Patent Publication Number: US-2018049877-A1

Title: Sizing device and method of positioning a prosthetic heart valve

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/788,631, filed Mar. 7, 2013, which claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/713,171, filed Oct. 12, 2012, the disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to heart valve replacement and, in particular, to collapsible prosthetic heart valves. More particularly, the present invention relates to devices and methods for positioning and sizing collapsible prosthetic heart valves. 
     Prosthetic heart valves that are collapsible to a relatively small circumferential size can be delivered into a patient less invasively than valves that are not collapsible. For example, a collapsible valve may be delivered into a patient via a tube-like delivery apparatus such as a catheter, a trocar, a laparoscopic instrument, or the like. This collapsibility can avoid the need for a more invasive procedure such as full open-chest, open-heart surgery. 
     Collapsible prosthetic heart valves typically take the form of a valve structure mounted on a stent. There are two types of stents on which the valve structures are ordinarily mounted: a self-expanding stent and a balloon-expandable stent. To place such valves into a delivery apparatus and ultimately into a patient, the valve must first be collapsed or crimped to reduce its circumferential size. 
     When a collapsed prosthetic valve has reached the desired implant site in the patient (e.g., at or near the annulus of the patient&#39;s heart valve that is to be replaced by the prosthetic valve), the prosthetic valve can be deployed or released from the delivery apparatus and re-expanded to full operating size. For balloon-expandable valves, this generally involves releasing the entire valve, and then expanding a balloon positioned within the valve stent. For self-expanding valves, on the other hand, the stent automatically expands as the sheath covering the valve is withdrawn. 
     Despite the various improvements that have been made to the collapsible prosthetic heart valve delivery process, conventional delivery devices, systems, and methods suffer from some shortcomings. For example, in conventional delivery devices for self-expanding valves, the clinical success of the valve is dependent on accurate deployment and anchoring, and on acceptable valve performance both acutely and chronically. Inaccurate sizing and positioning increases risks, such as valve migration, which may result in severe complications due to obstruction of the left ventricular outflow tract and may even result in patient death. Additionally, calcification of the aortic valve may affect performance. Specifically, the degree of calcification has been suggested to play a role in anchoring transcathether implants. The interaction between the implanted valve and the calcified tissue of the aortic valve is believed to be relevant to anchoring the valve in place and preventing valve migration. 
     Without being bound to any particular theory, it is believed that improper anchoring of the valve may occur due to a mismatch between the size of the native annulus and the size of the prosthetic valve (e.g., using a small size valve in a large annulus), lower calcification levels in the native tissue than actually predicted, or improper positioning of the valve resulting in insufficient expansion of the valve diameter. Thus, methods and devices are desirable that would reduce the likelihood of valve migration caused by improper anchoring. In addition, incorrect sizing of a valve due to anatomical variations between patients may require removal of a fully deployed heart valve from the patient if it appears that the valve is not functioning properly. Removing a fully deployed heart valve increases the length of the procedure and increases the risk of infection and/or damage to heart tissue. 
     There therefore is a need for further improvements in the devices, systems, and methods for transcatheter delivery and positioning of collapsible prosthetic heart valves. Specifically, there is a need for further improvements in the devices, systems, and methods for accurately measuring the native annulus dimensions and calcification levels in a patient. Such accurate measurement will help to reduce the risks associated with valve migration and improper valve positioning. Among other advantages, the present invention may address one or more of these needs. 
     SUMMARY OF THE INVENTION 
     In some embodiments, a sizing device for use in implanting a collapsible prosthetic heart valve in a native valve annulus includes a collapsible and expandable stent having an annulus section and an aortic section and a sensor coupled to the annulus section of the stent, the sensor being capable of collecting information related to the native valve annulus. 
     In some examples, the stent may be self-expandable. The stent may include nitinol and the sensor may be flexible. The information may include the diameter of the native valve annulus. The information may include data relating to the extent of calcification of tissue of the native valve annulus. The sensor may include at least one capacitor having variable capacitance, the capacitance corresponding to the information. The sensor may include at least one piezoelectric material. The sensor may include a polymer, polymide, fabric or polydimethylsiloxane. The sensor may be a microelectromechanical sensor and may include at least two electrodes mounted on a fabric. The sizing device may further include deployment device configured to expand the collapsible and expandable stent via a series of rotations. 
