Balloon sheath

Disclosed is a tuvular sheath which is adapted to fit coaxially over an inflatable balloon. The sheath functions to alter the expansion characteristics of the balloon. In one preferred embodiment, the sheath converts the inflation profile of a dilatation balloon from a compliant mode to a noncompliant mode. Advantageously, the balloon sheath may be used in stent placement procedures, to accurately size the stent within a body lumen, while providing protection against balloon rupture. In one embodiment, the balloon sheath comprises a two-layered tubular structure, with an outer elastic layer surrounding an inner inelastic layer. Also disclosed are methods of altering the expansion characteristics of a balloon using the balloon sheath.

BACKGROUND OF THE INVENTION 
The present invention relates to medical balloon catheters and, in 
particular, to a sheath which may be placed over a dilatation balloon to 
modify the expansion characteristics of the balloon. 
A wide variety of catheters have been developed in the prior art for 
percutaneous transluminal coronary or peripheral vascular applications. 
For example, balloon dilatation catheters for performing percutaneous 
transluminal coronary angioplasty ("PTCA") are well known in the art. 
In general, PTCA is one procedure for treating a narrowed region in an 
artery, which, in one form, uses a catheter having an expandable balloon 
thereon. The catheter is percutaneously inserted such as into a femoral 
artery, and advanced transluminally until the dilatation balloon is 
positioned within the restricted portion of the lumen. The balloon is 
thereafter inflated to radially outwardly displace the obstruction to 
restore some or all of the original interior diameter of the lumen. 
Intravascular stents are occasionally used following balloon angioplasty 
dilatation procedures to possibly reduce the occurrence of restenosis. 
Stents are expandable tubular supports for maintaining patency of the 
lumen following balloon dilatation or other treatment protocols. See, for 
example, U.S. Pat. No. 4,776,337 to Palmaz. 
Intravascular stents are typically deployed by a catheter having a stent 
prepositioned over an expandable balloon. The clinician percutaneously 
inserts the catheter and transluminally advances it to position the stent 
at the appropriate vascular treatment site. The balloon is then inflated 
to enlarge the stent against the arterial wall. 
When a stent is placed within a lumen, it is important that it be 
accurately "sized" with respect to the inside diameter of the lumen. The 
stent must generally be radially expanded to a sufficient diameter that 
the outer surface of the stent firmly contacts the interior lumen wall, 
thereby providing support to the lumen. It is also desirable to expand the 
stent such that the inside diameter of the stent approximates the native 
lumen diameter to minimize stent obstruction of fluid flow through the 
lumen. However, excessive radial expansion of the stent is undesirable, 
and may result in damage to the vessel wall. 
Conventional stent deployment catheters such as the SDS available from 
Johnson & Johnson are often provided with balloons which exhibit compliant 
expansion characteristics. A compliant balloon tends to exhibit an 
increasing radial diameter with increasing inflation pressure, until the 
balloon burst pressure or rated maximum is reached. Because of this 
expansion profile, compliant balloons may not provide the optimum degree 
of control desired for stent implantation or sizing procedures. Indeed, to 
reduce the risk of over-expansion of a stent, clinicians often use 
catheters with compliant balloons only to partially expand a stent, and 
then remove that catheter and insert a second less-compliant stent sizing 
catheter to appropriately size the implanted stent to the lumen. 
Conventional noncompliant stent sizing balloons present problems under 
certain circumstances as well. Many noncompliant balloons used for stent 
sizing are made of thin-walled materials, such as polyethylene 
terephthalate ("PET"). When catheters with these types of balloons are 
used to size metallic-mesh stents, there exists an increased risk of 
perforation of the balloon by a stent strut. Moreover, rupture of a stent 
sizing balloon under high pressures may lead to arterial trauma. 
Thus, there remains a need for a device which can be used in conjunction 
with stent implantation or sizing balloons, to improve control, increase 
burst pressure and also to reduce the risk of perforation of the balloon 
by the stent. 
