MINIMALLY-INVASIVE DIRECT CARDIAC MASSAGE APPARATUS AND METHOD

Abstract: A medical device (20) has a spring made up of a plurality of elongate members (42) in substantial juxtaposition so as to be substantially parallel. The spring is alternately deployed and stowed in use of the device (20), and has a relaxed state and a stressed state. The spring may be arcuate in its relaxed and deployed state and be generally linear in its stressed and stowed state. The elongate members may be configured so that each contributes to the overall bending strength of the spring but none are stressed beyond its respective yield stress in the stressed state of the spring. The elongate members may be flat in cross-section and together define a laminated structure. One of the elongate members (204a) may terminate short of a distal tip (206) of the spring so that it is more flexible at the tip (206) than at a proximal base (210). The device (20) may be a heart massager with plurality of springs deployed from the end of a tubular cannula to combine to form an inverted umbrella-shape with a fabric (44) stretched over the assembly. The material of the elongate members may be Nitinol and the maximum stress imposed on each is less than that which would induce a martensitic phase change.

Description: MINIMALLY-INVASIVE DIRECT CARDIAC MASSAGE APPARATUS AND METHOD FIELD OF THE INVENTION The present invention relates to medical devices for cardiac massage and, more particularly, to devices and methods for performing minimally invasive direct cardiac massage. BACKGROUND OF THE INVENTION Sudden cardiac arrest is a leading cause of death in most industrial societies. While in many cases it is possible to re-establish cardiac function after cardiac arrest, irreversible damage to vital organs, particularly the brain and heart itself, will usually occur prior to restoration of normal cardiac activity. Therefore, immediate cardiac massage is crucial to provide artificial circulation of blood to oxygenate the heart and brain during the period between cardiac arrest and restoration of normal cardiac activity. A number of techniques for cardiac massage have been developed. Open chest cardiac massage (OCM), involving opening of the patient's chest and manually squeezing the heart to pump blood to the body, was, prior to the 1960s, standard treatment for sudden cardiac arrest. More recently, closed cardiac massage (CCM), involving externally compressing the chest wall, has become the standard treatment. When CCM is combined with airway support, it is known as cardiopulmonary resuscitation (CPR). Although CPR is much less invasive than OCM, studies have shown that CCM provides significantly less cardiac output, less neuroperfusion, and less cardiac perfusion than that achieved with OCM. Methods and devices for performing minimally invasive direct cardiac massage (MID-CM) through an intercostal space have been described by Buckman, et al., and by Drs. Filiberto and Giorgio Zadini (e.g., U.S. Patent Nos. 5,582,580, 5,571 ,074, and 5,484,391 to Buckman, Jr., et al.; and 5,683,364, and 5,466,221 to Zadini et al.). While the methods of Buckman, et al., and Zadini, et al., differ in a number of respects, they generally rely on introducing a balloon, shoes, or other deployable member to engage the heart through a small incision in an intercostal space above the pericardium. The heart may then be pumped by directly engaging and compressing the pericardium, either by inflating and deflating the member or by reciprocat ng a shaft attached to the member. Although these approaches have been shown to be effective in animal models, the particular devices described in the patents would be awkward for emergency use on human patients. In particular, the balloon-type and shoe-type heart-engaging members described in the patents may be difficult to deploy in the potential space between the posterior surface of the rib cage and the pericardium. Moreover, inflatable balloon members may lack sufficient rigidity to impart the necessary compressive force, rigid shoe-like members risk damage to the pericardium, and cup-shaped compressive members may compress so much of the heart volume that the end diastolic volume is compromised. In addition to being somewhat awkward during deployment, and potentially damaging to the surrounding anatomy, the devices of the prior art do not appear to take into account potential fatigue of the materials used in the device. That is, the device requires deployment and use after a long period of storage which may overstress members that are pre-biased in their stowed configurations. Typically, as seen in U.S. patent No. 5,582,580 to Buckman, et al., an umbrella-like structure including a plurality of flexible spoke members is deployed from the distal end of a tubular cannula. The spoke members are biased to spring open and form the umbrella structure from a constrained position within the cannula. Repetitive pumping of the heart once the device is deployed may tend to create regions of localized high stress, potentially risking failure of the structure material. Additionally, the spokes, although encased in a flexible cup member, are directed toward the heart in use and thus present a potential danger of puncturing the heart. There is thus a need for an improved device for performing minimally invasive direct cardiac massage which is simple to deploy, and reliably deploys and expands within a relatively narrow region between the posterior rib cage and pericardium. The device should minimize risks to the surrounding anatomy, and should be designed so as to avoid exceeding the yield stress of the material. SUMMARY OF THE INVENTION The present invention provides an improved minimally-invasive direct cardiac massager apparatus that utilizes a plurality of spring members expandable from a delivery cannula into an atraumatic heart contacting portion. The delivery cannula is sized to fit between two ribs so that the heart contacting portion may be deployed into the mediastinum space between the patient's rib cage and heart. The spring members are configured such that the heart contacting portion is relatively flat so as not to occupy excessive space between the rib cage and heart. Desirably, the spring members are arranged in a circle and are biased outwardly so as to form a trumpet-like structure, and a biocompatible sheet is stretched across the spring members to provide a flexible heart contacting surface. The spring members are designed to be stowed within the delivery cannula in a stressed condition below the yield stress of the material, and are deployed into a relaxed, curved configuration. One aspect of the present invention is a medical device incorporating a spring that is alternately disposable in stowed and deployed configurations. The spring includes a plurality of elongate members each having a longitudinal axis, a bending strength, and a yield stress. The elongate members are arranged in substantial juxtaposition and are substantially parallel to one another. The spring has a spring bending strength to which the bending strengths of each of the elongate members contribute. The spring further has a relaxed state and a stressed state such that when disposed in one of the relaxed or stressed states the spring is in the stowed configuration, and when in the other of the relaxed or stressed states the spring is in its deployed configuration. Finally, none of the elongate members are stressed beyond their respective yield stresses when the spring is in its stressed state. Desirably, the spring is disposed in its stressed state when in the stowed configuration. Preferably, the spring is arcuate in the relaxed state and substantially straight in the stressed state. The medical device incorporating the spring preferably further includes a tubular cannula within which the spring is linearly displaceable. The cannula includes an open end wherein the spring extends from the open-end into the relaxed state/deployed configuration, and is located substantially within the cannula in the stressed state/stowed configuration. There may be a plurality of the springs arranged in a circle so as to form a trumpet-like shape extending from the open mouth of the cannula in the relaxed state/deployed configuration. A flexible, biocompatible sheet may connect the springs in the relaxed state/deployed configuration, the sheet being disposed generally perpendicular to the axis of tubular cannula. In one particular embodiment of the present invention, the medical device incorporating the open-mouthed tubular cannula further includes a blunt tip member that is initially positioned to cover the open mouth of the tubular cannula to facilitate advancement of the cannula through an access incision. In the configuration where the medical device incorporates a plurality of springs arranged in a trumpet-like shape extending from the open mouth of the cannula, and a biocompatible sheet spanning the springs, the blunt tip member is mechanically restrained so as not to contact either the springs or the sheet during transition of the springs from the relaxed state/deployed configuration to the stressed state/stowed configuration. In