RECOMMENDED REPLACEMENT TIME OF AN IMPLANTABLE MEDICAL DEVICE
Technical Field
The present invention relates to a method and a circuit for determining recommended replacement time, RRT, of a battery of an implantable medical device. The method comprises the steps of measuring the battery's internal impedance, comparing said measured impedance with a predetermined limit value, and determining RRT from the result of this comparison. The circuit includes an impedance measurement means for measuring the internal impedance of the stimulator battery, a comparator means for comparing the measured internal impedance with the predetermined limit value, and a determining means for determining RRT from the result of this comparison.
Background Art
It is of utmost importance to get reliable information about the status of batteries used in implantable medical devices, like heart stimulators, and in particular information about the remaining capacity or remaining charge of the battery. From this information remaining operation time of the device can be determined and this enables the physician to plan for replacement of the battery and/or the medical device at an appropriate time. Several techniques have therefore been proposed for monitoring battery depletion and determining remaining battery capacity.
One of the most commonly used methods consists in monitoring the internal impedance of the battery. US-A-5 , 620 , 474 discloses a method to calculate the RRT impedance depending on the operating conditions seen by the pacemaker . The disclosed method in US-A-5 , 620 , 474 calculates a new value of the RRT-impedance to be used in the future each time the operating conditions are changed. Further, US-A-5 , 800 , 472 describes determination of recommended replacement time, RRT, of an implantable multimode rate responsive pacemaker by monitoring the battery voltage.
To obtain a more reliable determination of the battery status it has been proposed to independently monitor or measure at least two different parameters indicating the battery depletion. Thus, to reject especially transients in the demand on the battery as criteria for an elective replacement indication, US-A-5 , 370 , 668 discloses an implantable medical device in which internal battery impedance measurements are combined with periodical assessments of the loaded terminal voltage of the battery to obtain an elective replacement indication. Another example of such a technique is described in US-A-5 , 741, 307. This publication discloses a method of determining RRT for an implantable cardioverter-defibrillator by measuring battery terminal voltage and capacitor charging time .
From a theoretical point of view the ideal way of determining the remaining capacity of a battery would be measurement of the charge drawn from the battery. Such techniques are proposed in e.g. US-A-4 , 715 , 381 and US-A-5 , 769, 873.
As mentioned above, one of the most commonly used methods for determining remaining capacity of the battery of an implanted medical device consists in measurement of the internal impedance of the battery. This impedance increases exponentially with the charge drawn from the battery. Thus, during depletion of the first 50% of the total charge of the battery the change in its impedance can hardly be measured, whereas during depletion of the subsequent 50% of the charge the impedance change will be more and more pronounced.
From information about when a predetermined limit value of the battery internal impedance is reached it is possible to determine how much current is consumed in the actual mode of operation of the device, and it is then also possible to determine for how long time the remaining charge of the battery will suffice. A recommended replacement time, RRT, for the battery of the medical device or for the medical device, can consequently be determined. As a safety measure RRT is in practice selected 3-6 months before the calculated end of life of the battery.
The current consumption is, however, depending on several factors, such as adjustable operating parameters of the medical device in question, like amplitude and width of stimulation pulses, programmed stimulation rate, and diagnostic data, like electrode lead impedance, actual stimulation rate and current consumption of the stimulator electronics, as well as mode of operation of the medical device. This means that the time from RRT till the battery reaches its end of life is also depending on these factors.
Disclosure of the Invention
The purpose of the present invention is to provide an improvement of the previously known technique for determining RRT from measurements of the battery internal impedance, whereby a sufficiently long safety period between RRT and the battery end of life is secured also when factors affecting the current consumption are changed.
This purpose is obtained by a method and a circuit of the kind set forth in the introductory portion of the description and having the characterizing features of claim 1 and claim 9 respectively .
