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BACKGROUND OF THE INVENTION
Coronary artery bypass graft (CABG) surgery has become a well known and
conventional procedure, often referred to as "heart bypass" surgery. Such
surgery is performed to relieve a condition in which a partially or fully
blocked artery is no longer effective to transport blood to the heart and
involves removing a portion of a vein from another part of the body,
frequently the saphenous vein, to use as a graft and installing this graft
at points at which bypass the obstruction to restore normal blood flow to
the heart. Common though this procedure has become, it is nevertheless
lengthy, traumatic and subject to patient risk. Among the risk factors
involved is the use of cardiopulmonary bypass equipment, i.e., the
so-called "heart-lung machine", to both pump blood and oxygenate the blood
so that the patient's heart may be stopped during the surgery, with its
function performed by the cardiopulmonary bypass equipment.
Prior to the present invention, it has been found possible to conduct CABG
surgery without stopping the heart, i.e., on a beating heart. In such a
beating heart procedure, the function of the heart is maintained and the
cardiopulmonary bypass equipment is not needed to replace that function.
However, Since the heart is beating in such a procedure, the surgeon must
cope with the movement of the heart, whether the surgery is a bypass
procedure or any other type of coronary surgery. The present invention
addresses this problem.
The performance of coronary surgery on the beating heart is described by
Benetti et al in "Coronary Revascularization With Arterial Conduits Via a
Small Thoracotomy and Assisted by Thoracoscopy, Although Without
Cardiopulmonary Bypass", Cor. Europatum, 4(1):22-24 (1995), which is
incorporated herein by reference and by Westaby, "Coronary Surgery Without
Cardiopulmonary Bypass" in the March, 1995 issue of the British Heart
Journal which is incorporated by reference herein. Additional discussion
of this subject matter can be found in Benetti et al, Chest, 100(2):312-16
(1991), Pfister et al, Ann. Thorac. Surg., 54:1085-92 (1992), and Fanning
et al, Ann. Thorac. Surg., 55:486-89 (1993). These articles discuss the
further details of grafting by anastomosis of a saphenous vein or mammary
artery to diseased coronary arteries including the LAD or the right
coronary artery (RCA), temporary occlusion of the coronary artery to
provide a bloodless anastomotic field, use of a double suture placed above
and below the point of anastomosis, and use of a running suture for the
anastomosis. These articles also contrast the beating heart procedure to
the more widely used CABG method performed on the non-beating heart with
cardiopulmonary bypass.
SUMMARY OF THE INVENTION
In heart surgery performed while heart is beating, surgeon is faced with a
moving organ which places increased demands on his skill in performing the
desired procedure, e.g., an anastomosis of the left anterior descending
artery (LAD) to the left internal mammary artery (LIMA) or anastomoses of
both ends of a free graft means to a target artery and a coronary artery.
If, without substantial harm to the patient, the heart could be
momentarily substantially stopped or slowed while the surgeon performed
the desired task, e.g., taking a stitch with a suturing needle, such task
would be less difficult to accomplish.
The purpose of the present invention is to provide a surgical procedure in
which the heart is momentarily substantially stopped or slowed in a
predictable and reliable manner to facilitate the surgery by electrically
stimulating the vagus nerve. This stimulation can be accomplished by
gaining access to the vagus nerve in the neck or in the chest and then
using a suitable device, e.g., a commercially available nerve stimulator
or insulated pacing wires with distally exposed conductors connected to a
current source, to briefly, e.g., for 1 to 5 seconds, apply electric
energy to the vagus nerve. A 50 millihertz current may be used, but the
present invention is not limited to any particular quantitative amount of
electrical energy.
The time of the stimulation and amount of current applied will vary
according to the type of surgery and the nature of the task for which
substantial stopping or slowing of the heart is desired. In any event, the
normal sinuous rhythm of the heart is rapidly restored by natural forces
once the stimulation is terminated. Thus, for repetitive tasks such as
stitching during suturing, the stimulation may be repeatedly applied for
brief intervals during which time the task can be performed in less
difficult conditions than would apply if the heart were beating in a
normal manner.
The role of the vagus nerves in the control of cardiac rate and rhythm has
been recognized for more than three centuries. Furthermore, soon after
electrical stimulating devices became available, it was shown that vagal
stimulation caused a reduction in heart rate and, as earlier as 1897, Hunt
carried out a quantitative study of the effects of vagal stimulation on
heart rate, Hunt, R., "Experiments on the Relation of the Inhibitory to
the Accelerator Nerves of the Heart", J. Exp. Med. 2:252-279 (1897). An
excellent discussions of this phenomenon is found in Chapter 4,
Parasympathetic Control of the Heart, by Levy and Martin, in Nervous
Control of Cardiovascular Function, edited by Randall, Oxford University
Press (1984), which is incorporated by reference herein. However, in spite
of the fact that the effect of vagal nerve stimulation on heart rate has
long been known, it is believed that this knowledge has not previously
been applied to coronary surgery. Rather, vagal nerve stimulation for
other purposes has been employed, e.g., as disclosed in Schwartz U.S. Pat.
