Source: http://www.google.com/patents/US6597950?dq=5,973,252
Timestamp: 2015-05-06 19:44:00
Document Index: 8119641

Matched Legal Cases: ['art.\n4', 'art.\n17', 'art.\n28', 'art 110', 'art 110', 'art 110']

Patent US6597950 - Cardiac rhythm management system with painless defibrillation lead impedance ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA cardiac rhythm management system includes a defibrillation lead impedance measurement system by which defibrillation lead impedance is measured using a test current source different from the defibrillation output supply. A resulting voltage is measured to determine the defibrillation lead impedance....http://www.google.com/patents/US6597950?utm_source=gb-gplus-sharePatent US6597950 - Cardiac rhythm management system with painless defibrillation lead impedance measurementAdvanced Patent SearchPublication numberUS6597950 B2Publication typeGrantApplication numberUS 09/776,306Publication dateJul 22, 2003Filing dateFeb 2, 2001Priority dateJan 25, 1999Fee statusPaidAlso published asUS6317628, US20010007056, WO2000043065A1Publication number09776306, 776306, US 6597950 B2, US 6597950B2, US-B2-6597950, US6597950 B2, US6597950B2InventorsWilliam J. Linder, Keith R. MaileOriginal AssigneeCardiac Pacemakers, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (21), Referenced by (5), Classifications (6), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetCardiac rhythm management system with painless defibrillation lead impedance measurement
US 6597950 B2Abstract
a defibrillation electrode; a test current source coupled to the defibrillation electrode; a voltage measurement circuit coupled to the defibrillation electrode; a defibrillation output supply coupled to the defibrillation electrode; a control circuit coupled to the test current source, the voltage measurement circuit and the defibrillation output supply, the control circuit having test and defibrillation configurations, wherein, when the control circuit is in the test configuration, the control circuit is configured to cause the test current source to source a test current to the defibrillation electrode and to cause the voltage measurement circuit to measure a voltage at the defibrillation electrode and, when the control circuit is in the defibrillation configuration, the control circuit is configured to cause the defibrillation output supply to supply a defibrillation voltage to the defibrillation electrode; and a second defibrillation electrode and a test current sink coupled to the second defibrillation electrode, wherein, when the control circuit is in the test configuration, the test current is sunk by the test current sink. 2. The apparatus of claim 1, wherein the test current source sources a test current to the defibrillation electrode having an amplitude of less than or equal to 20 milliamperes.
3. The apparatus of claim 1, wherein the defibrillation electrode is configured to apply the test current to a heart, and the test current source provides a test current having a test stimuli energy less than an energy required to cause a depolarization and contraction of the heart.
4. The apparatus of claim 1, further comprising a diode, wherein the test current source has an output node coupled to the defibrillation electrode through the diode.
5. The apparatus of claim 4, wherein the voltage measurement circuit has an input node coupled to the defibrillation electrode through the diode.
6. The apparatus of claim 5, further comprising a zener diode, wherein the output node of the test current source and the input node of the voltage measurement circuit are coupled to ground through the zener diode.
7. The apparatus of claim 1, further comprising a switch circuit coupled between the defibrillation output supply and the defibrillation electrode, wherein the switch circuit is configured to decouple the defibrillation voltage from the defibrillation electrode when the control circuit is in the test configuration, and to couple the defibrillation voltage to the defibrillation electrode when the control circuit is in the defibrillation configuration.
8. The apparatus of claim 7, further comprising a diode coupled in series with the switch circuit between the defibrillation output supply and the defibrillation electrode.
9. The apparatus of claim 1, wherein the test current has a level, and the control circuit is further configured to determine an impedance between the defibrillation electrode and the second defibrillation electrode based upon the measured voltage and the level of the test current.
10. The apparatus of claim 9, wherein the control circuit is configured to determine the impedance between the defibrillation electrode and the second defibrillation electrode by determining a quotient between the measured voltage and the level of the test current.
