Combined telemetry and magnetic field detector unit for a medical implant

Abstract: A combined telemetry and magnetic field detector unit in a medical implant, whereby the combined unit has control logic (9), a magnetic field indicator (8) and a telemetry circuit (7) containing a plurality of switches (S1-S6) and a coil unit (11-12) . The control logic (9) is arranged to operate the switches in such a way that switching between the telemetry function and the magnetic field detection function of the combined unit (2) is achieved so the coil unit (11, 12) is utilized for transmission of telemetry signals to and from a programmer (4) and for magnetic field detection. In magnetic field detection, the coil unit (11, 12), which consists of parts, is connected by the control logic (9) in such a way that the parts form a primary side and a secondary side respectively of a pulse transformer, whereby the magnetic field indicator (8) is connected by the control logic (9) to the pulse transformer's secondary side.

Description: The present invention relates generally to a medical implant. More specifically it relates to a device for detecting magnetic fields around such an implant. In a medical implant, such as a pacemaker, a magnetic field detector is used for non-invasive activation of different functions in the implant with the aid of a permanent magnet placed in the vicinity of the implant on the outside of the patient's body. Some of the functions which can be activated in e.g. a pacemaker are: disabling the pacemaker's demand function so the pacemaker adapts its operation to battery capacity and having the pacemaker operate in a special, temporary stimulation mode, e.g. in tachycardia and in conjunction with pacemaker programming. Outside the implant art, the detection of magnetic fields in a plurality of different ways, e.g. with the aid of reed switches, by changing the resonance frequency or inductance etc., is generally known. One device for determining the strength of a magnetic field is previously known through Lennart Grahm, "Elektrisk mätteknik, Analoga instrument och mätmetoder", part 2, 1977, Elektrisk mätteknik, Lund, pp 543-545. The voltage inducted in a small test body made from a ferromagnetic metal is examined with a Förster probe. The Förster probe consists of a small test body made of a ferromagnetic material with high permeability and provided with two windings, one of which used for alternating current magnetization and the other used for measuring the ensuing inducted voltage. The larger a constant magnetic field, the greater the amplitude of even harmonics when the probe is placed in a constant magnetic field. Thus, a phase detector with a reference voltage equal to twice the frequency of the excitation current can be used for supplying a signal which increases with an increase in the constant magnetic field. In the implant art, a conventional magnetic field detector consists of a reed switch. However, reed switches are sensitive and rather expensive components which also take up a relatively large amount of space in the implant. In order to eliminate the need for a reed switch, therefore, recent proposals have suggested utilization of the implant's telemetry unit so the unit can also be used for detecting the presence of a magnetic field, in addition to its telemetry function. US-4 541 431 shows one such proposal with a combined telemetry and magnetic field detector unit. It contains a conventional resonance circuit containing e.g. a coil used in telemetry for transmitting and receiving data. The resonance circuit is also used here for sensing the presence of a magnetic field whose strength exceeds a predefined value. The resonance frequency for the resonance circuit varies with the strength of the magnetic field. The resonance circuit is periodically activated, and the number of zero crossings for the signal are studied in a sensing window with a predefined duration. If a predetermined number of zero crossings occurs, this means that the strength of the magnetic field exceeds the predefined value. Thus, the known unit requires a plurality of components for detecting the frequency deviation and, thus, the magnetic field. The object of the present invention is to set forth a simplified and improved device for detecting magnetic fields around an implant. According to the invention, this object is achieved with the features set forth in the main claim. According to the invention, the reed switch is thereby eliminated, in contrast to the solution proposed in US-4 541 431 which concurs with the main claim's preamble, by the use of a pulse transformer for magnetic field detection, whereby the pulse transformer is devised so it can also be used for the implant's telemetry function. With a pulse transformer, magnetic field detection is performed by determining the change in the pulse transformer's response to an applied pulse. This can be achieved simply and with few components, and the reliability of detection accordingly increases simultaneously. The configuration in the above-mentioned device with two coils for determining the strength of a constant magnetic field requires a specific signal generator for the excitation signal and a further generator for the reference voltage frequency in the phase comparator. Thus, this known