     In some embodiments, a method for determining the proper fitment of a prosthetic heart valve within a native valve annulus includes (i) introducing a sizing device into the native valve annulus, the sizing device including (i) a collapsible and expandable stent having an annulus section and an aortic section and (ii) a sensor coupled to the annulus section of the stent, the sensor being capable of collecting information related to the native valve annulus, (ii) expanding the diameter of the stent within the native valve annulus and (iii) acquiring information related to the native valve annulus via the sensor. 
     In some examples, the information may include the diameter of the native valve annulus or data relating to an extent of calcification of tissue of the native valve annulus. The step of expanding the diameter of the stent may include rotating a first portion of a deployment device relative to a second portion of the deployment device within the native valve annulus. The stent may be self-expandable and the sizing device may further include a removable cannula disposed about the stent to maintain the stent in a collapsed configuration, and the step of expanding the diameter of the stent may include removing the cannula from around the stent. 
     In some examples, the method may further include expanding the diameter of the stent in-vitro to establish a relationship between the number of rotations of the first portion of the deployment device relative to the second portion of the deployment device and a diameter of the stent. The step of acquiring information related to the native valve annulus may include comparing the number of rotations within the native valve annulus to the relationship. The expanding step may include expanding the diameter of the stent within the native valve annulus until the sensor measures a radial force of predetermined value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention are disclosed herein with reference to the drawings, wherein: 
         FIG. 1  is a side elevational view of a conventional prosthetic heart valve; 
         FIG. 2A  is a side elevational view of a prosthetic heart valve having poor fitment; 
         FIG. 2B  is a side elevational view of a prosthetic heart valve that has improperly migrated; 
         FIG. 3  is a side elevational view of a self-expandable nitinol stent having a microelectromechanical sensor according to one embodiment of the present invention; 
         FIG. 4A  is a schematic view illustrating the principles of operation of a single microelectromechanical sensor; 
         FIG. 4B  is a schematic view illustrating the principles of operation of multiple sensors; 
         FIG. 5A  is a top plan view of a microelectromechanical sensor array in accordance with an embodiment of the present invention; 
         FIG. 5B  is a close-up of a sensor structure of  FIG. 5A  with separated layers in accordance with an embodiment of the present invention; 
         FIG. 5C  is a schematic view illustrating the principles of operation of a microelectromechanical sensor; 
         FIGS. 5D and 5E  are schematic views illustrating a microelectromechanical sensor formed of a capacitative pair; 
         FIG. 6A  is a side elevational view of a sizing device having a microelectromechanical sensor coupled to an inner deployment device; 
         FIG. 6B  is a side elevational view of a sizing device having a microelectromechanical sensor coupled to an outer deployment device; and 
         FIG. 7  is a pair of graphs showing the use of data from a microelectromechanical sensor in estimating annulus diameter and calcification levels. 
     
    
    
     Various embodiments of the present invention will now be described with reference to the appended drawings. It is to be appreciated that these drawings depict only some embodiments of the invention and are therefore not to be considered limiting of its scope. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, the term “proximal,” when used in connection with a prosthetic heart valve, refers to the end of the heart valve closest to the heart when the heart valve is implanted in a patient, whereas the term “distal,” when used in connection with a prosthetic heart valve, refers to the end of the heart valve farthest from the heart when the heart valve is implanted in a patient. 
       FIG. 1  shows a collapsible prosthetic heart valve  100  according to an embodiment of the present disclosure. The prosthetic heart valve  100  is designed to replace the function of a native aortic valve of a patient. Examples of collapsible prosthetic heart valves are described in International Patent Application Publication No. WO/2009/042196; U.S. Pat. No. 7,018,406; and U.S. Pat. No. 7,329,278, the disclosures of all of which are hereby incorporated herein by reference. As discussed in detail below, the prosthetic heart valve has an expanded condition and a collapsed condition. Although the invention is described herein as applied to a prosthetic heart valve for replacing a native aortic valve, the invention is not so limited, and may be applied to prosthetic valves for replacing other types of cardiac valves. 
