Apparatus and method for adjusting heart/pacer rate relative to cardiac pCO.sub.2 to obtain a required cardiac output

• US 4,716,887 A
• Filed: 04/11/1985
• Issued: 01/05/1988
• Est. Priority Date: 04/11/1985
• Status: Expired due to Term

• Current Assignee: Telectronics Pacing Systems, Inc (Abbott Laboratories Incorporated)
• Original Assignee: Telectronics Incorporated
• Inventors: Schroeppel, Edward A., Koning, Gerrit
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US06/722,575
• Time in Patent Office: 999 Days
• Field of Search: 128/635, 128/2, 128/419 PG
• US Class Current: 607/24
• 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 relative to the partial pressure of carbon dioxide in the blood, pCO₂, 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 heart and having a distal electrode adapted to engage and supply pacing pulses to a right ventricle of a heart and a pCO₂ sensor for sensing pCO₂ of the blood in the heart. An algorithm and routine utilizing same are stored in the control circuitry (microprocessor) and are adapted to relate pCO₂ with the required heart rate or change in rate, ΔR, 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 implantable apparatus for pacing a heart in accordance with the heart/pacer rate needed to produce a required cardiac output relative to the partial pressure of carbon dioxide in blood, pCO₂, while the person having the apparatus implanted within his body is exercising comprising:

a demand pacer for implantation in the human body and having a pulse generator and control circuitry mounted therein and operable to sense, through a pacing lead, when a heart is not naturally paced;

a pacing lead for implantation in a heart and coupled to said pacer, said pacing lead having a tip electrode for engaging and supplying pacing pulses to a ventricle of a heart and having an opening therein which is located in a portion of the lead which is received in a heart chamber when the lead is implanted in a heart;

pCO₂ sensing means mounted in said opening for sensing the partial pressure (tension) of CO₂ dissolved in the blood and generating signals related to the pCO₂ sensed; and

said control circuitry comprising a microprocessor including means for relating a signal from the pCO₂ sensing means to the partial pressure of carbon dioxide, pCO₂, in the blood, means for determining changes in the partial pressure of carbon dioxide, Δ

p_T CO₂, and relating such changes to a corresponding change in pacing rate, Δ

R_T, between a maximum rate increase allowed, Δ

R_Max.1, and a maximum rate decrease allowed, Δ

R_Max.2, means for adjusting the pacing rate, Δ

R_T-1, by adding Δ

R_T to the present pacing rate, R_T-1, to obtain a new pacing rate, R_T, between a minimum programmed pacing rate, R_Min. and a maximum programmed pacing rate, R_Max., needed to supply a required cardiac output relative to the pCO₂ measured, and means for causing the pacer to pace the heart at the newly calculated required rate when the heart is not naturally paced while the person is exercising.

2. The apparatus of claim 1 wherein said control circuitry comprises memory means having as look-up table therein of the relationship between Δ

R_T and Δ

p_T CO₂ and means for adjusting the pacing rate, R_T-1, including means for computing R_T from reference to the look-up table for a Δ

R_T corresponding to a calculated Δ

p_T CO₂, at a particular point in time for a measurement a pCO₂ during exercise.

3. The apparatus of claim 2 wherein said control circuitry includes means for smoothing out changes in Δ

p_T CO₂ and means for determining when there has been a change in sign between samplings of pCO₂ and calculations of Δ

p_T CO₂.

4. The apparatus of claim 1 wherein said pCO₂ sensing means comprises an ion sensitive field effect transistor (ISFET) which is mounted in the pacing lead beneath the opening and which includes:

a silicon bulk having a middle surface area defining a gate region of aid ISFET and facing the opening;

means forming side walls for a chamber above said gate region;

a liquid solution in said chamber over the gate region;

a CO₂ permeable membrane across the opening and the gate region sealing off the area above the gate region to hold the liquid solution in said chamber formed between the bulk and the membrane above the gate region;

a drain formed in the silicon bulk;

a source formed in the silicon bulk; and

a reference electrode mounted in the chamber;

and means for supplying voltage to said ISFET and to said reference electrode whereby the drain-source current through the ISFET (assuming the voltage between reference electrode and source is kept constant) is changed by free hydrogen ions in the liquid solution caused by CO₂ that passes from the blood through the permeable membrane into the liquid solution, such changer in drain-source current is directly related to the pCO₂ in the blood in the heart.