SUMMARY OF THE INVENTION 
The present invention is directed toward a sheath which can be used with a 
variety of conventional balloon catheters to enable those balloons to 
properly size a stent to a body lumen, while also protecting the balloons 
from perforation by a stent. 
In accordance with one aspect of the present invention, the balloon sheath 
has an inner tubular sleeve formed of a relatively noncompliant material. 
An outer tubular sleeve formed of a relatively elastic material surrounds 
at least a portion and preferably all of the inner sleeve. The sheath is 
adapted to be positioned concentrically over a dilatation balloon, such as 
a conventional compliant balloon. When inflation media is introduced into 
the balloon, the balloon is constrained to a relatively noncompliant 
inflation profile by the noncompliant layer on the sheath, while the 
elastic layer reduces the risk of perforation. 
In one preferred embodiment, the inner sleeve comprises polyethylene 
terephthalate. 
Additionally, the balloon sheath may have a push wire attached to and 
extending from the proximal end of the sheath. The push wire extends 
proximally to a point outside of the patient, so that the sheath can be 
advanced proximally or distally while the balloon is in position at the 
treatment site. In one embodiment, the push wire extends proximally from 
the sheath alongside the outside of the catheter body. In another 
embodiment, the push wire extends proximally through a lumen within the 
catheter body. 
In accordance with another aspect of the present invention, there is 
provided a method of sizing a stent to an interior body lumen. The first 
step of the method is to provide a tubular balloon sheath. In one 
embodiment, the sheath has an outer sleeve of a first, relatively elastic 
material and an inner sleeve of a second, relatively inelastic material. 
The balloon sheath is then positioned coaxially about a balloon on a 
balloon catheter. The balloon and sheath are positioned within a 
previously deployed tubular stent. The balloon is then inflated inside the 
stent, so that the sheath constrains the balloon to a substantially 
noncompliant inflation profile having a preselected diameter. Optimally, 
the substantially noncompliant inflation profile provides a sufficient 
radial diameter to expand the stent to an appropriate implanted diameter 
without causing excessive trauma to the lumen. 
In accordance with a another aspect of the present invention, there is 
provided a method of altering the expansion characteristics of a 
dilatation balloon. The dilatation balloon is of the type, which when 
fully inflated, has a first inflation diameter at a reference pressure. A 
balloon sheath is placed over the deflated dilatation balloon. The balloon 
sheath is formed in part of a relatively inelastic material. When the 
dilatation balloon is inflated to the reference pressure, the balloon 
sheath restricts expansion of the dilatation balloon to a controlled 
inflation diameter which is smaller than the first inflation diameter. 
In another aspect of the present invention, the balloon sheath may be used 
to increase the inflation pressure handling characteristics of a 
dilatation balloon. Dilatation balloons are normally rated to have a 
characteristic burst pressure. By covering the dilatation balloon with the 
balloon sheath of the present invention, structural support is provided to 
the dilatation balloon, enabling it to withstand pressures in excess of 
its previously rated burst pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is depicted a stent deployment and/or sizing 
catheter 10 incorporating the balloon sheath of the present invention. 
Although illustrated in the context of a simple over the wire vascular 
balloon dilatation catheter, having a single inflation lumen and a single 
guidewire lumen, it is to be understood that the balloon sheath of the 
present invention can be readily adapted to essentially any balloon 
catheter regardless of what additional functions or structures the 
catheter may have. 
For example, the present inventors contemplate the use of the balloon 
sheath in catheters having balloon dilatation and blood perfusion 
capabilities, as well as other combinations of functional features which 
may be desirable in a particular intended application. Moreover, the 
balloon sheath of the present invention may be used to dilate vessels 
and/or size stents in any of a variety of body lumen, such as in urethral, 
ureteral, bile duct, venous, and arterial stent placement applications. 
The manner of adapting the balloon sheath of the present invention to 
catheters having these various functionalities or intended uses will 
become readily apparent to those of skill in the art in view of the 
description which follows. 