     The prosthetic heart valve  100  includes a stent or frame  102 , which may be wholly or partly formed of any biocompatible material, such as metals, synthetic polymers, or biopolymers capable of functioning as a stent. Suitable biopolymers include, but are not limited to, elastin, and mixtures or composites thereof. Suitable metals include, but are not limited to, cobalt, titanium, nickel, chromium, stainless steel, and alloys thereof, including nitinol. Suitable synthetic polymers for use as a stent include, but are not limited to, thermoplastics, such as polyolefins, polyesters, polyamides, polysulfones, acrylics, polyacrylonitriles, polyetheretherketone (PEEK), and polyaramides. The stent  102  may have an annulus section  110 , an aortic section (not shown) and a transition section (not shown) disposed between the annulus section and the aortic section. Each of the annulus section  110 , the aortic section and the transition section of the stent  102  includes a plurality of cells  112  connected to one another around the stent. The annulus section  110  and the aortic section of the stent  102  may include one or more annular rows of cells  112  connected to one another. For instance, the annulus section  110  may have two annular rows of cells  112 . When the prosthetic heart valve  100  is in the expanded condition, each cell  112  may be substantially diamond shaped. Regardless of its shape, each cell  112  is formed by a plurality of struts  114 . For example, a cell  112  may be formed by four struts  114 . 
     The stent  102  may include commissure features  116  connecting at least two cells  112  in the longitudinal direction of the stent  102 . The commissure features  116  may include eyelets for facilitating the suturing of a valve assembly  104  to the sent  102 . 
     The prosthetic heart valve  100  also includes a valve assembly  104  attached inside the annulus section  110  of the stent  102 . U.S. Patent Application Publication No. 2008/0228264, filed Mar. 12, 2007, and United States Patent Application Publication No. 2008/0147179, filed Dec. 19, 2007, the entire disclosures of both of which are hereby incorporated herein by reference, describe suitable valve assemblies. The valve assembly  104  may be wholly or partly formed of any suitable biological material or polymer. Examples of biological materials suitable for the valve assembly  104  include, but are not limited to, porcine or bovine pericardial tissue. Examples of polymers suitable for the valve assembly  104  include, but are not limited to, polyurethane and polyester. 
     The valve assembly  104  may include a cuff  106  disposed on the lumenal surface of annulus section  110 , on the ablumenal surface of annulus section  110 , or on both surfaces, and the cuff may cover all or part of either or both of the lumenal and ablumenal surfaces of the annulus section. The cuff  106  and/or the sutures used to attach the valve assembly  104  to stent  102  may be formed from or include ultra-high-molecular-weight polyethylene.  FIG. 1  shows cuff  106  disposed on the lumenal surface of annulus section  110  so as to cover part of the annulus section while leaving another part thereof uncovered. The valve assembly  104  may further include a plurality of leaflets  108  which collectively function as a one-way valve. A first edge  122  of each leaflet  108  may be attached to the cuff  106  or the stent  102  by any suitable attachment means, such as suturing, stapling, adhesives or the like. For example, the first edge  122  of each leaflet  108  may be bonded to the cuff  106 , and the cuff may in turn be bonded to the stent  102 . Alternatively, the first edge  122  of each leaflet  108  may be sutured to the stent  102  by passing strings or sutures through the cuff  106  of the valve assembly  104 . A second or free edge  124  of each leaflet  108  may coapt with the corresponding free edges of the other leaflets, thereby enabling the leaflets to function collectively as a one-way valve. 
     Irrespective of the attachment means employed, the leaflets  108  may be attached to the cuff  106  or to the stent  102  along at least some struts  114  of the stent to enhance the structural integrity of the valve assembly  104 . As a consequence of this attachment, the struts  114  help support the leaflets  108  of the valve assembly  104  and may therefore reduce the strain in the leaflet-cuff junction. 
     The leaflets  108  may be attached directly to and supported by certain struts  114 , such as by suturing. In such event, the cuff  106  may perform little or no supportive function for the leaflets  108 . Hence, the cuff  106  may not be subjected to high stresses and is therefore less likely to fail during use. In light of this, the thickness of the cuff may be reduced. Reducing the thickness of the cuff  106  results in a decrease in the volume of the valve assembly  104  in the collapsed condition. This decreased volume is desirable as it enables the prosthetic heart valve  100  to be implanted in a patient using a delivery device that is smaller in cross-section than conventional delivery devices. In addition, since the material forming the stent struts  114  is stronger than the material forming the cuff  106 , the stent struts  114  may perform the supportive function for the leaflets  108  better than the cuff  106 . 