5. The apparatus of claim 1 wherein said control circuitry comprises memory means having a look-up table of required heart rate changes relative to various values of pCO₂ changes, an algorithm for operating said apparatus stored therein, and a microprocessor including:

means for meausring the pCO₂ sensed at a first point in time;

means for computing the incremental change in pCO₂ ;

space="preserve" listing-type="equation">Δ

pCO.sub.2 =pCO.sub.2T -(pCO.sub.2T-1);

means for looking up the corresponding Δ

R_T in the look-up table;

means for computing the required heasrt rate R_T =Δ

R+(R_T-1);

means for determining whether the computed heart rate, R_T, is equal to or above the maximum programmed pacer rate, R_Max., or is equal to or below the minimum programmed pacer rate, R_Min. ; and

means for replacing the pacer rate R_T-1 by either the maximum programmed pacer rate, R_Max., or minimum programmed pacer rate, R_Min., stored in the memory of the microprocessor and replacing the last sensed pCO₂ value with the newly sensed pCO₂ value if the calculated heart rate meets either of the two conditions set forth above;

means for replacing the last pacing rate, R_T-1, by the newly computed heart rate, R_T, and replacing the last pCO₂ by the newly measured pCO₂, if the heart rate, R_T, calculated is between the maximum, R_Max., and minimum, R_Min., pacer rates stored in the memory of the microprocessor, and

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

6. An implantable apparatus for pacing a heart in accordance with the heart/pacer rate needed to produce a required cardiac output relative to the partial pressure of carbon dioxide in the blood, pCO₂, while the person having the apparatus implanted within his body is exercising comprising:

a demand pacer for implantation in the human body and having a pulse generator and control circuitry mounted therein and operable to sense, through a pacing lead, when a heart is not naturally paced;

a pacing lead for implantation in a heart and coupled to said pacer, said pacing lead having a tip electrode adapted to engage and supply pacing pulses to a ventricle of the heart when the lead is placed in a heart and having an opening therein which is located in a portion of the lead which is received in a heart chamber when the lead is placed in a heart;

said opening containing a pCO₂ sensor comprising a CO₂ permeable membrane which will make contact with the blood in a heart when the lead is placed in a heart and forming one side of a chamber having a liquid solution therein, a pH electrode on the other side of said chamber, a solid state reference electrode positioned within said chamber, and means for supplying voltage to said pH electrode and to said reference electrode whereby the potential between said pH electrode and said reference electrode is changed by the amount of free hydrogen ions in the liquid solution and such change in potential is directly related to the pCO₂ of the blood in a heart; and

said control circuitry comprising a microprocessor for relating the drain-source current to the partial pressure of carbon dioxide, pCO₂ in the blood, means for determining changes in the partial pressure of carbon dioxide, Δ

pCO₂, and relating such changes to a corresponding change in pacing rate, Δ

R_T, between a maximum rate increase allowed, R_Max.1, and a minimum rate decrease allowed, R_Max.2, means for adjusting the pacing rate, R_T-1, by adding Δ

R_T to the present pacing rate, R_T-1, to obtain a new pacing rate, R_T, between a minimum programmed pacing rate, R_Min. and a maximum programmed pacing rate, R_Max., needed to supply a desired cardiac output relative to the pCO₂ measured, and means for causing the pacer to pace the heart at the newly calculated required rate when the heart is not naturally paced while the person is exercising.

7. The apparatus of claim 6 wherein said control circuitry includes a memory having a look-up table therein of the relationship between Δ

R_T and Δ

p_T CO₂ and said control circuitry includes means for computing R_T, by referencing the look-up table for a Δ

R_T corresponding to a calculated Δ

p_T CO₂, at a particular point in time for a measurement of pCO₂ during exercise.