Thus, contrary to the situation in the prior art solutions where one single fixed battery impedance limit is used for determining RRT, a limit value is used in the present invent- ion which is changed in response to changes of operating condition affecting the current consumption. When e.g. settings of the medical device in question are changed or the patient load towards which the device is stimulating is changed, the battery impedance limit value used as an RRT indicator is adjusted correspondingly. Thus, with a circuit according to the invention RRT is automatically changed when factors affecting the current consumption are changing. In this way there will always be a sufficiently long time between RRT and end of life of the battery.
According to an advantageous embodiment of the method according to the invention said RRT is determined from the result of the comparison of the measured battery internal impedance with a predetermined limit value according to pre- defined worst conditions of operation. Examples of such worst conditions could be e.g. 100% stimulation and e.g. 250 Ohms drop in the lead impedance. By using such a procedure for the RRT determination the safety of the patient is further increased.
According to an advantageous embodiment of the circuit according to the invention the limit value changing means includes a plurality of registers storing different preprogrammed impedance limit values and a pointer for selecting one register of said plurality of registers, which is storing an impedance limit value suitable for use in said comparator means under the actual operating conditions of the heart stimulator. The registers are preferably programmed when the heart stimulator is manufactured. By using such preprogrammed registers patient safety is improved since it eliminates the risk of erroneous programming of this vital parameter.
According to another advantageous embodiment of the circuit according to the invention an indicator is provided to be activated if, in reprogramming the heart stimulator, its operating parameters are changed such that a hazardous increase of the current consumption will result. This is an important safety increasing feature of the circuit according to the invention, since an indication is then immediately given if the heart stimulator is reprogrammed into a mode of operation with an increased current consumption that would result in a quick discharge of the battery. According to still other advantageous embodiments of the circuit according to the invention the limit value changing means is implemented in an external programmer devised for communication with the heart stimulator by a telemetry link, preferably also the RRT determining means is implemented in such an external programmer. Thus, an external programmer is provided with calculation capacity necessary for determining a suitable impedance limit value and this limit value is then automatically set by the programmer for RRT determination.
Brief Description of the Drawings
To explain the invention more in detail certain embodiments of the invention will now be described with reference to the drawings, on which figure 1 is a block diagram illustrating schematically a heart stimulator provided with an embodiment of the circuit according to the invention, and figure 2 is a flow chart illustrating an example of the operation of the circuit according to the invention, and figures 3a and 3b illustrate the RRT- impedance register 17 and pointer 19 which indicates actual RRT-impedance according to the invention.
Description of Preferred Embodiments
Figure 1 shows schematically in the form of a block diagram a heart stimulator connected through a lead 2 to the heart 4 of a patient. The heart stimulator comprises a battery 6 for supplying necessary electric energy to the stimulator electronics 8 and for charging a discharge capacitor 10 for delivery of stimulation pulses to the heart 4 by the lead 2.
An impedance measurement means 12 is connected to the battery 6 for measuring the internal battery impedance. A comparison means 14 is connected to the impedance measurement means 12 for comparing the measured internal impedance with a predetermined limit value. A determining means 16 is provided for determining RRT from the results of this comparison. Current measurement means 18, 20 are further provided to continuously measure the current supplied to the stimulator electronics 8 and the current delivered to the discharge capacitor 10 for stimulation pulse delivery.
Various controlling and timing functions of the heart stimulator are performed by a control unit 30. Thus, e.g. charging of the discharge capacitor 10 and delivery of stimulation pulses are controlled from the control unit 30 by schematically shown switches 32 and 34 respectively.
The control unit 30 also includes means 31 for measuring diagnostic data like electrode lead impedance, actual stimulation rate and current consumption of the stimulator electronics .
A limit value changing means 15 is provided to automatically change the limit value of the comparator means 14 in response to changes of operating conditions like measured diagnostic data of the heart stimulator, programmed operating parameters, including mode of operation of the stimulator, etc. This limit value changing means 15 includes a plurality of registers 17 storing different impedance limit values programmed at the manufacture of the device. A pointer 19 is provided to point out one specific register of this plurality of registers 17, which is storing an impedance limit value suitable for use in the comparator means 14 under the actual operating conditions of the heart stimulator. When measured diagnostic data or programmed parameters are changed the pointer 19 automatically points out another register 17 storing a value which is suitable for use for the changed operating condition.