a further aspect of the present invention wherein a plurality of springs of the medical device project distally from a tubular cannula, each spring has the tip extending from the cannula open mouth that, in the relaxed state/deployed configuration, forms an angle of greater than 90 degrees with respect to a ray extending along the axis of the tubular cannula. More preferably, each spring forms an angle of greater than 180 degrees with respect to the axial ray. In a still further aspect of the present invention, the medical device spring defines a base on one and a tip on the other end, wherein the tip is more flexible than the base. In one form, the elongate members each define a first end in the direction of the spring base and a second end in the direction of the spring tip. The second end of least one elongate member does not extend to the spring tip so that the spring tip is more flexible than the base. Preferably, there are more than two of the elongate members, and none of the second ends of the elongate members is coincident with another of the second ends. The elongate members may be generally flat in cross-section and may be juxtaposed with wide faces in contact to form a laminated spring structure. Still further, the present invention contemplates a medical device comprising springs as described above, wherein the elongate members are made of a material that is capable of exhibiting a stress-induced Martensitic phase. The elongate members may be made of a nickel-titanium alloy. In a particular embodiment of the present invention, the medical device is usable for performing minimally invasive direct cardiac massage, and further includes a handle, wherein a plurality of the springs extend from a distal end of the handle. The springs are initially disposed in their stowed configurations such that the plurality of the springs is sufficiently compact to allow least the distal portions thereof to be inserted into the mediastinum of a patient through a minimal access incision of less than one inch in length. Subsequently, springs are movable to their deployed configurations such that they will radially expand within the mediastinum. Distal portions of the springs will directly engage the patient's heart such that pressure will be exerted upon the heart when the handle of the device is advanced in the distal direction. Desirably, the elongate members comprising each spring are formed of a superelastic metal alloy capable of exhibiting stress-induced martensite, however the springs move back and forth between their stowed and deployed configurations without exhibiting stress-induced martensite. In a preferred embodiment, each of the springs in the heart massaging device comprises a main strut having a first radius of curvature when in its deployed configuration and unconstrained, and a sub-strut having a second radius of curvature when in its deployed configuration and unconstrained. The ends of the sub-strut are attached to the main strut. The main strut may comprise a plurality of the elongate members each having a longitudinal axis, a bending strength, and a yield stress. The elongate members of the main strut are arranged in substantial juxtaposition and substantially parallel to one another. In one embodiment, the sub-strut may comprise a single elongate member attached at either of its ends to the main strut. Alternatively, the sub-strut may comprise a plurality of single elongate members each having a longitudinal axis, a bending strength, and a yield stress. The plurality of single elongate members of the sub-strut are arranged in substantial juxtaposition and substantially parallel to one another. At least one of the elongate members of the sub-strut is attached to the main strut at either end of the sub-strut. In one form, the handle of the heart massaging device of the present invention includes a tube within which the springs are positioned while in their stowed configurations. The tube has a distal portion that is insertable into the thoracic cavity of the patient through a minimal access incision of less than one inch in length. The handle is relatively displaceable after the tube has been inserted into the patient so as to allow the springs to radially expand. Desirably, the tube has an open mouth at its distal end, the device further including a blunt tip member initially positioned to cover the open mouth of the tube to facilitate advancement of the tube through the minimal access incision. In a further aspect of the present invention, an expandable medical device delivery cannula having a blunt insertion tip is provided. The delivery cannula includes an expandable medical device received in a contracted state within a lumen of the generally tubular cannula. The cannula lumen has an open distal end through which the medical device can be deployed into an expanded state and then subsequently retracted within the cannula lumen into the contracted state. The blunt insertion tip covers the open distal end of the cannula lumen during insertion of the cannula into a body cavity. A connecting shaft rigidly attached to the insertion tip extends proximally within the cannula lumen and is adapted to slide longitudinally therewithin. A deployment mechanism is provided on a proximal end of the tubular cannula and includes structure operable to deploy and retract the expandable medical device, and limit retraction of the connecting shaft with respect to the medical device during retraction of the medical device. Preferably, the deployment mechanism includes a handle longitudinally displaceable with respect to the tubular cannula and directly coupled to the expandable medical device to enable 1 :1 displacement of the handle with respect to the medical device. Furthermore, the handle may be frictionally coupled to a proximal end of the connecting shaft to enable 1 :1 displacement of the handle with respect to the connecting shaft, unless displacement of the connecting shaft is limited by structure other than the handle that overcomes the frictional coupling between handle and connecting shaft. Desirably, the structure that limits displacement of the connecting shaft is fixed with respect to the tubular cannula and includes a control housing and deployment button. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a schematic cross-section of a chest of a patient showing a minimally invasive direct cardiac massager of the present invention prior to insertion into the chest cavity; Figure 1 B is a view as in Figure 1A showing the heart massager after insertion through an intercostal space between the patient's ribs and prior to deployment; Figure 1C is a view as in Figure 1 B showing the heart massager after deployment within the space between the posterior side of the rib cage and pericardium; Figure 1 D is a view as in Figure 1C after use of the heart massager with the heart contacting portions of the device having been partially retracted back into an insertion cannula; Figure 1 E is a view as in Figure 1 D showing the complete retraction of the heart contacting portions and withdrawal of a blunt dissector tip prior to withdrawal of the heart massager from within the chest cavity; Figure 2 is a perspective view of the heart massager of the present invention showing the rear side of the deployed heart contacting portion; Figure 3 is a close-up perspective view of the rear side of the heart contacting portion in use on a heart; Figure 4A is elevational view of the heart massager of the present invention with a heart contacting portion retracted within an insertion cannula; Figure 4B is an elevational view of the heart massager with the heart contacting portion deployed from within the cannula; Figure 5A is a sectional view of a distal length of the heart massager of the present invention showing the heart contacting portion deployed; Figure 5B is a sectional view of a control housing seen in Figure 5B; Figure 6 is a partial elevational view of a slotted deployment shaft used in the heart massager of the present invention; Figure 7 is an elevational view of a connector shaft used in the heart massager of the present invention; Figure 8A is a partial sectional view of the heart massager of the present invention with the heart contacting portion removed, and showing the position of the connecting shaft during retraction; Figure 8B is a partial sectional view of the heart massager as in Figure 8A, and showing the connecting shaft fully retracted prior to withdrawal of the heart massager from within the chest cavity; Figure 9 is a detailed sectional view of a distal portion of the heart massager of the present invention and showing strut members in their retracted, stressed positions; Figure 10 is an isolated elevational view of one of the struts of the heart contacting portion in its deployed condition; Figure 11 is a detailed view of the strut of Figure 10; Figure 12 is a detailed view of an alternative strut having a single thick proximal member and a plurality of thin distal members; Figure 13 is a detailed view of an alternative strut having both thick and thin proximal members, the thin members extending to the distal tip; Figure 14 is a