Implantable cardiac stimulator with electrode-tissue interface characterization

• US 6,141,585 A
• Filed: 05/08/1998
• Issued: 10/31/2000
• Est. Priority Date: 05/08/1998
• Status: Expired due to Term

• Current Assignee: Intermedics Incorporated
• Original Assignee: Intermedics Incorporated
• Inventors: Paul, Patrick J., Martin, Gregory R., Prutchi, David
• Primary Examiner(s): Kamm, William E.
• Assistant Examiner(s): Evanisko, George R.
• Application Number:
  US09/075,144
• Time in Patent Office: 907 Days
• Field of Search: 607/5, 607/7, 607/8, 607/9, 607/28, 600/547
• US Class Current: 607/8
• CPC Class Codes: A61M 5/142 Pressure infusion, e.g. using pumps
• CPC Class Codes: A61M 5/14244 adapted to be carried by the patient, e.g. portable on the body
• CPC Class Codes: A61M 5/16854 by monitoring line pressure
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance
• CPC Class Codes: A61N 1/3706 Pacemaker parameters stimulation threshold A61N1/371

Abstract

A cardiac stimulator capable of measuring pacing impedance includes a tank capacitor for delivering charge to the heart via device leads, a shunt resistor, and high-impedance buffers for measuring pacing current through the shunt resistor. Soon after the leading edge of the stimulation pulse, the voltage across the shunt resistor, as sampled by a high-impedance buffer, indicates lead and cardiac tissue resistance. Just prior to opening the pacing switch to terminate the stimulation pulse, the voltage across the shunt resistor is sampled by a high-impedance buffer and held once again to allow the capacitance of the lead/heart tissue to be calculated. In alternative embodiments, a high-impedance buffer measures the voltage between the tank capacitor and ground immediately following the stimulation pulse to allow estimation of the lead/heart tissue capacitance. In one alternative embodiment, a look-up table is created in main memory and searched to find the closest lead/heart tissue capacitance estimate to any arbitrary degree of accuracy. In another alternative embodiment, the lead/heart tissue capacitance is estimated by successive approximation to any arbitrary degree of accuracy. When the lead/heart tissue capacitance and lead resistance have been determined, a plurality of parameters of importance for analyzing and optimizing a cardiac stimulation system may be calculated, such as the instantaneous current, the average current, the charge, and the energy delivered to the cardiac tissue.

Claims

1. A method, comprising:

determining a lead/tissue resistance; and

using lead/tissue resistance to determine a Helmholtz capacitance of a patient'"'"'s heart.

2. The method of claim 1 wherein determining a lead/tissue resistance comprises:

applying a voltage V_i to a closed-loop current path comprising a lead, heart tissue, and components of known impedance;

measuring a voltage produced in the closed-loop current path by the voltage V₁ after applying the voltage V₁ to closed-loop current path, thereby producing a measured voltage; and

calculating the lead/tissue resistance based on the measured voltage.

3. The method of claim 2 wherein measuring a voltage comprises measuring a voltage across a resistor, thereby producing the measured voltage across the resistor.

4. The method of claim 3 in which the lead/tissue resistance is calculated using a formula which comprises the measured voltage across the resistor as a variable of the formula.

5. The method of claim 1 wherein using the lead/tissue resistance to determine a Helmholtz capacitance of a patient'"'"'s heart further comprises:

applying a voltage V_i for a fixed amount of time, T_PW seconds, to a closed-loop current path comprising a lead, heart tissue, and components of known impedance;

measuring a voltage produced in the closed-loop current path by the voltage V;

at a time t=T_PW the time t=T_PW occurring just before time t=T_PW, thereby producing a measured voltage; and

calculating the Helmholtz capacitance based on the measured voltage.

6. The method of claim 5 wherein the Helmholtz capacitance is calculated using a formula which comprises the measured voltage as a variable of the formula.

7. The method of claim 2 wherein using the lead/tissue resistance to determine a Helmholtz capacitance of a patient'"'"'s heart further comprises:

discharging a tank capacitor for a duration of T_PW seconds through a closed-loop current path comprising a lead, heart tissue, and components of known impedance;

measuring a voltage across the tank capacitor at a time t=T_PW⁺ just after the end of the duration of T_PW seconds, thereby producing a measured voltage; and

estimating the Helmholtz capacitance based on the measured voltage.

8. The method of claim 7 wherein estimating the Helmholtz capacitance includes calculating an empirical voltage value across the tank capacitor based on a predetermined empirical estimate of a Helmholtz capacitance C_L (empirical).

9. The method of claim 8 wherein estimating the Helmholtz capacitance further includes calculating additional empirical voltage values across the tank capacitor based on additional predetermined empirical estimates of the Helmholtz capacitance C_L (empirical).

10. The method of claim 9 wherein estimating the Helmholtz capacitance further includes estimating the Helmholtz capacitance to be the empirical estimate C_L (empirical) which is associated with the empirical voltage value that is closest to the measured voltage.

11. The method of claim 7 wherein estimating the Helmholtz capacitance comprises an iterative process of calculating an empirical voltage value across the tank capacitor based on a predetermined empirical estimate of a Helmholtz capacitance C_L empirical), calculating an approximation of a Helmholtz capacitance C_L (approx) based on the empirical voltage value, comparing C_L approx) with C_L (empirical) and adjusting C_L (empirical) for a subsequent iteration of the process, wherein the process is repeated until comparing C_L (approx) with C_L (empirical) indicates that C_L (approx) is within a predetermined limit of C_L (empirical).

12. The method of claim 11 wherein adjusting C_L (empirical) comprises adjusting C_L (empirical) by an amount equal to the predetermined limit.