Arrhythmias, disturbances of the normal rhythm of the heart, occur when the electrical conduction systems malfunction. The malfunction could result in a change in heart rate, rhythm, impulse generation, or conduction of electrical signals through the heart muscle. Nonpharmacological approaches to arrhythmias include the use of pacemakers, implantable defibrillators, and ablation of an aberrant conduction pathway.
To understand the action and classification of the antiarrhythmic drugs, it is first necessary to understand the ionic movements that underlie the cardiac action potential (Figure 13–1). It is also good to remember the normal flow of electricity in the heart. What controls the rate? What controls the rhythm? What takes over in emergencies?
The cardiac action potential is shown. The action potential has been divided into phases (indicated on the left). The shape of the action potential is determined by the ions that are flowing during that phase (indicated on the right).
The antiarrhythmic agents are classified into four groups according to the part of the cardiac cycle they influence. This is a universal system, but it is not entirely accurate. Several drugs have more than one effect, and others do not fall into any of the four categories.
CLASS I DRUGS (SODIUM CHANNEL BLOCKERS)
The class I drugs are essentially sodium channel blockers.
The class I drugs are characterized by their ability to block sodium entry into the cell during depolarization. This decreases the rate of rise of phase 0 of the action potential (Figure 13–2). These drugs also suppress automaticity of the Purkinje fibers and His bundle.
Class I antiarrhythmics block sodium entry into myocardial cells during depolarization. This decreases the rate of rise of phase 0.
The class I drugs have been further divided into three groups. The differences between classes A, B, and C are not of primary importance to beginning students of pharmacology; this information can be learned later. Class IA drugs slow the rate of rise of phase 0 and prolong the effective refractory period of the ventricle. Class IB drugs have less of an effect on phase 0, but shorten the action potential duration and refractory period of the Purkinje fibers. Class IC drugs have the greatest effect on the early depolarization and have less of an effect on the refractory period of the ventricle.
|Class IA Drugs ||Class IB Drugs ||Class IC Drugs |
Here we have a major stumbling block for pharmacology students. Notice that there appears to be little ...