Abstract:
A catheter assembly is disclosed. This assembly may include a first elongate member having an internal lumen and a distal portion, a second elongate member, slidable within the first elongate member, and a spacer positioned between the second elongate member and the first elongate member. In this example, the spacer may be positioned at bendable portions of the first elongate member and the second elongate member such that the spacer maintains a substantially uniform spacing between the second elongate member and the first elongate member when the first elongate member and the second elongate member are in a curved orientation.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a Continuation of application Ser. No. 09/635,083, filed on Aug. 8, 2000, now U.S. Pat. No. ______. That application is incorporated, in its entirety, by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to intravascular catheters for performing medical procedures. More particularly, the present invention relates to shaft assemblies for use in intravascular catheters. Still, more particularly, the present invention relates to catheter shaft assemblies for use in injection catheters.  
       BACKGROUND OF THE INVENTION  
       [0003]     Intravascular catheters are currently utilized in a wide variety of minimally invasive or percutaneous medical procedures. Generally, an intravascular catheter enables a physician to remotely perform a medical procedure by inserting the catheter into the vascular system of the patient at an easily accessible location and navigating the tip of the catheter to a desirable target site. By this method, virtually any target site in the patient&#39;s vascular system may be remotely accessed.  
         [0004]     Typically, a percutaneous procedure begins with the step of inserting a distal portion of the catheter into the patient&#39;s vasculature at a convenient location. Once the distal portion of the catheter has entered the patient&#39;s vascular system the physician may urge the distal tip forward by applying longitudinal forces to the proximal portion of the catheter. Frequently the path taken by a catheter through the vascular system is tortuous, requiring the catheter to change direction frequently. While advancing the catheter through the tortuous path of the patient&#39;s vasculature, the physician must steer the distal end of the catheter. During a percutaneous procedure, the physician typically is not able to manipulate the distal portion of the catheter directly. For this reason, physicians typically must steer the distal end of the catheter by applying torsional forces to the proximal portion of the catheter.  
         [0005]     Injection catheters are a type of catheter which may be used to inject therapeutic or diagnostic agents into various target tissues within the human body. An advantage of injection catheters is that the target tissue may be accessed utilizing minimally invasive surgical techniques. As with other types of catheters, the physician typically is not able to manipulate the distal portion of an injection catheter directly.  
         [0006]     In many applications the target tissue is within a wall of an organ such as the stomach or the heart. When the target tissue is within the wall of an organ it is often desirable to inject the therapeutic or diagnostic agent into the tissue proximate the center of the organ wall. If the needle of the injection catheter inadvertently passes through the wall, the therapeutic or diagnostic agents dispensed from the distal end of the needle will not be effectively delivered to the target tissue.  
         [0007]     One example of a medical procedure involving the delivery of a therapeutic and/or diagnostic agent to a targeted portion of a patient&#39;s body is the treatment of esophageal varices. This is a condition in which blood vessels of the esophagus are enlarged and may potentially burst. For such a procedure, a therapeutic agent is injected into the varix. When treating an esophageal varix, the agent may be a coagulant such as sodium morrhuate. When a coagulant is injected into a varix, it causes it to occlude. An injection catheter may be used to deliver the therapeutic agent in order to minimize the invasive nature of the procedure.  
         [0008]     In a similar procedure, an injection catheter may be utilized in the treatment of ulcers in the stomach lining. With such treatment, an injection catheter may be used to deliver drugs such as sclerosing or vasoconstrictive agents. These drugs typically clot or occlude the bleeding tissue to stop bleeding or to reduce the possibility of a blood vessel bursting.  
         [0009]     Injection catheters may also be used to inject therapeutic or diagnostic agents into the heart. Examples of agents delivered to the heart include genes, proteins, or drugs. In the case of injecting a therapeutic agent into the heart, 27 or 28 gauge needles are generally used to inject solutions carrying genes, proteins, or drugs directly into the myocardium. A typical volume of an agent delivered to an injection site is about 100 microliters.  
         [0010]     Therapeutic and diagnostic agents may be delivered to a portion of the heart as part of a percutaneous myocardial revascularization (PMR) procedure. PMR is a procedure which is aimed at assuring that the heart is properly oxygenated. Assuring that the heart muscle is adequately supplied with oxygen is critical to sustaining the life of a patient. To receive an adequate supply of oxygen, the heart muscle must be well perfused with blood. In a healthy heart, blood perfusion is accomplished with a system of blood vessels and capillaries. However, it is common for the blood vessels to become occluded (blocked) or stenoic (narrowed). A stenosis may be formed by an atheroma, which is typically a harder, calcified substance which forms on the walls of a blood vessel.  
