System and method for controlling the temperature of a catheter-mounted heater

• US 5,588,438 A
• Filed: 06/07/1995
• Issued: 12/31/1996
• Est. Priority Date: 01/29/1991
• Status: Expired due to Term

• Current Assignee: Edwards Lifesciences Corporation
• Original Assignee: Interflo Medical, Inc.
• Inventors: McKown, Russell C., Quinn, Michael D., Yelderman, Mark L.
• Primary Examiner(s): Sykes, Angela D.
• Assistant Examiner(s): NASSER, ROBERT L
• Application Number:
  US08/473,738
• Time in Patent Office: 573 Days
• Field of Search: 128/691-4, 128/736, 128/897, 128/898, 128/713, 73/4 R
• US Class Current: 600/505
• CPC Class Codes: A61B 18/14 Probes or electrodes therefor
• CPC Class Codes: A61B 2017/00482 with a code
• CPC Class Codes: A61B 2018/00166 Multiple lumina
• CPC Class Codes: A61B 2018/00178 Electrical connectors
• CPC Class Codes: A61B 2018/00666 using a threshold value
• CPC Class Codes: A61B 2018/00678 upper
• CPC Class Codes: A61B 2018/00708 switching the power on or off
• CPC Class Codes: A61B 2018/00791 Temperature
• CPC Class Codes: A61B 2018/00797 measured by multiple temperature sensors
• CPC Class Codes: A61B 2018/00815 measured by a thermistor
• CPC Class Codes: A61B 2018/00821 measured by a thermocouple
• CPC Class Codes: A61B 2018/00988 Means for storing information, e.g. calibration constants, or for preventing excessive use, e.g. usage, service life counter
• CPC Class Codes: A61B 2562/08 Sensors provided with means for identification, e.g. barcodes or memory chips
• CPC Class Codes: A61B 5/028 by thermo-dilution
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes
• CPC Class Codes: A61B 5/6855 with a distal curved tip
• CPC Class Codes: A61B 5/6856 with a distal loop
• CPC Class Codes: A61B 5/6857 with a distal pigtail shape
• CPC Class Codes: G05D 23/2401 using a heating element as a sensing element

Abstract

A system for keeping the surface temperature of an electric resistance-type heater element in a thermodilution catheter within safe physiological limits includes, in the preferred embodiment, a heater element core temperature monitor, a monitor for monitoring the power that is supplied to the heater element, and a surface temperature calculator for calculating the surface temperature of the heater element based on the core temperature, supplied power, and information representing the characteristics of the particular catheter under anticipated clinical conditions. A second aspect of the invention involves a system for determining the supply of power to the heater element based on the core temperature of the heater element. A third aspect of the invention involves a system readiness test for determining, in vivo, that the thermodilution catheter system is properly calibrated before the system is operational. Methods of operation for each of the above-referenced aspects of the invention are also disclosed.

Claims

1. A heater resistance verification system, comprising:

(i) means for measuring, in vivo, a temperature of blood when the blood is in contact with a thermodilution catheter;

(ii) a power supply for supplying electric power to an electric resistance-type heater element;

(iii) a power monitor for monitoring an amount of electric power that is supplied to the electric resistance-type heater element by the power supply;

(iv) a resistance monitor for monitoring an electrical resistance of the electric resistance-type heater element; and

(v) control means in communication with the means for measuring, the power supply, the power monitor, and the resistance monitor, for;

(a) empirically determining a relationship between the amount of electric power supplied and the electrical resistance of the electric resistance-type heater element under in vivo conditions;

(b) using the empirically determined relationship to estimate what electrical resistance the electric resistance-type heater element would have at a reference temperature;

(c) comparing the estimated resistance of the electric resistance-type heater element at the reference temperature with a known resistance of the electric resistance-type heater element at the reference temperature; and

(d) determining whether the difference between the estimated resistance and the known resistance of the electric resistance-type heater element exceeds a predetermined maximum.