device is more complex than the device set forth in the main claim, in addition to the fact that it is not intended for the implant art nor is it combinable with telemetry. Advantageous embodiments of the invention, in which e.g. the threshold value for magnetic field detection is programmable, are set forth in the dependent claims. The invention will now be explained in greater detail, referring to six FIGURES in which FIG. 1 is a block diagram of an implant incorporating the invention; FIG. 2 is a block diagram of the combined telemetry and magnetic field detector unit; FIG. 3 is a schematic circuit diagram of a first, preferred embodiment of the invention; FIG. 4 shows a number of curve morphologies for the coil signals in the first preferred embodiment; FIG. 5 is a schematic circuit diagram of a second, preferred embodiment of the invention; FIG. 6 shows a number of curve morphologies for the coil signals in the second preferred embodiment; In the FIGURES the same reference designations are used for similar or identical elements. The invention is primarily exemplified in the description by an implant in the form of a pacemaker. It will be obvious to anyone well-versed in the art that no limitation is to be assumed here and that the implant can also be e.g. a defibrillator, an insulin pump or any other medical device. In FIG. 1 is shown the implant 1 inserted under the skin (designated with a dashed line in the FIGURE), an organ 6 and a programmer 4. The implant 1 contains a telemetry unit 2 for duplex communications with the programmer 4, a programmable control device 3 for controlling and coordinating the units in the implant and a therapy unit 5 for supplying appropriate therapy to the organ 6, all in accordance with the prior art in the field. As previously noted, the implant 1 can be exemplified with a pacemaker, the organ 6 then being a heart and the therapy unit 5 a pulse generator, controlled by the control unit 3, for stimulating the heart 6 with stimulation pulses at a rate which can be set by a physician and transmitted to the implant with the aid of the programmer 4. FIG. 2 is a block diagram of the telemetry unit 2. According to the invention, this unit is a combined telemetry and magnetic field detector unit and contains telemetry circuits 7 used for transmitting and receiving signals to and from the programmer 4 and for detecting magnetic fields, a magnetic field indicator 8 which indicates the presence of a magnetic field, control logic 9 for controlling the telemetry unit 2 and a discriminator circuit 10 for signals received by the telemetry circuits 7. The function of the parts in the telemetry units will be apparent from the description below. In FIG. 3 is shown in greater detail a first preferred embodiment of the invention, whereby the telemetry circuits 7 contain a coil unit 11, 12 consisting of a first coil 11 and a second coil 12, a drive voltage source, V DD , a grounded level, V SS , a capacitor 13, a resistor 14 and a connection point 20. With the aid of a plurality of switches S1-S6 controlled by the control logic 9, the components in the telemetry circuits are enabled according to the function the telemetry circuits are to perform. The drive voltage source, V DD , is connected, via a switch S1. to the connection point 20. The first coil 11 and the second coil 12 are connected in series, whereby the first coil 11 is connected to the connection point 20 and the second coil 12 is connected, via a switch S3, to V SS . The capacitor 13 is connected to the connection point 20 and, via a switch S5, to V SS . The resistor 14 is connected to the connection point 20 and, via a switch S6, to V SS . The connection between the coils 11 and 12 is connected, via a switch S2, to V SS . Between S3 and the second coil 12 there is a connection, via a switch S4, to the first input terminal of the magnetic field indicator 8. The discriminator circuit 10 is connected to the connection point 20. The device senses at a specific interval, e.g. once a second, whether or not a magnetic field is present. When this magnetic field test is performed, the switches S2 and S4 are closed, whereas S3, S5 and S6 are open. Thus, the second coil 12 is connected by switch S4 to the magnetic field indicator 8. The indicator consists of a differential amplifier with a second input terminal 18 at which the desired threshold value for magnetic field detection is set. The setting can be made by telemetry from a programmer. The switch S1 is closed for a specific period of time, e.g. of about 30 µs. In this period, the first coil is therefore connected to the drive voltage, V DD , whereby a brief pulse 15 is applied to the first coil 11 (see FIG. 4). A signal 16 is thereby inducted in the second coil 12 which is inductively coupled to the first coil 11. The coils jointly form a pulse transformer. In FIG. 4 is shown certain curve morphologies for signals on the coils, without (dashed line) and with (solid line) an external magnetic field. In the FIGURE, the horizontal axis represents a time axis, graduated in µs, whereas the vertical axis represents voltage and is graduated in volts. If an external magnetic field is present, the permeability of the core declines. This leads in turn to a decline in mutual inductance between the coils. This can be seen in