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Last updated: June, 2002
Cardiac Assistance
From Skeletal Muscle:
Electrical
Stimulation In The Management Of Heart Failure
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Cardiovascular
disease is the major cause of mortality and morbidity
in the industrialized world. Although death rates from coronary heart
disease have been declining, heart failure, which is usually the consequence of
obstruction of the coronary arteries, is on the increase. Heart failure is a
progressive, debilitating condition that accounts for a high proportion of
hospital admissions and readmissions. For some years
the only surgical option for treating heart failure has been cardiac
transplantation. Unfortunately fewer donor hearts are becoming available, and
the number of transplantation operations performed has therefore been falling
year by year, despite rising demand. Many patients cannot meet the strict age
and other criteria for transplant surgery. Of those who are placed on the
waiting list, 25–30% die before a suitable donor is found. Even for those who
receive the operation, transplantation may not be a perfect solution.
Transplanted hearts often fail through rejection or accelerated narrowing of the
coronary arteries. Lifelong use of immunosuppressive drugs is expensive and not
without side-effects, which include susceptibility to infection.
Xenotransplantation—the use of donor hearts from other species, such as
genetically modified pigs—could relieve the problem of donor shortage but poses
risks of porcine virus infection and would call for massive immunosuppression.
Total artificial hearts and left ventricular assist devices carry a risk of
thrombus formation, suffer from a high infection rate, and depend on external
power supplies; they are therefore used only as a bridge-to-transplant. Thus
there is a growing need for a viable alternative to current surgical approaches
to heart failure.
The idea of
using muscle from a patient’s own body to assist his failing heart is far from
new. However, early attempts failed because skeletal muscle could not be made to
work incessantly without becoming fatigued. The discovery of the adaptive
capabilities of skeletal muscle [1, 2] has now made this approach feasible. Skeletal muscle that has
been transformed (or ‘conditioned’) by stimulation, is so fatigue-resistant that
it can perform cardiac levels of work on a continuous basis [3].
The surgical procedure known as cardiomyoplasty [4-7] uses the latissimus dorsi muscle. This thin, flat sheet of
muscle from the back is often chosen by plastic reconstructive surgeons, partly
because moving it does not leave the patient with a serious functional or
cosmetic deficit. It must be detached from the pelvis, ribs and spine but the
nerve and blood vessels that enter at its upper end are carefully preserved.
Electrodes are placed close to the nerve, and connected to an implantable
electrical stimulator that is triggered by the electrical activity of the
patient’s heart. The muscle is then transferred into the chest, and wrapped
around the heart. After a delay to allow for revascularization, the grafted
muscle is conditioned and finally stimulated, usually on alternate beats, at the
appropriate point in the cardiac cycle. The clinical benefits of this procedure,
which has now been carried out on some 1500 patients worldwide, are due mainly
to the reinforcing effect of the wrap, which reduces ventricular wall stress and
restricts enlargement [8, 9]. About 85% of patients show symptomatic improvement. Many of
the patients who are candidates for cardiomyoplasty are prone to arrhythmias,
and outcomes have been improved by selecting patients carefully, and in some
cases by implanting a defibrillator at the time of surgery. In an alternative
procedure, known as aortomyoplasty, the muscle is wrapped around the ascending
or descending aorta. Experience with aortomyoplasty is limited as yet, in terms
of the number of patients (only about 20 worldwide) and the duration of
follow-up [10].
In the procedures just described, the latissimus dorsi muscle is wrapped
around existing structures: the ventricles of the heart in cardiomyoplasty, and
the aorta in aortomyoplasty. Other configurations are being investigated that
could harness muscle power more effectively [11]. In particular, the latissimus dorsi muscle can be formed
into a separate auxiliary pump, or Skeletal Muscle Ventricle (SMV) [12, 13]. Such a device can be connected to the aorta and stimulated
to contract during the filling phase of the patient’s own heart cycle. This
unloads the heart, boosts the blood supply around the body, and enhances the
blood flow in the coronary arteries that supply the muscular wall of the heart
(myocardium). In another configuration it is connected in parallel with the left
heart, rather like a mechanical left ventricular assist device [14, 15]. SMVs constructed in experimental dogs have pumped in
circulation for months and years, and in one case for over 4 years [16, 17]. In terms of pumping power, SMVs could rival the best
mechanical artificial ventricles, but—unlike those devices—every component can
be implanted, so they offer no psychological challenge to the patient [18]. However, the procedure is less conservative than
cardiomyoplasty, because it places a new surface in the bloodstream, and
although rapid progress is being made, it is not yet ready for use in patients.
In all its forms, cardiac assistance from skeletal muscle has the great
advantage that patients do not have to wait for a donor heart. Because the
procedure makes use of their own tissue, there is no need for immunosuppression,
with its attendant costs and undesirable side-effects. The patient’s own heart
is not discarded, so it can still respond to the neural and hormonal signals
that regulate its function in response to physical activity. And because the
skeletal muscle graft shares the workload of the heart the conditions are
created for at least a partial recovery of the weakened myocardium.