detailed view of an alternative strut having a single elongated member and a single reinforcing member; Figure 15 is a detailed view of an alternative strut having a plurality of reinforcing members, one of them being thicker than the others; and Figure 16 is a detailed view of an alternative strut having a plurality of same thickness reinforcing members. Figure 17a is a side view of an alternative strut having a distal closed-loop segment and a proximal portion formed of a plurality of individual leaves. Figure 17b is an enlarged view of the distal closed loop segment of the strut of Figure 17a. Figure 18 is a side view of the distal portion of a cardiac massage device of the present invention, in its deployed configuration, such device incorporating closed loop struts of the type shown in Figures 17a and 17b. Figure 19a is a partial, cut-away, side view of the device of Figure 18 showing the proximal portion of one of its struts in a non-deployed position (i.e., housed within the cannula 24). Figure 19b is a partial, cut-away, side view of the device of Figure 18 showing the proximal portion of one of its struts in a deployed position (i.e., following retraction of the cannula 24 and outward splaying of the struts) DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides an improved minimally invasive direct cardiac massager which is both easier to use and less susceptible to breakage. A number of different configurations of the heart massager are shown herein, none of which should be construed as especially advantageous over the others. Moreover, the heart massager of the present invention utilizes an improved spring strut design which has a relaxed curvilinear shape that is bent further than struts of the prior art, but which remains below the yield stress of the material when straightened out. Such a spring strut could be utilized in other medical implements for a similar benefit. Therefore, the present invention is not deemed to be limited to only a minimally invasive heart massager, as shown. Cardiac Massage Apparatus With reference to Figures 1A-1 E, a minimally invasive cardiac massage apparatus or heart massager 20, in accordance with the present invention, is schematically shown in use. As with prior art heart massagers, the heart massager 20 comprises an elongate, generally tubular device which can be passed through a small aperture between ribs into proximity of the heart H. The ribs shown are indicated at R2-R6, and a distal end of the heart massager 20 can be seen passing through the intercostal space between ribs R4 and R5. Of course, depending on the anatomy of the patient, and the surgeon preference, a different approach could be used. With reference specifically to Figure 1 A, the heart massager 20 has a blunt tip 22 covering the distal end of a tubular cannula 24. The blunt tip 22 preferably has a spherical surface to facilitate passage through the layer 26 of muscle and cartilage between the ribs, and preliminary incision through the layer 26 to facilitate this passage is shown. In addition, the blunt tip 22 has a width about the same as an adult finger, or more precisely between about 0.5 to 1.0 inches. The heart massager 20 further includes a cylindrical stop flange 28 extending outward from the cannula 24. The entire heart massager 20 has been advanced, as indicated by ε rrow 30 in Figure 1A, into the position in Figure 1 B wherein the stop flange 28 abuts the exterior surface of the layer 26. The interference between the stop flange 28 and the layer 26 provides an indication to the surgeon that the heart massager 20 is properly inserted. At this stage, the blunt tip 22 has passed into a cavity 32 between the ribs and heart H and is in slight contact with the pericardial layer surrounding the heart. Now with reference to Figure 1C, a heart contacting portion 34 of the heart massager 20 is shown deployed into contact with the heart H. This deployment is accomplished by depressing a control button 36 and advancing a deployment shaft 38, as indicated by arrow 40. The specific details of the heart massager 20 will be described below, although it should be noted that the heart contacting portion 34 comprises a plurality of spring struts 42 surrounded by a protective cover 44 in direct contact with the heart H. The heart massaging operation takes place by periodically moving the heart contacting portion 34 toward and away from the heart H, as indicated by the dashed line position 46. The timing of the massage thrusts is determined by the physician or operator of the device, and is not the subject of the present invention. Once the massaging operation is complete, the heart massager 20 is retracted as seen in Figure 1 D. More specifically, the deployment shaft 38 is displaced proximally with respect to the cannula 24, as indicated by arrow 48. In doing so, the heart contacting portion 34 collapses through a mouth 50 of the cannula 24 in the direction of arrow 52. As will be described below, the blunt tip 22 is prevented from being collapsed inside the heart contacting portion 34. Figure 1 E illustrates the final retraction configuration of the heart massager 20 in which the heart contacting portion 34 is fully retracted within the mouth 50 of the cannula 24. Prior to withdrawing the heart massager 20 from the patient's chest cavity, the blunt tip 22 is retracted to the position shown leaving a slight gap 54 between a taper 56 on the underside the tip and the mouth 50. The gap 54 helps ensure that tissue is not pinched between the blunt tip 22 and mouth 50, which might cause complications. Preferably, during the entire massaging operation, the stop flange 28 has remained in contact with the layer 26 of muscle and cartilage between the ribs. At this stage, the entire heart massager 20 is withdrawn from within the patient's chest cavity, as indicated by arrow 58. Figure 2 illustrates the heart massager 20 in perspective with the heart contacting portion 34 fully deployed so that the multiple struts 42 are visible. As will be described in more detail below, the struts 42 bend radially outward when advanced distally past the mouth 50 of the cannula 24. Although not seen in Figure 2, the distal-most tips of each strut 42 curls back upon the strut and the protective cover 44 wraps around each tip to provide a smooth, toroidal or nearly toroidal outer periphery 60. The protective cover 44 can be a variety of flexible, biocompatible materials, but is preferably a polyester fabric, such as a mesh available from Jelliff having the characteristics of: 160 mesh x 0.002 inch monofilament diameter x 104 μm mesh openings. As a safety feature, a tether 64 attaches the protective cover 44 to one of the struts 42 at a location not shown. The heart massager 20 further includes a proximal handle 62 and a control housing 66 within which the control button 36 is operably mounted. The control housing 66 is rigidly affixed to the proximal end of the cannula 24 and includes a central aperture 68 through which the deployment shaft 38 slides. The deployment shaft 38 controls the movement of the heart contacting portion 34, by virtue of its rigid attachment to the proximal end of each strut 42, as will be explained below. Figure 3 illustrates the distal end of the deployed heart massager 20 in use on a heart H. This view illustrates the preferred orientation of heart massager 20 at the lower apex of the left ventricle LV, or pumping chamber of the heart. Oxygenated blood from the left ventricle is urged upward through the aortic valve and ascending aorta AA to the arterial system of the body. Most importantly, oxygenated blood is immediately urged through the coronary arteries (CA), primarily during diastole, to perfuse the muscle tissue of the heart H itself, which is critical in cardiac arrest situations. Figure 3 best illustrates how the rounded, toroidal periphery 60 of the heart contacting portion 34 avoids damage to the heart H and surrounding tissue. In addition, the multiple spring struts 42 act as a buffer between the displacement motion of the heart massager 20 and the soft heart muscle. And finally, the protective cover 44 stretches across the distal-most face of the heart contacting portion 34 and "gives" to a degree to provide a pocket within the diverging struts 42, further helping to cradle the irregular shaped apex of the left ventricle LV. The combination of the multiple struts 42 and protective cover 44 form an inverted umbrella-shaped heart contacting portion 34. Figures 4A and 4B illustrate the heart massager 20 in its retracted (4A) and deployed (4B) states. In these views, the distal direction is to the left, while the proximal direction is to the right. These views illustrate the relative lengths of each of the components, especially emphasizing the short length of the cannula 24 extending distally from the control housing 66. In Figure 4A, the heart massager 20 is fully retracted so that the blunt tip 22 lies flush against the mouth 50. A pair of indicators can be seen on the deployment shaft 38. Specifically, a retraction indicator 70 is visible when shaft 38 is fully retracted, or retracted to at least a safe point, within the control housing 