The QR segment corresponds to ventricular depolarization, which is Phase 0 of the myocyte action potential and is characterized by sodium influx.
The QR segment of an EKG tracing represents synchronized ventricular depolarization and contraction. This coordinated action of ventricular myocytes is caused by the opening of the fast Na+ channels that allow influx of Na+ ions into the cell in Phase 0 of the action potential, leading to rapid depolarization. This portion of the action potential is affected by Class I antiarrhythmics, such as propafenone and flecainide.
Siddoway discusses the use of amiodarone, a potent class III antiarrhythmic that prolongs the QT interval, slows heart rate and AV node conduction (via Ca++ channel and beta blockade), increases refractoriness (K+ and Na+ channel blockade), and slows intracardiac conduction (via Na+ blockade).
Roy et al. performed a randomized controlled trial comparing amiodarone to propafenone (a class I drug) and sotalol (a class III drug) in maintaining sinus rhythm in patients with atrial fibrillation. They found that 35% of patients on amiodarone had recurrence of atrial fibrillation, while 63% on sotalol or propafenone had recurrence of their atrial fibrillation.
Image A shows a labeled beat with a P wave, QRS complex, and T wave.
Image B shows a cardiac action potential with the phases labelled.
Illustration A demonstrates the movement of ions through the ventricular action potential.
Illustration B demonstrates the appearance of the action potential in different parts of the heart, and the correlation with the QRS complex.
Answer 2: Phase 0 is primarily driven by Na+ influx through the fast Na+ channels.
Answer 3-4: Phase 1 follows the closure of the fast Na+ channels, and the downward deflection is driven by a net outward current driven by potassium efflux from the cell.
Answer 5: Phase 3 is the repolarization phase that is driven by potassium efflux, and corresponds to the T wave on a typical EKG tracing.
Siddoway LA. Amiodarone: guidelines for use and monitoring. Am Fam Physician. 2003 Dec 1;68(11):2189-96.
PMID:14677664 (Link to Abstract)
Roy D, Talajic M, Dorian P, Connolly S, Eisenberg MJ, Green M, Kus T, Lambert J, Dubuc M, Gagné P, Nattel S, Thibault B. Amiodarone to prevent recurrence of atrial fibrillation. Canadian Trial of Atrial Fibrillation Investigators. N Engl J Med. 2000 Mar 30;342(13):913-20.
PMID:10738049 (Link to Abstract)
USMLE World Step 1 QBank Question #1508. Copyright © USMLEWorld, LLC 2012