Class II antiarrhythmics

Class II antiarrhythmics

Class II antiarrhythmics are composed of beta-adrenergic antag-onists, or beta-adrenergic blockers. Beta-adrendergic blockers used as antiarrhythmics include:

·                 acebutolol (not used very often)

·                 esmolol

·                 propranolol.

Pharmacodynamics

Acebutolol and propranolol are absorbed almost entirely from the GI tract after an oral dose. Esmolol, which can be given only by I.V., is immediately available throughout the body.

Pharmacotherapeutics

Class II antiarrhythmics slow ventricular rates in patients with atrial flutter, atrial fibrillation, and paroxysmal atrial tachycardia.

Fat-headed

Acebutolol and esmolol have low lipid solubility. That means that they can’t penetrate the highly fatty cells that act as barriers be-tween the blood and brain, called the blood-brain barrier.

Propranolol has high lipid solubility and readily crosses the blood-brain barrier.

No leftovers

Propranolol undergoes significant first-pass effect, leaving only a small portion of these drugs available to reach circulation and be distributed to the body.

Esmolol is metabolized exclusively by red blood cells (RBCs), with only 1% excreted in urine. Propranolol’s metabolites are ex-creted in urine.

Pharmacodynamics

Class II antiarrhythmics block beta-adrenergic receptor sites in the conduction system of the heart. As a result, the ability of the SA node to fire spontaneously (automaticity) is slowed. The abili-ty of the AV node and other cells to receive and conduct an electri-cal impulse to nearby cells (conductivity) is also reduced.

Class II antiarrhythmics also reduce the strength of the heart’s contractions. When the heart beats less forcefully, it doesn’t re-quire as much oxygen to do its work.

Pharmacotherapeutics

Class II antiarrhythmics slow ventricular rates in patients with atrial flutter, atrial fibrillation, and paroxysmal atrial tachycardia.

Drug interactions

Class II antiarrhythmics can cause a variety of drug interactions:

§    Administering these drugs with phenothiazines and other anti-hypertensives increases the antihypertensive effect.

§    When given with nonsteroidal anti-inflammatory agents, fluid and water retention may occur, decreasing the antihypertensive effect.

§    The effects of sympathomimetics may be reduced when taken with class II antiarrhythmics.

§    Beta-adrenergic blockers given with verapamil can depress the heart, causing hypotension, bradycardia, AV block, and asystole.

§    Beta-adrenergic blockers reduce the effects of sulfonylureas.

§    The risk of digoxin toxicity increases when digoxin is taken with esmolol. (See Adverse reactions to class II antiarrhyth-mics.)