Review Question - QID 100524

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.

Figure A demonstrates the characteristic EKG of third degree AV block.

Incorrect Answers:
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).