Catheter 10 generally comprises an elongate tubular flexible body 12 
extending between a proximal control end 14 and a distal functional end 
16. The length of the tubular body 12 depends upon the desired 
application. For example, lengths in the area of from about 120 
centimeters to about 140 centimeters are typical for use in coronary stent 
placement applications. In general, tubular body 12 will have a generally 
circular cross-sectional configuration with an external diameter within 
the range of from about 0.023 inches to 0.065 inches for most 
cardiovascular applications. Alternatively, a generally triangular 
cross-sectional configuration can also be used, depending upon the number 
of lumen in the catheter, with the maximum base to apex distance also 
generally within the range of from about 0.023 inches to about 0.065 
inches. Other noncircular catheter configurations, such as rectangular or 
oval, may also be used with the balloon sheath of the present invention. 
In peripheral vascular stent placement applications, tubular body 12 will 
typically have an outside diameter within the range of from about 0.039 
inches to about 0.065 inches. In coronary vascular stent placement or 
sizing applications, tubular body 12 will typically have an outside 
diameter within the range of from about 0.030 inches to about 0.045 
inches. 
Stent placement of sizing catheters having diameters outside the preferred 
range may also be used with the balloon sheath of present invention, as 
described below, provided that the functional consequences of the diameter 
are acceptable for the specified intended purpose of the catheter. For 
example, the lower limit of the diameter for tubular body 12 in a given 
application will be a function of the number of fluid or other functional 
lumen contained in the catheter, together with the acceptable flow rate of 
dilatation fluid or other fluid to be delivered through the catheter. 
Catheters having larger tubular body diameters generally have sufficient 
internal flow properties and structural integrity, but reduce perfusion in 
the artery in which the catheter is placed and cannot reach small diameter 
vessels. In addition, increased diameter catheter bodies tend to exhibit 
reduced flexibility, which can be disadvantageous in stent placement 
applications in a remote vascular location. 
Tubular body 12 must have sufficient structural integrity (i.e., 
"pushability") to permit the catheter to be advanced to distal arterial 
locations without buckling or undesirable bending of tubular body 12. The 
ability of tubular body 12 to transmit torque may also be desirable, such 
as in those embodiments where it may be desirable to rotate the tubular 
body 12 after insertion. 
The proximal end 14 of the stent sizing catheter 10 is generally provided 
with a manifold 18 having a plurality of access ports, as is known in the 
art. Generally, manifold 18 is provided with a guidewire port 20 in an 
over-the-wire embodiment and a balloon inflation port 22. Guidewire port 
20 is in communication with a guidewire lumen 35, which extends axially 
along the length of catheter 10. An opening 38 is provided at the distal 
end of the catheter for introduction of the proximal end of the guidewire 
(not illustrated) into guidewire lumen 35. The proximal guidewire port 20 
may be eliminated from manifold 18 in a rapid-exchange or "monorail" 
embodiment, in which embodiment the proximal opening of the guidewire 
lumen 35 is positioned along the side of tubular body 12. The proximal 
guidewire access port in a rapid exchange embodiment is typically within 
about 20 cm from the distal end of the catheter. 
Inflation port 22 is in fluid communication with the balloon 24 by way of 
an axially extending inflation lumen 26. 
Additional access ports may be provided on manifold 18 as needed, depending 
upon the functional capabilities of the catheter. For example, a push wire 
access port can be provided in an embodiment having an axially moveable 
sheath with an internal push wire as will be discussed. See FIG. 5. 
The distal end 16 of the catheter is preferably provided with an a 
traumatic distal tip 36, as is known in the art. Preferably, one or more 
radio opaque markers are also provided to facilitate positioning of the 
catheter. Suitable marker bands can be produced from any of a variety of 
materials, including platinum, gold, and tungsten.backslash.rhenium alloy. 