No. 5,330,507 where vagal nerve stimulation is used in connection with
treating arrhythmias and in Kendall U.S. Pat. No. 5,458,625 which
addresses the use of vagal nerve stimulation for the alleviation of
substance withdrawal symptoms or the provision of pain relief, stress
relief, and/or general muscle relaxation. Thus, the present invention is
believed to be the first use of vagal nerve stimulation to facilitate
coronary surgery.
DETAILED DESCRIPTION OF THE INVENTION
The vagal nerve stimulation technique of the present invention may be used
in open chest coronary surgery where a sternotomy is used to gain access
to the heart or in closed chest beating heart coronary surgery in which a
thoracotomy is used to gain access. The following is an exemplary usage of
the latter procedure.
The patient is intubated with a double-lumen endobronchial tube which
allows selective ventilation or deflation of the right and left lungs. The
left lung is deflated to provide access to the heart and the LIMA. The
preferred surgical position of the patient is right lateral decubitus,
30.degree. from horizontal, with the left arm above the head.
Surgery begins with a left anterior thoracotomy over the fourth intercostal
space. Other sites are suitable depending on the patient's physiology,
particularly the fifth intercostal space. A retractor is used to spread
the ribs to provide access to the beating heart. The size of the
thoracotomy varies depending on the patient, but generally is less than 12
cm. The parietal pleura is dissected and separated from the ribbons, to
permit the introduction of a thoroscope through a procar at the fourth
intercostal space, medial axillary line. The thoroscope may be introduced
through other areas such as the fifth through seventh intercostal spaces,
again depending on the patient's physiology. The thoroscope is positioned
to provide visualization of the LIMA. The LIMA is then dissected with
suitable instruments introduced through the thoracotomy. These instruments
generally comprise scissors, clippers, pliers, electrocauteries or other
conventional devices useful for the dissection. It is sometimes useful to
make a graft with a radial artery coming out from the LIMA in a T-form.
This allows formation of anastomoses with multiple coronary arteries such
as sequential grafts to the Dx and Cx arteries.
Following dissection of the LIMA, a small pericardial incision is made to
expose the LAD. Access to the LAD and Dx arteries is typically relatively
easy, requiring an incision of about 5 cm. Access to the Cx artery depends
on the patient's characteristics and location of the vessels. In some
cases, a graft to the Cx artery requires increased rotation of the patient
to the right lateral decubitus and some extension of the pericardial
incision. Heparin, or other suitable anticoagulant, may be administered to
the patient in an appropriate dose such as 1.5 mg/kg.
To prevent excess bleeding and partially stabilize the vessel, a segment of
LAD is occluded with ligating stay sutures comprising 2.5 cm lengths of
5/0 polypropylene or other appropriate ligature material. Applying tension
to the ligatures helps stabilize the LAD even though the heart is beating.
Other conventional means for occluding and stabilizing the artery may be
suitable as well. Furthermore, forceps are also introduced through the
thoracotomy so further stabilize and retract the LAD. A scalpel is then
introduced to perform an arteriotomy in the LAD.
An anastomosis between the LIMA and the LAD is then performed by suturing
with 7/0 polypropylene using a needle manipulated by a forceps.
It is at this point that vagal nerve stimulation is used to temporarily
stop or slow the heart to reduce motion in the anastomosis field such that
a suturing stitch is taken immediately after stimulation when the heart's
motion is temporarily stopped or substantially reduced. The number of
stitches required will, of course, dictate the number of occasions on
which electrical stimulation of the vagal nerve will be desirable.
Upon completion of the anastomosis, the anticoagulant is reversed by
suitable means such as protamine. The hemostasis should be carefully
controlled. The thoracotomy is closed by conventional means. If the pleura
is closed, a small tube for drainage may be left in place and removed the
same day as the surgery. If the pleura is open, a larger tube should be
left in place for 24 hours. All drainage tubes are introduced through the
small incision for the thoroscope.
In the foregoing procedure, the electric stimulator, e.g., of the type
disclosed in U.S. Pat. No. 5,458,625 and may be attached to the patient's
ear, neck, or other points of access to the vagal nerve. The electrodes
used to stimulate the vagal nerve may be non-invasive, e.g., clips, or
invasive, e.g., needles. The electrical energy supplied to the vagal nerve
will vary with the type of equipment used, the point in the body at which
access to the vagal nerve is obtained, etc. and it is to be understood
that the practice of the present invention is not limited to any
particular values. Rather, the appropriate amount of electrical energy
needed to achieve the desired result can be readily determined empirically
once the type of equipment, point of access, etc. are known.
The present invention is not limited to the scope of the foregoing detailed
description, but is of the full scope of the claims appended hereto.
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