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METHOD AND APPARATUS FOR
MONITORING AND DISPLAYING LEAD
IMPEDANCE IN REAL-TIME FOR AN
IMPLANTABLE MEDICAL DEVICE
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FIELD OF THE INVENTION
The invention generally relates to implantable medical devices and to external programmer devices used in connection therewith and in particular to methods and apparatus for detecting and processing the electrical impedance of one 10 or more electrical leads of the implantable medical device.
BACKGROUND OF THE INVENTION
A wide range of implantable medical devices are provided for surgical implantation into humans or animals. One common example is the cardiac pacemaker. Another is the implantable cardioverter defibrillator (ICD). Other examples include devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other 2Q body organs or tissues.
Many such implantable medical devices include one or more electrical leads for conducting sensed electrical signals away from a particular part of the body such as the heart or for conducting stimulating electrical signals to the particular 25 part of the body. In the case of a pacemaker, the sensed electrical signals are typically representative of P-waves and R-waves. The stimulating electrical signals are small pulses of electricity, on the order of micro-joules, for stimulating the heart in the event that the expected P-waves or R-waves 30 are not detected. In the case of an ICD, the stimulating signals are typically 10 to 30 joule pulses of electricity provided to terminate tachycardia or fibrillation.
The electrical leads, for various reasons, may cease to function properly. For example, the electrical lead may 35 suffer some minor damage during implantation that may affect the electrical insulation of the lead. This type of damage may not be initially detectable but may manifest itself after an extended period of time. In particular, stress imposed on the electrical lead as a result of the normal 40 movements of the body may further damage the lead resulting in a complete or otherwise significant breakdown in the electrical insulation of the lead. In other cases, the lead itself may fracture. If either type of damage occurs, serious or even disastrous consequences may result. For example, in 45 the case of a pacemaker or ICD electrical stimulation signals intended for the heart may be shunted to other parts of the body rendering the stimulation signals ineffective for pacing the heart or for terminating tachycardia or fibrillation.
Accordingly, various techniques have been developed for 50 testing implanted electrical leads to detect lead faults such as lead fractures or complete breakdowns in insulation. To this end, many pacemakers now include circuitry for periodically or continuously testing the impedance of electrical leads connected to the pacemaker and any significant devia- 55 tion from a range of acceptable impedance values is recorded within the pacemaker (subject to memory-space limitations) for subsequent downloading to an external monitoring device such as a pacemaker programmer. The downloaded data is analyzed by the external monitoring 60 device to determine if an unexpected impedance value had been recorded by the pacemaker. Typically, the external monitoring device determines whether any of the recorded impedance values exceeds a predetermined upper threshold, such as 2000 ohms, or any falls below a predetermined 65 lower threshold, such as 200 ohms. If the lead impedance exceeds the upper threshold, the lead is presumed to have
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fractured and must be replaced. If the impedance falls below the lower threshold, the insulation of the lead is presumed to have failed and the lead therefore also must be replaced. In either case, an audible warning signal or other simple notification is provided to the physician operating the external monitoring device. In other cases, the testing of the lead occurs while the pacemaker is in communication with the external monitoring device allowing such warning signals to be generated immediately. In still other cases, the pacemaker itself tests for unexpected impedance values and generates a warning signal within the patient by producing a highpitched audible tone or by providing a mild, but noticeable electrical shock, to the patient.
Although the testing of electrical leads and the generation of simple warning signals upon the detection of an unexpected impedance value represents an improvement over systems which do not provide for lead testing, considerable room for improvement remains. In particular, with the generation of only a simple warning signal, the physician may not be provided with sufficient information to readily determine the exact nature and seriousness of the lead fault. The physician may not be able to determine easily, for example, whether the fault is a permanent fault or merely an intermittent one. If intermittent, the physician may not be able to determine easily whether the intermittent fault lasts only momentarily or for a longer period of time. As can be appreciated, further information regarding the exact nature of the lead fault may be required by the physician before he or she can determine the seriousness of the fault and, in particular, determine whether the lead must be replaced immediately or whether such action can be deferred at least temporarily. Moreover, further information regarding the exact nature of the fault may even be required before the physician can properly diagnose the patient. For example, a patient exhibiting an intermittent arrhythmia may have a faulty lead. If the lead fault is intermittent, the arrhythmia may be triggered by the absence of pacing signals during the intermittent faults. However, if the fault is permanent, the intermittent arrhythmia may have some other cause which may need to he further investigated.
Indeed, without further information regarding the exact nature of the fault, the physician may not even be able to determine easily whether the fault actually exists or whether the unexpected impedance values that have been detected are caused by some malfunction in either the pacemaker or the external monitoring device. For example, the impedance detection system simply may not be calibrated properly and therefore may be generating erroneous warning signals even though there is no actual lead fault. Alternatively, the impedance detection system may not be generating warning signals at all—even though a lead fault is present—perhaps because an electrical malfunction causes the detection system to output a constant impedance value regardless of the actual impedance. This is a particularly serious problem as the physician may assume that the pacemaker is working properly even though a permanent lead fault has occurred.
As can he appreciated, it would be highly desirable to provide the physician with more complete information regarding the detected impedance values than merely a warning signal indicating that an unexpected impedance value had been detected of course, some systems may permit the physician to print out or otherwise display diagnostic information pertaining to a lead fault thereby allowing the physician to eventually come to an informed decision about the exact nature of the fault and to determine whether the fault actually exists or not. However, unless such information is provided quickly and efficiently and in a format that
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allows the physician to immediately determine the nature of the lead fault, it may be of little practical use to the physician, particularly in emergency situations. In general, the more difficult and time-consuming it is for the physician to access lead fault diagnostic information, the less likely he 5 or she will routinely access that information and the less likely he or she will be able to make an informed decision regarding possible lead faults.
