Abstract:
A method and apparatus implementing the method, which is not dependent on monitoring the electrical impedance of the lead, detects imminent structural failure of an electrical lead in an implanted medical device, such as an implantable cardioverter-defibrillator (ICD) or a pacemaker. The approach is to monitor directly the mechanical load loss of the lead (a measure of the loss of structural integrity of the lead) rather than, as in the prior art, to infer it from the electrical impedance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/268,699, filed 15 Jun. 2009. 
    
    
     REFERENCE REGARDING FEDERAL SPONSORSHIP 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     FIELD OF THE INVENTION 
     The present invention relates to a cardiac therapy medical device implanted in a patient, such as an implantable cardioverter-defibrillator (ICD) or pacemaker and, more particularly, to detecting imminent lead failure in such an implanted medical device by post-implant monitoring. 
     DESCRIPTION OF RELATED ART AND OTHER CONSIDERATIONS 
     A key component of an implanted medical device, such as an ICD or pacemaker, is the electrical lead. Such leads typically consist of electrical conductors, which are electrically isolated from each other by insulation. A set of such insulated conductors is encased in an outer sheath of insulation. 
     Failure of the lead can have adverse, even fatal, clinical consequences for the patient. Failure mechanisms include insulation failure and fracture of a conductor. 
     The medical device industry uses electrical measurements, such as lead electrical impedance, to determine that failure of an implanted medical device (e.g., ICD or pacemaker) lead is imminent. In the prior art, the impedance is monitored and is compared with the impedance at the time the device was implanted in the patient. If the impedance changes by a specified amount, the lead is considered to be on the verge of failure and corrective action is warranted to prevent a lead failure that could harm the patient. 
     Prior art methods for detection of lead related conditions based on impedance monitoring are disclosed in the following U.S. Patents: 
                                             4,899,750   February 1990   Ekwall   128/419 PG       5,549,646   August 1996   Katz et al.   607/8       5,741,311   April 1998   Mc Venes et al.   607/28       5,755,742   May 1998   Schuelke et al.   607/27       5,814,088   September 1998   Paul et al.   607/28       5,837,900   November 1998   Lipson    73/661       6,317,633   November 2001   Jorgensen et al.   607/28       7,369,893   May 2008   Gunderson   607/27                    
All of these patents are incorporated by reference.
 
     However, clinicians report that electrical impedance monitoring has only limited effectiveness for preventing adverse clinical events, specifically inappropriate shocks, resulting from fracture of ICD leads (Kallinen et al., “Failure of impedance monitoring to prevent adverse clinical events caused by fracture of a recalled high-voltage implantable cardioverter-defibrillator lead”,  Heart Rhythm , Vol. 5, pp. 775-779, 2008; Farwell et al., “Accelerating risk of Fidelis lead fracture”,  Heart Rhythm , published online 4 Jul. 2008). 
     Kallis explained these findings by pointing out that electrical impedance has been found to be an insensitive measure of imminent structural failure of an electrical connection (Kallis,  Heart Rhythm , Vol. 6, No. 1, pp. e5-e6, January 2009). 
     In 2008, device manufacturer Medtronic, Inc., tried to solve the problem by revising the algorithm for detecting lead failure. Medtronic calls this software upgrade, which it claims “ . . . gives patients advance notice of a potential lead fracture”, Lead Integrity Alert™ (Medtronic News Release, Sep. 4, 2008). 
     The Minneapolis Heart Institute states that the “ . . . new lead monitoring algorithm . . . incorporates both impedance and noise detection. When significant impedance changes or noise is detected, the pulse generator automatically changes its tachycardia detection criterion to 30 of 40 intervals and initiates a series of audible alerts. Based on its bench tests, the manufacturer expects that this new algorithm will provide a 2- to 3-day advance warning before a lead fracture causes an adverse event, such as inappropriate shocks. Although clinical data regarding the safety and efficacy of the algorithm are needed, we are encouraged by our initial experience with this new monitoring technique for detecting Sprint Fidelis pace-sense conductor fractures” (Hauser et al.,  Heart Rhythm , Vol. 6, No. 1, p. e6, January 2009). However, Hauser et al. also point out a disadvantage of the new lead monitoring algorithm: “Physicians should recognize that prolonging the number of intervals to detect a ventricular tachyarrhythmia will delay therapy, which, for some patients, may be unsafe.” 
     Therefore, a novel approach to how lead failure is detected by post-implant monitoring is warranted. A measure of imminent structural failure of an implanted lead more sensitive than impedance has the potential benefit of earlier detection of lead failure. 