66. A deployment indicator 72 is used to alert the operator when the heart massager 20 is fully deployed. So, as seen in Figure 4B, the deployment indicator 72 is just visible on the right side of control housing 66 when the heart contacting portion 34 is fully deployed. Heart Contacting Portion Structure Figures 5A and 5B are cross-sections showing details of the inner workings of heart massager 20. On the left side of Figure 5A, a pair of diametrically opposed struts 42 are shown deployed from within the mouth 50 of the cannula 24. Only two struts 42 are shown, and they are simplified to more clearly illustrate their overall shape and interaction with the rest of the heart massager 20. It will be understood that Figures 9-16 show multiple variations of struts that can be used in place of those seen in Figure 5A. Each strut 42 includes a curled tip 80, a middle spring portion 82, and a base portion 84. The base portion 84 of each of the struts 42 is secured relative to the deployment shaft 38. Specifically, the deployment shaft includes a strut mounting cylinder 86 at the left (distal) end around which the base portions 84 are secured. Each of the base portions 84 is secured to the cylinder 86 with a suitable adhesive, potting compound, sutures, or other similar expedient. The base portions 84 terminate adjacent to a flange 88 which acts as a manufacturing guide to assist in securing the strut base portions 84 with a potting compound (see 174 in Figure 9). One of the advantages of the present invention is the extremely curled tips 80 of each of the struts 42. These present a non-traumatic shape for the heart- contacting portion 34, further softened by their highly flexible nature. Desirably, the tips 80 are curled back more than 90° from the distal direction along the cannula 24. Preferably, the tips 80 are curled back more than 135°, more preferably more than 180°. In the embodiment illustrated in Figures 5A and 5B, the tips 80 are curled back between about 200° and 240°. It should be noted, however, that an embodiment of the present invention, illustrated in Figures 17 and 18, includes tips that are bent completely back upon the strut, in a closed loop having a tip bend angle of 360°, and thus the preferred tip bend angle is thus between 180° and 360°. Figure 5A best illustrates the shape of the protective cover 44, which as mentioned, wraps around each of the curled tips 80 at a skirt portion 90. The protective cover 44 stretches between the diverging struts 42 in a planar trampoline portion 92 having a central aperture 94 through which a connecting shaft 96 extends. The blunt tip 22 is secured to the left (distal) end of the connecting shaft 96 and is larger than the aperture 94 so that it remains out in front of the trampoline portion 92. Although not illustrated in Figure 5A, the plurality of struts 42 and protective cover 44 form an inverted-umbrella shape which is not entirely circular, but instead extends along chordal segments. The number of struts 42 must be sufficient to provide a fairly uniform pressing surface for massaging the heart H, yet not too numerous or else retraction of the struts into the introducer sheath becomes difficult or impossible. At a minimum there are three struts 42, although there are preferably at least eight struts 42 across which the protective cover 44 is secured, with a preferred range of between four and twelve struts 42. The greater the number of struts 42, of course, the more truly toroidal is the periphery 60 (Figure 3) of the heart contacting portion 34. Deployment Mechanism With reference to Figures 5A and 5B, the mechanism for deploying the heart contacting portion 34 is shown. As mentioned, each strut 42 includes a base portion 84 secured to a strut mounting cylinder 86. The strut mounting cylinder 86 forms a distal end of the deployment shaft 38 which extends longitudinally through a bore in the control housing 66. The connecting shaft 96, on which left ( distal) end is secured the blunt tip 22, passes through a longitudinal bore formed within the deployment shaft 38. A portion of the connecting shaft 96 can be seen in Figure 5A through an elongated slot 98 formed in the deployment shaft 38. An annular guide sleeve 100 is secured to the connecting shaft 96 with a pin 102. The guide sleeve 100 has an inner throughbore sized to surround the deployment shaft 38, the guide sleeve being adapted to slide longitudinally with respect to the deployment shaft. The guide sleeve 100 is generally tubular with a maximum outer diameter smaller than the inner diameter of the cannula 24. Therefore, the guide sleeve 100 is adapted to slide longitudinally along the slot 98 with respect to the deployment shaft 38 and within the cannula 24. Figure 5B is an enlarged sectional view through the control housing 66 with the cannula 24 and deployment shaft 38 illustrated in phantom. The control button 36 is mounted within a dead-end cavity 104 of the control housing 66 having a depth greater than the control button to permit movement therein. More specifically, a cut out 106 on the control button 36 receives one end of a screw 108 fastened within a threaded bore 110 in the housing 66. The cut out 106 is elongated in a direction transverse to the longitudinal axis of the heart massager 20 to permit the control button 36 to be displaced with respect to the screw 108. A spring 112 positioned in the bottom of the cavity 104 biases the control button 36 in a direction out of the cavity. For purposes of explanation, movement of the control button 36 will be described as either in or out, with reference to the deadend cavity 104. The control button 36 defines a through bore 114 for receiving the deployment shaft 38 and guide sleeve 100 positioned thereon. The throughbore 114 defines a number of surfaces which interact with surfaces upon the deployment shaft 38 and guide sleeve 100 to control deployment of the heart contacting portion 34 as well as the relative movement of the connecting shaft 96 and blunt tip 22. Looking from left to right in Figure 5B, a left-facing cam surface 120 defines an angle of approximately 45 degrees with respect to the longitudinal axis, a sleeve stop 122 is oriented perpendicular with respect to the longitudinal axis and faces to the left (distally), and a deployment stop 124 is also oriented perpendicular with respect to the longitudinal axis and faces to the right (proximally). The interaction of these surfaces with guide sleeve 100 and a lock collar 140 (Figure 6) will be described below. The control housing 66 further includes a bore segment 126 on the left receiving the cannula 24 and terminating in a lip 128. A counter bore 130 located just to the right (proximally) of the cavity 104 terminates in a step 132 leading to a relief chamber 134, which in turn ends at an inner flange 136 closely receiving the deployment shaft 38. Figures 6 and 7 illustrate the deployment shaft 38 and connecting shaft 96 in isolation. The deployment shaft comprises the aforementioned slot 98 terminating just to the right of a lock collar 140. The lock collar 140 projects outward from the shaft 38 and defines, on its right (proximal) end, a tapered retraction ramp 142, and on its left (distal) end a perpendicular deployment shoulder 144. The retraction indicator 70, deployment indicator 72, and strut mounting cylinder 86 are more clearly seen in Figure 6. In Figure 7, the guide sleeve 100 can be seen to have, from left to right, a tapered first ramp 150 on its left (distal) end, a perpendicular retraction shoulder 152, a tapered second ramp 154, and a tapered safety ramp 156 on its right (proximal) end. The retraction shoulder 152 and second ramp 154 face each other across a neck 158 formed in the middle of the guide sleeve 100. Figure 7 also illustrates the fixed position of the blunt tip 22 with respect to the guide sleeve 100, the two elements being fastened by respective pins to the connecting shaft 96. Use of Heart Massager In use, the heart massager 20 is inserted as described above in a chest cavity of the patient and deployed from the retracted position shown in Figure 4A to the deployed position shown in Figure 4B. Once the heart massage has been performed, the device must be retracted back to the condition where the heart contacting portion 34 is within the cannula 24, and withdrawn from the chest cavity. Certain features of the present heart massager 20 prevent it from being prematurely deployed, and insure that the heart contacting portion 34 can be folded up within the cannula 24 without collapsing around blunt tip 22. In addition, retraction of the device will not cause the blunt tip 22 to pinch tissue against the mouth 50, as previously mentioned. The interaction of the various structural components to enable these functions will now be described. With reference first to Figure 8B, the heart massager 20 is seen in section with the heart contacting portion 34 removed for clarity. Although the blunt tip 22 is seen slightly spaced from the mouth 50, it need only be pressed inward to seat against the mouth, as seen in Figure 4A. In this position, the deployment shaft 38 is fully withdrawn to the right (proximally) so