the curve morphology for the second coil which displays a reduced initial amplitude 17 (the peak amplitude of the first half-period) when an external magnetic field is present. Comparing this initial amplitude in the magnetic field indicator 8 with a first threshold value, V 1 , set on the magnetic field indicator and indicating when the initial amplitude drops below this value results in an indication of the presence of a magnetic field. A typical value for V 1 can be 20% below the amplitude obtained when no magnetic field is present. When the device is to be used for telemetry, the switches S2 and S4 are open, whereas S3 is closed. S5 is closed in transmission, i.e. the capacitor 13 is connected to V SS , whereas S6 is open, i.e. the resistor 14 is not connected to V SS . The coils 11, 12 jointly form, with the capacitor 13, a resonance circuit which, with a suitable choice of component values, has a preferred resonance frequency of 8 Hz and a high Q value. The switch S1 is quickly opened and closed, causing brief pulses to be applied to the resonance circuit in which the coils constitute the transmission antenna for the electromagnetic waves. The pulse trains generated in this manner contain the information to be transmitted to the programmer 4. When signals are received from the programmer 4, S1 and S5 are open and S6 is closed. The coils 11, 12 pick up the signals transmitted by the programmer, and these signals are sent to the control device 3 via the discriminator circuit 10. The discriminator circuit 10 contains e.g. a comparator in which the signal level for the received signal is compared to a predefined threshold level, and only signals exceeding this level are processed. In reception, the resistor 14 dampens the signal so no residual voltage remains in the coils when transmission or reception is to start. Referring to FIGURES 5 and 6, a second preferred embodiment for the telemetry circuits 7 will now be described. Exactly as in the first preferred embodiment, these circuits contain a first coil 11 and a second coil 12 wound around the same core, a capacitor 13, a resistor 14, a connection point 20, a drive voltage V DD and a grounded level, V SS . With the aid of a plurality of switches S1-S8, which are controlled by the control logic 9, the respective components are enabled according to the function the telemetry circuits are to perform. The switches S1 to S6 have the same placement and function as previously described. In addition, the first coil 11 is connected, via a switch S7, to the second coil 12, and the connection point 20 is connected, via a switch S8, to the connection between the switch S7 and the second coil 12. The switch S7 is located between the connection socket which is connected to V SS via S2, and the connection socket is connected to the connection point 20 via S8. The device senses at a specific interval, e.g. once a second, whether or not any magnetic field is present. When this magnetic field test is performed, the switches S2, S4 and S8 are closed, whereas S3, S5, S6 and S7 are open. Thus, the second coil 12 is connected by switch S4 to the magnetic field indicator 8. The switch S1 is closed for a specific period of time, whereby a pulse 15 is emitted, in the same way as previously described, and applied to the first coil 11, and a signal 16 is inducted in the second coil, whereby the coils jointly form a pulse generator. In FIG. 6 is shown curve morphologies for signals on the coils, without (dashed line) and with (solid line) an external magnetic field. The FIGURE uses the same designations for the axes as in FIG. 4. If no external magnetic field is present, the signal in the second coil 12 does not change when a pulse 15 is applied to the first coil 11. If a magnetic field is present, however, the curve morphology for the second coil will display a rising initial amplitude 17. Comparing this initial amplitude in the magnetic field indicator 8 with a second threshold value, V 2 , set on the magnetic field indicator and indicating when the initial amplitude exceeds this value results in an indication of the presence of a magnetic field. When the device is to be used for telemetry, the switches S2, S4 and S8 are open, whereas S3 and S7 are closed. Telemetry transmission is otherwise performed in the same way as described above for the first preferred embodiment. According to the main claim, the coil unit 11, 12 consists of two parts which in magnetic field detection are interconnected by the control logic 9 in such a way that they form the primary side and secondary side respectively of a, pulse transformer. In the described, preferred embodiments, the primary side consists of a first coil 11 and secondary side of a second coil 12. In other embodiments, not described here, the primary side and secondary side respectively can consist of two or a plurality of coils. In the two described embodiments, the coils are connected in series when the device is used for telemetry. In other embodiments of the invention, the coils can be connected in parallel, or only one of the coils is utilized for telemetry. There is no magnetic field detection while telemetry is in progress. The threshold values V 1 and V 2 with which the initial amplitude of the signal on the second coil is compared can, as noted above, be set by telemetry when the implant is in place in a patient's body or during implant fabrication. Reference designations 1 Medical implant 2 Combined telemetry and magnetic field detector unit 3. Programmable control device 4. Programmer 5. Therapy unit 6. Human organ 7. Telemetry circuits 8. Magnetic field indicator 9. Control logic 10. Discriminator unit 11. First coil 12. Second coil 13. Capacitor 14. Resistor 15. Curve morphology for the first coil 16. Curve morphology for the second coil 17. Initial amplitude 18. Input terminal for setting a threshold value 20. Connection point