8. The apparatus of claim 6 wherein said control circuitry includes means for smoothing changes in Δ

p_T CO₂ and means for determining when there has been a change in sign between samplings of pCO₂ and calculations of Δ

p_T CO₂ so that samplings can be restarted after a change in direction, up or down, of the changes in pCO₂.

9. The apparatus of claim 6 wherein said control circuitry comprises memory means having as look-up table of required pacing rates relative to various values of pCO₂ and an algorithm for operating said apparatus stored therein, and a microprocessor including:

means for measuring the pCO₂ sensed;

means for looking up the corresponding heart rate in the look-up table;

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

means for monitoring and repeating the step of measuring pCO₂ and changing the pacing rate as the pCO₂ changes.

10. A method for pacing a heart in accordance with the heart rate needed to produce a required cardiac output relative to the partial pressure of carbon dioxide in the blood, pCO₂, while a person, whose heart is being paced, is exercising, said method comprising the steps of:

implanting a demand pacer in a human body having a pulse generator and control circuit mounted therein;

implanting a pacing lead in a heart and coupling said lead to said pacer, said pacing lead having a tip electrode;

positioning said tip electrode to engage in and supply pacing pulses to a ventricle of the heart;

providing said pacing lead with an opening therein a portion of the lead which is received in the heart;

mounting pCO₂ sensing means in the opening in the lead;

sensing the pCO₂ in the heart with the pCO₂ sensing means;

determining with said control circuitry the partial pressure of carbon dioxide, pCO₂, in the blood from signals from asid PCO₂ sensing means;

determining the required pacing rate needed to supply a desired cardiac output relative to the sensed pCO₂ including the steps of;

determining values of pCO₂ sensed;

determining changes in pCO₂ sensed;

relating the change, Δ

p_T CO₂, to a corresponding change in heart rate, Δ

R_T, between a maximum rate increase allowed, R_Max.1, and a minimum rate decrease allowed, R_Max.2 ;

adjusting the pacing rate, R_T-1, by adding Δ

R_T to the present pacing rate, R_T-1, to obtain a new pacing rate, R_T, between a minimum programmed pacing rate, R_Min. and a maximum programmed pacing rate, R_Max ; and

causing the pacer to pace the heart at the newly calculated required pacing rate, R_T, when the heart is not naturally paced.

11. The method of claim 10 wherein said step of relating partial pressure of carbon dioxide to changes in heart rate, Δ

R_T, and determining the required pacing rate includes looking up in a look-up table in a memory the relationship between Δ

R_T and Δ

p_T CO₂ and said step of relating pCO₂ with a required heart rate includes the step of computing R_T by first referencing the look-up table to find a Δ

R_T corresponding to a measured Δ

p_T CO₂, at a particular point in time during exercise.

12. The method of claim 10 including the steps of:

smoothing changes in Δ

p_T CO₂ ; and

determining when there has been a change in sign between samplings of pCO₂ so that samplings can be restarted after a change in direction, up or down, of the changes in pCO₂.

13. The method of claim 10 including the step of supplying an alternating a current to said pCO₂ sensor to minimize power consumption by the pCO₂ sensor.

14. The method of claim 10 including the step of supplying pulses of current to said pCO₂ sensor to minimize power consumption by the pCO₂ sensor.