Accordingly, it would be desirable to provide an improved system for quickly and efficiently providing useful informa- 10 tion to a physician regarding possible lead faults and it is to that end that aspects of the invention are primarily directed.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a real-time 15 impedance monitoring system is provided for use with an implantable medical device having an implantable electrical lead. The impedance monitoring system comprises a means for determining the electrical impedance of the lead as a function of time, with the determination being made sub- 20 stantially in real-time, and a means for graphically displaying the electrical impedance of the lead as a function of time, with the display also being generating substantially in realtime. In one specific example, the implantable medical device is a pacemaker and the impedance monitoring system 25 is within an external programmer device separate from the pacemaker. The means for graphically displaying the impedance of the lead is a computer display screen or a computer printout device.
Hence, a system is provided for graphically displaying the 30 impedance of an electrical lead as it changes in real-time. By providing a graphical display of the impedance in real-time, a physician can easily view the impedance, perhaps in response to a lead fault alarm, to thereby immediately determine the presence and severity of any fault triggering 35 the alarm and to determine whether the fault is permanent or intermittent If intermittent, the physician can easily see whether the intermittent fault lasts only momentarily or for a longer period of time. Additionally, the physician can gain some insight into whether the system is operating correctly. 40 For example, if the time-varying display of impedance reveals a completely constant impedance value that includes no noise or other statistical variations consistent with actual sensed impedance values, the physician may thus suspect that the impedance detection system is malfunctioning. As 45 another example, if the time-varying display reveals impedance values consistent with actual sensed impedance values, but where the impedance values are all either inordinately high or low, the physician may suspect that the impedance detection system is merely miscalibrated. In each case, it is 50 particularly advantageous that the impedance is presented in a graphical form and substantially in real-time to allow the physician to see at a glance the time varying characteristics of the impedance and to thereby be able to make informed decisions promptly and effectively. 55
Other objects and advantages of the invention are provided as well. Method embodiments of the invention are also provided.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 shows an implantable pacemaker coupled to a heart via a pair of electrical leads.
FIG. 2 is a perspective view of an external programmer that may be used for communicating with the implantable pacemaker of FIG. 1. 65
FIG. 3 is block diagram of pertinent components of the implantable pacemaker of FIG. 1 and of the external pro
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grammer of FIG. 2 for use in generating and displaying impedance graphics.
FIG. 4 is an exemplary graphic display, generated by the external programmer of FIG. 2, showing impedance as a function of time for the pair of electrical leads of the implantable pacemaker of FIG. 1 with values presented in real-time and bracketed with predetermined upper and lower boundaries of acceptable impedance.
FIG. 5 is another exemplary graphic display, generated by the external programmer of FIG. 2, showing real-time impedance values as in FIG. 4, but re-scaled to cover a longer period of time.
FIG. 6 is yet another exemplary graphic display, generated by the external programmer of FIG. 2, showing the impedance as a function of time of the electrical leads of the implantable pacemaker of FIG. 1 bracketed with variable upper and lower boundaries of acceptable impedance calculated based upon a running average of the impedance rather than upon predetermined boundaries.
FIG. 7 is yet another exemplary graphic display, generated by the external programmer of FIG. 2, showing the impedance as a function of time of the electrical leads of the implantable pacemaker of FIG. 1 with values generated from previously recorded data.
DETAILED DESCRIPTION OF THE
INVENTION
The invention relates to improved techniques for providing information to a physician regarding the electrical impedance of leads within an implantable medical device. The invention will be described primarily with reference to a pacemaker used in conjunction with an external programmer device, but principles of the invention are applicable to other implantable medical devices or other external devices as well. As such, the examples described herein should not be construed as limiting the scope of the invention.
FIG. 1 illustrates an implantable pacemaker 10 coupled to a heart 12 by way of a ventricular lead 14 and an atrial lead 16. Ventricular lead 14 includes an electrode 18 positioned in the right ventricle 20 of the heart and atrial lead includes an electrode 22 positioned in the right atrium 24 of the heart. Internal components of the pacemaker, to be described in greater detail below, operate to periodically detect electrical characteristics of leads 14 and 16 from which the electrical impedance of the leads can be determined. The detected signals are either transmitted immediately to an external programmer (FIG. 2) or are stored within the pacemaker for subsequent transmission. Pertinent internal components of the pacemaker for detecting, storing and transmitting the electrical characteristics of the leads will be described in greater detail below. Other components of the pacemaker, such as components for monitoring signals received from the heart and for providing responsive therapy, are not directly pertinent to the invention and will not be described in detail herein. Further information regarding internal components of a pacemaker, particularly components directed to efficiently storing and processing data within a pacemaker using "event records, is provided in U.S. Pat. No. 5,431,691 to Snell et al., which is incorporated by reference herein.
FIG. 2 illustrates an external programmer 100 configured for receiving the aforementioned electrical signals from pacemaker 10 (FIG. 1), for determining the impedance of the electrical leads of the pacemaker and for generating a time-varying display or printout of the impedance. Programmer 100 includes a printer 102 for printing out a graphical representation of the time-varying impedance and a display
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