11. The apparatus of claim 9, wherein the test current level is a predetermined level.
12. The apparatus of claim 9, wherein the test current level is a level measured prior to implantation of the apparatus.
a first defibrillation electrode; a second defibrillation electrode; a first test current source coupled to the first defibrillation electrode; a second test current source coupled to the second defibrillation electrode; a first voltage measurement circuit coupled to the first defibrillation electrode; a defibrillation output supply coupled to the first and second defibrillation electrodes; and a control circuit coupled to the first and second test current sources, the first voltage measurement circuit and the defibrillation output supply, the control circuit having test and defibrillation configurations wherein, when in the test configuration, the control circuit causes the first test current source to source a first test current to the first defibrillation electrode, the first voltage measurement circuit to measure a voltage at the first defibrillation electrode when the first test current is being sourced, and the second test current source to source a second test current to the second defibrillation electrode when the first test current is not being sourced, and, when in the defibrillation configuration, the control circuit causes the defibrillation output supply to supply a defibrillation voltage to the first and second defibrillation electrodes. 14. The apparatus of claim 13, further comprising a first test current sink coupled to the first defibrillation electrode and a second test current sink coupled to the second defibrillation electrode, wherein the first test current sink is configured to sink the second test current and the second test current sink is configured to sink the first test current.
15. The apparatus of claim 13, wherein the first test current and the second test current each have an amplitude of less than or equal to 20 milliamperes.
16. The apparatus of claim 13, wherein the first and second defibrillation electrodes are configured to apply the first and second test currents to a heart, and the first and second test currents each have a test stimuli energy less than an energy required to cause a depolarization and contraction of the heart.
17. The apparatus of claim 13, further comprising a first diode and a second diode, the first test current source having an output node coupled to the first defibrillation electrode through the first diode and the second test current source having an output node coupled to the second defibrillation electrode through the second diode.
18. The apparatus of claim 17, wherein the first voltage measurement circuit has an input node coupled to the first defibrillation electrode through the first diode.
19. The apparatus of claim 18, further comprising a zener diode, wherein the output node of the first test current source and the input node of the first voltage measurement circuit are coupled to ground through the zener diode.
20. The apparatus of claim 13, further comprising a second voltage measurement circuit coupled to the second defibrillation electrode, wherein the control circuit further causes the second voltage measurement circuit to measure a second voltage at the second defibrillation electrode when the second test current is being sourced.
21. The apparatus of claim 20, wherein the first and second test currents each have a level, and the control circuit is further configured to determine an impedance between the first and second defibrillation electrodes based upon the first and second measured voltages and the levels of the first and second test currents.
22. The apparatus of claim 21, wherein the control circuit is configured to determine the impedance between the first and second defibrillation electrodes by determining a first impedance using a quotient between the first measured voltage and the level of the first test current, determining a second impedance using a second quotient between the second measured voltage and the level of the second test current, and averaging the first and second impedances.
23. The apparatus of claim 21, wherein the test current levels are predetermined levels.
24. The apparatus of claim 21, wherein the test current levels are levels measured prior to implantation of the apparatus.
25. A method of operating a defibrillation device having a defibrillation electrode, a test current source, a voltage measurement circuit, and a defibrillation output supply, comprising:
causing the test current source to source a test current to the defibrillation electrode and causing the voltage measurement circuit to measure a voltage at the defibrillation electrode when operating in a test current mode; causing the defibrillation output supply to supply a defibrillation voltage to the defibrillation electrode when operating in a defibrillation mode of operation; and receiving the test current using a second defibrillation electrode, and sinking the test current using a test current sink. 26. The method of claim 25, further comprising determining an impedance between the first and the second defibrillation electrodes based at least upon the measured voltage.
27. The method of claim 25, wherein causing the test current source to source a test current includes causing the test current source to provide a test current having a test stimuli energy less than an energy required to cause a depolarization and contraction of the heart.
28. A method of operating a defibrillation device having first and second defibrillation electrodes, first and second test current sources, a voltage measurement circuit, and a defibrillation output supply, comprising:
causing the first test current source to source a first test current to the first defibrillation electrode, causing the voltage measurement circuit to measure a voltage at the first defibrillation electrode, and causing the second test current source to source a second test current to the second defibrillation electrode when operating in a test current mode; and causing the defibrillation output supply to supply a defibrillation voltage to the first and second defibrillation electrodes when operating in a defibrillation mode of operation. 29. The method of claim 28, further comprising receiving the first test current using the second defibrillation electrode and sinking the first test current using a first test current sink.
30. The method of claim 29, further comprising receiving the second test current using the first defibrillation electrode and sinking the second test current using a second test current sink.
