Apparatus and method for the control of an implantable medical device

An implantable medical device includes a giant magnetoresistance ratio (GMR) sensor is used to detect the presence of a magnet in order to command the device to enter a predetermined mode of operation. The GMR responds to a modulated magnetic field generated by the programming of a command transmitter apparatus for non-invasive programming or controlling of the implanted device. The implantable medical device also monitors for the presence of a steady magnetic field to place the implanted device in a known, safe mode.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of implantable medical 
devices such as cardiac pacemakers, defibrillators, cardioverters, drug 
delivery devices, neural stimulators, and the like. More particularly, the 
present invention relates to a method and apparatus for controlling the 
output and mode of operation of such devices and changing the output 
and/or mode of operation of such devices with an external signal source, 
e.g., without an invasive process. 
FIELD OF THE INVENTION 
Many modern pacemakers adjust pacing rate in response to a patient's 
intrinsic electrical cardiac activity and/or other parameters such as a 
patient's metabolic demand for oxygen. Most state-of-the-art pacemakers 
are programmable or multi-programmable, such as with an external 
programmer which communicates with the implanted device via radio 
frequency (Rf) telemetry. A pacemaker may be programmable with respect to 
various parameters including pacing mode, pacing rate, stimulating pulse 
width, refractory period, sense amplifier sensitivity, rate-responsiveness 
to measured physiological parameters, and other parameters. 
Implantable devices, such as pacemakers, are typically programmed by 
broadcasting information to the device in the form of digital information 
identifying at least the parameter to be programmed and the desired 
parameter value that will be input as a result of the programming. This 
information is typically in the form of binary digital data which is 
radio-frequency modulated and transmitted to an antenna within the 
implanted device. In the implanted device, the signal is demodulated and 
the digital information decoded. Other information, such as an 
identification code for the implanted device, verification codes, error 
correction codes, access verification codes, and the like, may also be 
transmitted to the device during programming. In addition, the transmitted 
information may include initialization data to reduce the possibility of 
inadvertent programming (or re-programming) of the device. Alternatively, 
a reed switch may be provided which allows for limited external control, 
such as for actuation of a programming mode, by means of placing an 
external magnet in proximity to the switch for actuation thereof. 
The disadvantages of the radio-frequency system of such devices are 
characterized in detail in U.S. Pat. No. 5,292,342, and the present 
invention is intended in part to overcome those disadvantages, as well as 
certain shortcomings of the device disclosed in the '342 patent. 
The '342 patent describes a device incorporating a MAGFET sensor in a 
circuit including a logic circuit for generating two signals ("magnet 
present" or "magnet not present"), depending upon the orientation of the 
magnetic field, which signals comprise essentially digital data, sensing 
different numbers of removal/replacement cycles of the external magnet 
thereby identifying the different operating parameters of the device for 
programming that device. The present invention, however, adds to the 
programming and operating capabilities of such devices by providing 
additional programming options and/or input and allows marking by the 
patient of external stimuli and/or operating conditions. These objects of 
the present invention are achieved by using a so-called giant 
magnetoresistance ratio (GMR) sensor in the device and external magnetic 
fields to which that sensor is sensitive. 
The device disclosed in U.S. Pat. No. 5,292,342 is characterized by other 
disadvantages and limitations. For instance, exclusive use of that sensor 
and an external magnetic field for programming limits that device to 
receiving only communications from the outside; without radio-frequency 
circuitry, and thus the device is not capable of transmitting signals. 
Further, and perhaps more importantly, the programming options that are 
available for that device are limited. 
In accordance with the present invention, it has been discovered that GMR 
sensors are adaptable for a number of control functions other than 
re-programming of an implantable device including a MAGFET in the manner 
described in U.S. Pat. No. 5,292,342. Specifically, it is an object of the 
present invention to supplement the programming and operating modes of an 
implantable device such as a pacemaker by providing, in addition to 
radio-frequency telemetry, a second mode of communication to the 
implantable device which is less likely to be influenced by external 
electromagnetic fields and which offers greater operating and programming 
flexibility to the device. 
It is also an object of the present invention to include in this operating 
and programming flexibility the opportunity for patient input for 
subsequent downloading by radio-frequency telemetry for diagnostic and 
other purposes relating to the function of the device. 
SUMMARY OF THE INVENTION 
The present invention solves these shortcomings of the prior art and 
provides these objects. This invention includes an implantable biomedical 
device in which a giant magnetoresistance ratio (GMR) sensor is used to 
detect the presence of a magnet in order to command the device to enter a 
predetermined mode of operation. Further, the GMR responds to a modulated 
magnetic field generated by the programming of a command transmitter 
apparatus for non-invasive programming or controlling of the implanted 
device. 
