STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 360

TABLE 7.5 Typical Parameters Used in Cardiac Contractility Modulation Instruments

Clinical Method of Typical Typical Current

Application Current Delivery Waveform or Voltage

Enhancement of High-capacitance implantable Charge-balanced biphasic controlled-current 5 to 15 mA with 20-V

cardiac electrodes in contact with or capacitor-discharge pulse burst 5 to compliance or up to 8V

contractility cardiac muscle; typical 10 ms in duration per phase; one to three from 660- µ F capacitor

electrode impedance pulses per burst delivered 30 to 60 ms into

200 to 500 Ω the ventricular absolute refractory period

Impule Dynamics' implantable cardiac contractility modulation devices [Prutchi et al.,
1999] are currently undergoing clinical investigations to determine their safety and e ec-

tiveness as tools in the treatment of heart failure. Inside the implantable pulse generator's
titanium can, sense ampli

ff

ers detect the heart's electrical activity through standard pacing
leads. Specialized circuitry is used to generate and deliver the cardiac contractility modu-
lation (CCM) signals to the heart muscle during the ventricular absolute refractory period
[Mika et al., 2001]. An implantable-grade battery powers the device.
In principle, the same techniques can be applied to controlling other tissues that use cal-
cium as their main signaling mechanism. In gastroenterology, for example, application of
electric impulses is being researched by Impulse Dynamics to treat morbidly obese patient.
The hope is that application of these nonexcitatory signals to these organs will alter cellu-
lar function in a predictable and reproducible way while avoiding the systemic side e

fi

ff ects

of pharmacological agents.

Bone Growth Stimulators

Electrical bone growth stimulators have been proven to hasten the healing process for cer-
tain types of fractures and bone fusions. Noninvasive, semi-invasive, and invasive meth-
ods of electrical bone growth stimulation are available (Table 7.6). There are two
noninvasive bone growth stimulation techniques. The elds

(PEMF), involves the use of paired coils that are placed on either side of a fracture site.
It is believed that this triggers calci

fi rst, pulsed electromagnetic fi

fibrous cartilage tissue within the frac-
ture gap. Ten hours of treatment per day are usually necessary. Because of the relatively
strong

fi cation of the fi

fields that need to be generated, these units run on mains power and are not
portable, which forces the patient to spend a substantial part of the day next to a wall
power outlet.
The second type of noninvasive bone growth stimulator, the capacitively coupled stim-
ulator , involves the use of skin-surface gelled electrodes through which a constant cur-
rent 60-Hz sine-wave signal is delivered at 5 to 10 mA to produce an electrical

fi

eld
strength at the desired fusion site of some 2.0 V/m with a current density of approxi-
mately 300 mA/cm 2 . The probable mechanism of operation of this method is through
translocation of calcium into the cells at the fracture site through voltage-gated calcium
channels.
Semi-invasive and invasive bone growth stimulators utilize a dc source to generate a
weak electrical current in the underlying tissue. Semi-invasive or percutaneous bone
growth stimulators use an external power supply and electrodes (cathodes) that are inserted
through the skin and into the bone segment where growth is desired, while a self-adhesive
gelled anode electrode is placed directly on the skin. The electrodes are then connected to
a power pack that delivers 20

fi

A dc and is embedded within the cast. These units are typ-
ically designed to be applied for 12 weeks.

µ