     The volume of the valve assembly  104  may be further reduced by having the cuff  106  cover only a portion of the surface of annulus section  110 . With continued reference to  FIG. 1 , the first or proximal end of the cuff  106  may substantially follow the contour of the first or proximal end  119  of the stent  102 . As such, the proximal end of the cuff  106  may have a generally sinusoidal or zigzag shape. This eliminates any free edge of the cuff  106 , which otherwise might extend directly between the cusps of the cells  112  at the proximal end  119  of the stent  102 , and enables the entire length of the proximal end  118  of the cuff  106  to be secured to the stent  102 . The second or distal end  120  of the cuff  106 , on the other hand, may be disposed substantially along at least some struts  114 , but not necessarily the struts in a single annular row of cells  112 . More particularly, the distal end  120  of the cuff  106  may follow the stent struts  114  up to the commissure features  116 , such that the cuff covers all of the cells  112  in the bottom annular row  113  of cells and in a second annular row  115  of cells located between the commissure features and the proximal end  119  of the stent  102 , but covers a lesser area of cells in the annular regions between the commissure features. In other words, the distal end  120  of the cuff  106  may be disposed substantially along struts  114   a,    114   b,    114   e,    114   f,    114   g  and  114   h,  as shown in  FIG. 1 . Strut  114   g  may be connected at one end to strut  114   h,  and at the other end to the intersection of struts  114   b  and  114   c.  Strut  114   h  may be connected at one end to strut  114   g,  and at the other end to the intersection of struts  114   d  and  114   e.  Struts  114   c,    114   d,    114   g,  and  114   h  collectively form a single cell  112 . 
     As a result of the foregoing configuration, all of the cells  112  in the bottom annular row  113  of cells may be entirely covered by the cuff  106 . The cuff  106  may also entirely cover those cells  112  in the second annular row  115  that are located directly below the commissure features  116 . All of the other cells  112  in the stent  102  may be open or not covered by the cuff  106 . Hence, there may be no cells  112  which are only partially covered by the cuff  106 . 
     Since the edges of the valve leaflets  108  extend up to the second annular row  115  of cells  112  only in the regions of the commissure features  116 , there is little to no likelihood of leakage in the area of the cells between the commissure features in the second annular row of cells, and therefore no need for the cuff  106  to cover this area. This reduction in the area of the cuff  106 , both at the proximal end  118  and at the distal end  120  thereof, reduces the amount of material in the valve assembly  104 , thereby enabling the prosthetic valve  100  to achieve a smaller cross-section in the collapsed condition. 
     In operation, the embodiment of the prosthetic heart valve described above may be used to replace a native heart valve, such as the aortic valve. The prosthetic heart valve may be delivered to the desired site (e.g., near a native aortic annulus) using any suitable delivery device. Typically, during delivery, the prosthetic heart valve is disposed inside the delivery device in the collapsed condition. The delivery device may be introduced into a patient using a transfemoral, transapical, transseptal or other approach. Once the delivery device has reached the target site, the user may deploy the prosthetic heart valve. Upon deployment, the prosthetic heart valve expands into secure engagement within the native aortic annulus. When the prosthetic heart valve is properly positioned inside the heart, it works as a one-way valve, allowing blood to flow in one direction and preventing blood from flowing in the opposite direction. It will also be noted that while the inventions herein are predominantly described in terms of a tricuspid valve, the valve could be a bicuspid valve, such as the mitral valve, and the stent could have different shapes, such as a flared or conical annulus section, a less-bulbous aortic section, and the like, and a differently shaped transition section. 
     In certain procedures, collapsible valves may be implanted in a native valve annulus without first resecting the native valve leaflets. The collapsible valves may have critical clinical issues because of the nature of the stenotic leaflets that are left in place. Additionally, patients with uneven calcification, bi-cuspid aortic valve disease, and/or valve insufficiency could not be treated well, if at all, with the current collapsible designs. 
     The reliance on evenly calcified leaflets could lead to several problems such as: (1) perivalvular leakage (PV leak), (2) valve migration, (3) mitral valve impingement, (4) conduction system disruption, (5) coronary blockage, etc., all of which can have severely adverse clinical outcomes. To reduce these adverse events, the optimal valve would seal and anchor adequately without the need for excessive radial force, protrusion into the left ventricular outflow tract (LVOT), etc., that could harm nearby anatomy and physiology. 
