Chapter 38: Chemical Cardioversion

INTRODUCTION

Cardioversion is a procedure performed to restore a fast or irregular heartbeat back to normal rhythm.¹ There are two general types of cardioversion. The first is synchronized cardioversion with electricity, which is the most effective treatment.^2-7 The second is pharmacologic or chemical cardioversion. The decision on which type of cardioversion to use is governed by the patient’s clinical condition. For unstable patients with hypotension, altered mental status, signs of shock, chest pain, or signs of heart failure, synchronized cardioversion is indicated.^2-4 For patients who are hemodynamically stable without any of the above signs or symptoms, chemical cardioversion may be considered.⁸ The use of chemical cardioversion is less expensive than electrical cardioversion, but patients have more complications.⁹

The specific dysrhythmias for which chemical cardioversion is an option include supraventricular tachycardias (e.g., atrial fibrillation and atrial flutter) and ventricular tachycardia. This chapter will focus on chemical cardioversion for these tachyarrhythmias.

ANATOMY AND PATHOPHYSIOLOGY

The orchestrated contraction of the atria and ventricles of the heart is the result of an organized electrical impulse passing through the myocytes. This electrical impulse originates in the pacemaker myocytes in the sinoatrial (SA) node in the right atrium and spreads through the atria, resulting in contraction. The impulse arrives at the atrioventricular (AV) node and is subsequently passed down the His bundle and Purkinje fibers to the ventricles, resulting in ventricular contraction. This electromechanical coupling results in approximately 75 mL of oxygenated blood being propelled from the left ventricle into the aorta around 60 to 100 times per minute. A disruption in this sequence may result in cardiac dysrhythmias, which interrupt the delivery of oxygenated blood to vital organs and tissues.

SUPRAVENTRICULAR TACHYCARDIA (SVT)

SVT is an umbrella term that encompasses tachyarrhythmias originating in the atria. These can be further classified into those originating in the atria and those originating from the AV node (Table 38-1).¹⁰ Sinus tachycardia is the most common cause of tachycardia. It is usually an appropriate cardiac physiologic response to an underlying condition (e.g., hypovolemia, anemia, pain, fever, coronary ischemia, pulmonary embolism) and falls outside the scope of this discussion. The most common tachydysrhythmias are AV nodal reentrant tachycardia, AV reentrant tachycardia, atrial fibrillation, and atrial flutter.

AV NODAL REENTRANT TACHYCARDIA (AVNRT) AND AV REENTRANT TACHYCARDIA (AVRT)

AVNRT and AVRT result from the presence of abnormal electrical circuits. In most individuals, the AV node has a single pathway that conducts the impulse in anterograde fashion, resulting ...