The distal end 16 of the catheter 10 is additionally provided with an 
inflatable balloon 24, having a stent 28 mounted thereon, illustrated 
schematically in FIG. 1. Inflatable balloon 24 may be made from any of a 
variety of materials known by those of skill in the art to be suitable for 
dilatation balloon manufacture. For example, inflation balloon 24 may be 
formed of materials imparting relatively compliant expansion 
characteristics such as conventionally treated polyethylenes. When 
compliant materials are used, once fully inflated, the diameter of balloon 
24 will tend to increase in response to further increases in inflation 
pressure, until an elastic limit or the burst pressure of inflation 
balloon 24 is reached. 
Alternatively, inflation balloon 24 may be formed of materials imparting 
relatively noncompliant expansion characteristics such as conventional 
polyethylene terephthalate (PET) formulations. Balloons formed of 
noncompliant materials such as PET generally inflate to a predetermined 
inflation diameter, which is substantially maintained upon increasing 
inflation pressure, until the burst pressure of the balloon is reached. 
Referring to FIGS. 1 and 2, there is illustrated one embodiment of the 
balloon sheath 40 of the present invention. Balloon sheath 40 comprises a 
tubular body which is adapted to be positioned coaxially about any of a 
variety of conventional inflation balloons. Sheath 40 can be positioned 
over the balloons of catheters currently on the market to improve the 
predictability of the inflated diameter at preselected inflation 
pressures. In particular, the sheath can limit compliant growth of the 
underlying balloon in response to increased inflation pressure, thereby 
producing an essentially non-compliant balloon assembly. The sheath 40 can 
also improve the pressure handling characteristics of the underlying 
balloon by elevating the burst pressure of the balloon. The sheath 40 can 
be positioned over a balloon and secured in place such as by adhesives, 
thermal bonding and the like, or can be axially moveable with respect to 
the balloon as will be discussed. 
In the illustrated embodiment, balloon sheath 40 has a two layer structure, 
with an outer sleeve 42 and inner sleeve 44. A lumen 46 extending through 
balloon sheath 40 is defined by the interior surface of inner sleeve 44. 
Alternatively, a single layer sheath, or a sheath having three or more 
layers can also be desirable, depending upon the intended application. 
Sheath 40 is constructed to be radially outwardly expandable from a first, 
insertion diameter to a second, enlarged diameter such as for dilating a 
stenosis or sizing a stent. "Expansion" may occur by stretching the 
material of the sheath, or by unfolding or unwrinkling the sheath, or 
both, depending upon its construction. 
Preferably, the sheath 40 is provided with one or more radiopaque markers 
such as a radiopaque marker band at the proximal and/or distal regions of 
the sheath. In a two layer sheath, the marker band(s) may be sandwiched 
between the two layers. Marker bands will enable visualization of the 
axial position of the sheath relative to other radiopaque structures, such 
as a stent or an inflation balloon filled with contrast media. 
In one preferred embodiment, outer sleeve 42 is formed of a resilient or 
elastic material which tends to contract back to a preselected relaxed 
size after being expanded. Outer sleeve 42 may be formed from any of a 
wide variety of materials, such as latex, silicone, polyurethane 
elastomer, C-Flex, Tecoflex, nylon elastomers (PEBAX), as well as other 
materials known to those of skill in the art. 
In general, selection of material for sleeve 42 will be guided by the 
intended application of sheath 40. For example, in applications where 
metal-mesh stents will be placed or sized, it is preferable that sleeve 42 
be made of a relatively puncture-resistant elastic material, or of 
sufficient thickness, so as to provide an adequate protective covering to 
prevent puncture of balloon 24 by stent struts. 
Inner sleeve 44 is preferably made from a material which is relatively 
inelastic, and preferably a material which is substantially noncompliant 
at typical balloon inflation pressures. The material of inner sleeve 44 
should also be resistant to bursting at the contemplated inflation 
pressure of the intended inflation balloon. A variety of inelastic 
materials having these properties may be used to form sleeve 44, such as 
PET and nylon, in addition to other materials which can be determined by 
those of skill in the art. 
