Thermodilution catheter method using a safe, flexible heating element

• US 6,355,001 B1
• Filed: 05/13/1996
• Issued: 03/12/2002
• Est. Priority Date: 01/29/1991
• Status: Expired due to Fees

• 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:
  US08/645,391
• Time in Patent Office: 2,129 Days
• Field of Search: 128/692, 128/713, 600/500, 600/505, 600/526
• 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. A method for protecting blood and tissue from damage due to overheating caused by heat transfer from a catheter used to monitor cardiac output, said method comprising the steps of:

(a) introducing the catheter intravascularly into a patient'"'"'s heart;

(b) monitoring a temperature of a heated portion of the catheter and producing a signal indicative thereof; and

(c) controlling the temperature of the heated portion in response to the signal so as to keep the temperature within safe physiological limits to protect blood and tissue from damage due to overheating caused by heat transfer from the catheter.

2. The method of claim 1 wherein the step of monitoring the temperature comprises the steps of:

(a) measuring an electrical current flowing through a resistive heater used to provide heat;

(b) measuring a voltage drop across the resistive heater; and

(c) determining the temperature of the heated portion of the catheter by making use of the fact that the temperature of the heated portion of the catheter is proportional to the ratio of the voltage drop and the electrical current.

3. The method of claim 1 wherein:

(a) an electrical current flowing through a resistive heater is used to provide heat and

(b) the step of monitoring the temperature comprises the steps of;

(i) monitoring a potential difference developed across a circuit comprising the resistive heater and

(ii) determining the temperature of the heated portion of the catheter using the potential difference.

4. A method for protecting blood and tissue from damage due to overheating caused by heat transfer from a catheter used to monitor cardiac output, said method comprising the steps of:

(a) introducing the catheter intravascularly into a patient'"'"'s heart;

(b) establishing a nominal fixed power distribution for heating the blood;

(c) monitoring the temperature of a portion of the catheter where heat is transferred to the blood;

(d) producing a signal indicative thereof; and

(e) in response to the signal, establishing a decreased nominal fixed power dissipation for heating the blood so that the temperature of the portion of the catheter where heat is transferred to the blood is reduced to at least a predetermined maximum safe value to protect blood and tissue from damage due to overheating caused by heat transfer from the catheter.

5. A method for protecting blood and tissue from damage due to overheating caused by heat transfer from a catheter used to monitor cardiac output, said method comprising the steps of:

(a) introducing the catheter intravascularly into a patient'"'"'s heart;

(b) sensing the temperature of a resistive heater mounted on the distal end of the catheter after insertion of the catheter into the patient'"'"'s heart;

(c) producing a signal indicative of the temperature of the resistive heater; and

(d) controlling an electrical current flowing through the resistive heater to keep the temperature of the resistive heater within safe physiological limits to protect blood and tissue from damage due to overheating caused by heat transfer from the catheter.

6. The method of claim 5 wherein the step of sensing the temperature comprises the steps of:

(a) measuring the electric current flowing through the resistive heater;

(b) measuring a voltage drop across the resistive heater; and

(c) calculating the temperature using the fact that the temperature of the heated portion of the catheter is proportional to the ratio of the electric current and the voltage drop.

7. A method for determining cardiac output based upon a temperature rise of blood in a patient'"'"'s heart, said method comprising the steps of:

(a) placing a catheter including a resistive heater located adjacent the distal end thereof intravascularly into a patient'"'"'s heart;

(b) monitoring the temperature of the resistive heater;

(c) producing a signal indicative of the temperature of the resistive heater;

(d) controlling the temperature of the resistive heater in response to the signal to keep the temperature within safe physiological limits;

(e) measuring the electrical power dissipated in the resistive heater;

(f) producing a signal indicative of the dissipated electrical power; and

(g) determining cardiac output as a function of the dissipated electrical power.

8. The method of claim 7 wherein the step of monitoring the temperature comprises the steps of:

(a) measuring an electrical current flowing through the resistive heater;

(b) measuring the voltage drop across the resistive heater; and

(c) determining the temperature of the resistive heater using the fact that the temperature of the resistive heater is proportional to the ratio of the voltage drop across the resistive heater to the electrical current flowing through the resistive heater.

9. A method for protecting blood and tissue from damage due to overheating caused by heat transfer from a catheter used to monitor cardiac output, said method comprising the steps of:

(a) introducing the catheter intravascularly into a patient'"'"'s heart;

(b) supplying an electrical current to a resistive heater located at the distal end of the catheter;

(c) monitoring the temperature of the resistive heater;

(d) producing a signal indicative of the temperature of the resistive heater; and

(e) controlling the electrical current flowing through the resistive heater in response to the signal to keep the temperature of the resistive heater within safe physiological limits to protect blood and tissue from damage due to overheating caused by heat transfer from the catheter.

10. The method of claim 9 wherein the controlling step further comprises controlling the voltage drop across the resistive heater.

11. A method for monitoring cardiac output based upon a temperature rise of blood flowing through a heart and for protecting the blood and tissue from damage due to overheating, said method comprising the steps of:

(a) placing a catheter having a resistive heater located at a distal end intravascularly into a heart;

(b) providing an electrical current to the resistive heater in order to elevate the temperature of the blood;

(c) monitoring the temperature of the blood heated by the resistive heater;

(d) sensing the temperature of the resistive heater;

(e) producing a signal indicative of the temperature of the resistive heater;

(f) controlling the temperature of the resistive heater in response to the signal to keep the temperature of the resistive heater within safe physiological limits;

(g) measuring the electrical power dissipated in the resistive heater; and

(h) determining cardiac output as a function of the dissipated electrical power.

12. A method for protecting blood and tissue from damage due to overheating caused by heat transfer from a catheter used to monitor cardiac output, said method comprising the steps of:

(a) introducing the catheter intravascularly into a patient'"'"'s heart;

(b) establishing a power distribution for heating the blood;

(c) monitoring a heater temperature of a resistive heater of the catheter; and

(d) establishing a decreased power dissipation for heating the blood for reducing said heater temperature of said resistive heater so that said heater temperature does not rise to a temperature and for a period of time that would damage the blood and tissue.