Conditioning
protocol and stimulation device of the Vienna SMV project.
H. Lanmüller, U. Windberger*, S. Sauermann, M. Rab**, E.
Unger, W. Girsch***
Department of Biomedical Engineering and Physics,
University of Vienna
*Department of Biomedical Investigation, University of Vienna
**Department for Plastic and Reconstructive Surgery, University of Vienna,
*** Orthopaedic Hospital Speising, Vienna, Austria
SUMMARY
The Vienna SMV is designed to perform counterpulsation
parallel to the descending aorta. The neo-ventricle is created by enlarging a
segment of a thoracic aorta with pericardium, both from a donor, driven from
the latissimus dorsi muscle (LDM). This configuration has been proofed in acute
experiments and now in a ongoing chronic animal experiment.
In the chronic study the LDM is conditioned in situ
before the SMV is constructed. At the beginning of the study the clinically
approved stimulation protocol for cardiomyoplasty was used bilaterally in four
sheep. Conditioning is completed after 12 weeks and results in a completely
transformed muscle with slow-twitching type I fibres. In two of these sheep a
special concept of muscle prefabrication was applied for the left LDM. The left
LDM was detached from the thoracic wall, divided longitudinally and reattached
in situ to achieve vascular delay. This concept of muscle prefabrication proved
to be a failure. The histomorphological analysis showed a dramatic increase of
perimysial and endomysial connective tissue at the end of conditioning.
A shorted protocol for conditioning in situ has been
used up till now in three goats. Conditioning is started 1 week after electrode
implantation with 30 contractions per minute. The number of pulses per
contraction is incremented biweekly from 2 pulses up to 5 pulses. Conditioning
is completed after 8 weeks. The histomorphological analysis of the first animal
showed a completely transformed muscle with slow-twitching type I fibres.
At the beginning of the study an implantable nerve
stimulator was applied in four adult sheep. This device was developed at our
own department, powered from a Lithium Thionyl Chloride battery and
hermetically sealed in a titanium case. To achieve a reduction in costs this
device was modified into a semi-implantable version. In this version only the
electrode leads are implanted chronically. Three sensing leads and four
epineural stimulation leads are connected to a single skin button with an
integrated connector. The external ECG triggered pulse generator is fixed on
the animal by an elastic belt, its functionality is equal to the implantable
version. Up to now the semi-implantable stimulator was used in 4 adult goats.
An implantation period of 4, 7 and 6
month was reached without complications. An infection because of the
percutaneous link appears in one goat after 7 month. Overall, this resolution
turned out as a well useful and cost-saving device for animal experiments.
State of the Art
In the Vienna SMV project the skeletal muscle
ventricle (SMV) consists of hemocompatible biomaterials only and is designed to
perform counterpulsation parallel to the descending aorta. The
neo-ventricle is created by enlarging a segment of a thoracic aorta with
pericardium, both from a donor. The LDM is divided longitudinally and the two
resulting branches are wrapped around the neo-ventricle in counterrotating
direction. This concept was applied in a acute series in sheep /1/ and in a
chronic animal experiment. The chronic study starts in 1997 and is still
ongoing. This paper is focused on the protocol for preconditioning the LDM and
the stimulation device of the chronic SMV study.
MATERIAL AND
METHODS
Two different protocols had been used for preconditioning
of the LDM. In 4 adult female sheep (group 1) the conditioning protocol was set
according to the clinically approved stimulation protocol for cardiomyoplasty
by Chachques et al /2/. In this protocol the number of pulses (1,2,3,5) in one
burst is incremented biweekly, afterwards the number of bursts is incremented
from 35 to 70 contractions per minute. Overall, conditioning is completed after
12 weeks.
The surgical procedure was performed under general
anesthesia. After surgery the animals were kept in cages. The thoracodorsal
nerve was exposed under careful preservation of the vascular pedicle emerging
from the thoracodorsal artery. Four epineural electrodes were sutured to the
epineurium of the nerve in helical manner. This procedure was done bilaterally
and the electrode leads from the right and left side was connected to an
implantable stimulator /3/.