Claims: A combined telemetry and magnetic field detector unit (2) in a medical implant, whereby the combined unit (2) has control logic (9), a magnetic field indicator (8) and a telemetry circuit (7) containing a drive voltage source (V DD ), a plurality of switches (S1-S8) and a coil unit (11, 12), and whereby the control logic (9) is arranged to operate the switches in such a way that switching is achieved between the telemetry function and the magnetic field detection function of the combined unit (2) so the coil unit (11, 12) is utilized for transmission of telemetry signals to and from a programmer (4) and for magnetic field detection respectively, characterized in that the coil unit (11, 12) consists of parts which in magnetic field detection are interconnected by the control logic (9) in such a way that they form a primary side and a secondary side of a pulse transformer, whereby the magnetic field indicator (8) is connected by the control logic (9) to the pulse transformer's secondary side. A combined unit according to claim 1, characterized in that the part of the coil unit forming the primary side consists of a first coil (11), and the part of the coil unit forming the secondary side consists of a second coil (12). A combined unit according to claim 2, characterized in that the first coil (11) and the second (12) coil share the same core. A combined unit according to claim 2 or 3, characterized in that the second coil (12), when there is a switch to the magnetic field detection function, is arranged for connection to the magnetic field indicator (8) via a first switch (S4) of the switches (S1-S8). A combined unit according to claim 4, characterized in that the magnetic field indicator (8) has at least one threshold value (V 1 , V 2 ) and is arranged to compare the threshold value (V 1 , V 2 ) to a signal, inducted in the second coil (12) by a pulse in the first coil (11), whereby the pulse is generated when the first coil (11) is connected to the drive voltage source (V DD ), via a second switch (S1) of the switches (S1-S8), in a time period long enough for signal induction. A combined unit according to any of the preceding claims, characterized in that the control logic (9) is arranged to perform periodic magnetic field detection. A combined unit according to claim 5, characterized in that the magnetic field indicator (8) has an input terminal (18) for setting the threshold value (V 1 , V 2 ) for magnetic field detection, whereby the input terminal (18) is arranged to receive a telemetric, threshold value-determining signal from the programmer (4). A combined unit according to claims 1-7, characterized in that the drive voltage source (V DD ) is connected, via the second switch (S1), to a connection point (20), the first coil (11) and the second coil (12) are connected in series, whereby the first coil is connected to the connection point (20) and the second coil (12) is connected, via a third switch (S3), to a grounded level (V SS ), a capacitor (13) is connected to the connection point (20) and, via a fourth switch (S5), to the grounded level (V SS ), a resistor (14) is connected to the connection point (20) and, via a fifth switch (S6), to the grounded level (V SS ), the connection between the first coil (11) and the second coil (12) is connected, via a sixth switch (S2), to the grounded level (V SS ), whereby the first and sixth switches (S2, S4) are closed, and the third, fourth and fifth switches (S3, S5, S6) are open in magnetic field detection. A combined unit according to any of claims 5-8, characterized in that the magnetic field indicator (8) emits a signal in magnetic field detection if the initial amplitude (17) for a signal inducted in the second coil (12) is less than a first threshold value (V 1 ). A combined unit according to claim 8, characterized in that the first coil (11) is connected, via a seventh switch (S7), to the second coil (12) and the connection point (20) is connected, via an eighth switch (S8), to an output terminal on the connection between the seventh switch (S7) and the second coil, whereby the first, sixth and eighth switches (S2, S4, S8) are closed and the third, fourth, fifth and seventh switches (S3, S5, S6, S7) are open in magnetic field detection. A combined unit according to claim 10, characterized in that the magnetic field indicator (8) in magnetic field detection emits a signal if the initial amplitude (17) of the signal inducted in the second coil (12) is greater than a second threshold value (V 2 ).

Inventors: CARLSON MICHAEL, NILSSON KENTH, SUNDBOM INGA, CARLSON, MICHAEL, NILSSON, KENTH, SUNDBOM, INGA
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1995
Main classification: A61N1
IPC codes: A61N1, A61M1