Pacemaker Therapy

In patients with sinus rhythm in whom a substantial amount of pacing is anticipated, single-chamber ventricular pacemakers should be avoided, for three reasons: they may cause so-called pacemaker syndrome (i.e., symptoms related to AV dyssynchrony during ventricular pacing); they are associated with a higher likelihood of developing atrial fibrillation; and recent evidence indicates that excessive right ventricular pacing should be avoided if possible. Single-chamber atrial pacemakers can be used in patients with sinus node dysfunction and normal AV conduction. However, single-chamber atrial pacemakers are not widely used in the United States because of the possibility, often unpredictable, of the development of problems with AV conduction, resulting either from progression of conduction system disease or from necessary drug therapy. Dual-chamber pacing prevents pacemaker syndrome by preserving atrioventricular synchrony. Compared with single-chamber ventricular pacing, dual-chamber pacing has been shown to decrease the incidence of atrial fibrillation and heart failure hospitalization, especially in patients with sinus node dysfunction. However, excessive amounts of right ventricular pacing from dual-chamber pacemakers are associated with increased likelihood of atrial fibrillation and heart failure hospitalization.

Key Concept/Objective: To understand that type II AV block is an indication for pacing regardless of symptoms when associated with a wide QRS complex
High-degree (or advanced) type II AV block is defined as block of two or more consecutive P waves. Complete heart block, or third-degree block, denotes a complete absence of conduction from the atria to the ventricles. In advanced or complete heart block, there is often an escape rhythm that can originate from the area of the AV node (which generally results in a narrow QRS with typical escape rate 50 to 60 beats/min) or from one of the ventricles (wide QRS with typical escape rate 30 to 40 beats/min). A hallmark of complete heart block with escape rhythm is a regular ventricular rate. The anatomic location of AV block has important prognostic and therapeutic implications. Typically, a block occurring at the level of the AV node-such as first-degree block, type I second-degree block, and 2:1 block at the level of the AV node-does not lead to abrupt complete heart block, though gradual progression is common. A block occurring below the level of the AV node, on the other hand, can often progress quickly to complete heart block. In addition, high-degree or complete heart block at the level of the AV node is more likely to be ameliorated by an escape rhythm, whereas block at the level of the His-Purkinje system is associated with a greater risk of complete ventricular asystole. Pacing is indicated in second-degree AV block, regardless of type, when associated with symptomatic bradycardia. Type II second-degree AV block is also an indication for pacing when associated with a wide QRS complex, regardless of symptoms, because it likely indicates His-Purkinje system disease.

Key Concept/Objective: To understand the education of patients after pacemaker implantation
When 24 hours have passed after implantation, minimal range-of-motion restrictions are placed on the ipsilateral arm and shoulder. Patients are asked to refrain from raising the arm above shoulder level and to perform only limited heavy lifting for the first few weeks. After this period, patients may return to normal activity levels without having to be concerned about displacing the leads or the generator system. Follow-up care can be provided during office visits, via transtelephonic monitoring (TTM), or both. Several documents containing guidelines for follow-up have been published. Some recommend that patients either be seen in the office or undergo TTM every 3 to 6 months. To function appropriately, pacemakers must be able to sense a clean signal from the myocardium. A number of potential sources can interfere with such signals and thereby affect pacemaker function. The most significant of these is environmental electromagnetic interference (EMI) from devices in the patient's environment. Sources of EMI can be divided into household sources, industrial sources, and medical sources. MRI scanners utilize large magnetic fields that can affect pacemaker function in four different ways: (1) overheating of the generator, causing permanent damage to its electronic components, (2) heating of the leads, leading to tissue injury and affecting pacemaker performance, (3) generating a pacing current, which could possibly induce arrhythmias, and (4) causing unwanted programming change. For these reasons, the Food and Drug Administration and device manufacturers recommend against the use of MRI in patients with implanted pacemakers.

Key Concept/Objective: Conduction abnormalities are common in the acute infarction period but they tend to be transient and do not necessarily imply a need for permanent pacing
Conduction abnormalities are common in the setting of acute myocardial infarction. Pathophysiologic mechanisms include ischemia, necrosis, autonomic influences, and the neurohumoral response to injury. Temporary transvenous pacing is occasionally required during the acute phase of an infarction. The need for temporary pacing does not necessarily imply a need for permanent pacing, since many of the conduction abnormalities are transient and resolve after revascularization or upon recovery from the acute phase of the infarction. Patients with acute inferior infarction can manifest a variety of abnormalities, including SA node dysfunction, first-degree AV block, type I second-degree block, and third-degree block at the level of the AV node. It is uncommon for any of these conduction disturbances to persist after the acute phase of the infarction. These patients often require temporary pacing if they manifest hemodynamic instability, but they rarely require permanent pacing. Patients with anterior infarction can manifest bundle branch block, bifascicular block, trifascicular block, type II second-degree block, or complete heart block. These patients are much more likely to require permanent pacing than those with inferior infarction.

Key Concept/Objective: To understand CRT
Biventricular pacing, otherwise known as CRT, utilizes simultaneous or near simultaneous pacing through a transvenous right ventricular lead and a left ventricular lead implanted epicardially or through the coronary sinus. CRT allows for simultaneous activation of septal and lateral walls of the left ventricle and restores atrioventricular, interventricular, and intraventricular synchrony. Several studies have reported significant improvement in functional status, mitral regurgitation, left ventricular ejection fraction, and survival in patients treated with CRT. Candidates for CRT devices are currently required to have New York Heart Association functional class III symptoms despite adequate heart failure treatment, a left ventricular ejection fraction less than 35% and evidence of intraventricular conduction delay, usually in a left bundle branch pattern.