Apparatus and method for adjusting heart/pacer rate relative to ejection time to obtain a required cardiac output

• US 4,730,619 A
• Filed: 10/30/1986
• Issued: 03/15/1988
• Est. Priority Date: 04/11/1985
• Status: Expired due to Term

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories)
• Original Assignee: Telectronics Incorporated
• Inventors: Schroeppel, Edward A., Koning, Gerrit
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US06/924,764
• Time in Patent Office: 502 Days
• Field of Search: 128/419 PG, 128/419 D
• US Class Current: 607/23
• CPC Class Codes: A61N 1/056 Transvascular endocardial electrode systems
• CPC Class Codes: A61N 1/36557 controlled by chemical substances in blood
• CPC Class Codes: A61N 1/36564 controlled by blood pressure

Abstract

The apparatus for pacing a heart in accordance with the heart rate needed to produce a required cardiac output while the person is exercising, comprises a pacer adapted to be implanted in a human body and having a pulse generator and control circuitry (e.g. including a microprocessor) therein, a pacing lead adapted to be implanted in a right ventricle in a heart and having a distal electrode adapted to engage and supply pacing pulses to the apex of the right ventricle and a pressure sensor or impedance sensing electrode mounted in or on the lead. An algorithm and routine utilizing same are stored in the control circuitry (microprocessor) and are adapted to (a) relate right ventricular pressure to duration of contraction to determine ejection time and changes in ejection time or (b) to relate impedance and changes in impedance to blood volume or changes in volume, to relate blood volume and changes in blood volume to opening and closing of the pulmonary valve and to determine each time period between each opening and closing to determine ejection time and changes in ejection time and to correlate ejection time or changes in ejection time with the required heart rate needed to supply a desired cardiac output and to cause the pacer to pace the heart at the required heart rate when the heart is not naturally paced.

Claims

1. An apparatus for pacing a heart in accordance with the heart/pacer rate needed to produce a required cardiac output while the person is exercising relative to ejection time of the right ventricle and changes thereof, comprising:

a demand pacer for implantation in a human body, said pacer having a pulse generator and control circuitry, which may be realized by a microprocessor, mounted therein;

a pacing lead for implantation in a heart, said lead including a lead body having a tip electrode adapted to engage and supply pacing pulses to a right ventricle of a heart;

sensing means mounted on said lead body for sensing changes in the right ventricle of the heart for determining the ejection time of the right ventricle and changes in ejection time over time in the right ventricle, such sensing including measurements selected from the group consisting of measurements of changes in volume of the right ventricle, measurements of changes in pressure in the right ventricle and measurements of changes in impedance of the fluid in the right ventricle; and

said control circuitry including means for determining ejection time from the sensing of changes in the right ventricle and for relating ejection time and the changes in ejection time in the right ventricle with the required pacing rate and changes in the existing pacing rate needed to supply a desired cardiac output and for causing said pacer to pace the heart at the required rate when the heart is not naturally paced.

2. The apparatus of claim 1 wherein said means for sensing changes in the right ventricle includes pressure sensing means mounted on said pacing lead.

3. The apparatus of claim 2 wherein said pressure sensing means comprise a strain gauge mounted in said lead body and spaced from said tip electrode.

4. The apparatus of claim 2 wherein said pressure sensing means comprise a piezoresistive microelectronic pressure transducer mounted in said lead body and spaced from said tip electrode.

5. The apparatus of claim 2 wherein said means for determining ejection time includes means for relating changes in pressure over time to duration of contraction, which time period is related to ejection time.

6. The apparatus of claim 4 wherein said control circuitry includes a pulse generator and a microprocessor programmed to cause voltage pulses to be supplied from said pulse generator to said pressure transducer for pressure sampling to minimize the power drain of the pressure transducer on the power supply of the pacer.

7. The apparatus of claim 6 including means for supplying voltage pulses having a duty cycle of at least 1:

200.

8. The apparatus of claim 1 wherein said means for sensing changes in the right ventricle includes two electrodes on said lead whereby impedance can be measured across said electrodes when a current is applied through said electrodes.

9. The apparatus of claim 8 wherein said two electrodes are defined by said tip electrode and a ring electrode on said pacing lead a sufficient distance behind said tip electrode so as to be able to be received in the right ventricle.

10. The apparatus of claim 1 wherein said means includes four electrodes, of which two electrodes are ring electrodes one positioned behind but spaced from said tip electrode and another ring electrode spaced a sufficient distance so as to be able still to be received in the right ventricle and said pacer has a casing which serves as the fourth electrode, whereby impedance can be measured between said two ring electrodes on the lead when a current is passed between the pacer casing and said tip electrode.