that the heart contacting portion 34 would be entirely disposed within the cannula 24. Further retraction is prevented by the contact between the lock collar retraction ramp 142 and the step 132 of the control housing 66. Additionally, the deployment shoulder 144 of the lock collar 140 abuts the deployment stop 124 on the control button 36. This engagement, secured by the bias force of the spring 112, prevents leftward (distal) movement of the deployment shaft 38 with respect to the control housing 66. This is the fully retracted position shown in Figure 4A prior to use. The heart massager 20 will not be inadvertently deployed because of the interaction between the control button 36 and the lock collar 140. To release the deployment shaft 38, and deploy the heart contacting portion 34, the operator depresses the control button 36 against the force of the spring 112 to disengage the deployment stop 124 from the deployment shoulder 144. The operator can then displaced the handle 62 to the left (distally) while holding the control housing 66 to cause the entire heart contacting portion 34 to emerge from the mouth 50 of the cannula 24. The first and second ramps 150 and 154 cam the guide sleeve 100 past the spring-loaded deployment stop 124. The axial width of the neck 158 is sized smaller than or equal to the distance across the sleeve stop 122 and deployment stop 124 to help prevent binding of the guide sleeve 100. Because of the curvature of each of the struts 42 in their relaxed states, the heart contacting portion 34 splays outward into the position shown in Figure 5A. In addition, the emergence of the heart contacting portion 34 from the cannula 24 pushes the blunt tip 22 outward a short distance. Friction between the connecting shaft 96 and the collapsed struts 42 may cause the connecting shaft 96 and blunt tip 22 to continue outward until the blunt tip is restrained from further movement, such as by contact with the pericardium around the heart. The blunt tip 22 at all times remains forward of the protective cover 44, by virtue of its relatively larger size than the aperture 94. The operator continues movement of the handle 62 and shaft 38 to the left (distally) until the deployment indicator 72 reaches the control housing 66, which is seen in both Figures 4B and 5A. As can be seen by comparison between the retracted position of Figure 8B and the deployed position of Figure 5A, the deployment shaft 38 is displaced a distance approximately equal to the distance between the retraction indicator 70 and deployment indicator 72. This distance is sufficient to fully deploy the heart contacting portion 34. Figure 5A illustrates a dimension X representing the longitudinal "footprint" of the heart contacting portion 34. This dimension X is desirably minimized so that the heart contacting portion 34 can fully deploy within the cavity 32 between the ribs and heart, as shown in Figure 1C, without placing undue pressure on the heart or otherwise injuring surrounding anatomical structures. This dimension X is preferably between 1 and 5 cm, and more preferably is about 3 cm. Once the massaging operation has been completed, the operator begins retraction of the deployment shaft 38, as indicated by arrow 160 in Figure 8A. The tapered safety ramp 156 on the guide sleeve 100 ensures that in the highly unlikely scenario of the guide sleeve 100 projecting past the mouth 50 of the cannula 24, it can easily be pulled back into the cannula. That is, the elimination of a perpendicular edge on the right (proximal) end of the guide sleeve 100 avoids potentially catching the guide sleeve on the mouth 50. Movement of the strut mounting cylinder 86 to the right begins to pull the struts 42 into the cannula 24. Again, friction between guide sleeve 100 and deployment shaft 38 tends to pull the connecting shaft 96 and blunt tip 22 to the right (in a proximal direction) as well. To prevent the blunt tip 22 from interfering with the collapsing struts 42, the control button 36 prevents the guide sleeve 100 from displacement to the right beyond the position shown in Figure 8A. More specifically, retraction shoulder 152 on the guide sleeve 100 abuts against the sleeve stop 122 on the control button 36. Prior to this engagement, the safety ramp 156 cams the sleeve stop 122 inward, the tip stop ultimately being biased outward against the neck 158. The interaction between the retraction shoulder 152 and sleeve stop 122 restrains the guide sleeve 100, although the deployment shaft 38 is still free to slide within the guide sleeve. Restraining the guide sleeve 100 in this manner in turn restraints movement of the blunt tip 22, by virtue of their interconnection via the connecting shaft 96. As seen in Figure 8A, therefore, the blunt tip 22 remains spaced from the mouth 50 to permit complete and unobstructed collapse of the struts 42 within the cannula 24. Figure 8A also shows the retraction ramp 142 on the lock collar 140 approaching the control housing 66. Eventually, the retraction ramp contacts the cam surface 120 provided on the left end of the control button 36 and forces the control button inward. This inward movement of the control button 36 disengages the sleeve stop 122 from the retraction shoulder 152, and releases the guide sleeve 100 for further movement to the right. The deployment shaft 38 and connecting shaft 96 end up in the positions shown in Figure 8B from continued movement of the deployment shaft to the right. Ultimately, the retraction ramp 142 contacts the step 132 with the guide sleeve 100 positioned within the relief chamber 134. The length between the mouth 50 and step 132 in conjunction with the length between the blunt tip 22 and guide sleeve 100 is such that the blunt tip 22 remains spaced from the mouth across the gap 54, previously described with respect to Figure 1E. The gap 54 is desirably between about 10-30 mm wide. Although the blunt tip 22 can be easily pressed against the mouth 50 because of the space within the relief chamber 134, as indicated with arrow 162 in Figure 8B, the gap 54 ensures that no tissue is pinched between the blunt tip 22 and the mouth 50 when the heart massager 20 is in the chest cavity. The massager 20 is thus removed from the chest cavity without tearing any tissue. Strut Properties Figure 9 is a detailed view of the distal portion of the heart massager 20 with one embodiment of the struts 42 shown in the fully retracted positions. Only two diametrically opposed struts 42 are illustrated in isolation from the rest of the heart contacting portion 34 for the sake of clarity. As described previously, each of the struts 42 has a distal or tip portion 80, a middle spring portion 82, and a base portion 84. Each of the struts 42 comprises a primary member 170 and a secondary member 172. The primary member 170 has a length extending between the strut flange 88 and the mouth 50 of the cannula 24, while the secondary member 172 is shorter and only extends from the strut flange to the approximate midpoint of the primary member. In this embodiment, the primary member 170 and secondary member 172 have narrow rectangular cross-sections and are juxtaposed side-by-side, contacting along opposed flat surfaces. Figure 9 illustrates an anchoring sleeve 174 surrounding the base portions 84 of the struts 42 and holding them firmly against the strut mounting cylinder 86. In addition, a connecting sleeve 176 is provided for each strut 42 around both the primary member 170 and secondary member 172 at the location where the secondary strut terminates. The connecting sleeve 176 can be a variety of materials, but is preferably a ring welded to the ends of the secondary members 172 (seen below in Figures 19A and 19B), or may also be a biocompatible heat- shrink polymer, such as FEP. As will be more clear from the description of the alternative struts below, the primary member 170 is permitted to slip with respect to the secondary member 172 along their contact surfaces, except at the base portion 84 where the secondary member 172 is axially retained but allowed to flex outward. Looking at the deployed and retracted configurations of the struts 42 seen in Figures 5A and 9, respectively, the bending stress imposed in the retracted configuration is apparent. That is, the curvilinear deployed struts 42 seen in Figure 5A are in their relaxed, un-stressed shapes. In contrast, as the struts 42 are retracted within the cannula 24 and straightened out, internal stresses are generated because of the extreme relaxed curvature, especially at the tips 80. The struts of the present invention are designed so that these stresses imposed in the retracted, or straightened, configuration do not exceed the yield stress of the material used. Because the stresses are below the yield point, the struts 42 reliably deploy in their intended shapes, and the possibility of fatigue failure is substantially eliminated. Thus, the parts of the structure subject to the highest stress are constructed of one or more thinner members to minimize the stress on the individual layers. When more than one layer of material are laminated, the cladding serves to hold them together as a single unit. At the