The stimulator and the epineural electrodes were designed and manufactured at our Department.
The nerve pacing leads are made from stainless-steel stranded wire (Eticon
612P, Ethicon Co., Norderstedt, Germany), coiled and embedded in silicon
(Selastic, Dow Corning, S.A., France). The battery-powered pulse generator is
hermetically sealed in a titanium case. Implant dimensions are 65 x 17 mm
(diameter x height), the device weighs 88 g. This pulse generator can be used
for activating two skeletal muscles via the motor nerves, using
constant-current impulses with a maximum current of 4 mA at a pulse duration of
0,2 to 1 ms. Stimulation can be achieved by either single channel or
multichannel methods (with up to 4 electrodes for each nerve), i.e. carousel
stimulation and sequential stimulation. A stimulation burst can be activated
unsynchronized or synchronized to the ECG. Additionally, the system allows a
dynamic adaptation of the synchronization delay and the burst duration to the
heart rate. Both parameters are calculated by an internal microcontroller. The
current values are the sum of a constant and a percentage of the RR interval.
The value of the constant, the percentage of the RR interval and all other
parameters can be set by an external PC, which is used as a programmer unit.
In the first two sheep out of this series a special
concept of muscle prefabrication was applied for the left LDM. The left LDM was
detached from the thoracic wall, with all perforating vessels
deriving from the intercostal vascular bundles being ligated, while the LDM
insertion at the humeral bone was kept unaffected. After the intramuscular
vascular architecture of the left LDM had been identified by means of
translumination, the muscle was divided longitudinally from its distal end up
to the entry of the neurovascular bundle in order to create two muscle branches
of equal size. The LDM was reattached to the thoracic wall with absorbable
sutures in original position.
A shorted protocol for conditioning in situ was
applied in 3 adult female goats (group 2). Conditioning is started 1 week after
electrode implantation with 30 contractions per minute. The number of pulses
per contraction is incremented biweekly from 2 pulses up to 5 pulses.
Conditioning is completed after 8 weeks. In this group only the left the
thoracodorsal nerve was exposed and four epineural electrodes were sutured to
the epineurium of the nerve. The left LDM was left untouched, without
mobilization or splitting.
Table 1. Shorted protocol for conditioning (group 2).
|
Weeks |
Burst type |
Muscle
contractions per minute |
|
1 |
no stimulation |
no stimulation |
|
2-3 |
double
pulses |
30/min |
|
4-5 |
triple
pulses |
30/min |
|
6-7 |
four pulses |
30/min |
|
8-9 |
five pulses |
30/min |
Used stimulation parameters: frequency 30Hz, pulse
duration 0.6ms
The leads terminate in one skin button with an
integrated multipolar connector, which was placed in a subcutaneous pocket (see
figure 1). The connector (Redel Lemo Group, Ecublens, Switzerland) is screwed
into a disc with an axial flange covered entirely by a woven double velour
patch (Meadox Medical, Ratingen, Germany) which provides an infection barrier.
The electrode wires are crimped to the connector contacts and embedded in
silicone rubber (Silastic, Dow Corning, S.A., France). Additionally, three
temporary pacing leads ( Medtronic, Inc., Minneapolis, MN, USA) embedded in a
velour pouch are connected to the skin button. The pouch is opened by building
the SMV and the electrodes are placed on the heart and used for ECG sensing. An
external ECG triggered pulse generator was fixed on the animal by an elastic
belt. The functionality of this semi implanted device is equal to the fully
implantable version.
Figure 1. Electrodes used for conditioning (group 2)
and ECG triggered stimulation of the SMV. Four epineural leads and three
sensing leads are connected to an integrated connector in one skin button.
RESULTS
The histomorphological analysis was done for all sheep
of group 1. After conditioning the LDM was completely transformed to slow
twitching Type I fibers. The equivalent diameter of the Type I fibers was
decreased and the percentage of the connective tissue was slightly increased.