         [0011]     Historically, individual stenotic lesions have been treated with a number of medical procedures including coronary bypass surgery, angioplasty, and atherectomy. Coronary bypass surgery typically involves utilizing vascular tissue from another part of the patient&#39;s body to construct a shunt around the obstructed vessel. Angioplasty techniques such as percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA) are relatively non-invasive methods of treating a stenotic lesion. These angioplasty techniques typically involve the use of a guidewire and a balloon catheter. In these procedures, a balloon catheter is advanced over a guidewire such that the balloon is positioned proximate a restriction in a diseased vessel. The balloon is then inflated and the restriction in the vessel is opened. A third technique which may be used to treat a stenotic lesion is atherectomy. During an atherectomy procedure, the stenotic lesion is mechanically cut or abraded away from the blood vessel wall.  
         [0012]     Coronary by-pass, angioplasty, and atherectomy procedures have all been found effective in treating individual stenotic lesions in relatively large blood vessels. However, the heart muscle is perfused with blood through a network of small vessels and capillaries. In some cases, a large number of stenotic lesions may occur in a large number of locations throughout this network of small blood vessels and capillaries. The tortuous path and small diameter of these blood vessels limit access to the stenotic lesions. The sheer number and small size of these stenotic lesions make techniques such as cardiovascular by-pass surgery, angioplasty, and atherectomy impractical.  
         [0013]     When techniques which treat individual lesions are not practical, percutaneous myocardial revascularization (PMR) may be used to improve the oxygenation of the myocardial tissue. A PMR procedure generally involves the creation of holes, craters or channels directly into the myocardium of the heart. In a typical PMR procedure, these holes are created using radio frequency energy delivered by a catheter having one or more electrodes near its distal end. After the wound has been created, therapeutic agents are sometimes ejected into the heart chamber from the distal end of a catheter.  
         [0014]     Positive clinical results have been demonstrated in human patients receiving PMR treatments. These results are believed to be caused in part by blood flowing within a heart chamber through channels in myocardial tissue formed by PMR. Increased blood flow to the myocardium is also believed to be caused in part by the healing response to wound formation. Specifically, the formation of new blood vessels is believed to occur in response to the newly created wound. This response is sometimes referred to as angiogenesis. After the wound has been created, therapeutic agents which are intended to promote angiogenesis are sometimes ejected into the heart chamber. A limitation of this procedure is that the therapeutic agent may be quickly carried away by the flow of blood through the heart.  
         [0015]     In addition to promoting increased blood flow, it is also believed that PNM improves a patient&#39;s condition through denervation. Denervation is the elimination of nerves. The creation of wounds during a PMR procedure results in the elimination of nerve endings which were previously sending pain signals to the brain as a result of hibernating tissue.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention relates generally to intravascular catheters for performing medical procedures. More particularly, the present invention relates to shaft assemblies for use in intravascular catheters. Still, more particularly, the present invention relates to catheter shaft assemblies for use in injection catheters.  
         [0017]     This assembly may include a first elongate member having an internal lumen and a distal portion, a second elongate member, slidable within the first elongate member, and a spacer positioned between the second elongate member and the first elongate member. In this example, the spacer may be positioned at bendable portions of the first elongate member and the second elongate member such that the spacer maintains a substantially uniform spacing between the second elongate member and the first elongate member when the first elongate member and the second elongate member are in a curved orientation.  
         [0018]     A shaft assembly in accordance with an exemplary embodiment of the present invention may also comprise a first elongate shaft having an inner surface defining a lumen, a second elongate shaft slidingly disposed within the lumen of the first elongate shaft, and an interstitial member disposed between the inner surface of the first elongate shaft and the outer surface of the second elongate shaft.  
         [0019]     In one embodiment of a shaft assembly in accordance with the present invention a plurality of longitudinal ribs extend beyond an inner surface of the first elongate shaft. These longitudinal ribs preferably contact the outer surface of the second elongate shaft.  
         [0020]     In an additional embodiment of a shaft assembly in accordance with the present invention a radial rib extends beyond the inner surface of the first elongate shaft. The radial rib preferably contacts the outer surface of the second elongate shaft. When the radial rib is in contact with outer surface of second elongate shaft there is preferably no substantial gap between the radial rib and the outer surface of the second elongate shaft.  
         [0021]     An additional embodiment of a shaft assembly in accordance with the present invention includes a coil comprising a wire forming a plurality of turns. Each turn of the coil is disposed between the inner surface of the first elongate shaft and the outer surface of the second elongate shaft.  
