Battery-powered implantable nerve stimulator for chronic activation of two skeletal muscles using multichannel techniques
H. Lanmüller, S. Sauermann, E. Unger, G. Schnetz, W. Mayr, M. Bijak, D. Rafolt, W. Girsch*
Department of
Biomedical Engineering & Physics
Department for
Plastic and Reconstructive Surgery*,
University of Vienna, Vienna, Austria
SUMMARY
Chronic activation of skeletal muscle is used
clinically in representative numbers for diaphragm pacing to restore breathing
and for graciloplasty to achieve fecal continence. In both applications the
skeletal muscle is extremely strained; muscle fatigue is consequently one of
the exciting problems associated with these techniques. Various stimulation
methods have been developed to improve on this. It was our aim to make some of
these stimulation methods applicable
for the above FES applications by a battery-powered nerve stimulator.
The implantable nerve stimulator can be used for
activating two skeletal muscles. Stimulation of the motor nerve is achieved by
either single channel or multichannel methods with up to 4 electrodes for each
nerve. Carousel stimulation, sequential stimulation as well as optimized pulse
trains can be implemented. All stimulation parameters can be adjusted with a
high resolution using an external programmer. The system can be used for
graciloplasty as well as for diaphragm pacing.
Nerve pacing is effected by ring-shaped
stainless-steel electrodes. These epineurial electrodes are used clinically in
the ”Vienna Phrenic Pacemaker”. The multichannel pulse generator is hermetically
sealed in a titanium case and powered by a Lithium Thionyl Chloride battery.
For diaphragm pacing we calculated a life span of 4.1 years, based on the
stimulation parameters used by us clinically. The size of the pulse generator
is 65 x 17 mm (diameter x height); it weighs 88 g.
State of the Art
Chronic activation of skeletal muscle is used
clinically in representative numbers for diaphragm pacing to restore breathing
and for graciloplasty to achieve fecal continence. In both applications the
skeletal muscle is extremely strained; muscle fatigue is consequently one of
the exciting problems associated with these techniques. In clinical use the
phrenic pacemaker is set to an inspiration rate of 10/min at a stimulation
duration of 1 s over 24 hours per day. The gracilus muscle for treatment of fecal incontinence is stimulated continuously
over several hours each day.
For respiration in humans electrical pacing systems
have been developed by three different groups /1/. These systems consist of
electrodes placed on the phrenic nerves which are connected by leads to an
implanted stimulator. The single or multichannel stimulators are powered and
controlled via an external high frequency transmitter from a portable
programmer. The programmer coordinates
the overall timing, and stores the received stimulation
parameters. For
graciloplasty, battery-powered single channel implants, with the electrodes
usually positioned near the motor point of the skeletal muscle, are currently
in use /2/. Activation of two muscles is not yet applied clinically.
Over the last few years our group has worked on
developing a modular stimulation system usable for a variety of applications.
Important components of the stimulation system, such as the implant case, the
main module of the implant electronics, the stimulation electrodes, and the
hardware of the programmer unit, remain the same in all applications. Useful applications and the limitations of such a battery-powered
implant have been described in /3/ on the basis of first calculations and
prototyping. Additionally, the first specific application of this modular
concept, i.e. an ECG-triggered
stimulator for cardiac assistance by skeletal muscle, has already been tested
in an animal study and produced extremely satisfactory results.
The implantable device presented here was developed for use in electrophrenic respiration (EPR) and in graciloplasty. In developing the stimulator we had set ourselves the following goals: The system should allow the implementation of all stimulation techniques used so far in diaphragm pacing in humans (single channel or multichannel methods such as carousel stimulation /4/ and sequential stimulation /5/). The use of external devices should be limited to the time necessary for programming in order to maximize the patient's freedom of movement. Additionally, the system should allow alternating stimulation of two skeletal muscles as a further improvement in graciloplasty.
MATERIAL AND METHODS
The
stimulation device is composed of a programmer with a transmitter unit, an
implantable multichannel stimulator, and nerve pacing leads (Fig. 1).
The
multichannel stimulator is powered by a Lithium Thionyl Chloride battery
(WG8602, Wilson Greatbatch Ltd. NY, USA) and works as an independent system in the body. It includes an eight-channel
output stage, a transmitter unit, and a controller unit. A microcontroller
serves as the central unit of the controller. The number of pulses in
one burst, burst frequency (1¸50 Hz), pulse width (0.2¸1 ms),
pulse amplitude (0¸4 mA), and current
active output are set by the controller unit. All parameters and functions are
programmable via the bidirectional telemetry circuit. The eight output stages
provide a constant current pulse, each output channel can be switched as anode
or cathode or can be deactivated.
The
multichannel pulse generator is hermetically sealed in a titanium case and powered
by a Lithium Thionyl Chloride battery. Its size is 65 x 17 mm (diameter x
height) and it weighs 88 g. Nerve pacing is effected by ring-shaped
stainless-steel electrodes. These epineurial electrodes are used clinically in
the ”Vienna Phrenic Pacemaker” /4/.