     SUMMARY OF THE INVENTION 
     This invention is a method, not dependent on monitoring the electrical impedance of the lead, for detecting imminent structural failure of an electrical lead in an implanted medical device, such as an implantable cardioverter-defibrillator (ICD) or a pacemaker. The approach is to monitor directly the mechanical load loss of the lead (the decrease in the mechanical load required to deflect/elongate the lead by a specified amount, which is a measure of the loss of structural integrity of the lead) rather than, as in the prior art, infer it from the electrical impedance. The invention includes apparatus for implementing the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sketch of an ICD implanted into a patient&#39;s heart. 
         FIG. 2  is a flow chart describing the steps in the method. 
         FIG. 3  is a sketch of an embodiment of the apparatus of the invention, incorporated into a cardiac therapy medical device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principle is to exploit, that is, to take advantage of the stresses and strains placed on the lead which are produced by ordinary movements of the patient by monitoring and measuring the mechanical load versus the lead deflection as a function of time after implantation. A decrease in the mechanical load resulting from a deflection and/or elongation (that is, an event that results in a mechanical load loss) indicates that the structural integrity of the lead has decreased such as, for example, resulting from a crack occurring or growing in the structure. A decrease that exceeds a specified threshold indicates imminent structural failure and triggers an alarm to the patient. 
     These stresses and/or strains, which are produced by everyday movements of the patient, occur because of where the cardiac therapy medical device is implanted or placed in the patient. The United States Government websites show the placement of the ICD in the upper left chest of the patient, as shown by indicium  10  in  FIG. 1 . (The US NHLBI (National Heart, Lung, and Blood Institute, National Institutes of Health), states that an ICD is placed in the chest or abdomen (www.nhlbi.nih.gov/health/dci/Diseases/icd/icd_whatis.html), but shows only placement in the upper left chest.) Thus the upper left chest appears to be the typical location. 
     As shown in  FIG. 1 , ICD  10  is implanted in the upper left chest (as shown by indicium  12 ) of the patient, and a lead  20  is inserted into a vein  30  leading to the patient&#39;s heart  40  and from there into its right ventricle ( 42 ). 
     Verification that movements by the patient produced stresses on the lead is provided by the following quotations:
         1. “Cardiac lead bodies are continuously flexed by the beating of the heart . . . Movements by the patient can cause the route traversed by the lead body to be constricted or otherwise altered causing stresses on the lead body.” (Gunderson, U.S. Pat. No. 7,369,893 B2 (May 6, 2008))   2. “Almost all forms of physical activities can be performed by patients with an ICD. All forms of sports that do not pose a risk of damaging the ICD can be enjoyed by the patient. Special care should be placed not to put excessive strain on the shoulder, arms, and torso area where the ICD is implanted. Doing so may damage the ICD or the leads going from the unit to the patient&#39;s heart.” (“Implantable Cardioverter-Defibrillator”, Wikipedia, 23 Apr. 2009 (http://en.wikipedia.org/wiki/Implantable_cardioverter-defibrillator))       

     The method of the invention is to implement this principle by establishing two deflection or elongation versus mechanical load relationships, as obtained from the performance and measuring steps (enclosures  220  and  230 ) in  FIG. 2 . For this purpose, the lead assemblies preferably comprise insulated conductors; the purpose of using insulated conductors is to account for the stiffening effect of the insulation on the structural characteristics of the lead. 
     The first relationship is the threshold mechanical load loss (the difference between the baseline load and the value after stressing) that indicates imminent structural failure. 
     The first step (enclosure  210 ) is to select samples of cardiac therapy medical device leads of the model number of interest from the production line. The leads are subjected to destructive laboratory pull testing (enclosure  220 ) (as described, for example, in US Military Standard 883C, Notice 5, “Test Methods and Procedures for Microelectronics”, Method 2011.6“Bond Strength (Destructive Bond Pull Test)”, 29 May 1987). At the start of the testing, the leads are pulled, and the elongation E and the baseline mechanical load L B  are measured. Then flaws are made to grow in the leads, and the measurements are repeated. By this means, the tests result in evaluating the threshold mechanical load L T  and the threshold mechanical load loss ΔL T =L B −L T  as a function of the elongation E (enclosure  230 ). The threshold test data are fit to an empirical formula (enclosure  240 ), of the general form ΔL T =f T (E) (enclosure  250 ). The best-fit function and the best-fit parameters are determined by well known methods (for example, least squares (Ralston,  A First Course in Numerical Analysis , McGraw-Hill Book Co., New York, 1965, Chapter 6)). The empirical formula is stored in the cardiac therapy medical device computer (enclosure  360 ). The steps  210 - 250  and  360  have to be performed only once for a particular lead model. 