         [0022]     In an additional embodiment of a shaft assembly in accordance with the present invention the second elongate shaft has a plurality of projections extending beyond the outer surface of the second elongate shaft. When the second elongate shaft is disposed within the lumen defined by the first elongate shaft, these projections preferably contact the inner surface of the first elongate shaft.  
         [0023]     An injection catheter in accordance with an exemplary embodiment of the present invention includes a first elongate shaft having an inner surface defining a lumen. A second elongate shaft having an outer surface is slidingly disposed within the lumen of the first elongate shaft. In this exemplary embodiment, the second elongate shaft includes a point and an injection orifice proximate it&#39;s distal end. Also in this exemplary embodiment, an interstitial member is disposed between the inner surface of the first elongate shaft and the outer surface of the second elongate.  
         [0024]     In many applications it is desirable to advance the distal end of the second elongate shaft by a known distance relative to the distal end of the first elongate shaft. For example, when a physician wishes to inject a fluid into the wall of an organ. In one embodiment of a catheter in accordance with the present invention, a slider is fixed to the second elongate shaft proximate the proximal end thereof. Also in this embodiment, a portion of the slider is disposed within a cavity defined by a housing which is preferably fixed to the first elongate shaft proximate the proximal end thereof. Also in a preferred embodiment, a plurality of indicia are disposed on a face of the housing proximate the slider.  
         [0025]     A physician utilizing the catheter in a surgical procedure may move the distal end of the second elongate shaft a known distance relative to the distal end of the first elongate shaft. For example, a physician may urge the slider distally while visually observing the travel of the slider relative to the indicia of the housing. The movement of the slider is translated via the second elongate shaft to the distal end thereof.  
         [0026]     In a preferred embodiment there is substantially a one-to-one relationship between the movement of the slider relative to the indicia of the housing and the movement of the distal. end of the second elongate shaft relative to the distal end of the first elongate shaft. In a particularly preferred embodiment, the presence of an interstitial member between the outer surface of the second elongate shaft and the inner surface of the first elongate shaft directs the motion of the second elongate shaft in a substantially longitudinal direction. Also in a particularly preferred embodiment, the presence of the interstitial member between the outer surface of the second elongate shaft and the inner surface of the first elongate shaft substantially precludes lateral movement of the second elongate shaft relative to the first elongate shaft. Thus, it is assured that there will be substantially a one-to-one relationship between the movement of the slider relative to the indicia of the housing and the movement of the distal. end of the second elongate shaft relative to the distal end of the first elongate shaft.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a plan view of a catheter including a shaft assembly in accordance with the present invention;  
         [0028]      FIG. 2  is a diagrammatic view including the catheter of  FIG. 1  and a patient;  
         [0029]      FIG. 3  is a lateral cross section view of the shaft assembly of the catheter of  FIG. 1  and  FIG. 2 ;  
         [0030]      FIG. 4  is a partial cross section view of an additional embodiment of a shaft assembly in accordance with the present invention;  
         [0031]      FIG. 5  is a partial cross section view of an additional embodiment of a shaft assembly in accordance with the present invention;  
         [0032]      FIG. 6  is a partial cross section view of an additional embodiment of a shaft assembly in accordance with the present invention;  
         [0033]      FIG. 7  is a partial cross section view of an additional embodiment of a shaft assembly in accordance with the present invention;  
         [0034]      FIG. 8  is a plan view of an additional embodiment of a catheter in accordance with the present invention;  
         [0035]      FIG. 9  is a cross sectional view of a distal. portion of a catheter in accordance with the present invention;  
         [0036]      FIG. 10  is a plan view of a catheter including a shaft assembly in accordance with the present invention;  
         [0037]      FIG. 11  is a partial cross section view of a distal portion of the catheter of  FIG. 10 ; and  
         [0038]      FIG. 12  is a partial cross section view of a distal portion of the catheter of  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. In some cases, the drawings may be highly diagrammatic in nature. Examples of constructions, materials, dimensions, and manufacturing processes are provided for various elements. Those skilled in the art will recognize that many of the examples provided have suitable alternatives which may be utilized.  
         [0040]      FIG. 1  is a plan view of a catheter  120  including a shaft assembly  122  in accordance with the present invention. Shaft assembly  122  comprises a first elongate shaft  124  having a distal end  134 , a proximal end  144 , and an inner surface  154  defining a lumen  132 . Shaft assembly  122  also includes a second elongate shaft  126  slidingly disposed within lumen  132  of first elongate shaft  124 . Catheter  120  also includes a distal end  130 , and a proximal end  140 .  
