Medical lead impedance measurement system

• US 5,201,865 A
• Filed: 10/28/1991
• Issued: 04/13/1993
• Est. Priority Date: 10/28/1991
• Status: Expired due to Fees

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Kuehn, Kevin P.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): Jastrzab, Jeffrey R.
• Application Number:
  US07/783,651
• Time in Patent Office: 533 Days
• Field of Search: 128/419 PT, 128/419 P, 128/419 PL, 128/419 D
• US Class Current: 607/8
• CPC Class Codes: A61N 1/3706 Pacemaker parameters stimulation threshold A61N1/371

Abstract

A method and apparatus for measuring lead impedance during preselected test mode operation of an implantable body tissue stimulator. The analysis circuitry is periodically triggered into operation, such as on each reprogramming by the physician or periodically as a function of elapsed time or number of stimulation events counted from the preceding measurement. The actual lead impedance, measured from the output circuit from the body tissue-stimulator pulse generator, and taking into account impedance of the interconnection between the lead connector pin and the pulse generator connector block, the lead electrical conductor and its connections with the electrode and the connector pin and the electrode-tissue interface, is calculated as a function of the ratio of the elapsed times that it takes to discharge a capacitor from a first reference voltage to a second reference voltage through a precision resistor and through the lead impedance itself The calculated lead impedance may be stored in memory with a suitable time tag, employed to automatically effect a change in operating modes or change a lead and electrode selection, if measured lead impedance falls outside normal high and low impedance boundary values. In the pacing context, calculated lead impedance may be employed to adjust sense amplifier sensitivity and pacing output pulse parameters. The method and apparatus may also be employed to calculate cardioversion/defibrillation lead impedance through selective partial discharge of high voltage output capacitors.

Claims

1. A method of measuring the impedance of an implantable medical lead having an unknown impedance, comprising:

a. Coupling said implantable lead to a pulse generator which includes an output capacitor and a precision resistor having a known resistance;

b. Charging said output capacitor to a first predetermined voltage, discharging said output capacitor through said precision resistor and measuring the time interval during the discharge of said output capacitor from said first predetermined voltage to a second predetermined voltage;

c. Charging said output capacitor to said first predetermined voltage, discharging said output capacitor through said lead and measuring the time interval during discharge of said output capacitor from said first predetermined voltage to said second predetermined voltage; and

d. Determining the ratio of said measured time interval during discharge of said output capacitor through said lead impedance to said measured time interval during discharge of said output capacitor through said precision resistor and multiplying said ratio by said known resistance of said precision resistor to provide a measurement of the impedance of said lead.

2. In an apparatus for stimulating body tissue comprising a pulse generator for generating stimulation pulses, including an output capacitor, a lead coupled to said pulse generator for delivering said stimulation pulses to body tissue and means within said pulse generator for measuring the impedance of said lead, the improvement wherein said impedance measuring means comprises:

means for charging said output capacitor to a first voltage;

means for defining a second voltage, less than said first voltage;

a precision resistor of known resistance;

means for discharging said output capacitor through said precision resistor;

means for measuring the time interval during discharge of said output capacitor, from said first voltage to said second voltage, through said precision resistor;

means for discharging said capacitor into said lead;

means for measuring the time interval during discharge of said capacitor from said first voltage to said second voltage, through said lead; and

means for deriving a measurement of the impedance of said lead as a function of said measured time intervals during discharge of said capacitor through said precision resistor and through said lead.

3. An apparatus according to claim 2 wherein said deriving means comprises means for determining the ratio of said measured time interval during discharge of said output capacitor through said lead to said measured time interval during discharge of said capacitor through said precision resistor and for multiplying said determined ratio by said known resistance of said precision resistor to provide said measurement of said lead impedance.