Thermodilution catheter having a safe, flexible heating element

• US 6,387,052 B1
• Filed: 04/19/1993
• Issued: 05/14/2002
• Est. Priority Date: 01/29/1991
• Status: Active Grant

• Current Assignee: Edwards Lifesciences Corporation
• Original Assignee: Edwards Lifesciences Corporation
• Inventors: Quinn, Michael D., Yelderman, Mark L.
• Primary Examiner(s): Nasser, Robert L.
• Application Number:
  US08/049,231
• Time in Patent Office: 3,312 Days
• Field of Search: 128/690-3, 128/713, 219/497, 219/549, 219/535, 600/526, 600/505
• 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 2018/00178 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

Abstract

A thermodilution catheter having a heating filament which is fabricated so as to be thin and flexible enough to avoid contact with the patient'"'"'s blood. The heating filament is either inserted in a preformed catheter lumen, incorporated into a wall of the catheter body itself, or wrapped around the catheter body wall and surrounded by an external sheath. Generally, the covering of the heating filament is minimally thin so as to allow the heat from the heating filament to be transferred to the surrounding blood and to minimally increase the overall cross-sectional area. Since the heating filament does not directly touch the patient'"'"'s blood, the outer surface may be made smooth so as to prevent inducement of blood clots. In addition, the heating filament may be maintained at a safe temperature by forming the heating element of a flexible material having a high temperature coefficient of resistance, low thermal capacitance and high thermal conductivity. Preferably, the temperature coefficient of resistance of the material forming the heater filament is greater than 0.001 Ω/Ω-° C. and the resistance of the heating filament is proportional or inversely proportional to its temperature. By monitoring this resistance, the application of power to the heating filament may be regulated to prevent overheating of the heating filament. Calibration techniques are also disclosed whereby a memory containing encoded calibration information is disposed within or connected to a connector of the catheter. This memory may also include a program segment used by a cardiac output computer having a cardiac output calculation program, whereby the portion of code stored in the memory must be input into the cardiac output computer before the determination of the cardiac output can be performed. As a result of these improvements, the invention enables the thermodilution cardiac output calculation to be performed quite safely and accurately in a clinical setting.

Claims

1. Apparatus for heating blood flowing through a heart in order to determine cardiac output based upon a temperature rise of the blood, said apparatus comprising:

(a) a resistive heater connectable to a source of electrical current;

(b) a catheter having a distal end that can be inserted intravascularly into the heart, the distal end including a temperature sensor for sensing the temperature of blood heated by the resistive heater, the resistive heater being mounted on the catheter and displaced from the distal end thereof so that the temperature sensor is not directly affected by the temperature of the resistive heater via thermal conduction through said catheter;

(c) temperature sensing means for sensing the temperature of the resistive heater and producing a signal indicative thereof; and

(d) control means, connected to receive the signal indicative of the temperature of the resistive heater, for controlling an electrical current flowing from the source through the resistive heater to limit the temperature of the resistive heater.

2. The apparatus of claim 1, wherein the temperature sensor comprises a thermistor or a thermocouple.

3. The apparatus of claim 1, wherein the resistive heater has a resistance that varies with temperature.

4. The apparatus of claim 1, wherein the control means are operative to maintain a nominal fixed power dissipation in the resistive heater, but reduce the power delivered to the resistive heater to establish a different nominal fixed power dissipation if the temperature of the resistive heater otherwise exceeds the predetermined value.

5. The apparatus of claim 1, further comprising means for calculating the power delivered to the resistive heater and for calculating the temperature of the resistive heater for use in calculating cardiac output.

6. Apparatus for use in continuously monitoring cardiac output based upon a temperature rise of blood flowing through a heart due to heat transfer, said apparatus comprising:

(a) a catheter having a distal end and a temperature sensor disposed proximate the distal end;

(b) means for supplying heat to a heated portion of the catheter, heat transfer from the heated portion of the catheter serving to heat the blood within the heart, the heated portion being displaced from the distal end of the catheter so that the temperature sensor disposed proximate the distal end of the catheter can sense the temperature of blood heated by the means for supplying heat without being directly affected by the temperature of said means;

(c) temperature sensing means for monitoring the temperature of the heated portion of the catheter and for producing a signal indicative thereof; and

(d) control means, connected to receive the signal indicative of the temperature of the heated portion of the catheter, for controlling the temperature of the heated portion so that it does not rise to a level and for a period of time that is likely to damage the blood.