         [0041]     Second elongate shaft  126  has an outer surface  138 , distal end  136 , and a proximal end  146 . In many applications it is desirable to advance distal end  136  of second elongate shaft  126  by a known distance relative to distal. end  134  of first elongate shaft  124 . In the embodiment of  FIG. 1 , a slider  142  is fixed to second elongate shaft  126  proximate proximal end  146  thereof. In the embodiment of  FIG. 1 , a portion of slider  142  is disposed within a cavity  148  defined by a housing  150 . In a presently preferred embodiment, housing  150  is fixed to first elongate shaft  124  proximate proximal end  144  thereof. Also in a preferred embodiment, a plurality of indicia  152  are disposed on a face  154  of housing  150  proximate slider  142 .  
         [0042]     A physician utilizing catheter  120  in a surgical procedure may move distal end  136  of second elongate shaft  126  a known distance relative to distal end  134  of first elongate shaft  124 . For example, a physician may urge slider  142  distally while visually observing the travel of slider  142  relative to indicia  152  of housing  150 . The movement of slider  142  is translated via second elongate shaft  126  to distal end  136  thereof.  
         [0043]     In a preferred embodiment there is substantially a one-to-one relationship between the movement of slider  142  relative to indicia  152  of housing  150  and the movement of distal end  136  of second elongate shaft  126  relative to distal end  134  of first elongate shaft  124 . In a particularly preferred embodiment, the presence of an interstitial member  156  between outer surface  138  of second elongate shaft  126  and inner surface  154  of first elongate shaft  124  directs the motion of second elongate shaft  126  in a substantially longitudinal direction. Also in a particularly preferred embodiment, the presence of interstitial member  156  between outer surface  138  of second elongate shaft  126  and inner surface  154  of first elongate shaft  124  substantially precludes lateral movement of second elongate shaft  126  relative to first elongate shaft  124 . Thus, it is assured that there will be substantially a one-to-one relationship between the movement of slider  142  relative to indicia  152  of housing  150  and the movement of distal end  136  of second elongate shaft  126  relative to distal end  134  of first elongate shaft  124 .  
         [0044]     In the embodiment of  FIG. 1 , second elongate shaft  126  forms a point  158  proximate distal end  136  thereof. Second elongate shaft also defines an injection port  160  proximate point  158 . A hub  164  is disposed about second elongate shaft  126  proximate proximal end  146  thereof. Hub  164  defines a proximal port  166 . In a preferred embodiment, proximal port  166  is in fluid communication with injection port  160  via an injection lumen  162  defined by second elongate shaft  126 .  
         [0045]     Catheter  120  of  FIG. 1  may be generally referred to as an injection catheter. It is to be appreciated that a catheter in accordance with the present invention may comprise various types of catheters without deviating from the spirit and scope of the present invention.  
         [0046]     In a preferred embodiment, second elongate shaft  126  of catheter  120  comprises hypodermic tubing. Second elongate shaft  126  may comprise various metallic and non-metallic materials without deviating from the spirit and scope of the present invention. Examples of metallic materials which may be suitable in some applications include stainless steel, and nickel-titanium alloy. Examples of non-metallic materials which may be suitable in some applications are included in the list below which is not exhaustive: polycarbonate, poly(L-lactide) (PLLA), poly(D,L-lactide) (PLA), polyglycolide (PGA), poly(L-lactide-co-D,L-lactide) (PLLA/PLA), poly(L-lactide-co-glycolide) (PLLA/PGA), poly(D, L-lactide-co-glycolide) (PLA/PGA), poly(glycolide-co-trimethylene carbonate) (PGA/PTMC), polyethylene oxide (PEO), polydioxanone (PDS), polycaprolactone (PCL), polyhydroxylbutyrate (PHBT), poly(phosphazene), polyD,L-lactide-cocaprolactone) (PLA/PCL), poly(glycolide-co-caprolactone) (PGA/PCL), polyanhydrides (PAN), poly(ortho esters), poly(phoshate ester), poly(amino acid), poly(hydroxy butyrate), polyacrylate, polyacrylamid, poly(hydroxyethyl methacrylate), polyurethane, polysiloxane and their copolymers.  
         [0047]     In a preferred embodiment, first elongate shaft  124  of catheter  120  comprises an elongate tubular member including a reinforcement member (e.g., braided or coiled wire). Second elongate shaft  126  may comprise various metallic and non-metallic materials without deviating from the spirit and scope of the present invention. Examples of metallic materials which may be suitable in some applications include stainless steel, and nickel-titanium alloy. Examples of non-metallic materials which may be suitable in some applications include: polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), polyurethane, polytetrafluoroethylene (PTFE), polyether block amide (PEBA), polyamide, and polyimide.  
