Patent Publication Number: US-2021169368-A1

Title: Determining release of implant from sheath based on measuring impedance

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to monitoring invasive medical procedures, and particularly to monitoring deployment of implants. 
     BACKGROUND OF THE INVENTION 
     Various techniques were proposed in the patent literature to assist invasive procedures such as delivery and placement of an implant using a probe. For example, U.S. Patent Application Publication 2003/0187340 describes a first electrode positioned within an artery proximate an implanted intravascular stent. A second electrode is positioned at a separate location relative the position of the first electrode. Electrical energy is then delivered between the first and the second electrodes to produce an electrical field adjacent the implanted intravascular stent. When an intravascular stent is implanted in a coronary artery, the delivery of the electrical energy is coordinated to cardiac cycles detected in sensed cardiac signals, where the delivery of the electrical energy between the first electrode and the second electrode occurs during a predetermined portion of the cardiac cycle. 
     As another example, U.S. Patent Application Publication 2015/0038833 describes methods and systems for determining information about a vascular bodily lumen. An exemplary method includes generating an electrical signal, delivering the electrical signal to a plurality of excitation elements in the vicinity of the vascular bodily lumen, measuring a responsive electrical signal from a plurality of sensing elements in response to the delivered electrical signal, and determining a lumen dimension. Specific embodiments include using spatial diversity of the excitation elements. Diagnostic devices incorporating the method are also disclosed, including guidewires, catheters and implants. The methods and systems described herein are advantageous as they do not include injecting a second fluid for the measurements. 
     SUMMARY OF THE DISCLOSURE 
     An embodiment of the present invention provides an apparatus including an electrical conductor, a patch electrode and readout circuitry. The electrical conductor is configured to be connected to an implant fitted at a distal end of a shaft for insertion via a sheath into a liquid-filled lumen of an organ of a patient. the patch electrode that is configured to be attached to skin of the patient. The readout circuitry is configured to be connected to the electrical conductor and to the patch electrode, to measure, via the electrical conductor, an electrical impedance between the implant and the patch electrode, and to detect the implant exiting the sheath into the lumen by detecting a drop in the electrical impedance that is larger than a predefined threshold. 
     In some embodiments, the apparatus further includes a magnetic sensor fitted at the distal end of the shaft, adjacently to the implant, the magnetic sensor configured to transmit position signals indicative of a position of the implant when the implant exits the sheath into the lumen. 
     In an embodiment, the liquid is blood. In another embodiment, the organ is a heart. 
     In some embodiments, the implant is an artificial heart-valve. In other embodiments, the implant is a stent. 
     There is additionally provided, in accordance with an embodiment of the present invention, a method including connecting an electrical conductor to an implant fitted at a distal end of a shaft for insertion via a sheath into a liquid-filled lumen of an organ of a patient. A patch electrode is attached to skin of the patient. An electrical impedance is measured, via the electrical conductor, between the implant and the patch electrode, and the implant exiting the sheath into the lumen is detected by detecting a drop in the electrical impedance that is larger than a predefined threshold. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a catheter-based position-tracking system, in accordance with an embodiment of the present invention; 
         FIGS. 2A and 2B  are schematic side views of a stent positioned inside and outside of a distal end of a sheath, respectively, in accordance with an embodiment of the present invention; 
         FIG. 3  is a graph that schematically shows impedances measured between the stent of  FIGS. 2A and 2B  and a patch electrode, respectively, in accordance with an embodiment of the present invention; and 
         FIG. 4  is a flow chart that schematically describes a method to determine the status of the stent shown in  FIGS. 2A and 2B , in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Placement of medical implants, such as cardiac stents and valves, typically requires monitoring the deployment status of the implants, such as monitoring whether the implant is collapsed inside a distal end of a sheath of a delivery probe (e.g., a catheter) or released from the sheath. In the latter case, for example, the implant may already be at least partially in an expanded state and ready for placement. Present systems for tracking an implant, such as a stent, when it is positioned in a blood vessel, typically use X-ray fluoroscopy. However, for the health of the patient and operating personnel, it is preferable not to use a stent-tracking system that uses ionizing radiation. 
     In an invasive procedure involving placement of an implant, such as in a valve replacement or in a stenting procedure, a sheath is inserted into the blood vessel where the implant is to be located. Once the sheath is in place, the sheath&#39;s distal end is located using, for example, the CARTO™ system, produced by Biosense-Webster Inc. (Irvine, Calif.), and the implant is pushed through the sheath by the probe (e.g., catheter) which is to deliver the implant. 