31. The method of claim 28, further comprising determining an impedance between the first and the second defibrillation electrodes based at least upon the measured voltage.
32. The method of claim 28, wherein the first and the second test currents each have a test stimuli energy less than an energy required to cause depolarization and contraction of a heart.
This application is a continuation of U.S. application Ser. No. 09/236,911, filed on Jan. 25, 1999, now U.S. Pat. No. 6,317,628 the specification of which is incorporated herein by reference.
One problem that arises in cardiac rhythm management devices is in determining defibrillation or �shocking� lead impedance. The defibrillation lead impedance includes the effective resistance of the leadwire that couples the cardiac rhythm management device to the heart for delivering the electrical defibrillation countershock at defibrillation electrodes located at or near the heart. The defibrillation lead impedance also includes the effective resistance of the body tissue (e.g., the heart) and body fluids located between the defibrillation electrodes. The defibrillation lead impedance due to the leadwire and heart resistance is generally around 50 Ω, but can range from 15 Ω to 100 Ω.
The present system provides, among other things, a cardiac rhythm management system that provides a painless technique of measuring defibrillation lead impedances without delivering a painful high voltage defibrillation countershock. As a result, the defibrillation lead impedance measurement can be performed occasionally, periodically, or even routinely because performing the measurement does not cause discomfort to the patient. Moreover, the present technique allows a defibrillation countershock therapy to be dynamically adjusted, based at least partially on the measured values of defibrillation lead impedance. Furthermore, the present system uses test stimuli energies (e.g., amplitude and pulsewidth) that are less than the energy required to �capture� the heart (i.e., less than the �pacing threshold� energy that causes a resulting depolarization and heart contraction), ensuring that the present system is both painless and nondisruptive to the underlying paced or intrinsic heart rhythm.
In the embodiment of FIG. 1, cardiac rhythm management system 100 includes a cardiac rhythm management device 105 coupled to heart 110 via one or more endocardial or epicardial leadwires, such a pacing leadwire or a defibrillation leadwire 115. Defibrillation leadwire 115 includes one or more defibrillation electrodes, such as for delivering defibrillation countershock (�shock�) therapy via first defibrillation electrode 120A and/or second defibrillation electrode 120B. Defibrillation leadwire 115 may also include additional electrodes, such as for delivering pacing therapy via first pacing electrode 125A (e.g., a �tip� electrode) and/or second pacing electrode 125B (e.g., a �ring� electrode). Defibrillation electrodes 120A-B and pacing electrodes 125A-B are typically disposed in or near one or more chambers of heart 110.
FIG. 3 is a generalized signal waveform diagram illustrating generally, by way of example, but not by way of limitation, one embodiment of a technique of operating system 100, such as illustrated in part in FIG. 2, for performing a defibrillation lead impedance measurement. At time, t0 first switch 225, second switch 230, third switch 240, and fourth switch 245 are all turned off, such as by control signals from controller 235.
At approximately time t1, second switch 230 is turned on, and first current source/sink circuit 255 is activated to source a test current I250 of predetermined approximately constant magnitude I250A (e.g., approximately between 10 to 20 milliamperes) through first diode 250, first defibrillation electrode 120A, and heart resistance 200. The current I250 is received by second defibrillation electrode 120B, second switch 230, and returned through ground node 220. At approximately time t2, first current source/sink circuit 255 and second switch 230 are turned off.
During the time period between t1 and t2, a voltage V252 is generated at node 252 in response to the test current I250 flowing through an effective series resistance that includes the �on� resistance of first diode 250, the defibrillation leadwire resistance, the first defibrillation electrode 120A resistance, the heart resistance 200, the second defibrillation electrode 120B resistance, and the �on� resistance of second switch 230. In one embodiment, during this time, node 272 is coupled to the ground voltage at node 220 by second current source/sink circuit 275 such that second diode 270 is turned off.
At some time during the time period between t1 and t2, the voltage V252 stabilizes and is then sampled by voltage measurement circuit 260, obtaining the measured voltage V252A. A first indication of the defibrillation lead impedance, Z1, is obtained by dividing the measured voltage V252A by the known current magnitude I250A. In this embodiment, Z1 includes the �on� resistances of first diode 250 and second switch 230, as discussed above.