7. The apparatus of claim 6, wherein the temperature sensor comprises a thermistor or a thermocouple.

8. The apparatus of claim 6, wherein:

(a) the means for supplying heat delivers a fixed power to the heated portion of the catheter and

(b) the control means are operative to vary the temperature of the heated portion of the catheter, thereby causing a different fixed power to be delivered to the heated portion of the catheter only if the temperature of the heated portion of the catheter exceeds a level that is likely to damage the blood.

9. Apparatus for heating blood flowing through a heart in order to determine cardiac output based upon a temperature rise of the blood, said apparatus comprising:

(a) a resistive heater connectable to a source of electrical current, wherein the resistive heater has a resistance that varies with temperature;

(b) a catheter having a distal end that can be inserted intravascularly into the heart, the resistive heater being mounted on the catheter and displaced from the distal end thereof;

(c) temperature sensing means for sensing the temperature of the resistive heater and producing a signal indicative thereof, wherein the temperature sensing means comprise means for measuring an electrical current flowing from the source through the resistive heater and a voltage drop across the resistive heater; and

(d) control means, connected to receive the signal indicative of the temperature of the resistive heater, for controlling the electrical current flowing from the source through the resistive heater to limit the temperature of the resistive heater, wherein the signal indicative of the temperature of the resistive heater is proportional to a ratio of the voltage drop across the resistive heater and the electrical current flowing through the resistive heater.

10. The apparatus of claim 9, wherein the control means include:

(a) means for producing a reference signal corresponding to the predetermined value and

(b) a closed loop control means for monitoring the reference signal and for controlling the temperature of the resistive heater by controlling the application of power to the resistive heater.

11. Apparatus for heating blood flowing through a heart in order to determine cardiac output based upon a temperature rise of the blood, said apparatus comprising:

(a) a resistive heater connectable to a source of electrical current;

(b) a catheter having a distal end that can be inserted intravascularly into the heart, the resistive heater being mounted on the catheter and displaced from the distal end thereof;

(c) temperature sensing means for sensing the temperature of the resistive heater and for producing a signal indicative thereof;

(d) control means, connected to receive the signal indicative of the temperature of the resistive heater, for controlling the electrical current flowing from the source through the resistive heater to limit the temperature of the resistive heater; and

(e) means for measuring electrical power dissipated in the resistive heater as a function of the signal indicative of its temperature, producing a signal corresponding thereto for use in determining the cardiac output.

12. Apparatus for use in continuously monitoring cardiac output based upon a temperature rise of blood flowing through a heart due to heat transfer, said apparatus comprising:

(a) a catheter;

(b) means for supplying heat to a heated portion of the catheter, heat transfer from the heated portion of the catheter serving to heat the blood within the heart, wherein the means for supplying heat comprise a resistive heater disposed at the heated portion of the catheter, the resistive heater adapted to be connected to a source of an electrical current and having a resistance that varies with temperature;

(c) temperature sensing means for monitoring the temperature of the heated portion of the catheter and for producing a signal indicative thereof, wherein the temperature sensing means comprise means for measuring an electrical current flowing through the resistive heater and a voltage drop across it, the signal indicative of the temperature of the heated portion being proportional to a ratio of the voltage drop and the electrical current; and

(d) control means, connected to receive the signal indicative of the temperature of the heated portion of the catheter, for controlling the temperature of the heated portion so that it does not rise to a level and for a period of time that is likely to damage the blood.

13. Apparatus for heating blood flowing through a heart in order to determine cardiac output based upon a temperature rise of the blood, said apparatus comprising:

(a) a resistive heater connectable to a source of an electrical current, the resistive heater having a resistance that varies with its temperature;

(b) a catheter having a distal end that can be inserted intravascularly into the heart, the distal end including a temperature sensor for sensing a temperature of blood heated by the resistive heater, the resistive heater being mounted on the catheter and displaced from the distal end thereof so that the temperature sensor is not directly affected by the temperature of the resistive heater;

(c) temperature sensing means for sensing the temperature of the resistive heater and for producing a signal indicative thereof, wherein;

(i) the temperature sensing means comprise means for measuring the electrical current flowing through the resistive heater and a voltage drop across it and

(ii) the signal indicative of the temperature of the resistive heater is proportional to a ratio of the voltage drop and the electrical current flowing through the resistive heater; and

(d) control means, connected to receive the signal indicative of the temperature of the resistive heater, for controlling the electrical current flowing from the source through the resistive heater to limit the temperature of the resistive heater.