same time, however, the layers are permitted to slide relative to each other within the cladding. Alternative Strut Configurations Figures 10-16 illustrate a number of alternative strut configurations of the present invention that combine the desirable attributes of being highly curled for reduced trauma to the patient, stiff enough to perform adequate heart massage, and also configured so that the yield stress of the material is not exceeded when they are straightened out. As was seen above in Figure 9, each of the struts combines a number of strut members together to provide, in conjunction, these advantageous features. Although a number of variations are shown, still others are contemplated and within the scope of the present invention. Figures 10 and 11 illustrate an exemplary embodiment of strut 200 comprising a plurality of individual strut members having progressively different lengths and juxtaposed against one another. More specifically, the strut 200 includes a pair of primary strut members 202 extending the full length of the strut, and a series of progressively shorter secondary members 204a-d that are stacked in series against the primary strut members. The strut 200 can be functionally divided into a curled tip portion 206, a middle portion 208, and a base portion 210. Again, the base portion 210 attaches to a distal end of the deployment shaft 212, only partially shown in Figure 10. Although not shown, a connecting sleeve may be provided around part or all of the length of the primary and secondary members 202, 204, to hold them against one another but still permit slip therebetween. The secondary strut members 204 are sequentially stacked to the inside of the curvature of the strut 200, and gradually shorten in length in this direction. In particular, the first secondary strut member 204a lies against the primary strut member 202, terminates just short of the curled tip portion 206 and is the longest of the secondary strut members. Secondary strut member 204b is juxtaposed against and shorter than the first member 204a, and so on. As illustrated, the ends of each of the secondary strut members 204 are evenly spaced apart to provide a uniformly stepped configuration. In addition, each of the secondary strut members 204 are shown having approximately the same thickness, or cross-sectional size. This arrangement results in a spring portion 208 that is stiffest toward the base portion 210 and becomes gradually more flexible toward the curled tip portion 206. In other words, the spring stiffness decreases in a linear step function from the base portion 210 toward the tip portion 206. In use, the struts 200 will be pressed against the heart and tend to spread apart further, or bend to the left as seen in Figures 10 and 11. The spring portion 208 must be sufficiently stiff to perform the massaging function, but the tip portion 206 has to be extremely flexible to be curled into the non-traumatic orientation shown and then be straightened out without exceeding the yield stress of the material. The strut 200 provides these advantages by virtue of the stepped configuration of the strut members. Those of skill in the art will understand that varying the cross-sectional size and/or shape of any of the strut members, or their respective lengths, may be desirable to change the spring characteristics of the strut 200. As was mentioned above with respect to Figure 5A, the extreme curl of the tip portion 206 forms the toroidal or pseudo-toroidal periphery 60 that insures the heart massager 20 will not damage or catch on any surrounding anatomical structures, especially the heart. Because the cross-sectional size of the strut 200 is reduced to a minimum at the tip portion 206, the area moment of inertia is likewise reduced. Indeed, the strut 200 is designed to have an area moment of inertia in the tip portion 206 that ensures that the yield stress is not exceeded when the tip portion is flattened or straightened out. Likewise, each of the secondary strut members 204a-d are permit to slide with respect one another, and with respect to the primary strut members 202, so as not to superimpose each of their area moments of inertia in bending. In other words, the stress in each of the primary and secondary strut members is independent of the stress in the others. Figure 12 illustrate an alternative strut 220 having a relatively large proximal member 222 and a plurality of smaller distal members 224. One of the distal members 224 in shown positioned on the inside of the curve of the proximal member 222, while the other two are shown to the outside. Again, the strut 220 has a curled tip portion 226, a middle portion 228, and a base portion (not shown). In this embodiment, the middle portion 228 has a constant stiffness in bending along the proximal member 222, and has increased flexible in the region of the distal members 224. Although not shown, a connecting sleeve may be provided around the three distal members 224, to hold them against one another but still permit slip therebetween. Figure 13 illustrates a still further strut 230 having a pair of primary strut members 232 and a single secondary member 234 which is larger in cross-section than the primary members. The secondary member 234 is provided to the inside of the curve of the strut 230, while the primary members 232 extend the full length of the strut to the outside of the curve. The primary members thus define a curled tip portion 236, while the combination of the primary and secondary members define a spring portion 238. Figures 14-16 illustrate, respectively, alternative struts 250, 260, and 270, all of which comprise multiple members that separate in a mid portion of the strut. The strut 250 of Figure 14 includes a single primary member 252 and a single support or reinforcing member 254 disposed to the inside of curve of the primary member. The primary member 252 has a generally constant cross-section in a proximal region, but tapers down to a narrower cross-section in a tip portion 256. The strut 260 of Figure 15 comprises a single primary spring member 262 and a plurality of reinforcing members 264. One of the reinforcing members extends the full length of the strut 260 and defines the curled tip portion 266. The reinforcing members 264 are stacked to the inside of the curve of the primary spring member 262 and have varying lengths. Finally, the strut 270 in Figure 16includes a plurality of strut members having approximately the same cross-sectional size, one of which extends the entire length of the strut to define the fip portion 272. The remaining strut members terminate at different locations to the inside of the curve of the strut. In all of the configurations shown in Figures 14-16, the struts are designed to have sufficient stiffness to perform the heart massaging function, but also have a minimum area moment of inertia at their tips to avoid exceeding the yield stress from straightening out the highly curled, non-traumatic tip portion. Strut Materials The struts described herein can be made from a variety of biocompatible materials that provide the required strength and elastic properties. Preferably, the struts are made from a Nickel-Titanium (e.g., Nitinol) alloy having super-elastic properties in a region of the stress-strain curve below the transition from an austenitic phase to a martensitic phase. That is, certain Nickel-Titanium alloys exhibit a stress-induced martensitic (SIM) phase change at predetermined stress levels. This allows additional strain before yield occurs. The present invention provides struts that remain below the stress-induced martensite point of the material, such as the SIM point if SIM alloys such as nickel-titanium alloys are used. Other materials that can be used include stainless steel, Elgiloy, and even certain composites. Alternatively, the at least some of the struts described herein can be formed from a shape memory alloy exhibiting a Martensitic to Austenitic phase transition at a predetermined temperature, for instance a temperature less than or equal to 32° C. Therefore, the struts exhibit one shape at a normal ambient room temperature of around 25 °C and undergo a Martensitic to Austenitic phase transition and attendant shape change when inserted into a body cavity having a temperature of around 37°C. A further embodiment of a strut 280 is seen in Figures 17A and 17B. The strut 280 comprises a primary member 282 that extends substantially the length of the strut to a distal tip 284, and a secondary member 286 that extends on both sides of the primary member. More specifically, secondary member 286 extends along one face of the primary member 282 from a proximal end 288 of the strut 280 to the distal tip 284, wraps around in a closed loop 290, and continues adjacent the opposite face of the primary member back to the proximal end 288. The primary member 282 terminates short of the distal tip 284. The closed loop is seen in greater detail in figure 17B. Additionally, an embodiment of the cardiac massage device 294 of the present invention incorporating the closed loop struts 280 is seen in Figure 18. Figure 17A also illustrates a reinforcing member 300 that