Pacing occurs when a stimulus creates an electrical field at the interface between the pacing catheter and the adjacent myocardium. A wave of depolarization is created, which spreads across the heart and leads to myocardial contraction by means of excitation-contraction coupling. The earliest pacemakers were relatively primitive devices that produced pacing stimuli at regular intervals (at a rate that was preset by the manufacturer), but were not able to detect the heart's intrinsic rhythm. These "fixed-rate" pacemakers were soon supplanted by "demand" pacing units, equipped with circuitry to "sense" the local electrogram associated with intrinsic QRS complexes. As technology was further refined, dual-chamber pacing, rate-responsiveness, and program-mability were added to the armamentarium. The current generation of permanent pacemakers is powered by lithium batteries that may last as long as 12 yr. Traditionally, the amplitude (voltage) has been programmed to twice the threshold level to ensure an adequate safety margin at the expense of battery longevity. New design elements incorporate a feature that automatically tests capture threshold on a regular basis so that a lower voltage can be used, improving battery longevity. Many pacemaker models are equipped with multiple sensors (to detect body movement or minute ventilation), which allow fine-tuning of rate-responsiveness and mimic the response of a normal sinus node), mode-switching (which automatically inactivates atrial tracking when a supraventricular tachyarrhythmia is detected), and "rate-smoothing" mechanisms that prevent abrupt drops in pacing rate when the upper rate limit is exceeded. All of these features tend to be programmable, and can be adjusted or even turned off if necessary.
Pacemaker leads have also undergone important technological advances. Pacing leads have either passive (e.g., tines) or active (e.g., screw-in devices at the tip) fixation mechanisms to prevent dislodgment. There are small pacing electrodes that increase impedance, minimizing current drain and prolonging battery life. Most pacing electrodes contain steroid-eluting reservoirs that decrease inflammation and fibrosis at the electrode-tissue interface, with a resulting reduction in the chronic pacing threshold. Currently under development are single leads that will allow pacing and sensing in both the atrium and ventricle, which would simplify implantation and reduce cost.
AMERICAN COLLEGE OF CARDIOLOGY/ AMERICAN HEART ASSOCIATION GUIDELINES
There are a wide variety of conditions and disorders that can potentially be improved by pacing. In an effort to standardize their recommendations, the American College of Cardiology/American Heart Association (ACC/AHA) Task Force on Practice Guidelines used the following format in their most recent guidelines for pacemaker implantation, published in April 1998 (1):
Class I: there is general agreement that a pacemaker is beneficial, useful, and effective.
Class II: there is divergence of opinion about the usefulness of pacing. a: weight of the evidence favors usefulness/efficacy. b: usefulness/efficacy is more poorly established. Class III: it is generally agreed that a pacemaker is not useful and may actually be harmful.
These categories will be referred to in the discussion of each potential indication for pacing.
SINUS-NODE DYSFUNCTION ("SICK SINUS SYNDROME")
Sinus-node dysfunction is a heterogeneous group of disorders of diverse etiologies (see Fig. 1). This spectrum of arrhythmias includes sinus and junctional bradycardia, sinus arrest, sinoatrial (SA) block, alternating periods of sinus bradycardia and supraventricular tachyarrhythmias—such as atrial fibrillation (AF) (the "bradycardia-tachycardia syndrome), and AF with a slow ventricular response (4). With chronotropic incompetence, the resting heart rate may be adequate, but the heart rate fails to augment normally with exercise. Thus, the exercise heart rate is inappropriately slow, and the resulting cardiac output is insufficient to meet the metabolic demands of the body. Sinus-node dysfunction is seen with increasing frequency in the elderly, but has also been reported in children and young adults, especially after corrective surgery for congenital heart disease. Because of its abundant blood supply (the sinus node is essentially a specialized adventitia of the large sinus-node artery), the sinus node is relatively resistant to ischemia, and chronic sinus-node dysfunction is not a frequent sequelae of acute MI.
Sinus-node dysfunction is the most common reason for permanent pacemaker insertion, and accounts for up to 50% of permanent pacemakers implanted in the United States (3). The current ACC/AHA indications for permanent pacing in sinus-node dysfunction are as follows:
Class I: Documented symptomatic bradycardia (or symptomatic chronotropic incompetence), either spontaneous or as a result of necessary drug therapy. Class Ila: The presence of bradycardia—heart rate <40 beats per minute (BPM)—and symptoms without a documented association between the two. Class lib: Minimally symptomatic patients with very slow resting heart rates. Class III: Asymptomatic individuals or those in whom symptoms are documented not to be a consequence of a slow heart rate.
Although it is generally accepted that rate-responsive pacing is required in individuals with sinus-node dysfunction because of the high incidence of chronotropic incompetence, there is some controversy about the use of atrial vs ventricular pacemakers. Short-term crossover studies have demonstrated dual-chamber or atrial pacing to be superior to ventricular pacing in improving quality of life, but there is no clear difference in exercise performance when dual-chamber and ventricular rate-responsive units are compared (5-7). Long-term studies, mostly nonrandomized, have consistently demonstrated that dual-chamber pacemakers are associated with a lower incidence of AF, but mortality results have been more equivocal (8-14).
Several large, prospective, and randomized studies have recently been completed. In the Pacemaker Selection in the Elderly (PASE) study, Lamas and colleagues conducted a 30-mo, single-blind, randomized, controlled comparison of ventricular and dual-chamber rate-responsive pacing in 407 patients 65 yr of age or older (15). They also found improved quality of life in patients with sinus-node dysfunction, but could not document any benefit in mortality or cardiovascular events. Recently, Anderson and associates found a significant difference in both morbidity and mortality over a 3.3 (mean)-yr follow-up period in a randomized study of atrial vs ventricular pacing in 225 patients with sick sinus syndrome (16). Preliminary results of the Pac-A-Tach study also show a mortality benefit for physiologic pacing (DDDR) over single-chamber ventricular pacing (VVIR) pacing in 198 patients with bradycardia-tachycardia syndrome (17). In contrast, the Canadian Trial of Physiological Pacing (CTOPP) investigators found that dual-chamber pacing provided no benefit in cardiovascular mortality or incidence of stroke in 2,568 patients followed for a mean period of 3.1 yr (18). This trial did suggest, however, less AF during follow-up in the DDDR group compared to the VVIR group. The preliminary results from the MOST study were similar (18a).
There is also some debate about whether single-chamber atrial pacing (AAI or AAIR) may be superior to dual-chamber pacing in patients with sinus-node dysfunction and intact atrioventricular (AV) conduction (19). Although it would intuitively seem that dual-chamber pacing would be preferable because it would protect patients against the subsequent development of AV block, it has been shown that this complication is unusual in patients with intact AV conduction at the time of implant (20,21), and may not warrant the extra expense and complications associated with inserting an additional pacing lead. In addition, there may be adverse effects resulting from the altered activation pattern associated with ventricular pacing. The answer to this question may be provided by the results of several large ongoing clinical trials (see Table 1).
In summary, permanent pacemakers have played a major role in the therapeutic approach to sinus-node dysfunction. The indications for pacing are fairly well delineated, but the type of pacemaker that is most appropriate in a given clinical situation remains controversial. Atrial pacing has demonstrated superiority over ventricular pacing in terms of quality of life and the incidence of AF, but the mortality data are less convincing. Current trials focus on the role of AAI or AAIR in individuals with sinus-node dysfunction and intact AV conduction.
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