       FIG. 2A  illustrates a prosthetic heart valve  200  positioned within the native valve annulus, the heart valve  200  having poor fitment. Specifically, as seen in  FIG. 2A , the annulus section  210  of the stent  202  is distorted at portion  295  due to improper fitment of the stent  202  within annulus  290 . Improper fitment of the prosthetic heart valve  200  may lead to improper valve function, as well as any of the problems discussed above. For example, as the stent  202  of a collapsible prosthetic heart valve  200  distorts during implantation, during beating of the heart, or because of irregularities in the patient&#39;s anatomy or the condition of the native valve, such distortion may be translated to the valve assembly  204 , such that not all of the valve leaflets  208  meet to form effective coaptation junctions. This can result in leakage or regurgitation and other inefficiencies which can reduce cardiac performance. Moreover, if the prosthetic valve  200  is not placed optimally and the valve leaflets  208  are not coapting as intended, other long term effects, such as uneven wear of the individual leaflets  208 , can be postulated. Such improper fitment may be due to poor positioning, disregard for calcification or due to use of the wrong valve size. 
     Poor positioning, disregard for calcification or the use of the wrong valve size may also cause heart valve migration. As seen in  FIG. 2B , prosthetic heart valve  200  has partially translated into the ventricle from its intended location within native valve annulus  290  as indicated by arrows “A”, a condition that may lead to a host of problems as discussed above. Even a small shift in position, such as that seen in  FIG. 2B , may cause inadequate sealing and improper valve function. Migration may also result in regurgitation of blood passing through the valve. 
     In order to avoid these problems, a valve sizing device may be used to accurately determine the annulus diameter and the calcification levels in the aortic valve. The valve sizing device may be first deployed within the native valve annulus to determine the shape and condition of the annulus. After obtaining sufficient measurements, the valve sizing device may be removed from the native valve annulus and a suitable prosthetic heart valve may be chosen based on the obtained measurements. The selected prosthetic heart valve may then be implanted, reducing the risk of deformation and/or migration. 
       FIG. 3  illustrates a valve sizing device  300  according to one embodiment of the present invention. The valve sizing device  300  includes a self-expandable stent  302  similar to stent  102  described above, and may be made from the same materials. The stent  302  may have an annulus section  310 , an aortic section  320 , and a transition section  315  disposed between the annulus section and the aortic section. Each of the annulus section  310 , the aortic section  320  and the transition section  315  of the stent  302  includes a plurality of cells  312  connected to one another around the stent. The annulus section  310  and the aortic section of the stent  302  may include one or more annular rows of cells  312  connected to one another. For example, the annulus section  310  may have two annular rows of cells  312 . When the sizing device  300  is in the expanded condition, each cell  312  may be substantially diamond shaped. Regardless of its shape, each cell  312  is formed by a plurality of struts  314 . A cell  312  may be formed by four struts  314 , for example. 
     As seen in  FIG. 3 , the valve sizing device  300  may further include a sensor  350  coupled to stent  302 . Sensor  350  may be a microelectromechanical sensor and may include, but is not limited to, sensors capable of measuring capacitance between two electrodes. In some examples, sensors  350  may include piezoelectric sensors, optical sensors, electromagnetic sensors, capacitive sensors and the like positioned around the stent to measure a force applied to the sensor by the native valve annulus. By way of example, a FLEXIFORCE® sensor made by TEKSCAN® may be used to measure force. 
     Sensor  350  may be embedded within stent  302  or coupled to struts  314  of stent  302  in any suitable manner For example, as seen in  FIG. 3 , sensor  350  may be coupled to struts  314  at various attachment points  355  around the perimeter of the stent. Thus, deformation of stent  302  also causes a corresponding deformation of sensor  350 , and the sensor is assumed to comply with the intravascular geometry. It will be understood that more than one sensor  350  may be coupled to stent  302 . For example, two or three sensors  350  may be evenly disposed about the circumference of stent  302 . The sensors  350  may be disposed on the periphery of stent  302  so that they are capable of being in direct contact with body tissue. 