         [0048]      FIG. 2  is a diagrammatic view including catheter  120  of  FIG. 1  and a patient  20 . Patient  20  has a heart  22  and a vascular system  24  including a blood vessel  26  defining a blood vessel lumen  28 . Am access sheath  30  is partially disposed within a leg  32  of patient  20 . A distal end of access sheath  30  is disposed within blood vessel lumen  28  of blood vessel  26 . Access sheath  30  may aid in the introduction of catheter  120  into blood vessel lumen  28 .  
         [0049]     As shown in  FIG. 2 , a portion of catheter  120  is disposed within blood vessel lumen  28  of blood vessel  26 . Distal end  130  (not visible in  FIG. 2 ) of catheter  120  is disposed within heart  22  of patient  20 . In a preferred embodiment, distal end  130  of catheter  120  is disposed proximate a wall of heart  22 .  
         [0050]     In the embodiment of  FIG. 2 , a fluid source  34  is coupled to hub  164  disposed about second elongate shaft  126  of catheter  120 . In the embodiment of  FIG. 2 , fluid source  34  includes a variable volume chamber  36  defined by a body  38 . In a preferred embodiment, variable volume chamber  36  is in fluid communication with injection lumen  162  of second elongate shaft  126 . A plunger  40  is slidingly disposed within variable volume chamber  36 . Urging the plunger distally has the effect of urging fluid into injection lumen  162  of second elongate shaft  126 . A number of energy sources may be utilized to urge plunger  40  distally. Energy sources which may be suitable in some applications include springs, compressed gas, a human being, and electricity. Various additional embodiments of fluid source  34  are possible without deviating from the spirit and scope of the present invention. Examples of fluid sources which may be suitable in some applications include syringes, peristaltic pumps, and an I.V. bag with pressure applied to an outer surface thereof.  
         [0051]     A method of injecting a fluid into heart  22  of patient  20  may be described with reference to  FIG. 2 . The distal end of access sheath  30  may be inserted into blood vessel lumen  28  of blood vessel  26 . Distal end  130  of catheter  120  may be inserted into the lumen of access sheath  30 . Distal end  130  of catheter  120  may be advanced through access sheath  30  and into blood vessel lumen  28  of blood vessel  26 . Catheter  120  may be urged forward through vascular system  24  of patient  20  until distal end  130  is proximate the target tissue (e.g., a wall of heart  22 ). I n  FIG. 2  it may be appreciated that shaft assembly  122  of catheter  120  is bent in a plurality of locations to conform with a tortuous path defined by vascular system  24 .  
         [0052]     In a preferred method, distal end  136  of second elongate shaft  126  is disposed within lumen  132  of first elongate shaft  124  during the above steps. Once distal end  130  of catheter  120  is positioned proximate the target tissue, second elongate shaft  126  may be advanced so that point  158  penetrates the bodily tissue at the target site. With injection port  160  of second elongate shaft  126  disposed within the target tissue, fluid may be urged into the target tissue. For example, force may be applied to plunger  40  urging fluid out of fluid source  34  and into injection lumen  162  of second elongate shaft  126 . he addition of fluid from fluid source  34  results in the injection of fluid into the target tissue.  
         [0053]     In many applications it is desirable to advance point  158  and injection port  160  into the target tissue by a known distance. A physician may advance point  158  and injection port  160  into the target tissue by urging slider  142  distally. A physician may determine the depth of penetration by visually observing the travel of slider  142  relative to indicia  152  of housing  150 .  
         [0054]     The movement of slider  142  is translated via second elongate shaft  126  to point  158  formed by second elongate shaft  126  proximate the distal end  136  thereof. In a preferred embodiment there is substantially a one-to-one relationship between the movement of slider  142  relative to indicia  152  of housing  150  and the movement of distal end  136  of second elongate shaft  126  relative to distal end  134  of first elongate shaft  124 . In a particularly preferred embodiment, the presence of interstitial member  156  between outer surface  138  of second elongate shaft  126  and inner surface  154  of first elongate shaft  124  directs the motion of second elongate shaft  126  in a substantially longitudinal direction.  
         [0055]     The fluid injected into the target area may include various therapeutic or diagnostic agents adapted to treat the medical condition which the physician is treating. It is to be appreciated that methods in accordance with the present invention may be used in the treatment of a number of medical conditions. For example, methods and devices of performing percutaneous myocardial revascularization (PMR) in accordance with the present invention have been envisioned. For example, a plurality of wounds may be created in hibernating tissue of the heart. These wounds may be created by injecting a fluid into the tissue of the heart. As a result of these wounds, there will be increased blood flow to the myocardium caused in part by the body&#39;s healing response to the wound. One healing response of the body is sometimes referred to as angiogenesis. In addition to promoting increased blood flow, it is also believed that PMR improves a patient&#39;s condition through denervation. Denervation is the elimination of nerves. The creation of wounds during this procedure results in the elimination of nerve endings which were previously sending pain signals to the brain as a result of hibernating tissue.  