During the time period between t3 and t4, a voltage V272 is generated at node 272 in response to the test current I270 flowing through an effective series resistance that includes the �on� resistance of second diode 270, the defibrillation leadwire resistance, the second defibrillation electrode 120B resistance, the heart resistance 200, the first defibrillation electrode 120A resistance, and the �on� resistance of first switch 225. In one embodiment, during this time, node 252 is coupled to the ground voltage at node 220 by first current source/sink circuit 255 such that first diode 250 is turned off.
According to one aspect of the present technique, the defibrillation lead impedance measurement test currents are approximately �charge-balanced,� such that the charge sourced at first electrode 120A during the time period between t1 and t2 is approximately equal to the charge sunk at first electrode 120A during the time period between t3 and t4. Similarly, the charge sunk at second electrode 120B during the time period between t1 and t2 is approximately equal to the charge sourced at second electrode 120B during the time period between t3 and t4. In one embodiment, by way of example, but not by way of limitation, the time period t4−t3 is approximately equal to the time period t2−t1, and the test current magnitude I270A is approximately equal to the test current magnitude I250A. In a further embodiment, by way of example, but not by way of limitation, t4−t3≈t2−t1≈10 to 100 microseconds (e.g., 32 microseconds), I270A≈I250A≈10 to 20 milliamperes, and t3−t2≈1.5 milliseconds. However, it is understood that other time periods or test current levels could also be used.
In a further embodiment, a second defibrillation lead impedance measurement is optionally obtained. In one such example, at some time during the time period between t3 and t4, the voltage V272 stabilizes and is then sampled by voltage measurement circuit 280, obtaining the measured voltage V272A. A second indication of the defibrillation lead impedance, Z2, is obtained by dividing the measured voltage V272A by the known current magnitude I270A. In this embodiment, Z2 includes the �on� resistances of second diode 270 and first switch 225, as discussed above. In one embodiment, the defibrillation lead impedance is determined by averaging Z1 and Z2. However, it is understood that system 100 does not require the use of more than one defibrillation lead impedance measurement.
At some time during the time period between t1, and t2, the voltage V252 stabilizes and is then sampled by first voltage measurement circuit 260, obtaining first measured voltage V252A. Similarly, at some time during the time period between t2 and t3, the voltage V252 stabilizes and is then sampled by first voltage measurement circuit 260, obtaining second measured voltage V252B. A differential first indication of the defibrillation lead impedance, Z1D, is obtained according to Equation 1. In one example, the operations described by Equation 1 are carried out in controller 235 which, in one embodiment, includes a peripheral analog-to-digital (A/D) converter for converting the measured voltage differences into digital values before performing the calculation illustrated by Equation 1. Z 1  D = V 252  A - V 252  B I 250  A - I 250  B ( 1 ) In Equation 1, first voltage measurement circuit 260 performs a differential voltage measurement using the two sampled voltages V252A and V252B. The difference between these two sampled voltages is digitized by the A/D converter. In one embodiment, the difference I250A−I250B is a known difference between predetermined current magnitudes, such that it need undergo measurement and A/D conversion. Instead, the known digital value corresponding to the difference I250A−I250B is stored digitally in device 105, or alternatively in an external programmer, for carrying out the calculation of Equation 1. In another embodiment, the values of I250A and I250B are measured during manufacture of device 105, and these measured values (or the difference between them) are stored digitally in device 105, or alternatively in an external programmer, for use in the calculation of Equation 1. This provides a more accurate measurement because it accounts for variability in the test current amplitudes between particular ones of devices 105.
The current magnitudes I250A and I250B are selected to be sufficiently small as to avoid significant discomfort to the patient (e.g., I250A≈20 milliamperes and I250B≈10 milliamperes, or vice-versa). The selected current magnitudes I250A and I250B are sufficiently large so that the series combination of the �on� resistances of first diode 250 and second switch 230 are suitably small (e.g., 5 Ω total series �on� resistance) compared to the smallest expected actual lead impedance (e.g., 15 Ω) of the leadwire and heart. By using the differential lead impedance measurement of Equation 1, the effect of first diode 250 and second switch 230 is canceled or substantially reduced.
