Method and device for performing transluminal angioplasty

A novel method and device for performing percutaneous transluminal angioplasty to treat occlusive artery disease is disclosed. The method involves simultaneous measurement and display of the fluid pressure and volume existing within the balloon catheter as the procedure is performed. Information is produced which is useful in determining the efficacy of the procedure as it is performed which obviates the need for arbitrary repeated inflations. The information is also useful in the subsequent management of the patient's disease.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention constitutes an improved technique for performing 
percutaneous transluminal angioplasty. Angioplasty is a medical procedure 
used to treat patients whose arteries have become occluded due to the 
disease process call atherosclerosis. 
Arteriosclerosis is a general term which refers to any of a group of 
diseases in which the lumen of an artery becomes narrowed or blocked. The 
most common and important form of arteriosclerosis, especially in Western 
societies, is the disease known as atherosclerosis. In atherosclerosis, 
there is an accumulation of lipids in the intimal, or inner, layer of the 
affected artery. The resulting intimal thickening restricts the flow of 
blood so as to hinder the functioning of, or permanently damage, the organ 
which the artery feeds. These accumulations of lipids tend to be localized 
and can occur in coronary, cerebral, or peripheral arteries. They will 
hereinafter be referred to synonymously as lesions, plaques, or atheromas. 
The lipid accumulation is made up of free lipid and smooth muscle cells 
which have proliferated and taken up lipid. As the disease progresses, the 
lesion may begin to absorb calcium which causes it to harden and may also 
be composed of blood which has clotted in response to the presence of the 
atheroma. Although the process of plaque formation is not completely 
understood, it is known to be progressive, and atherosclerotic plaques may 
vary greatly in their physical characteristics. 
Treatment of atherosclerosis is aimed at alleviating the diminished blood 
flow. This can sometimes be done by medical means which cause the smooth 
muscles of the arterial walls to relax and thereby dilate the artery. 
Other treatment methods are directed toward physiological compensation for 
the reduced blood flow. In cases where the artery is severely occluded, 
however, there is no reasonable alternative but to try to re-establish a 
lumen of proper diameter. A number of surgical procedures have been 
developed toward this end. These include endarterectomy, in which the 
plaque is surgically removed, and by-pass grafts, in which a segment of 
artery or vein from elsewhere in the body is removed and reattached in 
place of the occluded artery. These procedures are major surgical 
operations and present a number of disadvantages to a patient including 
financial cost, inconvenience, and the risk of complications associated 
with any major surgery. Therefore, in the past several years, methods of 
re-establishing the patency of an occluded artery have been developed 
which are relatively non-invasive and present less risk to a patient than 
conventional surgery. One such method is transluminal angioplasty. 
2. Description of the Prior Art 
The conventional method of performing transluminal angioplasty uses a 
special double lumen catheter. The first, or inner, lumen allows passage 
of a guide wire. Concentric with this lumen is a second lumen which 
connects to a sausage-shaped segment or balloon at the distal end of the 
catheter. The second lumen and balloon are generally filled with diluted 
contrast media. Contrast media is radio-opaque liquid which makes 
visualization of the catheter possible by means of X-rays. The procedure 
first involves selecting a convenient place to introduce the catheter into 
the arterial system of the patient, such as the femoral artery of the leg. 
Next, the catheter is guided to the blocked artery. This is done manually 
and with the aid of an X-ray monitor. When the catheter is appropriately 
positioned, the guide wire is advanced to and past the point of 
obstruction. The balloon catheter, which surrounds the guide wire, is then 
advanced along with the guide wire until it is surrounded by the occluding 
plaque. The balloon, made of material with high tensile strength and low 
elasticity, is inflated to a pressure as high as twelve atmospheres. As 
the balloon expands it creates a larger inner diameter within the occluded 
artery. It is not known with certainty what physical processes occur 
within the occluded artery in response to the balloon inflation, but the 
usual method is to inflate the balloon to a certain predetermined pressure 
and repeat the inflation an arbitrary number of times. The balloon is then 
collapsed and retracted. The site of the obstruction is then examined 
angiographically and, if the artery is still occluded, a decision is made 
either to repeat the angioplasty procedure or to resort to some other 
option. 
As aforementioned, the procedure involves inflating the balloon to a 
predetermined pressure. Although the operator may observe the size of the 
balloon during the inflation by means of the X-ray monitor, unless the 
pressure is measured, the bursting pressure of the balloon may be exceeded 
causing rupture. Therefore, practitioners have realized the need for 
continuous monitoring of the fluid pressure within the balloon. As it is 
conventional to inject fluid into the balloon with a syringe, the most 
obvious method is to interpose a T-fitting between the delivery end of the 
syringe and the balloon catheter. A standard pressure transducer can then 
be connected to the T-fitting and the fluid pressure within measured. U.S. 