     Embodiments of the present invention that are described hereinafter provide a technique to monitor the status of the implant on a verge of exiting the sheath in order to determine its subsequent exit. In some embodiments, an electrical conductor (typically a wire) is attached to the implant (e.g., stent), and this electrical conductor is used to measure the impedance between the implant and a patch electrode attached to the patient&#39;s skin. Once the stent is deployed, the wire is cut at the stent using standard tooling (e.g., a clamp). 
     The impedance between the implant and the patch electrode is initially measured while the implant is in a collapsed configuration inside the sheath. As long as the implant is at least partially in the sheath and in a collapsed state, the impedance is expected to indicate that the implant surface has only minimal contact with blood, and is mostly in contact with the surrounding electrically insulating sheath. As the implant exits the sheath and becomes fully immersed in blood, the impedance is expected to reduce by an amount above a predefined threshold, since the implant is now in the blood pool (and possibly also in a partially expanded configuration that allows more contact area with the electrically conducting blood). This reduction (e.g., drop) in impedance can therefore be used as an indication that the implant is positioned distally just beyond (i.e., outside) the distal end of the sheath. 
     The total impedance measured between the patch electrode and the stent includes a local changing impedance between the stent and blood pool. As the implant exits the sheath, a reduction (e.g., a step-wise drop) in impedance, of several tens of ohms is expected. Compared with a baseline impedance to the patch of several hundreds of ohms, the change is measured as few tens of percent of that baseline impedance, and such magnitude of change is expected to be a reliable indicator of the implant exiting the sheath. 
     The disclosed technique may reduce exposure to excessive amounts of X-ray radiation to the patient and medical staff deploying an implant in a lumen of the patient. 
     System Description 
       FIG. 1  is a schematic, pictorial illustration of a catheter-based position-tracking system  20 , in accordance with an embodiment of the present invention. System  20  comprises a catheter  21 , wherein, as seen in inset  25 , a distal end  22   a  of shaft  22  of catheter  21  is inserted through a sheath  23  into a heart  26  of a patient  28  lying on a table  29 . As further shown in inset  25 , distal end  22   a  comprises a magnetic sensor  39 , fitted at distal end  22   a  just proximally to a stent  40 . 
     The proximal end of catheter  21  is connected to a control console  24 . In the embodiment described herein, catheter  21  is used for placement of stent  40  in a blood-filled vessel of heart  26 . 
     During navigation of distal end  22   a  in heart  26 , console  24  receives signals from magnetic sensor  39  in response to magnetic fields from external field generators  36 , for example, for the purpose of measuring the position of stent  40  in the heart and, optionally, presenting the tracked position on a display  27 . Magnetic field generators  36  are placed at known positions external to patient  28 , e.g., below patient table  29 . Console  24  also comprises a driver circuit  34 , configured to drive magnetic field generators  36 . 
     In an embodiment, position signals received from position sensor  39  are indicative of the position of stent  40  in the coordinate system of position tracking and catheter-based position-tracking system  20 . The method of position sensing using external magnetic fields is implemented in various medical applications, for example, in the CARTO™ system, produced by Biosense-Webster Inc. (Irvine, Calif.), and is described in detail in U.S. Pat. Nos. 5,391,199 and 6,332,089, and in PCT Patent Publication WO 96/05768, whose disclosures are all incorporated herein by reference. 
     Physician  30  navigates the distal end of shaft  22  to a target location in heart  26  by manipulating shaft  22  using a manipulator  32  near the proximal end of the catheter and/or deflection from the sheath  23 . During the insertion of shaft  22 , stent  40  is maintained in a collapsed configuration by sheath  23 . By containing stent  40  in a collapsed configuration, sheath  23  also serves to minimize vascular trauma along the way to target location. A wire  44  (or other suitable electrical conductor) in shaft  22  electrically connects the implant (e.g., stent) to a first pole  63  of a readout circuitry  65  in console  24 . A patch electrode  50  is attached to the patient&#39;s skin and wired to a second pole  64  of readout circuitry  65  with a wire  52 . Readout circuitry  65  is configured to indicate a reduction (e.g., a step-wise drop) in an impedance measured between first pole  63  and second pole  64  of the readout circuitry, so as to indicate a status of stent  40 , as described below. 