14. Apparatus for heating blood flowing through a heart in order to determine cardiac output based upon a temperature rise of the blood, said apparatus comprising:

(a) a resistive heater connectable to a source of an electrical current;

(b) a catheter having a distal end that can be inserted intravascularly into the heart, the distal end including a temperature sensor for sensing a temperature of blood heated by the resistive heater, the resistive heater being mounted on the catheter and displaced from the distal end thereof so that the temperature sensor is not directly affected by the temperature of the resistive heater;

(c) temperature sensing means for sensing the temperature of the resistive heater and for producing a signal indicative thereof; and

(d) control means, connected to receive the signal indicative of the temperature of the resistive heater, for controlling the electrical current flowing from the source through the resistive heater to limit the temperature of the resistive heater, wherein the control means include;

(i) means for producing a reference signal corresponding to the predetermined value and

(ii) a closed loop control means for monitoring the reference signal and for controlling the temperature of the resistive heater by controlling the application of power to the resistive heater.

15. Apparatus for use in continuously monitoring cardiac output based upon a temperature rise of blood flowing through a heart, said apparatus comprising:

(a) a catheter having a heater mounted thereon;

(b) means for supplying heat to the heater that is insertable intravascularly into a heart, heat transfer from the heater serving to heat the blood within the heart, wherein;

(i) the means for supplying heat comprises a source of an electrical current coupled electrically to the heater so that the electrical current flows through the heater and

(ii) the heater has a resistance that varies with its temperature;

(b) temperature sensing means for monitoring the temperature of the heater and for producing a signal indicative thereof; and

(c) control means, connected to receive the signal indicative of the temperature of the heater, for controlling the temperature of the heater so that it does not rise to a level and for a period of time that is likely to damage the blood.

16. The apparatus of claim 15, wherein the temperature sensing means comprise means for measuring the electrical current flowing through the heater and a voltage drop across it, the signal indicative of the temperature of the resistive heater being proportional to a ratio of the voltage drop and the electrical current.

17. The apparatus of claim 15, wherein the temperature sensing means comprise a network that includes:

(i) the resistive heater and

(ii) means for sensing voltage differential across the resistive heater to produce the signal indicative of the temperature of the resistive heater.

18. The apparatus of claim 17, wherein the electrical heating current flowing through the heater provides the signal indicative of the temperature of the resistive heater.

19. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introducing into a blood vessel;

a flexible heating filament disposed with respect to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel, said flexible heating filament having a resistance which is proportional or inversely proportional to its temperature and a temperature coefficient of resistance whose absolute magnitude is at least about 0.001 Ω

/Ω

-°

C.;

means for measuring the resistance of said flexible heating filament;

means, coupled to the means for measuring, for controlling the temperature of said flexible heating filament based upon the measured resistance of said flexible heating filament; and

temperature detecting means, disposed downstream of said flexible heating filament relative to the direction of flow of the blood in the blood vessel, for detecting the temperature of the blood at a location that is downstream of said flexible heating filament.

20. An apparatus as in claim 19 wherein said means for controlling includes means for reducing power to said flexible heating filament when the resistance of said flexible heating filament exceeds a predetermined resistance.

21. An apparatus as in claim 20 wherein said means for reducing power is adapted to reduce the power to said flexible heating filament when the temperature of said flexible heating filament is equal to or greater than approximately 52°

C.

22. An apparatus as in claim 19 wherein the temperature coefficient of resistance of said flexible heating filament is about 0.004 Ω

/Ω

-°

C.

23. An apparatus as in claim 19 wherein said flexible heating filament is comprised of approximately 70% nickel and 30% iron.

24. An apparatus as in claim 19 wherein said flexible heating filament is comprised of approximately 29% nickel, 17% cobalt, and 54% iron.

25. An apparatus as in claim 19 wherein said flexible tubular catheter member comprises

a substantially cylindrical body wall portion and

an outer sheath having an outer surface, said flexible heating filament being in a thin layer, being wrapped about said substantially cylindrical body wall portion, and being enclosed by said outer sheath.

26. An apparatus as in claim 25 further comprising:

a layer of a material with a high thermal conductivity disposed about said flexible heating filament so as to create a more uniform surface temperature for the outer surface of said outer sheath.