The fully implantable stimulation device performed without complications. The
electrodes of one nerve had to be exchanged in one instance after they were
dislocated.
The concept of muscle prefabrication, which was
applied for the left LDM in the first two sheep out of this series proved to be
a failure. A high increase in the perimysial-and endomysial connective tissue
together with a complete loss of muscle fiber architecture was evident. Beside
the high increase of the connective and fatty tissue the muscle fibers showed typical
signs of degeneration or necrosis (for details see /4/).
The histomorphological analysis of the first animal in
group 2 showed a completely transformed muscle of the left LDM with
slow-twitching type I fibres. The percentage of the connective tissue was
slightly increased. Further analysis had not be done up till now.
An infection due to the skin button of the electrode
leads appears in one goat after a implantation time of 7 month. An implantation
period of 4, 7 and 6 month was reached
without complications.
DISCUSSION
Two clear massages can be drawn from the
histomorphological results. The concept of muscle prefabrication caused a
disastrous outcome. As a consequence, muscle splitting and mobilization
followed by vascular delay and in situ conditioning as a concept of muscle
prefabrication should be strictly avoided. The stimulation protocol applied in
group 1 without mobilization of the muscle showed the expected outcome. The
muscle was in a good condition and completely transformed to slow-twitching
type I fibres.
The first histomorphological analysis of the shortened
protocol (group 2) did not show the result we had hoped for. The protocol ends
with a stimulation frequency of 2.5 Hz in average per day. Nevertheless, we
could not found a relevant part of type 2A fibres, as it was found in rabbit by
Jarvis JC et. al. /5/ by continuous stimulation at 2.5Hz.
The stimulation device, particularly the functionality
turned out to be useful. The dynamic adaptation of the synchronization delay
and the burst duration to the heart rate enables a fast and simple adjustment.
This feature was very helpful during the chronic employment of the SMV, which
was achieved in three goats for 4, 5 and 4 month up till now.
The semi implantable device turned out as a useful and
cost-saving device for this animal experiment. The skin button seems to be
resistant again an infection for an implant period of six month. However, this
setup is not resistant again the animal itself, which leads to interruptions of
the stimulation for some hours. The cable which was used to connect the skin
button with the stimulation device was often found in the cages used as a
chewing gum. By that reason and additionally to prolong the implant period we
are now working on a full implantable and cost-saving device.
REFERENCES
/1/ Girsch W, Koller R, Lanmüller H, Rab M, Avanessian
R, Schima H, Wolner E, Seitelberger R. Experimental development of an
electrically stimulated biological skeletal muscle ventricle for chronic aortic
counterpulsation . Eur J Cardio Thorac. JAN 1998; 13 (1) : 78-83
/2/ Chachques JC, Carpentier A. Postoperative
Managment. In A Carpentier, JC Chachques, P Grandjean (eds.): Cardiomyoplasty.
Mount Kisco, NY, Futura Publishing Co., Inc., 1991, pp 131-138
/3/ Lanmüller H., Sauermann S., Unger E., Schnetz G.,
Mayr W., Bijak M., Girsch W. Multifunctional implantable nerve stimulator for
cardiac assistance by skeletal muscle. Artif-Organs Apr 1999; 23 (4): 352-359
/4/ Lanmüller H. ,Girsch W., Rab M., Sauermann S.,
Kamolz LP, Seitelberger R, Wolner E.: Preparation of a skeletal muscle
ventricle in sheep: severe damage to the latissimus dorsi muscle due to
mobilization before preconditioning.
European Surgical Research 2000;32:129-134
/5/ Jarvis JC; Sutherland H, Mayne CN, Gilroy SJ,
Salmons S. Induction of a fast-oxidative phenotype by chronic muscle
stimulation: mechanical and biochemical studies. Am J Physiol. 1996; 270(1 Pt
1):C306-12.
AUTHOR’S ADDRESS
|
Hermann
Lanmüller Ph.D. |
e-mail:
Hermannn.Lanmueller@univie.ac.at home page: www.bmtp.akh-wien.ac.at |