         [0056]     Suitable wounds may be created by injecting a fluid such as water, saline, or ringers solution into the heart tissue. Wound formation and revascularization of myocardial tissue may enhanced by injecting a fluid including a therapeutic agent into the tissue of the heart. Examples of therapeutic agents which may be suitable include growth factors, drugs and caustic agents. The fluid injected into the heart tissue may also include a radiopaque material. Injecting a radiopaque material into the wound effectively marks the locations which have been treated. This will aid the physician in procedures which are being performed percutaneously using fluoroscopic equipment.  
         [0057]     In the exemplary embodiment of  FIG. 2 , catheter  120  may be utilized to inject fluid into heart  22  of patient  20 . It is to be appreciated that catheter  120  may be utilized in the treatment of various medical conditions occurring in various locations in the body. For example, catheter  120  may be used in the treatment of esophageal varices, a condition in which blood vessels of the esophagus are enlarged and may potentially burst. For such a procedure, injection port  160  would be disposed proximate the enlarged varix and an appropriate agent would be injected into the varix. When treating an esophageal varice, the agent may be a coagulant such as sodium morrhuate. When a coagulant is injected into a varix, it causes the occlusion thereof.  
         [0058]      FIG. 3  is a lateral cross section view of shaft assembly  122  of catheter  120  of  5   FIG. 1  and  FIG. 2 . As described above, shaft assembly  122  includes second elongate shaft  126  which is disposed within lumen  132  defined by first elongate shaft  124 . In  FIG. 3  it may be appreciated that a plurality of interstitial members  156  extend between first elongate shaft  124  and second elongate shaft  126 . In the embodiment of  FIG. 3 , each interstitial member comprises a longitudinal rib  168 . Each longitudinal rib  168  extends beyond an inner surface  154  of first elongate shaft  124  and contacts an outer surface  138  of second elongate shaft  126 . Since longitudinal ribs  168  are in contact with outer surface  138  of second elongate shaft  126  it may be appreciated that there is substantially no gap between longitudinal ribs  168  and second elongate shaft  126 . In a presently preferred embodiment, the gap between second elongate shaft  126  and each longitudinal rib  168  is, for example, between zero and 0.05 mm. Embodiments of the present invention are possible in which there is an interference fit between longitudinal ribs  168  and second elongate shaft  126 . In  FIG. 3 , it may also be appreciated that second elongate shaft  126  defines an injection lumen  162 .  
         [0059]      FIG. 4  is a partial cross section view of an additional embodiment of a shaft assembly  222  in accordance with the present invention. Shaft assembly  222  includes a second elongate shaft  226  disposed within a lumen  232  defined by an inner surface  254  of a first elongate shaft  224 . First elongate shaft  224  also includes a plurality of interstitial members  256  extending beyond inner surface  254  of first elongate shaft  224 . In the embodiment of  FIG. 4 , each interstitial member comprises a radial rib  270 . As shown in  FIG. 4 , each radial rib  270  contacts an outer surface  238  of second elongate shaft  226 . Since radial ribs  270  are in contact with outer surface  238  of second elongate shaft  226  it may be appreciated that there is substantially no gap between radial ribs  270  and second elongate shaft  226 . In a presently preferred embodiment, the gap between second elongate shaft  226  and each radial rib  270  is, for example, between zero and 0.05 mm. Embodiments of the present invention are possible in which there is an interference fit between radial ribs  270  and second elongate shaft  226 .  
         [0060]      FIG. 5  is a partial cross section view of an additional embodiment of a shaft assembly  322  in accordance with the present invention. Shaft assembly  322  includes a first elongate shaft  324  having a lumen  332  defined by an inner surface  354  thereof. A second elongate shaft  326  and an interstitial member  356  are disposed within lumen  332  of first elongate shaft  324 . In the embodiment of  FIG. 5 , interstitial member comprises a coil  372  comprising a wire  376  forming a plurality of turns  374 . As shown in  FIG. 5 , each turn  374  of coil  372  is disposed between inner surface  354  of first elongate shaft  324  and outer surface  338  of second elongate shaft  326 . I n the embodiment of  FIG. 5 , each turn  374  is in contact with inner surface  354  of first elongate shaft  324  and outer surface  338  of second elongate shaft  326 .  