2. A system according to claim 1, wherein the control means empirically determines the relationship between the amount of electric power supplied and the resistance of the electric resistance-type heater element under said in vivo conditions by:

(1) instructing the power supply to supply power at a first power level;

(2) transmitting values for supplied power and resistance for the first power level from the power monitor and the resistance monitor, respectively, to the control means;

(3) instructing the power supply to supply power at a second power level; and

(4) transmitting values for supplied power and resistance for the second power level from the power monitor and the resistance monitor, respectively, to the control means, whereby empirical data relating power to resistance under the in vivo conditions is obtained.

3. A system according to claim 1, wherein the control means uses the empirically determined relationship to estimate what electrical resistance the heater element would have at the reference temperature by using the formula:

space="preserve" listing-type="equation">Ro.sub.b =R.sub.b /{(T.sub.b -T.sub.o)*TCR+1};

where Ro_b is the estimated heater element resistance at the reference temperature;

R_b is a zero power intercept value from the empirical determination made in step (a);

T_b is the blood temperature measured by the means for measuring temperature of the blood;

T_o is the reference temperature; and

TCR is the temperature coefficient of resistance for the material from which the heater element is fabricated.

4. A system according to claim 1, wherein the control means functions to cease the supply of power to the electric resistance-type heater element when the control means determines that the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds the predetermined maximum.

5. A heater resistance verification method for verifying, in vivo, the calibration of a thermodilution catheter system of the type which utilizes an electric resistance-type heater element, comprising the steps of:

(a) empirically determining a relationship between power supplied to the electric resistance-type heater element and resistance of the electric resistance-type heater element under in vivo conditions;

(b) using the empirically determined relationship to estimate what resistance of the electric resistance-type heater element would be at a reference temperature;

(c) comparing the estimated resistance of the electric resistance-type heater element at the reference temperature with a known resistance of the electric resistance-type heater element at the reference temperature; and

(d) determining whether the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds a predetermined maximum.

6. A method according to claim 5, wherein the step of using the empirically determined relationship between power supplied to the electric resistance-type heater element and resistance of the electric resistance-type heater element under in vivo conditions comprises the sub-steps of:

(1) supplying power at a first power level to the electric resistance-type heater element;

(2) storing values representing the first power level and storing values representing a resistance of the electric resistance-type heater element when the electric resistance-type heater element is powered at the first power level;

(3) supplying power at a second power level to the electric resistance-type heater element; and

(4) storing values representing the second power level and storing values representing a resistance of the electric resistance-type heater element when the electric resistance-type heater element is powered at the second power level, whereby empirical data relating power to resistance under the in vivo conditions is obtained.

7. A method according to claim 6, further comprising the step of measuring blood temperature for blood adjacent the electric resistance-type heater element, wherein the step of using the empirically determined relationship to estimate what resistance of the electric resistance-type heater element would be at a reference temperature comprises using the formula:

space="preserve" listing-type="equation">Ro.sub.b =R.sub.b /{(T.sub.b -T.sub.o)*TCR+1};

where Ro_b is the estimated heater element resistance at the reference temperature;

R_b is a zero power intercept value from the empirical determination made in step (a);

T_b is the blood temperature for the blood adjacent the electric resistance-type heater element;

T_o is the reference temperature; and

TCR is the temperature coefficient of resistance for the material from which the heater element is fabricated.

8. A method according to claim 6, further comprising the step of ceasing the supply of power to the electric resistance-type heater element if step (d) indicates that the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds the predetermined maximum.

9. A heater resistance verification system, comprising:

(i) means for measuring a temperature of blood when the blood is in contact with a thermodilution catheter;

(ii) a power supply for supplying electric power to an electric resistance-type heater element;

(iii) a power monitor for monitoring an amount of electric power that is supplied to the electric resistance-type heater element by the power supply;

(iv) a resistance monitor for monitoring an electrical resistance of the electric resistance-type heater element; and

(v) control means in communication with the means for measuring, the power supply, the power monitor, and the resistance monitor, for;

(a) empirically determining a relationship between the amount of electric power supplied and the electrical resistance of the electric resistance-type heater element;

(b) using the empirically determined relationship to estimate what electrical resistance the electric resistance-type heater element would have at a reference temperature;

(c) comparing the estimated resistance of the electric resistance-type heater element at the reference temperature with a known resistance of the electric resistance-type heater element at the reference temperature; and

(d) determining whether the difference between the estimated resistance and the known resistance of the electric resistance-type heater element exceeds a predetermined maximum.

