Drugs for Arrhythmias. A. Pathophysiology of arrhythmias. Arrhythmias are a group of conditions in which the muscle contraction of the heart is irregular, and they occur in both healthy and diseased hearts. .
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Phase 0: This is the rapid depolarization phase and occurs with the opening of the Na1+ channels and the rapid influx of Na1+ into the cell.
Phase 1: This phase of the action potential occurs with the closure of the Na1+ channels. There is a transient net outward current due to the movement of K1+ and Cl1- ions. Phase 0 and 1 together correspond to the QRS complex of the ECG.
Phase 2: This is the plateau phase of the action potential and occurs because of the balanced inward movement of Ca2+ ions through calcium channels, and outward movement of K1+ ions through potassium channels. It corresponds to the ST segment of the ECG.
Phase 3: In this phase of the action potential, the K1+ channel is still open, allowing more K1+ to leave the cell, resulting in a net loss of positive charge, which causes the cell to repolarize. Phase 3 corresponds to the T wave of the ECG.
a. Class IAThey slow the rate of rise of phase 0 and lengthen the action potential and are indicated for managing a variety of atrial and ventricular arrhythmias.
It’s most common adverse effects are GI disturbances. Some individuals experience lupus-like symptoms (rash, muscle pain).
b. Class IBThey shorten the action potential and are indicated for the treatment of ventricular tachycardia and prevention of ventricular fibrillation
c. Class ICThey have no effect on the length of the action potential, but reduce the rate of rise of phase 0. These drugs are indicated for the treatment of ventricular tachycardia and supraventricular tachycardia.
In addition, these drugs decrease the force of myocardial contractions (they would not be recommended in patients with CHF).
ALL CCB’s produce vasodilation, however, only verapamil and diltiazem have direct actions on the heart.