15. The method of claim 10 wherein said step of relating the pCO₂ with the required heart rate comprises the steps of:

measuring the pCO₂ at a first point in time;

computing the incremental change in pCO₂ ;

space="preserve" listing-type="equation">Δ

pCO.sub.2 =pCO.sub.2 T-(pCO.sub.2T-1);

looking up the coresponding Δ

R in a look-up table of heart rates relative to various values of pCO₂ ;

computing the required heart rate R_T =Δ

R+(R_T-1);

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

if the calculated heart rate meets either of these conditions, replacing the pacer rate R_T-1 by either the maximum programmed pacer rate or minimum programmed pacer rate stored in a memory and replacing the last sensed pCO₂ with the newly sensed pCO₂ value;

if the heart rate calculated is between the maximum and minimum programmed pacer rates stored in a memory, replacing the last pacing rate by the newly computed heart rate and replacing the last pCO₂ value by the newly measured pCO₂ value; and

then

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

16. The method of claim 15 including the step of smoothing changes in Δ

pCO₂ ; and

determining whether there has been a change in sign between samplings of Δ

p_T CO₂ so that samplings can be restarted after a change in direction, up or down, of the changes in pCO₂.

17. The method of claim 10 wherein said step of relating pCO₂ with the required heart rate comprises the steps of:

measuring pCO₂ in the right ventricle;

looking up the corresponding heart rate in a look-up table of heart rates relative to various values of pCO₂ ;

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

monitoring and repeating the step of measuring pCO₂ and changing the pacing rate as the pCO₂ changes.

18. A method for pacing a heart in accordance with the heart rate needed to produce a required cardiac output, while a person, whose heart is being paced, is exercising, relative to the partial pressure of carbon dioxide in blood, pCO₂, said method comprising the steps of:

implanting a demand pacer in a human body and having a pulse generator and control circuitry mounted therein;

implanting a pacing lead in a heart and coupling said lead to said pacer, said pacing lead having a tip electrode;

positioning said tip electrode to engage in and supply pacing pulses to a ventricle of the heart;

providing said pacing lead with an opening therein in a portion of the lead which is received in the heart;

mounting a pCO₂ sensor in said opening comprising a chamber having a liquid solution therein, a CO₂ permeable membrane on one side of said chamber which will make contact with the blood in a heart when the lead is placed in a heart, a pH electrode on the other side of said chamber, a solid state reference electrode positioned within said chamber, and, means for supplying a voltage to said pH electrode and to said reference electrode whereby the potential between said pH electrode and said reference electrode is changed by the amount of free hydrogen ions in the liquid solution and such change in potential is directly related to the pCO₂ of the blood in the heart;

sensing the pCO₂ in the heart with the pCO₂ sensor; and

determining with said control circuitry the drain-source current to the partial pressure of carbon dioxide, pCO₂, in the blood;

determining the required pacing rate needed to supply a desired cardiac output relative to the sensed pCO₂ including the steps of;

determining values of pCO₂ sensed;

determining changes in pCO₂ sensed

relating the change, Δ

p_T CO₂, to a corresponding change in heart rate, Δ

R_T, between a maximum rate increase allowed, R_Max.1, and a maximum rate decrease allowed, R_Max.2 ;

adjusting the pacing rate, R_T-1, by adding Δ

R_T to the present pacing rate, R_T-1, to obtain a new pacing rate, R_T, between a minimum programmed pacing rate, R_Min., and a maximum programmed pacing rate, R_Max. ; and

causing the pacer to pace the heart at the newly calculated required pacing rate, R_T, when the heart is not naturally paced.

19. The method of claim 18 wherein said step of relating pCO₂ sensed with a required heart rate includes looking up in a look-up table of the relationship between Δ

R_T and Δ

p_T CO₂ in a memory and said step of relating pCO₂ with a required heart rate including the step of computing R_T from a Δ

R_T corresponding to a measured Δ

p_T CO₂ by referencing the look-up table at a particular point in time during exercise.

20. The method of claim 18 including the steps of smoothing changes in pCO₂ ;

and determining when there has been a change in sign between samplings of Δ

p_T CO₂ so that samplings can be restarted after a change in direction, up or down, of the changes in pCO₂.

21. The method of claim 18 including the step of supplying an alternating current to said pCO₂ sensor, including said pH electrode and said reference electrode, to minimize the power consumption by the pCO₂ sensor.

22. The method of claim 18 including the step of supplying pulses of current to said pCO₂ sensor, including said pH sensor and said reference electrode to minimize the power consumption by the pCO₂ sensor.