The present invention also uses sensed, time varying magnetic signals which 
indicate low frequency electromagnetic noise. Such electromagnetic noise 
may adversely influence the behavior of an implantable medical device 
which has sensitive signal sensing circuits, as described in concurrently 
filed U.S. application Ser. No. 08/475,489 entitled Electromagnetic Noise 
Detector For Implantable Medical Devices. The GMR sensor signal, when 
appropriately conditioned and demodulated, may be used for the detection 
of extraneous low-frequency electromagnetic fields that may cause 
unreliable or unsafe operation of the implantable device. For example, a 
pulsed magnetic field from a electronic article surveillance system (EAS) 
may interfere with normal operation of an implantable medical device. The 
implantable medical device may then activate special circuitry to enter a 
safe mode of operation. A plurality of spaced GMR sensors may be employed 
in the present invention to counteract the effects of multiple axis 
magnetic fields encountered by a mobile recipient of such an implantable 
device. 
In the present invention, a GMR sensor is excited by an excitation voltage 
source. The GMR sensor develops a modulated signal in response to an 
applied magnetic field and provides this modulated signal to a sensor 
signal conditioning circuit. The sensor signal conditioning circuit 
amplifies and filters the GMR sensor signal such that the circuit's output 
is a demodulated version of the applied magnetic field. The demodulated 
version of the applied field is used by a command decoder circuit, which 
decodes modulated signals into commands for the implantable device. Also, 
the present invention detects the application of a permanent magnet for 
appropriate reversion to a magnet response mode of operation. 
These and other objects and advantages of the present invention will be 
immediately apparent to those of skill in the art from a review of the 
following detailed description along with the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1a is a block diagram representing a rate adaptive pacemaker 10 for 
control in accordance with a presently preferred embodiment of the present 
invention. Those skilled in the art who have the benefit of this 
disclosure, however, will recognize that the present invention is 
adaptable for use in controlling other types of pacemakers and many other 
types of implantable, microprocessor-controlled medical devices. The 
pacemaker 10 is illustrated merely for purposes of exemplifying a 
presently preferred embodiment of the invention. 
Briefly, U.S. Pat. No. 4,967,746 describes a pacemaker controlled by a 
microprocessor 50 such as the microprocessor described in detail in U.S. 
Pat. No. 4,404,972, incorporated herein in its entirety by this specific 
reference thereto and assigned to the Assignee of the present invention. 
The basic pacemaker described in the '746 patent has been modified and 
improved by the present invention as described below. 
The microprocessor 50 is provided with input/output ports connected in a 
conventional manner via bi-directional bus 55 to memory 60, an A-V 
interval timer 65, and a pacing interval timer 70. In addition, the A-V 
timer 65 and pacing interval timer 70 each has an output connected 
individually to a corresponding input port of the microprocessor 50 by 
lines 67 and 72, respectively. 
The microprocessor 50 preferably also has an input/output port connected to 
a telemetry interface 62 by line 52. The pacemaker 10 when implanted is 
thus able to receive pacing and rate control parameters from an external 
programmer 120 (FIG. 2) and send data to an external receiver as known in 
the art, and described below with regard to an antenna 53. One such system 
and encoding arrangement is described in U.S. Pat. No. 4,539,992, which is 
also assigned to the assignee of the present invention and which is 
incorporated herein in its entirety by this specific reference thereto. 
The microprocessor 50 may also be provided with a radio-frequency link 
through the antenna 53. The antenna 53 is coupled to a telemetry interface 
62', which may be the same interface as interface 62, or it may provide a 
distinct interface. 
The microprocessor output ports are connected to the inputs of a dual 
chamber stimulus pulse generator 90 by control line 89. The microprocessor 
50 transmits pulse parameter data, such as amplitude and pulse width, as 
well as enable/disable initiation codes to the generator 90 on control 
line 89. 
The microprocessor 50 also has input ports connected to outputs of an 
atrial sense amplifier 80 and a ventricular sense amplifier 85 by lines 78 
and 87, respectively. The atrial and ventricular sense amplifiers 80 and 
85 detect occurrences of P-waves and R-waves, respectively. The atrial 
sense amplifier 80 outputs a signal on line 78 to the microprocessor 50 
when it detects a P-wave and the ventricular sense amplifier 85 outputs a 
signal on line 87 to the microprocessor 50 when it detects an R-wave. 
The input of the atrial sense amplifier 80 and the output of the stimulus 
pulse generator 90 are connected to a first conductor 92 which is inserted 
in a first conventional lead 96. Lead 96 is inserted into the heart 100 
and has an electrically conductive pacing/sensing tip 98 at its distal end 
which is electrically connected to the conductor 92. The pacing/sensing 
tip 98 is preferably lodged in the right atrium 105. 
The input of the ventricular sense amplifier 85 and the output of stimulus 
pulse generator 90 are connected to a second conductor 95. The second 
conductor 95 is inserted in a second conventional lead 110 which is 
inserted intravenously or otherwise in the right ventricle 107 of the 
heart 100. The second lead 110 has an electrically conductive 
pacing/sensing tip 112 at its distal end. The pacing/sensing tip 112 is 
electrically connected to the conductor 95. The pacing)sensing tip 112 is 
preferably lodged on the wall of the right ventricle. 