11. The apparatus of claim 10 wherein said ejection time determining means includes means for supplying current pulses from said control circuitry to said electrodes and, means for receiving voltage pulses from said two ring electrodes and for supplying same to said control circuitry, and means for determining impedance values from the voltage and the current pulses, for relating changes in impedance to changes in blood volume in the right ventricle and changes in blood volume to openings and closings of the pulmonary valve and for determining the time period between each opening and closing which is related to, if not equal to, ejection time.

12. The apparatus of claim 1 wherein said control circuitry includes a microprocessor and a memory, and said means for relating the right ventricular ejection time and/or changes thereof with the required pacer rate include an algorithm stored in said memory.

13. The apparatus of claim 12 wherein said microprocessor includes means for implementing said algorithm including means for:

determining the ejection time in the right ventricle at a point in time T;

computing the heart/pacer rate R_T from a programmed slope "A" of a graph of heart rate versus ejection time between a minimum heart rate and a maximum heart rate;

determining whether the heart rate calculated is equal to or above the programmed maximum pacer rate or equal to or below the programmed minimum pacer rate; and

if the calculated heart rate is at or above the programmed maximum pacer rate or at or below the programmed minimum pacer rate, replacing the pacer rate value stored in said memory with either the programmed maximum pacer rate or the programmed minimum pacer rate respectively;

or

if the calculated heart rate is between the programmed minimum pacer rate and the programmed maximum pacer rate, replacing the pacer rate value stored in the memory with the newly calculated rate in said memory;

replacing the ejection time value stored in said memory at time T-1 with the newly determined ejection time value; and

altering the pacer rate of the pacer by the newly calculated rate.

14. The apparatus of claim 13 including means for programming in said microprocessor a family of slopes parallel to the first named slope whereby the implemented algorithm functions independent of a particular ejection time and in response to changes in ejection time so as to be substantially insensitive to any drift in the means for determining ejection time.

15. The apparatus of claim 12 wherein said memory has stored therein a look-up table of required pacing rates relative to various values of right ventricular ejection time and said microprocessor includes means for implementing said algorithm including means for:

determining the ejection time in the right ventricle;

looking up the corresponding heart rate in the look-up table; and

monitoring and repeating the step of determining ejection time and changing the pacing rate as the pressure changes.

16. The apparatus of claim 12 wherein said memory has stored therein a look-up table of required heart rate changes relative to various values of right ventricular ejection time changes and said microprocessor includes means for implementing said algorithm including means for:

determining the ejection time at a first point in time;

computing the incremental change in ejection time Δ

ET=ET_T -ET_T-1 ;

looking up the corresponding R in the look-up table;

computing the required heart rate R_T =Δ

R+R_T-1 ;

determining whether the computed heart rate is equal to or above the programmed maximum pacer rate or is equal to or below the programmed minimum pacer rate; and

if the calculated heart rate meets either of these conditions, replacing the pacer rate R_T-1 by either the programmed maximum pacer rate or programmed minimum pacer rate stored in said memory and replacing the last determined ET value;

if the heart rate calculated is between the maximum and minimum pacer rates stored in said memory, replacing the last pacing rate by the newly computed rate and replacing the last ejection time value by the newly determined ejection time value; and

adjusting the pacer rate to the value of the heart rate just calculated.

17. The apparatus of claim 16 including means for smoothing the heart/pacer rate changes by smoothing the changes in ejection time.

18. The apparatus of claim 16 including means for comparing the sign of the result of Δ

ET_T =ET_T -ET_T-1 to the sign of the N-1 previously computed differences which have been stored and if there is no sign change in N consecutive samples, means for causing the algorithm to determine the appropriate rate change, if the sign of the present increment Δ

ET is equal to the sign of the previous N-1 increments, the algorithm increments another increment of Δ

ET for the next comparison, and if the sign changes, the new sign is stored in the memory and the said microprocessor returns to the step of determining ejection time.

19. The apparatus of claim 16 including means for basing the rate change on the average value of Δ

ET over the N most recent samples if no sign change has occurred.

20. A method for pacing a heart in accordance with the heart rate needed to produce a required cardiac output while a person is exercising and relative to ejection time in the right ventricle and changes thereof, comprising the steps of:

sensing changes in the right ventricle from about the end of right atrial systole through right ventricular systole during each heart cycle, such sensing including measurements selected from the group consisting of measurements of changes in volume in the right ventricle, measurements of changes in pressure in the right ventricle, and measurements of changes in impedance of the fluid in the right ventricle;

determining ejection time (E₂ or E₃) and changes in ejection time in the right ventricle from the sensing of changes in the right ventricle;

relating ejection time (E₂ or E₃) and the changes in ejection time in the right ventricle with the required heart rate and change in heart rate needed to supply a desired cardiac output; and

pacing the heart at the required heart rate when the heart is not naturally paced.