         [0061]      FIG. 6  is a partial cross section view of an additional embodiment of a shaft assembly  422  in accordance with the present invention. Shaft assembly  422  includes a first elongate shaft  424  having a lumen  432  defined by an inner surface  454  thereof. A second elongate shaft  426  and an interstitial member  456  are disposed within lumen  432  of first elongate shaft  424 . In the embodiment of  FIG. 6 , interstitial member comprises a coil  472  comprising a wire  476  forming a plurality of turns  474 .  
         [0062]     In the embodiment of  FIG. 6 , coil  472  has a first pitch diameter A and a second pitch diameter B. In the embodiment of  FIG. 6 , coil  472  is preferably adapted to contact outer surface  438  of second elongate shaft  426  with a plurality of turns  474 . Also in the embodiment of  FIG. 6 , coil  472  is preferably adapted to contact inner surface  454  of first elongate shaft  424  with a plurality of turns. For example, first pitch diameter A may be pre-selected such that a plurality of turns  474  of coil  472  will contact outer surface  438  of second elongate shaft  426 . By way of a second example, second pitch diameter B may be pre-selected such that a plurality of turns  474  of coil  472  will contact inner surface  454  of first elongate shaft  424 .  
         [0063]      FIG. 7  is a partial cross section view of an additional embodiment of a shaft assembly  522  in accordance with the present invention. Shaft assembly  522  includes a first elongate shaft  524  having a lumen  532  defined by an inner surface  554  thereof. A second elongate shaft  526  is disposed within lumen  532  of first elongate shaft  524 . Second elongate shaft  526  includes an outer surface  538  and a plurality of interstitial members  556 . In the embodiment of  FIG. 7 , each interstitial member  556  comprises a projection  578  extending beyond outer surface  538  of second elongate shaft  526 .  
         [0064]      FIG. 8  is a plan view of an additional embodiment of a catheter  620  in accordance with the present invention. Catheter  620  has a distal end  630 , a proximal end  640 , and a shaft assembly  622 . Shaft assembly  622  comprises a first elongate shaft  624  having a distal end  634 , a proximal end  644 , and an inner surface  654  defining a lumen  632 . Shaft assembly  622  also includes a second elongate shaft  626  slidingly disposed within lumen  632  of first elongate shaft  624 .  
         [0065]     Second elongate shaft  626  has an outer surface  638 , a distal end  636 , and a proximal end  646 . In many applications it is desirable to advance distal end  636  of second elongate shaft  626  by a known distance relative to distal end  634  of first elongate shaft  624 . In the embodiment of  FIG. 8 , a plurality of indicia  652  are disposed on outer surface  638  of second elongate shaft  626  proximate a point  658  of second elongate shaft  626 . In a preferred embodiment, indicia  652  are comprised of a radiopaque material. Examples of materials which may be suitable in some applications include gold, platinum, tungsten, iron, silver, and thermoplastic material loaded with a radiopaque filler. Examples of radiopaque filler which may be suitable in some applications include barium sulfate, bismuth subcarbonate, bismuth trioxide, bismuth oxychloride, tungsten, and depleted uranium.  
         [0066]     A radiopaque reference  680  is disposed proximate distal end  634  of first elongate shaft  624 . During surgery a radiopaque reference  680  and indicia  652  may be viewed on a fluoroscopy screen. The image viewed on the fluoroscopy screen may be utilized to determine the depth which point  658  has penetrated into the target tissue.  
         [0067]      FIG. 9  is a cross sectional view of a distal portion  782  of a catheter  720  in accordance with the present invention. Catheter  720  comprises a first elongate shaft  724  having an inner surface  754  defining a lumen  732 . A ferrule  784  is disposed within lumen  732  proximate a distal end  734  of first elongate shaft  724 . In a preferred embodiment, ferrule  784  is fixed to first elongate shaft  724 . A needle  786  is slidingly disposed within a ferrule lumen  794  defined by ferrule  784 . A piston member  788  is disposed about a proximal portion  791  of needle  786 . Piston member  788  forms a sliding seal with inner surface  754  of first elongate shaft  724 . A spring  792  is disposed within lumen  732  of first elongate shaft  724 . In the embodiment of  FIG. 9 , the distal end of spring  792  is seated against ferrule  784  and the proximal end of spring  792  is seated against piston member  788 .  
         [0068]     In  FIG. 9 , a fluid  790  is disposed within lumen  732  of first elongate shaft  724  and a needle lumen  796  defined by needle  786 . Catheter  720  of  FIG. 9  may be utilized to inject fluid  790  into a target tissue. A fluid source may be utilized to urge additional fluid  790  into lumen  732  of first elongate shaft  724  and a needle lumen  796  defined by needle  786 . In a preferred method, the additional fluid  790  is urged into lumen  732  with a velocity which is sufficient to create a pressure differential across piston member  788 . In this preferred method, the pressure differential across piston member  788  is sufficient to compress spring  792  and urge an injection port  760  of needle  786  into the target tissue. In this manner a dose of fluid  790  may be injected into the target tissue. When the flow of fluid  790  stops, spring  792  will urge piston member  788  back to the position shown in  FIG. 9 .  
