Chapter 37
Cardiac Pacemakers and Resynchronization Therapy
1. What are the components of a pacing system?
2. What is the accepted pacing nomenclature for the different pacing modalities?
Letter 1: chamber that is paced (A = atria, V = ventricles, D = dual chamber)
Letter 2: chamber that is sensed (A = atria, V = ventricles, D = dual chamber, 0 = none)
Letter 3: response to a sensed event (I = pacing inhibited, T = pacing triggered, D = dual, 0 = none)
Letter 4: rate-responsive features (an activity sensor), for example, an accelerometer in the pulse generator that detects bodily movement and increases the pacing rate according to a programmable algorithm (R = rate-responsive pacemaker)
3. What is the most important clinical feature that establishes the need for cardiac pacing?
4. What are the three types of acquired AV block?
Second-degree block is divided into two types. Mobitz type I (Wenckebach) exhibits progressive prolongation of the PR interval before an atrial impulse fails to stimulate the ventricle. Anatomically, this form of block occurs above the bundle of His in the AV node. Type II exhibits no prolongation of the PR interval before a dropped beat and anatomically occurs at the level of the bundle of His. This rhythm may be associated with a wide QRS complex.
5. What is the anatomic location of bifascicular or trifascicular block?
Complete block and symptomatic bradycardia
Alternating bundle branch block
Intermittent type II second-degree block with or without related symptoms
Symptoms suggestive of bradycardia and an HV interval greater than 100 ms on invasive electrophysiology study
6. When is pacing indicated for asymptomatic bradycardia?
There are few indications for pacing in patients with bradycardia who are truly asymptomatic:
Third-degree AV block with documented asystole lasting 3 or more seconds (in sinus rhythm) or escape rates below 40 beats/min in patients while awake
Third-degree AV block or second-degree AV Mobitz type II block in patients with chronic bifascicular and trifascicular block
Congenital third-degree AV block with a wide QRS escape rhythm, ventricular dysfunction, or bradycardia markedly inappropriate for age