The dimensions of the sheath can be varied as appropriate to fit the 
intended balloon catheter and desired inflation characteristics. In one 
embodiment, the sheath has an axial length of about 6 cm, and a total wall 
thickness of about 0.004" in the relaxed state. The relaxed outside 
diameter of the unmounted sheath is about 0.065" and the outside diameter 
at an inflation pressure of 60 psi is about 3 mm. The inner PET layer has 
a thickness of about 0.0003" and the outer urethane layer has a thickness 
of about 0.004". 
As a consequence of the structure described above, the outer sleeve 42 will 
tend to elastically constrict to its first, relaxed diameter. The inner 
sleeve 44 will be forced to fold or wrinkle to accommodate the radial 
reduction of outer sleeve 42. Expansion of a balloon within sheath 40 
causes the inner sleeve to unfold to its preset maximum diameter while 
elastically expanding the outer sleeve 42 to a corresponding diameter. 
Thus, sleeve 44 can be used to set a precise inflated maximum diameter at 
a preselected pressure and sleeve 42 can improve puncture resistance and 
assist during deflation and placement and removal steps by minimizing 
winging of the sleeve 44. In applications where these latter features are 
unnecessary, the order of inner sleeve 44 and outer sleeve 42 can be 
reversed, or either the inner or the outer sleeve can be eliminated. 
In one preferred embodiment, outer sleeve 42 and inner sleeve 44 are bonded 
together. Sleeve bonding may be performed by any of a variety of means 
known to those of skill in the art, such as heat sealing or by use of 
adhesives. Moreover, the extent of the bond between the two sleeves may be 
varied considerably and still form the balloon sheath of the present 
invention. For example, in some embodiments, it may be desirable to bond 
the entire outer surface of inner sleeve 44 to the inner surface of outer 
sleeve 42. Alternatively, the two sleeves may be bonded together along 
only portions of their surfaces, as for example, along the inner and outer 
circumferences near the edges of the two sleeves or at discrete spot 
welds. 
The two-layer embodiment of the sheath described above can be manufactured 
by bonding the inner and outer sleeves proximal and distal to the region 
of expansion by a suitable adhesive. 
In the embodiment illustrated in FIGS. 1 and 2, balloon sheath 40 has an 
attached push wire 50 extending from its proximal end. Advantageously, 
push wire 50 facilitates axial movement of balloon sheath 40 along tubular 
body 12 and balloon 24, so that balloon sheath 40 may, for example, be 
positioned proximally of the balloon 24 along the catheter shaft during 
catheter placement, and then advanced distally over deflated balloon 24 
and within a partially expanded stent prior to stent sizing. Push wire 50 
may also be used to retract balloon sheath 40 after a stent sizing 
procedure, to ensure removal of sheath 40 along with the catheter from the 
blood vessel following deflation of balloon 24. Although the sleeve can 
for some uses be secured to the balloon permanently thereby reducing the 
need for a push wire, the ability to axially advance the sleeve over the 
balloon following placement of the balloon can be advantageous in a number 
of applications. 
Push wire 50 may be attached to sheath 40 in a variety of ways. For 
example, push wire 50 may be attached to the proximal end of either outer 
sleeve 42 or inner sleeve 44 by molding the respective sleeve thereon 
during manufacture. Alternately, push wire 50 may be adhesively bonded to 
either sleeve. In another embodiment, push wire 50 may be attached to 
balloon sheath 50 by inserting push wire 50 between outer sleeve 42 and 
inner sleeve 44, and then bonding the two sleeves together to secure push 
wire 50 therebetween. 
Push wire 50 may be formed of any medical grade material with sufficient 
structural integrity to permit sheath 40 to be advanced or retracted over 
balloon 24 and along tubular body 12. Presently preferred materials for 
the manufacture of push wire 50 include stainless steel and nitinol. In 
one embodiment, push wire 50 comprises stainless steel and has an outside 
diameter of about 0.016". 
