Chapter 19: Pharmacology of Antiarrhythmics and Antihypertensives

INTRODUCTION

This chapter reviews common antiarrhythmic and antihypertensive medications administered in the ED. Acute medical conditions presenting with systemic hypertension and the specific recommended antihypertensive agents are presented in Chapter 57, “Systemic Hypertension.” Antiarrhythmic medications treat cardiac rhythm abnormalities by modifying autonomic function or myocardial ion channels, leading to changes in conduction velocity or duration of the effective refractory period.¹ Long-term use of these agents to prevent arrhythmias has not been proven to reduce mortality^2-5; however, this chapter focuses on medications used for acute pharmacologic conversion or rate control of common arrhythmias. In general, electrical cardioversion is preferable to pharmacologic conversion in patients who are hemodynamically unstable. The majority of antiarrhythmics are organized based on the Vaughan-Williams classification system (classes I to IV) (Table 19-1). This chapter also discusses the emergency applications of atropine, adenosine, magnesium, and isoproterenol.

CLASS I ANTIARRHYTHMICS: FAST SODIUM CHANNEL BLOCKERS

Class I agents block fast sodium channels and are further categorized based on their degree of blockade into classes Ia (moderate blockade), Ib (weak blockade), and Ic (strong blockade). They increase the excitability threshold, requiring more sodium channels to open in order to overcome the potassium current and generate an action potential. This effect increases the refractory period and can be useful in terminating reentry currents. In addition, some class I agents block potassium channels and exhibit antimuscarinic effects.

PROCAINAMIDE

Actions

Procainamide increases the refractory period, decreases automaticity and conduction, and prolongs cardiac action potentials through intermediate blockade of open sodium and potassium channels. N-Acetylprocainamide, the active metabolite of procainamide, lacks sodium channel effects but does block potassium channels, which can lead to QT prolongation.

Pharmacokinetics

See Table 19-2.⁶

Indications

Procainamide is indicated for life-threatening ventricular arrhythmias and supraventricular arrhythmias. Although it can be used to treat supraventricular tachycardia, the proarrhythmic nature of ...