Much of the early clinical development of cardiomyoplasty was supported by
Medtronic Inc., who made the special stimulators needed for the procedure. Their
withdrawal from the field has, for the moment, denied patients access to this
form of treatment. However, new stimulators are becoming available through
another company, Illini Group Ltd. of Chicago. Because of the regulatory
environment in North America these are being used at the moment by surgeons in
other parts of the world. There is a positive aspect to this hiatus in the
access to treatment. Since the introduction of cardiomyoplasty 15 years ago
there have been major advances in our understanding of the relevant basic
science. We have a better idea of how the procedure benefits the heart. We know
more about the conditioning process, and how this should be modified to give the
muscle the required combination of power and endurance. We have a more accurate
picture of the blood supply to the latissimus dorsi muscle, how it is affected
when the muscle is mobilised, and how best to maintain graft viability. These
and other issues have been the subject of recent reviews [19, 20]. There is now an opportunity to incorporate these important
advances into the surgical protocols for cardiomyoplasty, with the prospect that
both the benefits to the individual patient and the success rate of the
procedure will be improved.
Transplantation, mechanical pumps, cardiomyoplasty and SMVs are not in
competition. Rather they will form part of an enlarged armamentarium available
to the cardiothoracic surgeon. By taking less severely affected heart failure
patients off the transplant waiting list, techniques based on skeletal muscle
assistance can help to ensure that scarce donor hearts are available to those
for whom transplantation remains the only option.
References
1.
Salmons S, Vrbová G (1969) The influence of activity on some contractile
characteristics of mammalian fast and slow muscles. J Physiol 201: 535-549.
2. Salmons
S, Sréter FA (1976) Significance of impulse activity in the transformation of
skeletal muscle type. Nature 263: 30–34.
3. Acker MA,
Hammond RL, Mannion JD, Salmons S, Stephenson LW (1987) Skeletal muscle as the
potential power source for a cardiovascular pump: assessment in vivo. Science
236: 324-327.
4.
Carpentier A, Chachques J-C (1985) Myocardial substitution with a stimulated
skeletal muscle: first successful clinical case. Lancet i: 1267.
5. Hagege
AA, Desnos M, Chachques JC, Carpentier A, Fernandez F, Fontaliran F, et al.
(1990) Preliminary report: follow-up after dynamic cardiomyoplasty.
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TL, Salmons S (1993) Skeletal muscle assistance in heart failure. Cardiovasc Res
27: 1404-1406.
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van der Veen FH, Lorusso R, Schreuder JJ, Wolf T, David JB, et al. (1998)
Aortomyoplasty. Basic Appl Myol 8: 59-65.
11. Salmons S,
Jarvis JC (1992) Cardiac assistance from skeletal muscle: a critical appraisal
of the various approaches. Br Heart J 68: 333-338.
12. Acker MA,
Hammond R, Mannion JD, Salmons S, Stephenson LW (1986) An autologous biologic
pump motor. J Thorac Cardiovasc Surg 12: 733-746.
13. Girsch W,
Koller R, Lanmuller H, Rab M, Avanessian R, Schima H, et al. (1998) Experimental
development of an electrically stimulated biological skeletal muscle ventricle
for chronic aortic counterpulsation. Eur J Cardiothorac Surg 13: 78-83.
14. Guldner
NW, Eichstaedt HC, Klapproth P, Tilmans MHI, Thuaudet S, Umbrain V, et al.
(1994) Dynamic training of skeletal muscle ventricles. A method to increase
muscular power for cardiac assistance. Circulation 89: 1032-1040.
15. Thomas GA,
Baciewicz FA, Jr, Hammond RL, Greer KA, Lu H, Bastion S, et al. (1998) Power
output of pericardium-lined skeletal muscle ventricles, left ventricular apex to
aorta configuration: up to eight months in circulation. J Thorac Cardiovasc Surg
116: 1029-1042.
16. Thomas GA,
Isoda S, Hammond RL, Lu HP, Nakajima H, Nakajima HO, et al. (1996)
Pericardium-lined skeletal muscle ventricles: up to two years' in-circulation
experience. Ann Thorac Surg 62: 1698-1706; discussion 1706-1697.
17. Thomas GA,
Hammond RL, Greer KA, Lu H, Jarvis JC, Shortland AP, et al. (1999) Functional
assessment of skeletal muscle ventricles after pumping for up to four years in
circulation. Ann Thorac Surg (in press):
18. Salmons
PH, Salmons S (1992) Psychological costs of high-tech heart surgery (guest
editorial). Br J Hosp Med 48: 707-709.
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(1998) Latissimus dorsi stimulation in dynamic cardiomyoplasty: how should we
proceed? Basic Appl Myol 8: 41-50.
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(1999) Permanent cardiac assistance from skeletal muscle: a prospect for the new
millenium. Artif Org 23: 380-387.
Contributor:
Stanley
Salmons, Ph.D., 2001
See:
References:
Electrical Stimulation for Cardiac
Assistance
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