Method and apparatus for heart transplant monitoring and analog telemetry calibration

• US 5,402,794 A
• Filed: 07/01/1992
• Issued: 04/04/1995
• Est. Priority Date: 07/01/1992
• Status: Expired due to Term

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Thompson, David L., Wahlstrand, John D., Mulier, Peter M. J.
• Primary Examiner(s): Kamm, William E.
• Assistant Examiner(s): Parker, Marianne
• Application Number:
  US07/907,259
• Time in Patent Office: 1,007 Days
• Field of Search: 128/419 P, 128/419 PG, 128/419 T, 128/696, 128/700, 128/782, 128/903, 340/573, 340/502-505, 371/2.1, 371/4, 607/19, 607/32, 607/60, 607/145, 607/27
• US Class Current: 600/508
• CPC Class Codes: A61B 5/0002 Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network A61B5/07 takes precedence; transmission systems for measured values G08C; transmission H04B; transmission of digital information H04L; wireless communication networks H04W
• CPC Class Codes: A61B 5/0006 ECG or EEG signals
• CPC Class Codes: A61B 5/413 Monitoring transplanted tissue or organ, e.g. for possible rejection reactions after a transplant
• CPC Class Codes: A61N 1/37211 Means for communicating with stimulators
• CPC Class Codes: A61N 1/37254 Pacemaker or defibrillator security, e.g. to prevent or inhibit programming alterations by hackers or unauthorised individuals

Abstract

A cardiac pacemaker with an analog telemetry system. A calibration circuit within the pacemaker is adapted to provide a reference signal of known character to the pacemaker'"'"'s telemetry system. The reference signal is transmitted across the telemetry link as if it were an actual cardiac signal, and received by an external programmer. Since the reference signal has known, predetermined qualities, the programmer can automatically calibrate and scale the telemetry signal from the pacemaker, thereby increasing the accuracy of the telemetry channel. The increased accuracy is particular useful in assessing rejection of a transplanted heart, which is known to be associated with a 15% decline in the peak R-wave amplitude of the cardiac signal.

Claims

1. An implantable medical device having a telemetry system for communicating analog signals to an external peripheral device, said telemetry system being coupled to a source of electrical cardiac signals, and said telemetry system comprising:

a calibration circuit means for generating a reference signal having at least one known attribute;

a filter circuit, comprising means for alternately coupling to said calibration circuit and said cardiac signal source, and means for filtering said reference signal and said cardiac signal;

an encoding circuit means coupled to said filter circuit, for digitally encoding said filtered signals; and

a transmitter means coupled to said encoding circuit, for transmitting said digitally encoded signals to said external peripheral device to allow for determination of a difference with respect to said attribute between said transmitted reference signal and said reference signal;

wherein said reference signal is a square wave comprising a stream of square pulses and wherein said at least one known attribute of said reference signal is the amplitude of said pulses.

2. An implantable medical device having a telemetry system for communicating analog signals to an external peripheral device, said telemetry system being coupled to a source of electrical cardiac signals, and said telemetry system comprising:

a calibration circuit means for generating a reference signal having at least one known attribute;

a filter circuit comprising means for alternately coupling to said calibration circuit and said cardiac signal source, and means for filtering said reference signal and said cardiac signal;

an encoding circuit means coupled to said filter circuit, for digitally encoding said filtered signals; and

a transmitter means coupled to said encoding circuit, for transmitting said digitally encoded signals to said external peripheral device to allow for determination of a difference with respect to said attribute between said transmitted reference signal and said reference signal;

wherein said reference signal is a square wave comprising a stream of square pulses, and wherein said at least one known attribute is pulse width.

3. An implantable medical device having a telemetry system for communicating analog signals to an external peripheral device, said telemetry system being coupled to a source of electrical cardiac signals, and said telemetry system comprising:

a calibration circuit means for generating a reference signal having at least one known attribute;

a filter circuit comprising means for alternately coupling to said calibration circuit and said cardiac signal source, and means for filtering said reference signal and said cardiac signal;

an encoding circuit means coupled to said filter circuit, for digitally encoding said filtered signals; and

a transmitter means coupled to said encoding circuit, for transmitting said digitally encoded signals to said external peripheral device to allow for determination of a difference with respect to said attribute between said transmitted reference signal and said reference signal;

wherein said reference signal comprises stream of alternating positive and negative spikes, and wherein said at least one known attribute is peak spike magnitude.

4. An implantable medical device having a telemetry system for communicating analog signals to an external peripheral device, said telemetry system being coupled to a source of electrical cardiac signals, and said telemetry system comprising:

a calibration circuit means for generating a reference signal having at least one known attribute;

a filter circuit comprising means for alternately coupling to said calibration circuit and said cardiac signal source, and means for filtering said reference signal and said cardiac signal;

an encoding circuit means coupled to said filter circuit for digitally encoding said filtered signals; and

a transmitter means coupled to said encoding circuit, for transmitting said digitally encoded signals to said external peripheral device to allow for determination of a difference with respect to said attribute between said transmitted reference signal and said reference signal.

wherein said reference signal comprises a sinusoidal waveform and wherein said at least one known attribute is amplitude.

5. A method for quantifying an unpredictable attenuation factor in a telemetry channel between an implanted medical device and an external receiver, comprising the steps of:

(a) generating, in said implantable device, a reference signal having a known amplitude;

(b) transmitting said reference signal through said telemetry channel;

(c) comparing, in said receiver, an amplitude of said reference signal transmitted through said telemetry channel in step (b) with said known amplitude.

6. A method in accordance with claim 5, wherein said reference signal is a square wave.

7. A method in accordance with claim 5, wherein said reference signal is a stream of alternating positive and negative spikes.

8. A method in accordance with claim 5, wherein said reference signal is a sinusoidal wave.

9. A heart transplant monitoring system, comprising:

a plurality of implanted epicardial leads, electrically coupled to a patient'"'"'s heart, comprising means for conducting electrical cardiac signals;

an implanted telemetry system means coupled to said plurality of leads, for transmitting said electrical cardiac signals to an external receiver;

signal processing means coupled to said external receiver for detecting a reduction in said electrical cardiac signals indicative of heart rejection; and

an implanted activity sensing circuit means for producing an activity signal corresponding to detected levels of patient activity;

wherein said implanted telemetry system means comprises means for transmitting said electrical cardiac signals to said external receiver only when said activity signal indicates that said patient is at rest.