Referring to FIGS. 5 and 6, there is disclosed an internal push wire 
embodiment of the present invention. A balloon dilatation catheter 52 is 
provided with an inflatable balloon 54 on the distal end of an elongate 
flexible tubular body 56. An axially movable sheath 58 is positioned on 
the tubular body 56 proximally of inflatable balloon 54. A push wire 60 is 
secured to the tubular sleeve 58 and extends proximally throughout the 
length of the catheter. In this embodiment, at least a portion of push 
wire 60 is axially movably disposed within an elongate push wire lumen 66. 
Push wire lumen 66 is provided with a distal aperture 68, through which 
push wire 60 can exit the catheter body. Aperture 68 is positioned 
proximally of the balloon 54 by a sufficient distance to permit the 
tubular sheath 58 to be withdrawn proximally of the balloon 54. For 
example, in a balloon catheter having a 2 cm long dilatation balloon, the 
tubular sheath 58 will be approximately 3 cm long, and the aperture 68 
will be positioned within the range of from about 3.5 cm to about 6 cm 
proximally of the proximal end of the balloon 54. 
Referring to FIG. 6, the catheter body 56 is provided with a push wire 
lumen 66 as has been discussed, as well as an inflation lumen 62 and a 
guidewire lumen 64 as is known in the art. As with previous embodiments 
herein, additional lumen may be provided as needed, depending upon the 
desired functionality of the catheter. Alternatively, the guidewire lumen 
64, may be eliminated from proximal portions of the catheter such as in a 
rapid exchange embodiment. 
Whether the push wire extends proximally internally or externally to the 
catheter body, the push wire embodiment of the present invention permits a 
variety of dilatation, stent implantation and stent sizing methods. For 
example, in accordance with one aspect of the method of the present 
invention, a tubular sheath is mounted on a dilatation balloon. An 
expandable implantable stent is coaxially mounted on the tubular sheath 
either at the point of manufacture or at the clinical site. The assembly 
may thereafter be positioned within a treatment site, and the balloon 
inflated to expand the stent. The tubular sheath both improves the 
predictability of the inflated diameter of the balloon, as well as 
increases the pressure capacity of the balloon. This method can be 
accomplished with a balloon sheath of the present invention with or 
without a push (or safety) wire. 
In accordance with another aspect of the present invention, a tubular 
sheath having a push wire attached thereto is mounted on a balloon 
catheter, either at the point of manufacture or at the clinical site. The 
sheath is positioned initially on the catheter shaft proximally of the 
balloon. An expandable stent may optionally be positioned over the 
balloon. The assembly is percutaneously inserted and transluminally 
advanced to a treatment site in a body lumen. The balloon is thereafter 
inflated to enlarge the stent at the treatment site. The balloon is then 
deflated by aspiration of inflation media, and the tubular sheath is 
thereafter advanced distally by distal pressure on the push wire, so that 
the tubular sheath is positioned over the deflated balloon. The balloon 
may thereafter be reinflated, optionally to a higher pressure than the 
pressure used to implant the stent, to size the stent to the body lumen. 
In accordance with a further aspect of the method of the present invention, 
a tubular sheath having a proximally extending push wire 50 is positioned 
on a balloon dilatation catheter proximally of the balloon. The catheter 
may thereafter be transluminally positioned such that the balloon is 
within a treatment site, and the balloon dilated to predilate the 
stenosis. The balloon is thereafter deflated, and the tubular sheath 
advanced distally over the balloon. The balloon may then be inflated to a 
higher pressure, to produce a predetermined inflation diameter. The 
foregoing method may be utilized either to size a previously implanted 
stent, or to dilate a stenosis without the use of a stent. 
In certain circumstances, it may be desirable to eliminate push wire 50 and 
attach sheath 40 directly to catheter 10 over balloon 24. Attachment of 
the sheath to the balloon in an embodiment without a push wire can be 
accomplished at the original point of manufacture, or as a "retrofit" to 
improve or alter the characteristics of a commercially available catheter. 
Sheath 40 can have an axial length of substantially the same length or 
less than the length of the balloon. In these embodiments, the sheath is 
preferably bonded directly to the balloon. Bonding may be achieved by any 
manner known to those of skill in the art, such as heat sealing, spot 
welding, solvent bonding or by use of adhesives. The weld or other 
adhesion points between sheath 40 and balloon 24 may be varied as desired. 