27. An apparatus as in claim 25 wherein said substantially cylindrical body wall portion has an at least approximately constant first diameter over a majority of its length and a reduced second diameter in a heating region, said flexible heating filament is wrapped around said heating region such that the resulting total diameter in said heating region is approximately equal to the first diameter.

28. An apparatus as in claim 25 wherein said outer sheath is made of a flexible material formed by one of extrusion and blow molding.

29. An apparatus as in claim 25 wherein said outer sheath is made of a flexible material which has been heat shrunk to form-fit said flexible heating filament and said flexible tubular body wall portion.

30. An apparatus as in claim 25 wherein said flexible tubular catheter member has a distal end and said temperature detecting means comprises

a thermistor or a thermocouple proximate to the distal end of said flexible tubular catheter member and disposed distally with respect to said flexible heating filament.

31. An apparatus as in claim 25 wherein said temperature detecting means comprises:

a thermistor or a thermocouple disposed proximally with respect to said flexible heating filament.

32. An apparatus as in claim 31 wherein said flexible tubular catheter member has a distal end which comprises a pigtail tip.

33. An apparatus as in claim 19 further comprising;

at least one set of electrical leads having a first end and a second end and being attached to said flexible heating filament at the first end for applying power to said flexible heating filament.

34. An apparatus as in claim 33 further comprising a cardiac output computer, wherein the second end of said set of electrical leads is connected to said cardiac output computer for applying a power signal to said electrical leads for transmitting heat to the blood.

35. An apparatus as in claim 19 wherein said means for measuring the resistance includes means for measuring current through said flexible heating filament and means for measuring voltage across said flexible heating filament.

36. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel, said blood vessel having a direction of blood flow;

a flexible heating filament wrapped in a thin layer about said flexible tubular catheter member and enclosed by a sheath so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel, said flexible heating filament having a resistance which is proportional or inversely proportional to its temperature and a temperature coefficient of resistance whose absolute magnitude is at least about 0.001 Ω

/Ω

-°

C.;

means for measuring the resistance of said flexible heating filament;

means, coupled to the means for measuring, for controlling the temperature of said flexible heating filament based upon the measured resistance of said flexible heating filament; and

temperature detecting means, disposed downstream of said flexible heating filament relative to the direction of blood flow in the blood vessel, for detecting temperature of the blood at a location that is downstream of said flexible heating filament.

37. An apparatus as in claim 36 wherein said means for measuring the resistance includes means for measuring current through said flexible heating filament and means for measuring voltage across said flexible heating filament.

38. A method of applying heat to blood in a blood vessel of a patient for thermodilution measurement, comprising the steps of:

forming a flexible heating element having a resistance which is proportional or inversely proportional to its temperature and a temperature coefficient of resistance whose absolute magnitude is at least about 0.001 Ω

/Ω

-°

C.;

disposing said flexible heating element beneath an outer sheath of a flexible tubular catheter member so that said flexible heating element does not contact the blood when said flexible heating element is in the blood vessel;

inserting said flexible tubular catheter member into the blood vessel of the patient;

applying electric power to said flexible heating element, thereby converting said electric power to heat, whereby the heating is conducted from said flexible heating element to the blood;

measuring the resistance of said flexible heating element; and

controlling the temperature of said flexible heating element based upon the measured resistance of said flexible heating element.

39. A method as in claim 38 comprising the further step of:

reducing power to said flexible heating element when the resistance of said flexible heating element exceeds a predetermined resistance.

40. A method according to claim 38, wherein the step of measuring the resistance of said flexible heating element further includes the steps of:

measuring current through said flexible heating element;

measuring voltage across said flexible heating element; and

calculating the resistance of said flexible heating filament based upon the measured current and voltage of said flexible heating element.

41. A method of thermodilution measurement, comprising the steps of:

forming a flexible heating element having a resistance which is proportional or inversely proportional to its temperature, and a temperature coefficient of resistance whose absolute magnitude is at least about 0.001 Ω

/Ω

-°

C.;

disposing said heating element beneath an outer sheath or a flexible tubular catheter member so that said flexible heating element does not contact blood when said flexible heating element is in a blood vessel;

inserting said flexible tubular catheter member into the blood vessel;

applying electric power to said flexible heating element, thereby converting the electric power to heat, whereby the heating is conducted from said flexible heating element to the blood;

measuring the resistance of said flexible heating element;

controlling the temperature of said flexible heating element based upon the measured resistance of said flexible heating element; and

detecting the temperature of the blood at a location that is downstream of said flexible heating element relative to the direction of flow of the blood in the blood vessel, in order to measure temperature variations in the blood that result from application of the heat to the blood.