An external programmer 22 is devised for communication with the heart stimulator electronics 8 by a telemetry link 25, 27. By this programmer 22 programmable operating conditions of the heart stimulator can be reprogrammed via the telemetry link 25, 27 and the controlling unit 30. Such a reprogramming also results in a change of the limit value used in the comparison means 14 by selection of another register 17 by the pointer 19.
Results from the RRT determining means 16 are read by the programmer 22 via the telemetry link 25, 27 as well as other operating data of the stimulator determined and stored in the control unit 30.
If one or more parameters are changed in a reprogramming operation, e.g. the pulse rate is changed such that the current consumption is drastically increased which would result in a quick discharge of the battery, an indicator 21 is activated to draw the physicians attention to this circumstance .
As an alternative the external programmer 22 can include necessary calculating means 23 for determining RRT directly from the result of the comparison performed by the comparison means 14, i.e. the RRT determining means is contained in the programmer 22. This is indicated in figure 1 by the dashed line 24 between the comparison means 14 and the implanted part 27 of the telemetry link for further communication with the external programmer 22.
As an additional safety measure RRT is preferably determined based on a worst case parameter values, e.g. 100% pacing, a resistance drop of the lead impedance of 250 Ohms, together with programmed parameters such as programmed rate, programmed mode of operations of the heart stimulator, etc.
Thus, by choosing between preprogrammed register values and pointing out a suitable value, a sufficient time between RRT and end of life of the battery is always secured based on a worst case calculation, and when a physician makes a reprogramming of the implanted medical device, the limit value for RRT determination is automatically adjusted if necessary. As an example the heart stimulator illustrated in figure 1 could be a dual chamber pacemaker having five registers 17 preprogrammed with battery impedance limit values.
Before a follow up procedure the pacemaker is programmed according to the following shipped settings: DDD-mode, basic rate 75, pulse amplitude 3,9 V, pulse width 0.5 msec, and RRT battery impedance limit value is set to 13 kOhm, see figure 3a.
During the follow-up the heart stimulator is reprogrammed to VDD-mode with basic rate 45, pulse amplitude 2.4 V, and pulse width 0.25 msec. The electrode lead impedance is measured to 750 Ohm. Based on the information above the register containing 15 kOhm value is determined with the aid of the invention to be used for RRT determination as indicated in fig 3b.
The function of one embodiment of the invention applied to a pacemaker is illustrated by the flow chart of figure 2.
Thus, the telemetry channel is opened and interrogation of the implanted pacemaker is started, block 30. Pacing para- meters having impact on RRT determination, e.g. pulse amplitude, pulse width, programmed rate, pacing mode, and if rate response function is activated, etc. are then interrogated, block 32. Also diagnostic data having impact on RRT determination, e.g. lead impedance, actual paced rate (if other than the programmed rate) , and electronics' current consumption, are interrogated, block 34. If no such values are stored measurements of necessary values are performed. A RRT limit value is then calculated based on the data retrieved and taking a worst case into account e.g. a lead impedance decrease of 250 Ohm, 100% pacing, 10% of the time at maximum sensor rate or maximum tracking rate depending on the programming of the pacemaker, and also taking a higher measured rate into account if the pacemaker is programmed to a tracking mode, i.e. XDD, block 36. Based on the calculation in block 36 it is determined which preprogrammed RRT-criteria, i.e. which preprogrammed register 17 should be used and it is then programmed into an active state (by pointer 19), block 38. If any of the pacing parameters that have impact on the RRT criteria, e.g. pulse amplitude, pulse width, programmed rate, pacing mode, etc. are changed during the follow-up session due to reprogramming of the pacemaker by the physician, a new RRT-criteria should be calculated and chosen on the basis of these new data, and other unchanged data retrieved at the beginning of the session, block 40. Finally the telemetry channel is closed and the follow-up session is ended, block 42.