Page 96 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 75

Rate-adaptive pacing requires some method of rate modulation, preferably without patient inter-
vention, that mimics as closely as possible the normal behavior of the intact sinus node during exer-
tion (Leung and Lau, 2000). A very large number of sensors, control techniques, and algorithms have
been proposed in an effort to provide optimal rate responses to exercise and activity in those patients
unable to generate an adequate heart rate increase (Leung and Lau, 2000). Because no single sensor
and associated circuitry can perfectly replace the sinus node, dual sensor systems have been intro-
duced to accommodate for particular deficiencies in either sensor (Mitrani et al., 1999). The most
popular sensors are mechanical devices that measure vibration, which roughly indicates that body
movement is taking place (Morley-Davies and Cobbe, 1997; Leung and Lau, 2000). These units have
the benefit of being compatible with existing lead technology (Leung and Lau, 2000).
The current standard pacemaker lead utilizes many of the innovations described above and pos-
sesses a low coil resistance coupled with a high electrode impedence and is encased in a steriod-eluting
covering (de Voogt, 1999). The steroid reduces the inflammatory reaction to the implanted lead that
can increase the stimulation threshold over time (Mitrani et al., 1999; Crossley, 2000).
Programmer. The programmer allows the physician to adjust pacemaker and defibrillator settings
to meet the particular needs of the patient. Modern microcomputer-based systems use radiofrequency
waves or magnetic fields to communicate with the EP device noninvasively (Kusumoto and
Goldschlager, 1996; Morley-Davies and Cobbe, 1997). The programmer can retrieve device settings
and performance data, including failure episodes, electrocardiograms, and battery function, allowing
the physician to optimize device performance or analyze an existing problem (Kusumoto and
Goldschlager, 1996). A recent review divided the most common programmable features into six cat-
egories, including pacing mode selection, energy output and characteristics, electrical sensitivity,
rate limits, refractory periods and their duration, and the various rate-adaptive features and functions
(Kusumoto and Goldschlager, 1996). The ever-growing range of features and functions on the mod-
ern EP device complicates patient management but does allow tailored therapy.

3.4.4 Complications
Complications associated with electrophysiology devices can be divided into those that are a conse-
quence of device failure or malfunction and complications secondary to device implantation, extraction,
or patient care. In general, complications are reported to occur at a greater rate with those physicians who
implant pacemakers less frequently (Bernstein and Parsonnet, 1996b; Bernstein and Parsonnet, 2001).
Clinically significant perioperative complications such as hemothorax (blood in the chest cavity), pneu-
mothorax (air in the chesty cavity), infection, and hematoma (blood collection around insertion site) are
relatively rare at l to 2 percent (Morley-Davies and Cobbe, 1997; Bernstein and Parsonnet, 2001).
Generators. Approximately 20 percent of generator sales are for replacement of an existing device
(Bernstein and Parsonnet, 2001). Although there are a number of problems that could necessitate
generator replacement, the most common reason is a depleted battery at the end of its service life,
which indicates that proper generator function and longevity is the norm rather than the exception.
According to recent surveys, electrical component failure continues to decline as an indication for
generator replacement (Bernstein and Parsonnet, 2001).
Electrical Leads. Pacing (de Voogt, 1999) and defibrillator electrical leads are the most problem-
atic components in their respective electrophysiology systems. As a common site of malfunction and
a significant factor in device power consumption, the electrical pacemaker lead has been subject to
a variety of design and usage improvements (de Voogt, 1999). A recent survey of cardiac pacing
revealed that the most common reason for lead replacement was insulation failure, followed by a
high stimulation threshold for cardiac depolarization and displacement of the lead electrode itself
(Bernstein and Parsonnet, 2001). Bipolar leads suffered from a higher complication rate in both the
atrial and ventricular positions, and ventricular leads secured with passive fixation devices also expe-
rienced a higher reported complication rate (Bernstein and Parsonnet, 2001). Results from the
Danish Pacemaker Register (Moller and Arnsbo, 1996) revealed significantly lower reliability in
bipolar leads versus unipolar leads, but unipolar leads do have shortcomings. Unipolar sensing units