42. A method according to claim 41 wherein the step of measuring the resistance of said flexible heating element further includes the steps of:

measuring current through said flexible heating element;

measuring voltage across said flexible heating element; and

calculating the resistance of said flexible heating element based upon the measured current and voltage of said flexible heating element.

43. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel;

a flexible heating filament, having a temperature coefficient of resistance whose magnitude is at least 0.001 Ω

/Ω

-°

C., designed for heating the blood in the blood vessel for thermodilution measurements, and mounted to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel;

means for measuring flexible heating filament resistance of said flexible heating filament, said means for measuring providing a flexible heating filament resistance signal corresponding to said flexible heating filament resistance; and

,

means, coupled to said means for measuring, for calculating flexible heating filament temperature of said flexible heating filament from said flexible heating filament resistance signal, said means for measuring providing a flexible heating filament temperature signal corresponding to said flexible heating filament temperature.

44. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel;

a flexible heating filament, having a temperature coefficient of resistance whose magnitude is at least 0.001 Ω

/Ω

-°

C., designed for heating the blood in the blood vessel for thermodilution measurements, and mounted to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel;

means for measuring flexible heating filament resistance of said flexible heating filament, said means for measuring providing a flexible heating filament resistance signal corresponding to said flexible heating filament resistance; and

means, coupled to said means for measuring, for controlling said flexible heating filament temperature based on a function of at least said flexible heating filament temperature, said means for controlling providing a flexible heating filament temperature control signal.

45. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel;

a flexible heating filament, having a temperature coefficient of resistance whose magnitude is at least 0.001 Ω

/Ω

-°

C., designed for heating the blood in the blood vessel for thermodilution measurements, and mounted to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel;

means for measuring flexible heating filament resistance of said flexible heating filament, said means for measuring providing a flexible heating filament resistance signal corresponding to said flexible heating filament resistance; and

means, coupled to said means for controlling, for supplying an amount of electric power to said flexible heating filament corresponding to said flexible heating filament temperature control signal.

46. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel;

a flexible heating filament, having a temperature coefficient of resistance whose magnitude is at least 0.001 Ω

/Ω

-°

C., designed for heating the blood in the blood vessel for thermodilution measurements, and mounted to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel;

wherein said flexible heating filament is disposed along a flexible heating filament region of said flexible tubular catheter member;

a catheter sheath disposed along at least said flexible heating filament region of said flexible tubular catheter member and covering said flexible heating filament; and

wherein said catheter sheath comprises an elastic material and has a relaxed catheter sheath diameter when said elastic material is in a relaxed state, a combined structure of said flexible heating filament region of said catheter member and said flexible heating filament have a flexible heating filament region diameter, and said relaxed diameter is less than said flexible heating filament region diameter, whereby said elastic material of said catheter sheath is in a stretched state on said flexible heating filament region.

47. A thermodilution catheter apparatus, comprising:

a flexible tubular catheter member adapted for introduction into a blood vessel;

a flexible heating filament, having a temperature coefficient of resistance whose magnitude is at least 0.001 Ω

/Ω

-°

C., designed for heating the blood in the blood vessel for thermodilution measurements, and mounted to said flexible tubular catheter member so that said flexible heating filament does not contact blood when said flexible heating filament is in the blood vessel;

a blood temperature sensor, mounted to said flexible tubular catheter member;

a cardiac computer and control apparatus, electrically coupled to the flexible heating filament and blood temperature sensor, and comprising

(a) means for applying voltage and current to said flexible heating filament,

(b) means for continuously measuring the current and voltage delivered to said flexible catheter filament,

(c) means for calculating the heat delivered to the blood based upon the product of the measured current and voltage,

(d) means for calculating the resistance of said flexible heating filament based upon the ratio of the measured current delivered to said flexible heating filament to the measured voltage across said flexible heating filament,

(e) means for calculating blood temperature based upon the output of the blood temperature sensor,

(f) means for calculating cardiac output based upon the blood temperature calculated by the means for calculating blood temperature and the heat calculated by the means for calculating the heat delivered to the blood, and

(g) means for reducing voltage and current to said flexible heating filament when the resistance of said flexible heating filament exceeds a predetermined resistance for a predetermined time.