According to one aspect of the present technique, the defibrillation lead impedance measurement test currents are approximately �charge-balanced,� such that the charge sourced at first electrode 120A during the time period between t1, and t3 is approximately equal to the charge sunk at first electrode 120A (sourced by electrode 120B) during the time period between t4 and t6. In one embodiment, by way of example, but not by way of limitation, t6−t5≈t3−t2, and t5−t4≈t2−t1, and I270A≈I250A, and I270B≈I250B. In a further embodiment, by way of example, but not by way of limitation, t6−t5≈t3−t2≈32 microseconds, and t5−t4≈t2−t126 32 microseconds, and I270A≈I250A≈20 milliamperes, and I270B≈I250B≈10 milliamperes, and t4−t3≈1.5 milliseconds. However, it is understood that other time periods or test current levels could also be used.
In one further embodiment, a second differential defibrillation lead impedance measurement is optionally obtained. At some time during the time period between t4 and t5, the voltage V272 stabilizes and is then sampled by second voltage measurement circuit 280, obtaining third measured voltage V272A. Similarly, at some time during the time period between t5 and t6, the voltage V272 stabilizes and is then sampled by second voltage measurement circuit 280, obtaining fourth measured voltage V252B. A differential second indication of the defibrillation lead impedance, Z2D, is obtained according to Equation 2. In one example, the operations described by Equation 2 are carried out in controller 235 which, in one embodiment, includes a peripheral analog-to-digital (A/D) converter for converting the measured voltage differences into digital values before performing the calculation illustrated by Equation 2. Z 2  D = V 272  A - V 272  B I 270  A - I 270  B ( 2 ) In Equation 2, second voltage measurement circuit 280 performs a differential voltage measurement using the two sampled voltages V272A and V272B. The difference between these two sampled voltages is digitized by the A/D converter. In one embodiment, the difference I270A−I270B is a known difference between predetermined current magnitudes, such that it need not undergo measurement and A/D conversion. Instead, the known digital value corresponding to the difference I250A−I250B is stored digitally in device 105, or alternatively in an external programmer, for carrying out the calculation of Equation 2. In another embodiment, the values of I270A and I270B are measured during manufacture of device 105, and these measured values (or the difference between them) are stored digitally in device 105, or alternatively in an external programmer, for use in the calculation of Equation 2. This provides a more accurate measurement because it accounts for variability in the test current amplitudes between particular ones of devices 105.
The current magnitudes I270A and I270B are selected to be sufficiently small as to avoid significant discomfort to the patient (e.g., I270A≈20 milliamperes and I270B≈10 milliamperes, or vice-versa). The selected current magnitudes I270A and I270B are sufficiently large so that the �on� resistances of second diode 270 and first switch 225 are suitably small (e.g., 5 Ω total series �on� resistance) compared to the smallest expected actual lead impedance (e.g., 15 Ω) of the leadwire and heart. By using the differential defibrillation lead impedance measurement of Equation 2, the effect of second diode 270 and first switch 225 is canceled or substantially reduced. In this embodiment, the defibrillation lead impedance, ZM, is determined by averaging Z1D and Z2D. This advantageously minimizes effects on the defibrillation lead impedance measurement resulting from the different directions of the test current through R200 during the two separate defibrillation lead impedance measurements. However, it is understood that system 100 does not require the use of more than one defibrillation lead impedance measurement.
First zener diode 500 protects first current source/sink circuit 255 and first voltage measurement circuit 280 from dangerously large positive and/or negative voltages at node 252, such as during a defibrillation countershock delivered to heart 110 by system 100 or by an external defibrillator. During such a defibrillation countershock delivered by system 100, for example, first zener diode 500 clamps node 252 at a safe voltage (i.e., at approximately the reverse breakdown voltage of first zener diode 500) and ensures that first diode 250 remains off. Moreover, if a sufficiently large negative voltage is present at first defibrillation electrode 120A, such as during a defibrillation countershock delivered by an external defibrillator, for example, first diode 250 and first zener diode 500 each turn on. This clamps the voltage at first defibrillation electrode 120A (i.e., at a negative voltage that is equal to the series �on� voltage of first diode 250 and first zener diode 500), thereby protecting first switch 225 against dangerously large negative voltages. This also clamps the voltage at node 252 (i.e., at a negative voltage that is equal to the �on� voltage of first zener diode 500), thereby protecting first current source/sink 255 and first voltage measurement circuit 260 against dangerously large negative voltages. Second zener diode 505 operates analogously to first zener diode 500, protecting second current source/sink circuit 275 and first voltage measurement circuit 280 from dangerously large positive and/or negative voltages at nodes 272, and protecting second switch 230 from dangerously large negative voltages at second defibrillation electrode 120B.