     By inserting sizing device within a native valve annulus, the radial force against the sensors may be measured.  FIG. 4A  illustrates use of a force sensor according to this embodiment. Though  FIG. 4A  illustrates a sensor having a spring, this example is merely illustrative and it will be understood that the sensor may be any of those described above as well as other sensors known in the art. A sensor  350  may include a contacting member  502 , a spring  504  and a base layer  506 . Spring  504  may be connected to both the contacting member  502  and the base layer  506  and disposed between the two. The sensor  350  may be positioned near target tissue  500  and, as can be appreciated from  FIG. 4A , brought in contact with tissue  500 , with contacting member  502  abutting the tissue. As the sensor  350  is gradually advanced, spring  504  begins to compress. Knowing the spring constant kl of spring  504 , the force against contacting member  502  may be measured. 
     This measured radial force may be compared against valves in a lookup table or database that provides adequate radial force for valves of varying diameter. These values may be obtained by in vitro testing. In at least some examples, the table or database may also include information relating to blood pressure to adjust for variations in blood pressure. Specifically, patients with higher blood pressure (e.g., 200 mm Hg) may suggest the need for greater radial forces for adequate anchoring while patients with lower blood pressure (e.g., 100 mm Hg or less) may call for lower radial forces. 
     In a second embodiment, multiple sensors may be located near one another to acquire information relating to elasticity of the surrounding tissue.  FIG. 4B  shows the concept of using a sensor  350  to measure calcification of tissue by measuring the tissue elasticity. A sensor  350  may include a contacting member  502 , a spring  504  and a base layer  506 . A second sensor  350  may include a contacting member  502 ′, a spring  504 ′ and a base layer  506 ′. Each spring  504 , 504 ′ may be connected to its respective contacting member  502 , 502 ′ and base layer  506 , 506 ′ and disposed between the two. Moreover, sensors  350 , 350 ′ may be positioned near target tissue  500  and, as can be appreciated from  FIG. 4B , brought in contact with tissue  500 , with contacting members  502 , 502 ′ abutting the tissue. As the sensors  350 , 350 ′ are gradually advanced, springs  504  and  504 ′ begin to compress. 
     Springs  504  and  504 ′ may have different spring constants. As shown in  FIG. 4B , spring  504  has a spring constant of k 1  and spring  504 ′ has a spring constant of k 2 . Additionally, the stiffness of tissue  500  may be represented by a spring having a spring constant k T . By pushing contacting members  502 , 502 ′ against tissue  500 , the springs  504  and  504 ′ will have different amounts of deflection based on the different spring constants. Specifically, spring  504 ′ having a lower spring constant will suffer a greater deflection compared to its counterpart as shown in the figure on the right. The relative deflection of the springs may then be used to calculate the tissue stiffness represented by k 2 . This may then be used to analyze the extent of calcification of the tissue and, to decalcify the tissue to a suitable level and to choose the appropriate prosthetic heart valve for implanting in the patient. Thus, by examining the force exerted on springs  504  and  504 ′ and the displacement of both springs, the stiffness of tissue  500  may be determined. The stiffness of the tissue may then be used to select the appropriate valve or appropriate level of calcification needed as will be described in greater detail with reference to the algorithms and methods below. 
     In a third embodiment, microelectromechanical sensors may be used to measure the extent of calcification of a tissue. Details of these sensors will be fully discussed with reference to  FIGS. 5A-E . In this embodiment, sensor  350  may be a microelectromechanical sensor and may include, but is not limited to, sensors capable of measuring capacitance, piezoelectricity or any other suitable parameter. Sensor  350  may also include a flexible tactile microelectromechanical sensor. One example of such sensor is known in the art and described in “Flexible Tactile Sensor For Tissue Elasticity Measurements,” Journal of Microelectromechanical Systems, Vol.19, No.6, December 2009,the contents of which are hereby incorporated in its entirety as if fully recited herein. 
       FIGS. 5A and 5B  illustrate one possible configuration of a suitable microelectromechanical sensor  350 . Sensor  350  may be flexible and deformable in order to collect information about size, shape and calcification of the native aortic valve. In that regard, sensor  350  may be fashioned from fabric or flexible polymer layers such as polydimethylsiloxane (PDMS) or a polyimide having capacitors. 
