A 60-year-old man comes to the emergency department with dizziness and palpitations. He woke up this morning with an intermittent pounding sensation in his chest. The patient has occasionally experienced a similar sensation in the past and did not think it was serious. Later, while mowing the lawn, he became lightheaded and had to sit down to avoid passing out. After receiving the appropriate workup, the patient is diagnosed with a cardiac arrhythmia and started on a medication known to prolong the QT interval. However, the drug is associated with a lower incidence of torsade de pointes than other QT-prolonging agents. Which of the following medications was most likely used in this patient?

Question

A 60-year-old man comes to the emergency department with dizziness and palpitations.  He woke up this morning with an intermittent pounding sensation in his chest.  The patient has occasionally experienced a similar sensation in the past and did not think it was serious.  Later, while mowing the lawn, he became lightheaded and had to sit down to avoid passing out.  After receiving the appropriate workup, the patient is diagnosed with a cardiac arrhythmia and started on a medication known to prolong the QT interval.  However, the drug is associated with a lower incidence of torsade de pointes than other QT-prolonging agents.  Which of the following medications was most likely used in this patient?

Quizplus · Accepted Answer

11ec160b_3b78_da9a_9249_cd2c34621b69_MD0001_00 Amiodarone is a class III antiarrhythmic drug used for the management of a variety of supraventricular and ventricular arrhythmias. Class III drugs predominantly block potassium channels and inhibit the outward potassium currents during phase 3 of the cardiac action potential, prolonging repolarization and total action potential duration (APD). The QT interval represents the time required for ventricular depolarization and repolarization; it can be regarded as a rough estimate of the APD. Drugs that significantly prolong the cardiac action potential (eg, class IA and III antiarrhythmics) cause prolongation of the QT interval. Prolonged QT is associated with a form of ventricular tachycardia called torsades de pointes (polymorphic ventricular tachycardia). However, unlike other drugs that cause QT prolongation, amiodarone has very little risk of inducing torsades de pointes. This effect is thought to be due to it having a more homogeneous effect on ventricular repolarization compared to other drugs (ie, less QT dispersion). (Choice A) Adenosine interacts with A1 receptors on cardiac cells and activates potassium channels, increasing potassium conductance. It causes a transient conduction delay through the atrioventricular (AV) node and is used for acute termination of paroxysmal supraventricular tachycardia (PSVT). Adenosine is rapidly metabolized in the blood and tissues and has a very brief duration of action. (Choice C) Digoxin slows AV nodal conduction speed by augmenting vagal parasympathetic tone and increases cardiac contractility by inhibiting Na⁺/K⁺-ATPase (increases intracellular calcium). It also decreases the APD and can cause QT interval shortening. (Choice D) Esmolol is a rapidly acting, short-duration beta blocker (class II antiarrhythmic) that slows the rate of discharge of sinus or ectopic pacemakers and increases the refractory period of the AV node. It has no effect on QT interval. (Choice E) Lidocaine is a class IB antiarrhythmic drug that acts by blocking sodium channels and inhibiting phase 0 depolarization. It does not prolong the QT interval. (Choice F) Procainamide is a class IA antiarrhythmic drug that inhibits sodium-dependent (phase 0) depolarization. It also has moderate potassium channel blocking activity, leading to prolongation of APD and the QT interval with an increased risk of torsades de pointes. (Choice G) Verapamil is a cardio-selective calcium channel blocker (class IV antiarrhythmic drug). It slows the sinus rate and prolongs the PR interval by slowing conduction through the AV node. It does not prolong the QT interval. Educational objective: Amiodarone (and other class III and class IA antiarrhythmic agents) causes lengthening of the cardiac action potential, which manifests as QT interval prolongation on ECG. QT prolongation caused by amiodarone, in contrast to other drugs, is associated with a very low risk of torsades de pointes. __________ References: Chronic amiodarone evokes no torsade de pointes arrhythmias despite QT lengthening in an animal model of acquired long-QT syndrome. (http://www.ncbi.nlm.nih.gov/pubmed/11723026)

A 60-Year-Old Man Comes to the Emergency Department with Dizziness