21. The method of claim 20 wherein said step of determining ejection time includes the steps of:

measuring pressure in the right ventricle at short time intervals over time;

determining changes in pressure;

relating the pressure sensed over time and changes in pressure to the duration of contraction;

determining the duration of contraction during the time the pressure increases and decreases; and

determining each ejection time (E₂) from each said time period.

22. The method of claim 21 wherein pressure is sensed by a piezoresistive pressure transducer and said method includes the step of supplying voltage pulses to said pressure transducer for pressure sampling to minimize the power drain of the pressure transducer on a power supply for the transducer.

23. The method of claim 22 wherein said voltage pulses have a duty cycle of at least 1:

200.

24. The method of claim 20 wherein said steps of sensing changes in the right ventricle and determining ejection time (E₃) includes the steps of:

determining blood volume and changes in blood volume in the right ventricle over time;

relating the blood volume and changes in blood volume to openings and closings of the pulmonary valve;

determining the time period between each opening and closing during the time period when the volume changes from a maximum value to a minimum value; and

determining each ejection time (E₃) from each said time period.

25. The method of claim 24 wherein blood volume and changes in blood volume are determined by the steps of:

measuring at short intervals over time the impedance between two electrodes in the right ventricle; and

correlating the impedance measurements and changes in impedance with blood volume and changes in blood volume over time.

26. The method of claim 20 wherein said steps of relating the right ventricular ejection time and/or the time derivative therof with the required pacer rate comprises the steps of:

determining the ejection time in the right ventricle at a point in time T;

computing the heart rate R_T from a slope "A" of a graph of heart rate versus ejection time between a programmed minimum pacer rate and a programmed maximum pacer rate;

determining whether the heart rate calculated is equal to or above the programmed maximum pacer rate or equal to or below the programmed minimum pacer rate; and

if the calculated heart rate is at or above the programmed maximum pacer rate or at or below the programmed minimum pacer rate, replacing the pacer rate value stored in a memory with either the programmed maximum pacer rate or the programmed minimum pacer rate, respectively;

or

if the calculated heart rate is between the programmed minimum pacer rate and the programmed maximum pacer rate, replacing the pacer rate value stored in the memory with the newly calculated rate in the memory;

replacing the ejection time value stored in the memory at time T-1 with the newly measured ejection time value; and

altering the pacer rate of the pacer by the newly calculated rate.

27. A method of claim 26 includes the step of computing the heart rate from a family of slopes parallel to the first named slope whereby said method is independent of a particular ejection time and alters pacer rate in response to changes in ejection time so as to be substantially insensitive to drift in any means used for determining ejection time.

28. The method of claim 26 wherein different slopes A, A₁, A₂, etc. are programmed in the microprocessor and said method includes the step of selecting a slope most closely related to the natural change in ejection for changes in heart rate for the patient.

29. The method of claim 20 wherein said step of relating the right ventricular ejection time with the required heart rate comprises the steps of:

determining the ejection time in the right ventricle;

looking up the corresponding heart rate in a look-up table of heart rates relative to various values of right ventricular systolic ejection time;

adjusting the pacer rate to the value looked up in the look-up table; and

monitoring and repeating the step of determining ejection time and changing the pacing rate as the ejection time changes.

30. The method of claim 26 wherein said step of relating the right ventricular ejection time with the required heart rate comprises the steps of:

determining the ejection time at a first point in time;

computing the incremental change in ejection time, Δ

ET=ET_T -(ET_T-1);

looking up the corresponding changes in heart rate, Δ

R in, a look-up table of heart rates relative to various values of right ventricular ejection time;

computing the required heart rate R_T =Δ

R+(R_T-1);

determining whether the computed heart rate is equal to or above the programmed maximum pacer rate or is equal to or below the programmed minimum pacer rate; and

if the calculated heart rate meets either of these conditions, replacing the pacer rate R_T-1 by either the programmed maximum pacer rate or programmed minimum pacer rate stored in a memory and replacing the last sensed ejection time value with the newly sensed ejection time value;

if the heart rate calculated is between the programmed maximum and minimum pacer rates stored in the memory, replacing the last pacing rate by the newly computed rate and replacing the last ejection time value by the newly determined ejection time value; and

adjusting the pacer rate to the value of the rate just calculated.

31. The method of claim 30 including the step of smoothing the heart/pacer rate changes by smoothing the changes in ejection time.

32. The method of claim 31 including the step of comparing the sign of the result, Δ

ET_T =ET_T -(ET_T-1) to the sign of the N-1 previously computed differences which have been stored and if there is no sign change in N consecutive samples, determining the appropriate rate change, if the sign of the present increment in ejection time Δ

ET is equal to the sign of the previous N-1 increments in ejection time, incrementing another increment of ejection time Δ

ET for the next comparison, and if the sign changes, storing the new sign in the memory and returning to the initial step of determining ejection time.

33. The method of claim 32 including the step of basing the rate change on the average value of Δ

ET over the N most recent samples if no sign change has occurred.