The conductors 92 and 95 conduct the stimulus pulses generated by the 
stimulus pulse generator 90 to the respective pacing/sensing tips 98 and 
112. The pacing/sensing tips 98 and 112 and corresponding conductors 92 
and 95 also conduct sensed cardiac electrical signals in the right atrium 
appendage and right ventricle to the atrial and ventricular amplifiers 80 
and 85, respectively. 
The implantable device 10 further includes a magnetic sensor 74. The sensor 
74 may be any appropriate sensor capable of sensing a time-varying 
magnetic field, in a manner described herein, and is preferably a GMR 
sensor. The sensor 74 is excited by an excitation voltage source V.sub.1. 
The GMR is preferably an integrated GMR magnetic sensor from Nonvolatile 
Electronics, Inc. in Eden Prairie, Minn. Other resistive sensor geometries 
are equally applicable to the present invention, such as those described 
in The Constant Current Loop: A New Paradigm for Resistance Signal 
Conditioning, Anderson, K. F., Sensors, April 1994, so long as such 
resistive sensors are sensitive to a time-varying magnetic field. 
As shown in FIGS. 1a and 1b, the GMR may be arranged as a bridge circuit 
with bridge elements R.sub.1 -R.sub.4, inclusive. The GMR sensor provides 
an output signal to a sensor signal conditioning circuit 76 via signal 
lines 77 and 79. This signal conditioning circuit 76 demodulates the 
signal from the GMR sensor 74, as well as providing filtering and signal 
shaping so that the signal is in condition for use by the microprocessor 
50. The sensor signal conditioning circuit provides a further feature of 
defining a minimum threshold level from the sensor 74 to eliminate the 
deleterious effects of magnetic noise. 
The bridge elements R.sub.1 -R.sub.4 are preferably integrated circuit 
field effect transistors that have been biased to operate in the resistive 
region. Two of the bridge elements, for example R.sub.3 and R.sub.4, are 
shielded against magnetic influence, by shields S.sub.3 and S.sub.4 while 
magnetically-sensitive R.sub.1 and R.sub.2 are unshielded. FIG. 3 
illustrates typical operating characteristics of the GMR sensor 74 in the 
presence of a magnetic field, with the output of the sensor between lines 
77 and 79. As shown, the output voltage of the sensor is independent of 
the polarity of the magnetic field; that is, the voltage output depends 
only on the absolute value of the magnetic field. 
The magnetic field is provided to the implantable device 10 from a source 
outside the patient. In a preferred embodiment, the magnetic signal is 
provided by a programming device/command transmitter 120, shown in greater 
detail in FIG. 2. The command transmitter 120 powers a coil 122 which is 
magnetically coupled to a core 124. The command transmitter 120 develops a 
modulated digital data stream which is magnetically coupled to the sensor 
74 for communication to the microprocessor 50. The digital data stream may 
include a code sequence to define a specific parameter, which then follows 
the code sequence in the data stream. In this way, commands or information 
may be selectively directed to the microprocessor. If desired this may 
include re-programming the implantable device 10, to suit the needs of the 
patient. 
The implantable device 10 is also sensitive to the presence of a magnetic 
field from a permanent magnet 126, shown in FIG. 1b. Thus, the sensor 74 
may be used to revert the operation of the implantable device to the 
magnet mode of operation. In magnet mode, a predetermined, typically 
asynchronous, ventricular pacing rate is issued. This mode eliminates 
complex timing and synchronizing signals to assist in monitoring the 
functioning of a remotely stimulated organ. As used herein, the term 
"magnet mode" may also be referred to as a "known, safe mode" because this 
mode is dependent only on the voltage of the battery in the implanted 
device, and is not dependent on either heart activity or on any adapted 
rate factors which can alter operation of the implanted device. 
FIG. 2 depicts a preferred embodiment of the programming device/command 
transmitter 120 of the present invention. An input device 130, such as a 
keyboard, switch array, modem, or other means of assembling a character 
stream, is coupled to an encoding element 132 which develops the digital 
bit string. This bit string is communicated to a control logic element 134 
which drives a modulator 136. The modulator modulates the signal from the 
control logic to a condition to drive an output stage or amplifier 138. 
The amplifier 138 matches the impedance of a conductor 140 and amplifies 
the signal sufficiently to drive the coil 122 and core 124 to develop a 
magnetic signal that is detected by the device 10. The control logic 134 
also provides monitoring of the transmitter 120 for display indicators 
142. 
The principles, preferred embodiment, and mode of operation of the present 
invention have been described in the foregoing specification. This 
invention is not to be construed as limited to the particular forms 
disclosed, since these are regarded as illustrative rather than 
restrictive. Moreover, variations and changes may be made by those skilled 
in the art without departing from the spirit of the invention.