10. A system according to claim 9, wherein the control means empirically determines the relationship between the amount of electric power supplied and the resistance of the electric resistance-type heater element by:

(1) instructing the power supply to supply power at a first power level;

(2) transmitting values for supplied power and resistance for the first power level from the power monitor and the resistance monitor, respectively, to the control means; and

(3) instructing the power supply to supply power at a second power level;

(4) transmitting values for supplied power and resistance for the second power level from the power monitor and the resistance monitor, respectively, to the control means, whereby empirical data relating power to resistance is obtained.

11. A system according to claim 9, wherein the control means uses the empirically determined relationship to estimate what electrical resistance the electric resistance-type heater element would have at the reference temperature by using the formula:

space="preserve" listing-type="equation">Ro.sub.b =R.sub.b /{(T.sub.b -T.sub.o)*TCR+1};

where Ro_b is the estimated heater element resistance at the reference temperature;

R_b is a zero power intercept value from the empirical determination made in step (a);

T_b is the blood temperature measured by the means for measuring temperature of the blood;

T_o is the reference temperature; and

TCR is the temperature coefficient of resistance for the material from which the heater element is fabricated.

12. A system according to claim 9, wherein the control means functions to cease the supply of power to the electric resistance-type heater element when the control means determines that the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds the predetermined maximum.

13. A heater resistance verification method for verifying the calibration of a thermodilution catheter system of the type which utilizes an electric resistance-type heater element, comprising the steps of:

(a) empirically determining a relationship between power supplied to the electric resistance-type heater element and resistance of the electric resistance-type heater element;

(b) using the empirically determined relationship to estimate what resistance of the electric resistance-type heater element would be at a reference temperature;

(c) comparing the estimated resistance of the electric resistance-type heater element at the reference temperature with a known resistance of the electric resistance-type heater element at the reference temperature; and

(d) determining whether the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds a predetermined maximum.

14. A method according to claim 13, wherein the step of using the empirically determined relationship between power supplied to the electric resistance-type heater element and resistance of the electric resistance-type heater element comprises the sub-steps of:

(1) supplying power at a first power level to the electric resistance-type heater element;

(2) storing values representing the first power level and storing values representing a resistance of the electric resistance-type heater element when the electric resistance-type heater element is powered at the first power level; and

(3) supplying power at a second power level to the electric resistance-type heater element;

(4) storing values representing the second power level and storing values representing a resistance of the electric resistance-type heater element when the electric resistance-type heater element is powered at the second power level, whereby empirical data relating power to resistance is obtained.

15. A method according to claim 14, further comprising the step of measuring blood temperature for blood adjacent the electric resistance-type heater element, wherein the step of using the empirically determined relationship to estimate what resistance of the electric resistance-type heater element would be at a reference temperature comprises using the formula:

space="preserve" listing-type="equation">Ro.sub.b =R.sub.b /{(T.sub.b -T.sub.o)*TCR+1};

where Ro_b is the estimated heater element resistance at the reference temperature;

R_b is a zero power intercept value from the empirical determination made in step (a);

T_b is the blood temperature for the blood adjacent the electric resistance-type heater element;

T_o is the reference temperature; and

TCR is the temperature coefficient of resistance for the material from which the heater element is fabricated.

16. A method according to claim 14, further comprising the step of ceasing the supply of power to the electric resistance-type heater element if step (d) indicates that the difference between the estimated resistance of the electric resistance-type heater element and the known resistance of the electric resistance-type heater element exceeds the predetermined maximum.