Chapter 23 - Antiarrhythmic agents

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What is an arrhythmia?

an abnormality in the rate, regularity, or site of origin of the cardiac impulse or a disruption in impulse conduction such that the normal sequence of atrial and ventricular activation is changed

What are arrhythmias associated with?

the imbalance of the parasympathetic and sympathetic branches of the ANS; changes in serum electrolyte concentrations (K+ and Ca++); hypoxemia; acidosis; changes in concentration of carbon dioxide; excessive stretch of cardiac tissue; mechanical trauma; myocardial disease states (CHF and viral myocarditis); numerous drugs; ischemia and infarction of the heart muscle

What are Class I drugs?

local anesthetic agents and membrane stabilizers

How can Class I drugs control arrhythmias?

by enhanced automaticity

What is the original and prototypical Class I drug?

quinidine

What are Class II drugs?

B-blockers

What do Class III drugs produce?

a prominent prolongation of the action potential, thereby extending the refractory period

What are Class IV drugs?

calcium channel blockers

Class I drugs are most frequently used to treat what?

ventricular tachyarrhythmias

What is the common mechanism of action of Class I drugs?

blockade of fast sodium channels in the myocardial cell membrane

What are the Class IA agents?

quinidine, procainamide, and disopyramide

What do Class IA agents do?

depress conductino in normal and abnormal cardiac tissue and prolongs repolarization

What are the Class IB agents?

lidocaine and its derivatives (tocainide and mexiletine) along with phenytoin

What are Class II drugs useful for treating?

supraventricular and ventricular tachyarrhythmias

What are Class III drugs commonly used to prevent?

sudden death due to ventricular tachyarrhythmias

Class III drugs can be effective in suppressing what?

ventricular arrhythmias

How do Class IV drugs act?

by inhibiting the function of the slow L-type calcium channels on cardiac cell membranes

What type of inotropic agents are calcium channel blocking drugs?

negative

What is the obvious drug of choice when severe sinus bradycardia or sinus arrest is presented secondary to vagal discharge and accumulation of acetylcholine?

atropine

What is epinephrine indicated for?

in attempts to restart the heart after cardiac arrest

Does quinidine have direct or indirect effects on cardiac rhythmicity?

both

What is quinidine?

a Class I antiarrhythmic agent

The capability of quinidine to directly prolong the refractory period of atrial fibers is thought to account for what?

its ability to convert atrial fibrillation to sinus rhythm

What has limited the use of quinidine?

formulations are difficult to obtain

Quinidine has generally been less successful in treatment of what in small dogs?

atrial fibrillation

Quinidine has been used in the treatment of atrial fibrillation in what animal?

horses

Procainamide is more effective in controlling what?

ventricular arrhythmias rather than atrial arrhythmias

What is procainamide?

a Class I antiarrhythmic agent

What is phenytoin?

a Class I antiarrhythmic agent

What characteristic does phenytoin exert in the heart?

antiarrhythmic activity

Why is phenytoin classified as a Class IB drug?

because it minimally shortens the refractory period

In general, phenytoin is considered to be effective in controlling what?

digitalis-induced arrhythmias of all types and is useful in treating ventricular arrhythmias

What is the IV dose for phenytoin?

5-10 mg/kg at an infusion rate of about 25-50 mg/min

What is lidocaine?

a Class I antiarrhythmic agent

Lidocaine is a local anesthetic drug that has been found to exert what effect?

antidysrhythmic action

Lidocaine can be used in treating what?

ventricular tachyarrhythmias

Lidocaine is not recommended for controlling what?

supraventricular arrhythmias

Which species is particularly susceptible to adverse effects of lidocaine and therefore need to be dosed carefully?

cats

IV infusions of lidocaine are used in horses to treat what?

postoperative ileus

What is mexiletine?

a Class I antiarrhythmic agent

What is mexiletine indicated for treatment of?

chronic treatment of ventricular tachyarrhythmias in dogs

What are the toxic effects of mexiletine?

vomiting and disorientation/ataxia

What is propranolol?

a Class II antiarrhythmic agent

What is the mechanism of action for propranolol?

it slows the rate of spontaneous discharge of the SA and ectopic pacemakers and slows both antegrade and retrograde conduction through anomalous pathways of the heart

Propranolol is well absorbed from what and is eliminated largely by what?

the gut; the liver

What is atenolol?

a Class II antiarrhythmic agent

What is atenolol used for in dogs and in combination with what?

slow the heart rate in patients with atrial fibrillation combined with digoxin

What is atenolol used for?

slow heart rate with atrial fibrillation, treat supraventricular tachycardia and ventricular tachyarrhythmias and is used in attempts to prevent sudden death in dogs with severe subaortic stenosis

What is the dose for atenolol in dogs?

6.25-50 mg every 12 hours orally

What is esmolol?

a Class II antiarrhythmic agent

What is esmolol used for?

acute termination of supraventricular tachycardia

Which Class II antiarrhythmic agent is ultrashort-acting and used commonly for intravenous administration?

esmolol

What is metoprolol?

a Class II antiarrhythmic agent

What is metoprolol more effective in blocking?

B1 receptors of the heart

Metoprolol may be the B-blocking agent in patients with a history of what?

COPD

Beta-blocking agents should be administered with caution in patients with what?

reduced cardiac reserve

What is carvedilol?

a Class II antiarrhythmic agent (a 3rd generation beta blocker)

What is the generic of carvedilol?

coreg

What is the generic of metoprolol?

lopressor

What is the generic of propranolol?

inderal

What is amiodarone?

a Class III antiarrhythmic agent

What is the generic of amiodarone?

cordarone

What should be monitored during treatment with amiodarone and why?

thyroid function because amiodarone inhibits T3 and T4 secretion from canine thyroid glands

What is sotalol?

a Class III antiarrhythmic agent

What is sotalol as effective as quinidine in doing?

treating various arrhythmias

What is a major indication of sotalol?

treating severe ventricular tachyarrhythmias and syncope

How much of sotalol is metabolized?

less than 1%

Elimination of sotalol is via what?

renal clearance and is linearly related to the GFR

The drug dose of sotalol must be decreased in which animals?

animals that have compromised renal function

Sudden cessation of sotalol can produce what?

fatal ventricular arrhythmias

What is verapamil?

a Class IV antiarrhythmic agent

What is diltiazem?

a Class IV antiarrhythmic agent

Verapamil has application in several types of what?

atrial arrhythmias

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