is disposed on the inside curve of the closed loop strut 280. In accordance with the explanation above, the reinforcing member 300 facilitates bending of the strut 280 and helps maintain stresses in the strut below the yield port of the material. A preferred connection between a reinforcing member 310 and a primary, multi-layer strut 312 of the present invention is seen in Figures 19A and 19B. The reinforcing member 310 is welded at either end to small rings or bands 314a, 314b disposed around the strut 312. The band 314a on the left or proximal end of the reinforcing member 310 is fixed axially with respect to the strut 312. The band 314b on the right or distal end of the reinforcing member 310, on the other hand, is permitted to slide with respect to the strut 312. Therefore, as seen in Figure 19B, when the strut 312 is deployed, the band 314b is permitted to slide with respect thereto. At the same time, the reinforcing member 310 flexes with respect to the curvature of strut 312 such that a space 316 therebetween appears. While a particular embodiment of the invention has been described above, for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A medical device incorporating a spring that is alternately disposable in a stowed configuration and a deployed configuration, wherein the spring comprises: a plurality of elongate members each having a longitudinal axis, a bending strength and a yield stress, said elongate members being arranged in substantial juxtaposition and substantially parallel to one another; said spring having a spring bending strength to which the bending strengths of each of the elongate members contribute; said spring having a relaxed state and a stressed state such that when in one of said relaxed or stressed states the spring is in stowed configuration and when in the other of its relaxed or stressed states the spring is in its deployed configuration, and wherein when the spring is in its stressed state none of the elongate members are stressed beyond their respective yield stresses. 2. The device of claim 1 , wherein the stowed configuration corresponds to the stressed state of the spring. 3. The device of claim 2, wherein the spring is arcuate in the relaxed state and substantially straight in the stressed state. 4. The device of claim 3, further including a tubular cannula within which the spring is linearly displaceable, the cannula having an open end, wherein the spring extends from the open end in the relaxed state/deployed configuration and is located substantially within the cannula in the stressed state/stowed configuration. 5. The device of claim 4, wherein there are a plurality of the springs arranged in a circle so as to form a trumpet-like shape extending from the open mouth of the cannula in the relaxed state/deployed configuration. 6. The device of claim 5, further including a flexible, biocompatible sheet connecting the springs in the relaxed state/deployed configuration, the sheet being disposed generally perpendicular to the axis of the tubular cannula. 7. The device of Claim 4 further comprising: a blunt tip member that is initially positioned to cover the open mouth of the tubular cannula to facilitate advancement of the tubular cannula through an access incision in a patient. 8. The device of Claim 7 wherein there are a plurality of the springs, and further including a flexible, biocompatible sheet connecting the springs in the relaxed state/deployed configuration, the sheet being disposed generally perpendicular to the axis of the tubular cannula, and wherein the blunt tip member is mechanically restrained so as not to contact the springs and sheet during transition of the springs from the relaxed state/deployed configuration to the stressed state/stowed configuration. 9. The device of claim 5, wherein each spring has a tip extending from the cannula open mouth that, in the relaxed state/deployed configuration, forms an angle of greater than 90 degrees with respect to a ray extending out the open mouth of the tubular cannula and along its axis. 10. The device of claim 9, wherein the tip of each spring forms an angle of > 180 degrees with respect to a ray extending out the open mouth of the tubular cannula and along its axis. 11. The device of claim 1 , wherein the spring defines a base on one end and a tip on the other end, and wherein the tip is more flexible than the base. 12. The device of claim 11 , wherein the elongate members each define a first end in the direction of the spring base and a second end in the direction of the spring tip, wherein the second end of least one elongate member does not extend to the spring tip. 13. The device of claim 12, wherein there are more than two of the elongate members. 14. The device of claim 13, wherein none of the second ends of the elongate members is coincident with another of the second ends. 15. The device of claim 14, wherein the second ends are located approximately the same distance apart along the spring so that the spring becomes more flexible in stages in the direction of its tip. 16. The device of claim 12, wherein the elongate members are generally flat in cross-section with opposing faces substantially greater in dimension than opposing edges, and wherein the members are juxtaposed with respective faces in contact to form a laminated structure. 17. The device of claim 1 , wherein the elongate members are generally flat in cross-section with opposing faces substantially greater in dimension than opposing edges, and wherein the members are juxtaposed with respective faces in contact to form a laminated structure. 18. The device of claim 17, wherein the spring is arcuate in the relaxed state and substantially straight in the stressed state. 19. The device of claim 18, wherein the spring defines a base on one end and a tip on the other end, and the elongate members each define a first end in the direction of the spring base and a second end in the direction of the spring tip, wherein an elongate member located on the inside of the curve of the relaxed arcuate spring and has a second end that does not extend to the spring tip so that the tip is more flexible than the base. 20. The device ofclaim 17, wherein at least one ofthe elongate members has a larger area moment of inertia than another of the elongate members. 21. The device of claim 20, wherein the spring is arcuate in the relaxed state and substantially straight in the stressed state, and wherein the spring defines a base on one end and a tip on the other end, and the elongate members each define a first end in the direction of the spring base and a second end in the direction of the spring tip, and wherein the at least one of the elongate members having a larger area moment of inertia than another ofthe elongate members does not extend to the spring tip so that the tip is more flexible than the base. 22. The device ofclaim 17, wherein the spring defines a base on one end and a tip on the other end and is arcuate in the relaxed state and substantially straight in the stressed state, and wherein each spring tip, in the relaxed state, forms an angle of > 90 degrees with respect to the base. 23. The device of claim 22, wherein the tip of each spring forms an angle of > 180 degrees with respect to the base. 24. The device of claim 1 , wherein the elongate members are made of a material that is capable of exhibiting a stress-induced martensitic phase. 25. The device of claim 24, wherein the elongate members are made of nickel-titanium alloy. 26. The device of claim 1 , wherein the elongate members are made of a material exhibiting a plastically-deforming behavior, and wherein the yield stress corresponds to the commencement of plastic deformation of the material. 27. A device according to Claim 1 that is useable for performing minimally invasive direct cardiac massage, said device comprising: a handle having a proximal end and a distal end; and, a plurality of said springs attached to and extending from the distal end of said handle; said springs being a) initially disposed in their stowed configurations such that the plurality of said springs is sufficiently compact to allow at least the distal portions of said springs to be inserted into the mediastinum of a mammalian patient through a minimal access incision that is less than one inch in length and b) subsequently moveable to their deployed configurations such that the springs will radially expand within the mediastinum and distal portions of the springs will directly engage the patient's heart such that pressure will be exerted upon the heart when the handle of the device is advanced in the distal direction. 28. The device of Claim 27 wherein the elongate members that comprise each spring are formed of a metal alloy that is superelastic and capable of exhibiting stress-induced martensite, and wherein the springs may be moved back and forth between their stowed and deployed configurations without exhibiting stress-induced martensite. 29. The device of Claim 28 wherein the elongate members are formed of nickel-titanium alloy. 30. The device of Claim 28 wherein a single one of the elongate members that comprise a given spring protrudes distally beyond the remaining elongate members that comprise that spring to form a flexible distal portion on that spring. 