Alternatively, the sheath 40 can have an axial length of greater than the 
axial length of the underlying balloon. Such sheaths can be secured to the 
catheter shaft at the proximal side and/or the distal side of the balloon. 
Bonding of the sheath to the catheter shaft can be accomplished by any of 
the techniques discussed above, as well as by shrinking proximal and 
distal necks on the sheath such as by the application of heat. 
The balloon expansion-altering and protection properties of balloon sheath 
40 are illustrated schematically in FIGS. 3 and 4. Considering an 
embodiment in which balloon 24 is of the compliant variety, as inflation 
media is introduced into balloon 24 though inflation lumen 26, the radial 
diameter of balloon 24 will increase from that shown in FIG. 3 to the 
diameter of FIG. 4. As seen in FIG. 4, the outer surface of balloon 24 
will be in substantially complete contact with noncompliant inner sleeve 
44 of sheath 40 as balloon 24 fully inflates. As inflation pressure 
increases beyond the point of full inflation of balloon 24, inner sleeve 
44 restrains further expansion of balloon 24, so that balloon 24 has a 
substantially constant expanded diameter rating at a increased pressures. 
Thus, in effect, sleeve 44 serves to convert the expansion characteristics 
of balloon 24 from a compliant mode to a noncompliant mode. Moreover, the 
added structural support of sleeve 44 in contact with balloon 24 improves 
the pressure handling characteristics underlying balloon 24, permitting 
balloon 24 to be exposed to higher inflation pressures without bursting. 
Outer sleeve 42 surrounds inner sleeve 44, and being formed of an elastic 
material, will expand in conjunction with inner sleeve 44. A stent (not 
shown), may be positioned over outer sleeve 42 prior to catheter 
insertion, or sheath 40 and balloon 24 may be inserted within a partially 
expanded stent already positioned within a lumen. As balloon 24 expands, 
outer sleeve 42 will also expand and contact the stent, thereby increasing 
the stent's radial diameter, which will result in the sizing of the stent 
to the lumen. Being formed of a material resistant to puncture, outer 
sleeve 42 also tends to protect balloon 24 and inner sleeve 44 from 
perforation by stent struts. In addition, because of its elastic nature, 
sleeve 42 will also exert a compressive force on balloon 24 as it is 
deflated. This will facilitate balloon collapsing on deflation, to 
minimize the profile of the deflated balloon prior to catheter withdrawal 
and prevent "winging out" of the balloon. Advantageously, this minimizes 
the risk that the newly implanted stent will be dislodged as balloon 24 
and sheath 40 are removed from the implantation site. 
Furthermore, should balloon 24 rupture during a high pressure stent 
implantation procedure, the two sleeves of sheath 40 may prevent or 
minimize any trauma to the lumen. Moreover, the compressive nature of 
sleeve 42 should ensure removal of balloon material without stent 
entanglement. 
Optimally, balloon sheath 40 is selected so that the inflation diameter of 
the sheath-covered balloon is appropriate for the particular stent 
placement application. Thus, it is contemplated that the present invention 
can be used to adapt a particular catheter to a number of different stent 
placement and sizing applications, simply by selecting an appropriate 
sheath from a number of sheaths each having a unique inflated diameter 
rating at a reference pressure. As will be appreciated by those of skill 
in the art, varying the materials and manufacturing processes used to form 
inner sleeve 44 allows creation of sheaths with variety of noncompliant 
expansion profiles. 
Any of the foregoing methods can alternatively be accomplished using a 
single layer sheath, comprising any of the materials disclosed herein, 
depending upon the desired functional characteristics of the balloon and 
sheath assembly. 
It will be appreciated that certain variations of the present invention may 
suggest themselves to those skilled in the art. The foregoing detailed 
description is to be clearly understood as given by way of illustration, 
the spirit and scope of this invention being limited solely by the 
appended claims.