When system 100 is operated as illustrated in FIG. 4A, switches 920A-D are operated as described below. Before time t1, switches 920A-D are all off, S1*=S2*=S3*=S1*=VCC=(high). At time t1, switches 920A and 920B are turned on (i.e., S1*=S2*=VSS=(low)), such that current mirror transistors 915A-B together source a test current of amplitude I250A to first defibrillation electrode 120A. At time t2, switch 920A is turned off (i.e., S1*=high, and S2*=low), such that current mirror transistor 915B alone sources a test current of amplitude I250B to first defibrillation electrode 120A. At time t3, switch 920B is turned off (i.e., S1*=S2*=S3*=S1*=high); interrupting the test current to first defibrillation electrode 120A. At time t4, switches 920C and 920D are turned on (i.e., S3*=S4*=low), such that current mirror transistors 915C-D together source a test current of amplitude I270A to second defibrillation electrode 120B. At time t5, switch 920C is turned off (i.e., S3*=high, and S4*=low), such that current mirror transistor 915D alone sources a test current of amplitude I270B to second defibrillation electrode 120B. At time t6, switch 920D is turned off (i.e., S1*=S2*=S3*=S1*=high), interrupting the test current to second defibrillation electrode 120B.
When system 100 is operated as illustrated in FIG. 4B, switches 920A-D are operated as described below. Before time t1, switches 920A-D are all off, S1*=S2*=S3*=S1=VCC=(high). At time t1, switch 920A is turned on (i.e., S1*=low), such that current mirror transistor 915A alone sources a test current of amplitude I250A to first defibrillation electrode 120A. At time t2, switch 920B is turned on (i.e., S1*=S2*=low), such that current mirror transistors 915A-B together source a test current of amplitude I250B to first defibrillation electrode 120A. At time t3, switches 920A-B are turned off (i.e., S1*=S2*=S3*=S1*=high); interrupting the test current to first defibrillation electrode 120A. At time t4, switch 920C is turned on (i.e., S3*=low), such that current mirror transistor 915C alone sources a test current of amplitude I270A to second defibrillation electrode 120B. At time t5, switch 920D is turned on (i.e., S3*=S4*=low), such that current mirror transistors 915C-D together source a test current of amplitude I270B to second defibrillation electrode 120B. At time t6, switches 920C-D are turned off (i.e., S1*=S2*=S3*=S1*=high), interrupting the test current to second defibrillation electrode 120B.
Operation of system 100 according to FIGS. 2, 4A, 9 and 11 is described below. During the time period t2−t1 (i.e., φ1) a first voltage measurement is performed at node 252, as described with respect to FIG. 10. During the time period t3−t2 (i.e., φ2) a second voltage measurement is performed at node 252, as described with respect to FIG. 10.
In one embodiment, the lead impedance measurement circuitry in device 105 is calibrated during manufacturing to improve the precision of the measurement of defibrillator lead impedance. According to one such calibration technique, two different calibration resistances of known values (e.g., R1=30 Ω and R2=80Ω) are measured as Z1M and Z2M, respectively, using one of the above-described defibrillation lead impedance measurement circuits and methods. Controller 235 performs a calibration/correction according to Equation 3 to obtain a corrected measured value, ZCM, of an unknown defibrillation lead impedance from its measured value, ZM, the measured values of the known resistances, Z1M and Z2M, and the known values of the calibration resistances, R1 and R2. Z CM = R 1 + Z 2  M - Z 1  M R 2 - R 1  ( Z M - Z 1  M ) ( 3 ) Equation 3 represents one form of performing a point-slope calibration. There are many other possible techniques of performing point-slope calibrations, and such other particular implementations are included within the scope of the present system. In one embodiment, for example, the calibration is performed using the measured voltage differences in Equations 1 and 2 without undergoing the computational step of converting the measured voltage differences into corresponding impedances. Furthermore, it is understood that calibration/correction is performed either in the implanted device 105, or is alternatively performed in an external programmer operating on data that is communicatively coupled from the implanted device 105 to the external programmer.
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