Pat. No. 4,370,982 discloses a method for measuring fluid pressure without 
the transmitter coming in contact with the working medium. The '982 patent 
also discloses an injection device which uses a threaded member which when 
rotated produces translational motion of the syringe plunger. The 
relatively slow inflation is supposed to reduce further the risk of 
balloon rupture. 
Another relevant patent is U.S. Pat. No. 4,446,867 which discloses a method 
and apparatus for generating pulses of pressure within the balloon 
catheter. As discussed above, some atheromas become hard due to 
calcification and therefore resist dilation by the balloon. The '867 
patent represents an attempt to deal with this problem by inflating the 
balloon so rapidly that the plaque is broken. Although the specification 
of the '867 patent recites that pieces of broken plaque will be removed by 
normal cardiovascular processes, it seems obvious that such fragments may 
flow downstream and become lodged in a smaller artery, thereby completely 
blocking blood flow. As pieces of plaque may break off during conventional 
angioplasty procedures, even without using the pulsed pressure method of 
the '867 patent, it is important to know when this has occurred so that 
remedial steps may be taken. 
SUMMARY OF THE INVENTION 
One major problem with transluminal angioplasty is that there has 
heretofore been no means of evaluating the efficacy of the procedure 
contemporaneous with the performing of it. This has resulted in the 
establishment of arbitrary performance protocols whereby the balloon is 
inflated repeatedly an arbitrary number of times. Because the pressures 
involved are necessarily high, each subsequent inflation presents a risk 
of balloon rupture. It would be advantageous if the operator had some 
means of judging when the procedure had succeeded or failed and whether a 
subsequent inflation could be expected to succeed. As atherosclerotic 
plaques vary greatly in their physical characteristics, what is needed is 
a means of monitoring the underlying physical events occurring within the 
occluded artery as the balloon in inflated. Not only would this be helpful 
during the performance of the procedure itself, but it would make possible 
a more accurate prognosis of the course of the patient's disease and aid 
in evaluating other treatment options. 
The present invention accomplishes this objective by providing for the 
simultaneous monitoring of both pressure and volume changes occurring 
within the balloon as the angioplasty procedure is performed. By the use 
of basic physical principles, the pressure-volume curves thus generated 
can be correlated with the physical changes taking place within the 
occluded artery.

DESCRIPTION OF THE INVENTION 
The best mode and preferred embodiment of the invention is illustrated in 
FIG. 4. The proximal end of the balloon catheter 1 is attached to the 
inflation syringe 3. The syringe is of standard type but modified for 
reasons which will be apparent below. A plunger 10 moves through the 
barrel of the syringe 3 displacing liquid, such as diluted contrast media, 
into the balloon catheter 1. The plunger shaft 7 is finely threaded all 
along its length so that when turned, the shaft moves longitudinally 
through an oppositely threaded annular member 8. The annular member 8 is 
attached to the syringe 3. In this way, slow and even displacement of 
liquid into the balloon catheter is produced by rotating the shaft 7. The 
more fine the threads, of course, the slower will be the fluid 
displacement. A hand crank 4 has been added to facilitate the balloon 
inflation process. 
Interposed between the balloon catheter 1 and inflation syringe 3 is an 
electronic pressure transducer 2 of conventional type. An electronic 
signal proportional to the fluid pressure existing within the catheter is 
then fed to an oscilloscope 9 for real-time display. Any type of 
electronic recording device could also be used. A linear displacement 
transducer 6, which produces an electronic signal proportional to its 
length at any given time also feeds into the oscilloscope 9. The ends of 
the linear displacement transducer 6 are connected by means of coupling 
bars 5a and 5b to the plunger shaft 7 and inflation syringe 3 
respectively. In this way the signal produced by the linear displacement 
transducer 6 is proportional to the volume of fluid displaced from the 
syringe 3 and hence residing in the balloon catheter 1. Thus, there are 
two electronic signals fed to the oscilloscope 9 which represent the 
volume and pressure of the fluid contained by the balloon at any given 
time. By displaying the pressure and volume inputs simultaneously a curve 
is generated by the oscilloscope wherein one axis corresponds to pressure 
and the other axis corresponds to volume. The information contained in 
this curve enables one to draw certain conclusions regarding the physical 
process taking place during the dilation process as will now be explained. 
