited amount of sodium ions to enter, which leads to controlled depolarization. Ischemic myocardial cells allow a rapid infusion of sodium ions. This causes the cells to depolarize much more quickly than normal and then begin firing spontaneously. The result: a ventricular arrhythmia