The correct answer is third degree (complete) AV block. In this type of atrioventricular block, the atria and ventricles beat independently of each other because of disruption in the conduction pathway in the heart.
Normal conduction of electricity through the heart starts at the SA node, travels to the AV node, and then to the ventricles via the bundle of His and Purkinje system. The SA node sets the heart rate because its cells have the fastest firing rate of all conductive cells (60-100 beats per minute). If there is some breach in the normal conductive pathway, other cells in the heart can set an alternative heart rate, albeit at a slower rate. In this case, the patient’s symptoms can be attributed to bradycardia. Given the patient’s exposure to wooded areas in the Northeast, a potential cause of this patient’s block is Lyme disease. Patients with suspected Lyme carditis require hospitalization for cardiac monitoring and treatment with intravenous antibiotics such as ceftriaxone.
Wright et al. discuss diagnosis and treatment of Lyme disease. The treatment of Lyme disease is determined mainly by the clinical manifestations of the disease. Doxycycline is often the preferred agent for oral treatment because of its activity against other tick-borne illnesses. Carditis occurs in 4% to 10% of cases of Lyme disease and usually begins 3 to 6 weeks after the initial illness.
Vogler et al. review the common bradyarrhythmias and conduction blocks. They note that if reversible extrinsic causes of bradyarrhythmias such as drugs (most often beta-blockers, glycosides and/or calcium channel blockers) or underlying treatable diseases are ruled out, cardiac pacing is usually the therapy of choice in symptomatic bradyarrhythmias.
Image A demonstrates the characteristic EKG of third degree AV block.
Answer 1: In Wolff-Parkinson-White syndrome, an there is an accessory conduction pathway from the atria to the ventricle (bundle of Kent) that bypasses the AV node resulting in earlier ventricular depolarization and a characteristic delta wave on EKG.
Answer 2: In second degree type I AV block (Wenckebach), there is progressive prolongation of the PR interval until a beat is lost.
Answer 3: In second degree type II AV block, dropped beats are not preceded by prolongation of the PR interval (as in second degree type I block).
Answer 4: First degree AV block is characterized by prolongation of the PR interval (above 200 msec).
Wright WF, Riedel DJ, Talwani R, Gilliam BL. Diagnosis and management of Lyme disease. Am Fam Physician. 2012 Jun 1;85(11):1086-93.
PMID:22627074 (Link to Abstract)
Vogler J, Breithardt G, Eckardt L. Bradyarrhythmias and conduction blocks.Rev Esp Cardiol (Engl Ed). 2012 Jul;65(7):656-67. doi: 10.1016/j.recesp.2012.01.025. Epub 2012 May 23.
PMID:22962880 (Link to Abstract)
USMLE World Step 1 QBank Question #1976. Copyright © USMLEWorld, LLC 2012