FIGS. 1-3 depict expansion curves generated by dilating models of arterial 
lesions with three different types of behavior. Superimposed on all three 
figures is the expansion curve 10 of the balloon expanded by itself. This 
represents the compliance of the balloon alone and will be used as the 
reference curve. 
Referring first to FIG. 1, expansion curve 12 shows that as the pressure is 
raised initially, there is little change in the volume of the balloon as 
compared with the reference curve 10. This indicates that the 
atherosclerotic plaque which surrounds the balloon is preventing the 
balloon from expanding. As the pressure is increased further, however, the 
pressure within the balloon becomes great enough to overcome the 
resistance of the plaque material. At this point the occluded artery 
begins to dilate as the balloon expands. It is not clear whether the 
plaque material is actually compressed so as to occupy less volume or is 
deformed so as to be redistributed along the length of the artery, but 
what is important is that the expansion takes place at relatively constant 
pressure. At any given point along the curve, the pressure of the fluid 
within the balloon is exactly balanced by the pressure exerted by the 
surrounding plaque. A region of constant pressure, or isobaric, expansion 
indicates that the plaque material is exerting the same force irrespective 
of the extent of the plaque's deformation. The theory of the properties of 
materials would predict that the stress exerted on the plaque had exceeded 
the yield point of the plaque material. This would mean that the plaque 
material is being deformed plastically rather than elastically. This is 
consistent with a young or at least still malleable atheroma which can be 
expected to retain the deformation produced by the expanded balloon. Thus, 
when an expansion curve like that of FIG. 1 is obtained, the operator may 
infer that the angioplasty procedure has been relatively successful and no 
further inflation cycles are necessary, especially if a repeat inflation 
yields a curve superimposed on curve 10. Furthermore, the knowledge that 
the atheroma responded to the procedure in this way is useful in the 
subsequent management of the patient's atherosclerotic disease. 
Next, in FIG. 2, is an expansion curve 14 which indicates that as the 
balloon expands against the occluded artery, the artery exerts increasing 
force against the balloon. This would lead one to conclude that the 
occluded artery is acting like a spring and storing the work of expansion 
only to return to its former occluded shape when the balloon is deflated. 
This has been found experimentally to be the case although with repeated 
inflations the curve sometimes moves closer to the reference curve 
indicating that the artery is becoming more compliant. Unlike the case in 
FIG. 1, the atheroma in this example has probably been deformed very 
little by the expanding balloon. Since plaque is known not to be composed 
of elastic, or energy storing, material the likely source of the 
elasticity is the medial layer of the arterial wall itself. In any case, 
an expansion curve like that in FIG. 2 indicates a less desirable result 
for the patient than that in the first example above. The increased 
compliance of the arterial wall following repeated inflations may also 
indicate plastic changes such as thinning and microscopic tearing, such 
that it would be hazardous to try another inflation cycle. 
Finally, FIG. 3 shows an expansion curve 16 exhibiting sharp drops in 
pressure as the balloon expands. A sudden decrease in the pressure exerted 
against the balloon by the occluded artery can only mean that a stress 
relieving fracture of some kind has occurred. One can then infer that the 
plaque was hard and brittle, presumably due to calcification, and was 
fractured by the expanding balloon. Not only does this indicate that 
angioplasty is not likely to be successful in dilating the artery, but 
remedial steps may need to be taken to prevent the plaque fragments from 
separating from the rest of the plaque causing complications at some point 
downstream. One such remedial step might be to inflate the balloon a 
second time, although at a lower pressure, in order to "tack" the plaque 
fragments down and prevent their dislodgment. Anticoagulant therapy may 
also be indicated. 
In generating the expansion curves discussed above, the particular 
instrumentation used must be able to respond to the extremely small 
changes in volume involved when the balloon expands as well as pressures 
reaching twelve atmospheres. The inflation syringe described in the 
preferred embodiment was also constructed with a shaft possessing 
screw-type threads fine enough so that many rotations are necessary to 
move the shaft through the oppositely threaded annular member. A slow and 
even displacement of fluid into the balloon is necessary to avoid 
introducing artifacts into the pressure signal and obscuring the 
information contained therein. That is, a properly constructed expansion 
curve only contains pressure values which have been obtained after any 
transient pressure waves in the fluid have died out. 
It should be understood that the embodiment disclosed hereinabove is not 
meant to limit the invention in any manner. On the contrary, it is 
intended to cover all modifications, alternatives, and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
following claims.