Page 92 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 71

3.4 PACEMAKERS AND IMPLANTABLE DEFIBRILLATORS

3.4.1 Market Size
The market for pacemakers and other cardiac electrophysiologic (EP) devices has grown over the past
five decades due to improved device function, advances in technology and implantation techniques, and
a rapidly expanding set of clinical indications. Approximately 1 million patients in the United States had
permanent pacemakers in 1996 (Kusumoto and Goldschlager, 1996), many of whom will need either a
lead or generator replacement sometime in the future. Recent incidence data from the CDC suggest that
at least 167,000 permanent cardiac pacemakers were implanted in United States in 2005, with a
minimal estimate of 79,000 automatic implanted cardioverter-defibrillator (AICD) units being placed
during the same year (DeFrances et al., 2007). An additional 39,000 combination devices intended for
coordinated contraction of the ventricles, termed cardiac resynchronization, were also implanted in 2005
(DeFrances et al., 2007). This last group in particular is expected to have significant future growth.

3.4.2 Indications

The American College of Cardiology/American Heart Association/Heart Rhythm Society consensus
practice guideline lists current indications for artificial pacemaker and implanted cardioverter-
defibrillator use (Epstein et al., 2008). In general, the purpose of a pacemaker is to deliver an elec-
trical impulse of sufficient magnitude to depolarize the heart chamber in a spreading, coordinated
fashion as occurs in a normal heartbeat. In contrast, defibrillators are used to depolarize the entire
heart at once in an effort to terminate uncoordinated contractions. The natural refractory period of
the myocardial tissue usually prevents erratic residual electrical activity from propagating for a short
period of time, restoring coordinated muscular contraction.
In general, a pacemaker is warranted in certain cases where electric impulse conduction or initia-
tion in the heart is blocked, slowed, or triggered in an irregular, variable fashion. Specific diseases for
which pacemaker therapy is employed include certain forms of atrioventricular and fascicular con-
duction block, sinus node dysfunction, and some forms of neurocardiogenic syncope (Gregoratos
et al., 1998). The most popular indications for first time implantation of cardiac pacemakers have
changed over time. In 1997, almost 50 percent of new pacemakers were placed to treat sinus node
dysfunction, the most common indication for pacemaker implantation in the United States (Bernstein
and Parsonnet, 2001). Congenital or acquired atrioventricular (AV) conduction block was second,
accounting for approximately 30 percent of primary implantations, followed by AV block secondary
to radiofrequency ablation, drug-induced bradycardia, neurocardiogenic causes, and tachyarrhythmia
(Bernstein and Parsonnet, 2001). Emerging indications, particularly pacemaker treatment of conges-
tive heart failure, could significantly alter the percentages indicated above (Cazeau et al., 2001; Gerber
et al., 2001). Worldwide results indicate fewer implantations for sinus node dysfunction when com-
pared to U.S. data, with a significant percentage implanted for atrial fibrillation (Mond et al., 2004).
The indications for implantation of a cardioverter-defibrillator are based primarily on the past
presence of a potentially fatal ventricular arrhythmia due to nontransient causes, regardless of the
specific illness (Gregoratos et al., 1998). Three indications, spontaneous ventricular tachycardia
(VT) or fibrillation (VF), aborted sudden death, and syncope with inducible VT or VF accounted for
around 95 percent of the devices implanted in the United States for which an indication was reported
(Bernstein and Parsonnet, 2001).

3.4.3 Device Design
The wide range of electrophysiologic disorders treated with pacemakers and defibrillators require
devices with various capabilities and settings. Generic classification codes have been developed to ease
the identification of the different pacemakers, defibrillators, and associated leads presented both in
the literature and medical practice. One such code is the North American Society for Pacing and
Electrophysiology/British Pacing and Electrophysiology Group Generic Pacemaker Code, which
contains five positions and defines the chambers that are paced and sensed, the potential electrical