Cardiac Pacemakers and Resynchronization Therapy

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Chapter 37

Cardiac Pacemakers and Resynchronization Therapy

1. What are the components of a pacing system?

    Pacing systems consist of a pulse generator and pacing leads, which can be placed in either the atria or the ventricles. Pacemakers provide an electrical stimulus to cause cardiac depolarization during periods when intrinsic cardiac electrical activity is inappropriately slow or absent. The battery most commonly used in permanent pacers has a life span of 5 to 9 years.

2. What is the accepted pacing nomenclature for the different pacing modalities?

    The North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group have developed a code to describe various pacing modes. It usually consists of three letters, but some systems use four or five:

A pacemaker in VVI mode denotes that it paces and senses the ventricle and is inhibited by a sensed ventricular event. The DDD mode denotes that both chambers are capable of being sensed and paced.

3. What is the most important clinical feature that establishes the need for cardiac pacing?

    The most important clinical feature consists of symptoms clearly associated with bradycardia. Symptomatic bradycardia is the cardinal feature in the placement of a permanent pacemaker in acquired atrioventricular (AV) block in adults. Reversible causes, such as drug toxicity (digoxin, beta-adrenergic blocking agents [β-blockers], and calcium channel blockers), electrolyte abnormalities, Lyme disease, transient increases in vagal tone, and sleep apnea syndrome, should be sought, and the offending agents should be discontinued. The clinical manifestations of symptomatic bradycardia include fatigue, lightheadedness, dizziness, presyncope, syncope, manifestations of cerebral ischemia, dyspnea on exertion, decreased exercise tolerance, and congestive heart failure.

4. What are the three types of acquired AV block?

    There are three degrees of AV block: first, second, and third (complete). This classification is based on both the electrocardiogram (ECG) and the anatomic location of the conduction disturbance.

    First-degree block refers to a stable prolongation of the PR interval to more than 200 ms and represents delay in conduction at the level of the AV node. There are no class I indications for pacing in isolated asymptomatic first-degree 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.

    Third-degree or complete block defines the absence of AV conduction and refers to complete dissociation of the atrial and ventricular rhythms, with a ventricular rate less than the atrial rate. The width and rate of the ventricular escape rhythm help to identify an anatomic location for the block: narrow QRS is associated with minimal slowing of the rate, generally at the AV node, and wide QRS is associated with considerable slowing of rate at or below the bundle of His. Permanent pacemaker implantation is indicated for third-degree and advanced second-degree AV block at any anatomic level associated with bradycardia with symptoms (including heart failure) or ventricular arrhythmias presumed to be due to AV block.

5. What is the anatomic location of bifascicular or trifascicular block?

    Bifascicular block is located below the AV node and involves a combination of block at the level of the right bundle with block within one of the fascicles of the left bundle (left anterior or left posterior fascicle).

    Trifascicular block refers to the presence of a prolonged PR interval in addition to a bifascicular block. Based on the surface ECG, it is impossible to tell whether the prolonged PR interval is due to delay at the AV node (suprahisian) or in the remaining conducting fascicle (infrahisian, hence the term trifascicular block). Pacing is indicated when bifascicular or trifascicular block is associated with the following:

6. When is pacing indicated for asymptomatic bradycardia?

    There are few indications for pacing in patients with bradycardia who are truly asymptomatic:

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