The
stimulation data are input using a laptop computer (IBM-PC or compatible)
instead of a specially designed programmer. A graphical user interface
facilitates the setting and modification of parameters and reduces training
time. Each modification of the parameters is stored automatically and can be
processed in conjunction with additional data such as respiration flow, or the
patient's medical history. Data are transferred between the PC and the
implanted pulse generator in both directions by a radio-frequency transmitter
unit linked to the serial port (RS-232) on the PC. The transmission link
operates over a vertical displacement of up to 50 mm at a data rate of
1200 bit/s and a carrier frequency of 100 kHz.
Fig.1
stimulation device, programmer (laptop computer), transmitter unit, implantable
multichannel stimulator, and nerve pacing leads
RESULTS
This
implantable neurostimulator can be used for activating two skeletal muscles.
Stimulation of the motor nerve can be achieved by either single channel or
multichannel methods with up to 4 electrodes for each nerve. The
stimulation wave-form can be adapted to the requirements of EPR and of
graciloplasty. Two skeletal muscles can be activated simultaneously,
alternately or in un-interrupt mode.
Diaphragm
pacing to restore breathing
The
stimulation system allows implementation of all stimulation techniques
currently in use in EPR, which differ greatly from each other in their use of
either single channel or multichannel electrodes and in their selection of
impulse parameters. Among the three groups that have developed phrenic pacing
systems the group from Yale (USA) uses unipolar stimulation for each nerve with
a remote indifferent electrode at low stimulation frequencies of 7-10 Hz and
inspiration rates between 7-10/min. The other two groups use multichannel
techniques with 4 bipolar electrodes applied to the phrenic nerves. The group
from Tampere (Finland) implements sequential stimulation /5/, the active
electrodes are changed for each impulse and the stimulation frequency decreases
from 25 to 16 Hz during the inspiration burst. The Vienna (Austria) group
implements carousel stimulation /4/, the active electrodes are changed after
each inspiration burst, a constant stimulation frequency of 26 Hz is used. Furthermore,
the stimulation current increases during an inspiration burst from the
threshold current to adequate maximum current to achieve a smooth contraction.
All
these functions are mastered by the developed device, which also allows all
parameters to be adjusted within a large range with a high resolution. As an
additional feature optimized pulse trains, a stimulation technique published in
1980 by Zajac FE and aimed at increasing fatigue resistance, can be
implemented.
Graciloplasty
for fecal continence
The
developed device allows the application of all advanced stimulation techniques
described above for graciloplasty, too. As a further
improvement of this particular FES application the system makes it possible to
alternately stimulate two skeletal muscles.
DISCUSSION
This stimulation system can be used for graciloplasty and for
diaphragm pacing. The developed device is based on a
modular system architecture, which is advantageous for several reasons. Time and effort necessary
for developing a specific application are reduced, by integrating components
already tested, such as the implant case, the risk of design errors is reduced.
The
integration of different stimulation techniques in one stimulation system has
to be considered the main achievement in the development of this device. Due to
its versatility the stimulator is especially suitable for in vivo studies, in
particular for the investigation of new methods for optimizing muscle output by
electrical stimulation. Furthermore, all parts of the implant that have direct
contact with body tissue use the latest pacemaker technology (titanium,
silicone-rubber, epoxy resin) and have been tested in animal studies.
REFERENCES
/1/ Creasey G, Elefteriades J, DiMarco A, Talonen P,
Bijak M, Girsch W, Kantor C. Electrical
stimulation to restore respiration J Rehabil Res Dev. 33/2 (123-132) 1996
/2/ Baeten C, Spaans F, Fluks A. An implanted neuromuscular stimulator for fecal continence following previously implanted gracilis muscle. Report of a case. Dis Colon Rectum. 1988 Feb; 31(2): 134-7
/3/ Lanmüller H, Bijak M, Mayr W, Rafolt D, Sauermann S, Thoma H. Useful applications and limits of battery powered implants in functional electrical stimulations Artif-Organs. MAR 1997; 21 (3) : 210-212
/4/ Mayr W, Bijak M, Girsch W, et al. Multichannel stimulation of phrenic nerves by epineural electrodes. Clinical experience and future developments. ASAIO J. 1993; 39(3) M729-35.
/5/ Talonen PP, Baer GA, Hakkinen V, et al.
Neurophysiological and technical considerations for the design of an
implantable phrenic nerve stimulator. Med Biol Eng Comput. 1990; 28(1):31-7.
AUTHOR’S ADDRESS
Hermann Lanmüller Ph.D. Department of Biomedical Engineering and Physics, AKH 04L, Währinger Gürtel 18-20, A1090 Vienna, Austria, Tel: + 43-1-40400-3985, Fax: +43-1-40400-3988, E-Mail: H.Lanmueller@bmtp.akh.ac.at