         [0069]     Those of skill in the art will appreciate that many embodiments of the fluid source are possible without deviating from the spirit and scope of the present invention. For example, the fluid source may include a variable volume chamber in fluid communication with lumen  732  of first elongate shaft  724 . In this exemplary embodiment, the fluid source may further include a plunger slidingly disposed within the variable volume chamber. Urging the plunger distally preferably has the effect of urging fluid into lumen  732  of first elongate shaft  724 . A number of energy sources may be utilized to urge the plunger distally. Energy sources which may be suitable in some applications include springs, compressed gas, a human being, and electricity.  
         [0070]      FIG. 10  is a plan view of a catheter  920  in accordance with the present invention. Catheter  920  includes a distal. end  930 , a proximal end  940 , and a shaft assembly  922 . Shaft assembly  922  comprises a first elongate shaft  924  having a distal end  934 , a proximal end  944 , and an inner surface  954  defining a lumen  932 . Shaft assembly  922  also includes a second elongate shaft  926  slidingly disposed within lumen  932  of first elongate shaft  924 , which may function as the interstitial member described above. A third elongate shaft  970  is slidingly disposed within a lumen defined by second elongate shaft  926 . In the embodiment of  FIG. 10 , a proximal portion  945  of second elongate shaft  926  extends beyond proximal end  944  of first elongate shaft  924 . Proximal portion  945  of second elongate shaft  926  terminates with a proximal end  946 . Also in the embodiment of  FIG. 10 , a slider  942  is fixed to second elongate shaft  926  proximate proximal end  946  thereof. A portion of slider  942  is disposed within a cavity  948  (also referred to as a chamber) defined by a housing  950  (also referred to as a hub). In a presently preferred embodiment, housing  950  is fixed to first elongate shaft  924  proximate proximal end  944  thereof. Also in a preferred embodiment, a plurality of indicia  952  are disposed on a face  954  of housing  950  proximate slider  942 .  
         [0071]      FIG. 11  is a partial cross section view of a distal portion of catheter  920  of  FIG. 10 . As described previously, catheter  920  includes a first elongate shaft  924 , a second elongate shaft  926 , and a third elongate shaft  970 . In a preferred embodiment, third elongate shaft  970  forms a point  958  proximate a distal. end  976  thereof. Third elongate shaft  970  also defines an injection port  960  in fluid communication with an injection lumen. A flange  972  is disposed about third elongate shaft  970 . Flange  972  cooperates with a mechanical stop  974  in order to limit the travel of third elongate shaft  970 . In a preferred embodiment, mechanical stop  974  is fixed to second elongate shaft  926  proximate to distal end  936  thereof, forming an interstitial member between first elongated shaft  924  and the third elongate shaft  970 . The depth which third elongate shaft  970  will penetrate into a target tissue (e.g., a heart wall) may be adjusted by moving distal end  936  of second elongate shaft  926  a known distance relative to distal end  934  of first elongate shaft  924 . For example, a physician utilizing catheter  920  may urge slider  942  distally while visually observing the travel of slider  942  relative to indicia  952  of housing  950 . In a preferred embodiment there is substantially a one-to-one relationship between the movement of slider  942  relative to indicia  952  of housing  950  and the movement of distal end  936  of second elongate shaft  926  relative to distal end  934  of first elongate shaft  924 . In the embodiment of  FIG. 11 , there is, preferably, interference fit between first elongate shaft  924  and second elongate shaft  926  to eliminate any slop, whether second elongated shaft  926  is tubular or comprises radial ribs, as illustrated in  FIG. 11 .  
         [0072]      FIG. 12  is a partial cross section view of a distal portion of catheter  920  of  FIG. 10  and  FIG. 11 . In the embodiment of  FIG. 12 , distal end  936  of second elongate shaft  926  has been moved to a new position relative to distal. end  934  of first elongate shaft  924 . The position of second elongate shaft  926  illustrated in  FIG. 12  may be referred to as a second position, and the position of second elongate shaft illustrated in  FIG. 11  may be referred to as a first position. It is to be appreciated that second elongate shaft  926  may be urged proximally and distally to a plurality of positions. An injection may be performed by urging point.  958  of third elongate shaft  970  distally into a target tissue. The advancement of third elongate shaft  970  into the target tissue may be stopped when flange  972  seats against mechanical stop  974 .  
         [0073]     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.