     In one example, PDMS may be chosen as the structural material due to its advantageous properties such as flexibility, ductility, and biocompatibility. The biological and medical compatibility of the material has been well documented. Moreover, PDMS devices can be readily sterilized for medical applications. In addition, PDMS is mechanically much softer than other polymer materials commonly utilized in microfabrication. 
       FIG. 5A  illustrates a PDMS sensor array consisting of 5×5 capacitors  360 , the operation of which will be described in greater detail with reference to  FIGS. 5D and 5E . In order to minimize the wiring interfaces, the top and bottom electrodes may be oriented in orthogonal directions. 
     As seen in  FIG. 5A , the intersection of wires forms each capacitor  360 . A close-up of the sensor structure with separated layers is shown in  FIG. 5B . Embedded electrodes are built on a top PDMS layer  412  and a bottom PDMS layer  414 . A spacer layer  416  is sandwiched between the electrodes and defines air gaps  556 . An insulation layer  418  may also be used to prevent the shorting of electrodes which could be the consequence when large deflection of sensing diaphragms occurs. Finally, a bump layer  420  is utilized to transfer contact forces through the air gap to be measured by capacitive change. 
     In order to illustrate the principle of operation of the invention,  FIG. 5C  shows the concept of using a sensor  350  to measure calcification of tissue by measuring the tissue elasticity. A sensor  350  may include a contacting member  502 , a pair of springs  504  and  504 ′ and a base layer  506 . Springs  504  and  504 ′ may be connected to both the contacting member  502  and the base layer  506  and disposed between the two. The sensor  350  may be positioned near target tissue  500  and, as can be appreciated from  FIG. 5C , brought in contact with tissue  500 , with contacting member  502  abutting the tissue. As the sensor  350  is gradually advanced, springs  504  and  504 ′ begin to compress. 
     Springs  504  and  504 ′ may have different spring constants. As shown in  FIG. 5C , spring  504  has a spring constant of kh and spring  504 ′ has a spring constant of k s . Additionally, the stiffness of tissue  500  may be represented by a spring having a spring constant k T . By pushing contacting member  502  against tissue  500 , the springs  504  and  504 ′ will have different amounts of deflection based on the different spring constants. Specifically, spring  504 ′ having a lower spring constant will suffer a greater deflection compared to its counterpart as shown in the figure on the right. The relative deflection of the springs may then be used to calculate the tissue stiffness represented by k T . This may then be used to analyze the extent of calcification of the tissue and, to decalcify the tissue to a suitable level and to choose the appropriate prosthetic heart valve for implanting in the patient. Thus, by examining the force exerted on springs  504  and  504 ′ and the displacement of both springs, the stiffness of tissue  500  may be determined. 
     In one embodiment of implementing this concept, a capacitor pair for the sensors  350  may be used, as shown in  FIGS. 5D and 5E . As shown in these figures, capacitor  550  includes a first top electrode  552 , a first bottom electrode  554  and a first air gap  556  to form a first capacitor. A second capacitor is formed of a second top electrode  552 ′, a second bottom electrode  554 ′ and a second air gap  556 ′ disposed between the second top electrode and the second bottom electrode. As seen in  FIG. 5D , air gaps  556  and  556 ′ are formed of varying areas analogous to the different springs discussed above with reference to  FIG. 5C . When the sensor is contacted by tissue  500  as seen in  FIG. 5D , relative deflection may be precisely measured by the capacitive change of each element as shown in  FIG. 5E . The ratio of deflection (based on the capacitive change of each capacitor) may then be compared against valves in tables or graphs of known relationships between deflection change ratios and tissue stiffness to classify the tissue stiffness and determine the presence and degree of calcification. 
       FIG. 6A  is a side elevational view of a sizing device  300  having a microelectromechanical sensor  350 . A deployment device  610  for deploying sizing device  300  may be disposed inside the annulus section of the sizing device and may be coupled to the struts of the sizing device. Actuating the deployment device  610  may serve to gradually expand the sizing device  300 . For example, rotating a first portion of the deployment device  610  in a first direction relative to a second portion thereof may expand the sizing device  300 , while rotating the first portion of the deployment device relative to the second portion in a second direction, counter to the first, may collapse the sizing device  300 . 