31. The device of Claim 28 wherein the distally protruding elongate member is curled. 32. The device of Claim 28 wherein the distally protruding elongate member is looped. 33. The device of Claim 27 wherein at least some of the elongate members are formed from a shape memory alloy exhibiting a Martensitic to Austenitic Phase transition at a predetermined temperature. 34. The device of Claim 33 wherein the predetermined temperature is less than or equal to 32° C. 35. The device of Claim 27 wherein each of said springs comprises a) a main strut having a first radius of curvature when in its deployed configuration and unconstrained and b) a sub-strut having a second radius of curvature when in its deployed configuration and unconstrained, the ends of said sub-strut being attached to the main strut. 36. The device of Claim 35 wherein: the main strut comprises said plurality of elongate members each having a longitudinal axis, a bending strength and a yield stress, said elongate members being arranged in substantial juxtaposition and substantially parallel to one another; and, the sub-strut comprises a single elongate member attached at either of its ends to the main strut. 37. The device of Claim 35 wherein: the main strut comprises a plurality of elongate members each having a longitudinal axis, a bending strength and a yield stress, said elongate members being arranged in substantial juxtaposition and substantially parallel to one another; and, the sub-strut also comprises a plurality of single elongate members each having a longitudinal axis, a bending strength and a yield stress, said elongate members being arranged in substantial juxtaposition and substantially parallel to one another, at least one of the elongate members of said sub-strut being attached to the main-strut at either end of the sub- strut. 38. The device of Claim 35 wherein: the main strut comprises a single elongate member; and, the sub-strut also comprises a plurality of single elongate members each having a longitudinal axis, a bending strength and a yield stress, said elongate members being arranged in substantial juxtaposition and substantially parallel to one another, at least on of the elongate members of said sub-strut being attached to the main-strut at either end of the sub-strut. 39. The device of Claim 27 wherein: said handle comprises a tube within which the springs are positioned while in their stowed configurations, said tube having a distal end that is insertable into the thoracic cavity of the patient through the incision that is less than one inch in length. 40. The device of Claim 39 wherein: a proximal portion of the handle is relatively displaceable toward the tube after the tube distal end has been inserted into the mediastinum of the patient so as to allow the springs to radially expand within the patient's mediastinum. 41. The device of Claim 39 wherein said tube has an open mouth at the distal end, and further comprising: a blunt tip member that is initially positioned to cover the open mouth ofthe tube to facilitate advancement ofthe tube through the minimal access incision. 42. The device delivery cannula of claim 41 , further including: a connecting shaft extending proximally within the tube and rigidly attached to the blunt tip member, the connecting shaft being adapted to slide longitudinally within the tube; a deployment mechanism located on a proximal end ofthe tube and including structure operable to deploy and retract the springs and limit retraction ofthe connecting shaft with respect to the springs during retraction of the springs; and wherein the proximal portion of the handle is directly coupled to the springs to enable 1 :1 displacement of the proximal portion with respect to the springs. 43. The device delivery cannula of claim 42, wherein the deployment mechanism comprises: a control housing rigidly attached to the tube, and wherein the proximal portion of the handle includes a tubular shaft sized to slide within the control housing, and the connecting shaft is sized to slide within the tubular shaft, the tubular shaft further including a longitudinal slot and a guide sleeve slidably disposed about the tubular shaft, the guide sleeve being rigidly attached to the connecting shaft via the longitudinal slot and having outer surface features sized and shaped to interact with inner surface features ofthe control housing to limit retraction ofthe connecting shaft with respect to the springs during retraction of the springs. 44. A n expandable medical device delivery cannula having a blunt insertion tip, comprising: an expandable medical device; a generally tubular cannula having a lumen sized to receive the expandable medical device therein in a contracted state, the cannula lumen having an open distal end through which the medical device can be deployed into an expanded state and then subsequently retracted within the cannula lumen into the contracted state; a blunt insertion tip adapted to cover the open distal end of the cannula lumen during insertion of the cannula into a body cavity; a connecting shaft extending proximally within the cannula lumen and rigidly attached to the insertion tip, the connecting shaft being adapted to slide longitudinally within the cannula lumen; and a deployment mechanism located on a proximal end of the tubular cannula and including structure operable to deploy and retract the expandable medical device and limit retraction of the connecting shaft with respect to the medical device during retraction of the medical device. 45. The device delivery cannula of claim 44, wherein the deployment mechanism comprises a handle longitudinally displaceable with respect to the tubular cannula and directly coupled to the expandable medical device to enable 1 :1 displacement of the handle with respect to the medical device. 46. The device delivery cannula of claim 45, wherein the handle is frictionally coupled to a proximal end of the connecting shaft to enable 1 :1 displacement of the handle with respect to the connecting shaft unless displacement of the connecting shaft is limited by structure other than the handle that overcomes the frictional coupling between the handle and the connecting shaft. 47. The device delivery cannula of claim 46, wherein the structure other than the handle is fixed with respect to the tubular cannula. 48. The device delivery cannula of claim 45, further including a control housing rigidly attached to the tubular cannula, and wherein the handle includes a tubular shaft sized to slide within the control housing, and the connecting shaft is sized to slide within the tubular shaft, the tubular shaft further including a longitudinal slot and a guide sleeve slidably disposed about the tubular shaft, the guide sleeve being rigidly attached to the connecting shaft via the longitudinal slot and having outer surface features sized and shaped to interact with inner surface features of the control housing to limit retraction of the connecting shaft with respect to the medical device during retraction of the medical device. 49. The device delivery cannula of claim 44, wherein the expandable medical device comprises a plurality of spring struts arranged generally in a circle and surrounding a space, ard wherein the connecting shaft extends through the space. 50. The device delivery cannula of claim 49, wherein the expandable medical device is a cardiac massager and further comprises a flexible sheet connecting the spring struts, the sheet being disposed generally perpendicular to the axis of the tubular cannula when the expandable medical device is in the expanded state, and wherein the connecting shaft extends through an aperture smaller than the blunt insertion tip provided in the sheet such that the blunt insertion tip is maintained on the distal side of the sheet. 51. A medical device including a spring that is alternately in a deployed configuration and a stowed configuration, comprising: an elongate spring comprising a plurality of biocompatible elongate members each having a longitudinal axis and being generally flat in cross- section with opposing faces substantially greater in dimension than opposing edges, and wherein the members are juxtaposed with respective faces in contact to form a laminated structure, the spring having a combined bending strength to which the bending strengths of each elongate member contributes, the spring having a relaxed state and a stressed state, wherein the stowed configuration corresponds to either the relaxed state or the stressed state of the spring and the deployed configuration corresponds to the other of the relaxed state or the stressed state of the spring. 52. The device of claim 51 , wherein none of the members are stressed beyond its respective yield stress when the spring is in its stressed state.

Inventors: BRENNEMAN RODNEY A, KOLEHMALNEN DONALD, LENKER JAY A, TRAN MINH, BRENNEMAN, RODNEY, A., KOLEHMALNEN, DONALD, LENKER, JAY, A., TRAN, MINH
Applicants: THERACARDIA INC, THERACARDIA, INC.
Application year: 2000
Main classification: A61H
IPC codes: A61B17, A61H31