     The second is performed for each serial number of a lead in an implanted device. A baseline is established immediately after the lead is installed into the cardiac therapy medical device during fabrication of the device (enclosure  310 ). The lead is subjected to a nondestructive laboratory pull test (enclosure  320 ) (as described, for example, in Manson and Halford,  Fatigue and Durability of Structural Materials , ASM International, Materials Park, Ohio, pp. 416-422, 2006), in which the elongation E and the baseline mechanical load L B  are measured (enclosure  330 ). The baseline test data are fit to a formula (enclosure  340 ), of the form L B =f B (E) (enclosure  350 ). The best-fit function and the best-fit parameters are determined by well known methods (for example, least squares (Ralston,  A First Course in Numerical Analysis , McGraw-Hill Book Co., New York, 1965, Chapter 6)). The empirical formula is stored in the serial number of the cardiac therapy medical device computer (enclosure  360 ). 
     Then the cardiac therapy medical device is implanted in the patient (enclosure  410 ) and operated (enclosure  420 ). The cardiac therapy medical device computer continuously monitors elongation of the lead resulting from patient movements (enclosure  430 ). When an elongation is sensed, the elongation E and corresponding mechanical load L are measured (enclosure  440 ). The cardiac therapy medical device computer determines whether the threshold for imminent structural failure has been reached by comparing the mechanical load loss ΔL=L B (E)−L with the threshold mechanical load loss ΔL T (E) (enclosure  450 ). If ΔL&gt;ΔL T (E), that is, if L&lt;f B (E)−f T (E), then the cardiac therapy medical device alerts the patient that structural failure of the lead is imminent. If ΔL&lt;ΔL T (E), that is, if L&gt;f B (E)−f T (E), then the cardiac therapy medical device continues to operate. This is the same algorithm as with the prior art, except that the parameter employed is mechanical load loss rather than electrical impedance. 
     The cardiac therapy medical device computer (enclosure  360 ) can be an existing cardiac therapy medical device computer, like that shown in  FIG. 2  of the prior art patent by Gunderson, U.S. Pat. No. 7,369,893 B2 (May 6, 2008). With the present invention, the computer processes mechanical (elongation and the mechanical load) data on the lead, rather than electrical impedance data. 
     In addition to the method, the invention includes apparatus: gauges to measure the elongation and mechanical load of the lead in the implanted device. Cardiac therapy medical devices in the prior art have no such gauges. An exemplary embodiment of the invention is shown in  FIG. 3 . The gauges are installed at the point where the lead (e.g., lead  20 ), consisting of a conductor  23  and insulation  26 , is connected to the cardiac therapy medical device connector block. The connector block, as indicated by indicium  122 , includes, inter alia, end walls  124  and  125 . When the connector block is moved, such as by the patient, e.g., in a direction to the right as viewed in the drawing and as denoted by indicium  122 ′ (with walls  124  and  125  moving to their new positions as denoted by dashed lines  124 ′ and  125 ′), lead  20  is elongated. This elongation is measured by a strain gauge  110 , which is bonded to the lead and which is positioned adjacent to and secured to wall  125 . As illustrated in  FIG. 3 , such elongation  126  (which is exaggerated for illustrative purposes) is denoted by the spacing respectively between walls  124  and  124 ′ and walls  125  and  125 ′. The corresponding mechanical load on the conductor is measured by a force gauge  100 . Gauges that can be modified to serve the purposes of gauges  100  and  110 , as are described and embodied herein, are commercially available. For example, Endevco Corporation manufactures piezoelectric accelerometers that are implanted in pacemakers to detect patient activity, such as motion. Endevco also manufactures silicon piezoresistive pressure gauges, which are designed to measure fluid pressure; they are modifiable to measure force and deflection/elongation and sealed for use in implanted medical devices. Vishay Micro-Measurements manufactures metal foil piezoresistive strain gauges, suitable for the elongation gauge  110 . FUTEK Advanced Sensor Technology, Inc., manufactures force gauges (also called force sensors, force transducers, or load cells), consisting of arrays of Vishay® (registered trademark of Vishay Intertechnology, Inc.) metal foil strain gauges that are electrically connected to form a Wheatstone bridge circuit. The sensors, including the associated electronics, can be miniaturized to the diameter of a shirt button. FUTEK sensors have been implanted in human knees and in bladders for urinary control. The implanted bladder device is a blocking oscillator electrical circuit consisting of (1) a silicon nitride membrane pressure sensor with integrated piezoresistors and (2) an integrated circuit, both mounted on a printed circuit board. The device is coated with silicone to render it biocompatible. 
     Although the invention has been described with respect to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.