       FIG. 6B  is a side elevational view of a sizing device  300  having a microelectromechanical sensor  350 , with the sizing device coupled to an outer deployment device  620 . In contrast to the “inner” deployment device  610  described above, the “outer” deployment device is disposed on the outside of the annulus section of the sizing device  300 , and may be coupled to the struts  314  thereof. Like inner deployment device  610 , outer deployment device  620  serves to gradually expand the sizing device  300 . This may be accomplished by rotating two portions of the delivery device  620  relative to one another, as with the delivery device  610 . Alternatively, outer deployment device  620  may be configured as a sheath that progressively exposes the sizing device  300 . In examples in which sizing device  300  includes a self-expandable stent  302 , as the sizing device is unsheathed from outer deployment device  620 , the stent is able to expand to its maximal diameter. 
       FIG. 7  shows the use of data from a microelectromechanical sensor  350  in estimating annulus diameter and calcification levels. The diameter of the annulus may be estimated using a three-step process. 
     The graph on the left illustrates the first step in this process. In the first step, the sizing device  300  is expanded in-vitro using a deployment device, such as one of the deployment devices described above with reference to  FIGS. 6A and 6B . Regardless of the deployment device used, it may include a rotating mechanism for gradually expanding the sizing device  300 . A plot of the number of rotations of the deployment device and the outer diameter of the sizing device  300  may be formed to illustrate the relationship between the two. For example, by examining the plot of  FIG. 7 , at number of rotations R A , the outer diameter is determined to be D A . 
     In a second step, the sizing device  300  may be collapsed and inserted into the patient body at the target size. Using the same deployment device of the first step, the sizing device  300  may be gradually expanded. As the device expands, measurements of the force against the sensor  350  may be collected and the stiffness of the tissue calculated. The user may stop expanding the sizing device  300  once the measured force is had reached a predetermined value. The calculated stiffness may then be plotted against the number of rotations of the deployment device. As seen in  FIG. 7 , a steep increase in stiffness to stiffness S A  appears at R A  rotations of the deployment device. This sudden increase in stiffness indicates to the user that the sensor  350  has been brought into contact with tissue  500 . 
     In a third step, the two graphs can be compared and the information may in turn be used to determine the appropriate size and/or shape of the prosthetic heart valve to be implanted. Specifically, the user may identify the number of rotations R A  at which stiffness increased and compare this to the in-vitro experiment. By identifying the same number of rotations R A  in the in-vitro step (the first graph), the corresponding outer diameter D A  of the sizing device  300  may be obtained and the appropriate size and shape of the prosthetic heart valve chosen. It will be understood that this technique of measurement and comparison may be done with multiple sensors  350 , each sensor  350  collecting data at various locations within the annulus of the valve. With enough data points, the desired shape and size of the prosthetic heart valve may be determined. 
     To use the sizing device  300  for sizing, positioning and selecting an appropriate prosthetic heart valve, the sizing device  300  may be deployed in-vitro using a deployment device to establish the relationship between rotations of a component of the deployment device during deployment and the outer diameter of the sizing device. 
     The sizing device  300  may then be collapsed and inserted into the patient transfemorally or transapically and advanced to the desired site for valve replacement. That is, the sizing device  300  may be advanced from the femoral vein through the iliac vein, the inferior vena cava, and the right atrium until reaching the deployment site, which will depend on the valve being replaced. This route requires the least amount of bending or turning. Minimizing the number of turns may facilitate control of the sizing device  300 . If the sizing device  300  includes echogenic materials, it may be guided to the appropriate position using the assistance of three-dimensional echocaradiography to visualize the sizing device within the patient. 
     Once sizing device  300  has reached the desired site of measurement, it may be unsheathed or otherwise deployed using the deployment device to assume its fully expanded shape. With the sizing device  300  in its expanded condition, measurements relating to the tissue stiffness and thus, calcification, may be taken using sensor  350 . After sufficient data has been collected, the sizing device  300  may be resheathed or otherwise collapsed and removed from the patient&#39;s body. 
     The collected data and the in-vitro data may then be used to select the appropriate valve size. A suitable prosthetic heart valve may be chosen, deployed and anchored at the desired site using any technique known in the art. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. 
     It will be appreciated that the various dependent claims and the features set forth therein can be combined in different ways than presented in the initial claims. It will also be appreciated that the features described in connection with individual embodiments may be shared with others of the described embodiments.