     Control console  24  comprises a processor  41 , typically a general-purpose computer, with suitable front end and interface circuits  38  for receiving signals from catheter  21 , as well as for applying treatment via catheter  21  in heart  26  and for controlling the other components of system  20 . Processor  41  typically comprises a general-purpose computer with software programmed to carry out the functions described herein. The software may be downloaded to the computer in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     The example configuration shown in  FIG. 1  is chosen purely for the sake of conceptual clarity. The disclosed techniques may similarly be applied using other system components and settings. For example, system  20  may comprise other components and perform non-cardiac stent implants. 
     Determining Release of Stent from Sheath Based on Measuring Impedance 
       FIGS. 2A and 2B  are schematic side views of stent  40  positioned inside and outside of a distal end of sheath  23 , respectively, in accordance with an embodiment of the present invention.  FIG. 2A  shows sheath  23  inside a lumen, such as a blood vessel  35 , and a distal end  22   a  of shaft  22  inside sheath  23 . As seen, a stent  40  is attached at a distal edge of shaft  22 , where stent  40  is in a collapsed configuration inside sheath  23 . An insulated electrical wire  44  is electrically connected at its distal edge to stent  40 , while patch electrode  50 , with a wire  52 , is attached to the skin of the patient. Wires  44  and  52  are connected to first pole  63  and second pole  64 , respectively, of readout circuitry  65  (as seen in  FIG. 1 ) that measures an impedance  66   a  between the collapsed stent and electrode patch  50 , for example, by flowing electrical current and measuring a voltage falling between the two poles. 
       FIG. 2B  shows distal end  22   a  after being slightly advanced distally as compared to its shown position in  FIG. 2A . As a result, stent  40  is just outside sheath  23  and is expanded. In stent  40  position of  FIG. 2B , readout circuitry  65  (seen in  FIG. 1 ) measures a lower impedance  66   b  (i.e., lower than impedance  66   a ) between (the possibly slightly expanded) stent  40  in contact with blood  10  and patch electrode  50 . 
       FIG. 3  is a graph plot  69  that schematically shows impedances  66   a  and  66   b  measured between the stent of  FIGS. 2A and 2B  and a patch electrode  50 , respectively, in accordance with an embodiment of the present invention. Graph plot  69  shows a step-wise drop in impedance between a region  70  that corresponds to stent  40  being collapsed inside sheath  23  and a region  74 , in which stent  40  is just outside the sheath, i.e., in contact with blood (and possibly also slightly expanded). The drop in the impedance occurs at a relatively narrow region  72  of implant status, which corresponds to the implant being abruptly exposed to surrounding blood  10  as stent  40  is being advanced and exits sheath  23 . Impedances  66   a  and  66   b  are usually much larger than the drop, with impedances  66   a  and  66   b  measured in hundreds of Ohms each, while the drop is measured in Ohms. The example graph plot shown in  FIG. 3  is chosen purely for the sake of conceptual clarity. In practice, an electrical measurement may show a distribution of values, and impedance data points may be fitted with a step-wise curve to assist the indication of the exit of stent  40 . 
       FIG. 4  is a flow chart that schematically describes a method to determine the status of the stent shown in  FIGS. 2A and 2B , in accordance with an embodiment of the present invention. The process begins with physician  30  inserting the implant (e.g., stent  40 ) into sheath  23  of catheter  21 , at an implant insertion step  80 . Readout circuitry  65  reads and records the impedance when stent  40  is still well inside sheath  23  (i.e., in region  70  of impedance readings), at an initial impedance reading step  82 . 
     Physician  30  advances the implant distally through sheath  23 , at implant advancement step  84 . Readout circuitry  65  reads and records the up-to-date impedance accordingly, at an up-to-date impedance reading step  86 . 
     Either physician  30 , or a processor, regularly checks if the impedance has dropped in a step-wise fashion, at an impedance monitoring step  88 . If the impedance hasn&#39;t yet dropped, the process continues by physician  30  advancing the implant further distally (i.e., process returns to step  86 ). If there is an indication of a step-wise drop in impedance, the readout circuitry  65 , or the processor, indicates to the user that the implant is exiting the sheath, at an implant status indication step  90 . In an embodiment, readout circuitry  65  indicates the implant exiting by an audiovisual alert that impedance has made a step-wise drop by at least a prespecified value. The processor may show a visual indication on display  27 . 
     Although the embodiments described herein mainly address cardiac applications, the methods and systems described herein can also be used in other applications, such as in invasive gastroenterology and neurology procedures. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.