Method and apparatus for continuous cardiac output monitoring

• US 6,325,762 B1
• Filed: 08/23/1999
• Issued: 12/04/2001
• Est. Priority Date: 12/09/1996
• Status: Expired due to Fees

• Current Assignee: Sif Universal Pte., Ltd.
• Original Assignee: Swee Chuan Tjin
• Inventors: Tjin, Swee Chuan
• Primary Examiner(s): Lacyk, John P.
• Assistant Examiner(s): Natnithithadha, Navin
• Application Number:
  US09/319,708
• Time in Patent Office: 834 Days
• Field of Search: 600/505, 600/504, 600/488, 600/486, 600/485, 600/561, 600/342, 606/15, 606/16, 356/336, 356/338, 356/342, 385/116, 385/117, 385/119
• US Class Current: 600/505
• CPC Class Codes: A61B 5/01 Measuring temperature of body parts ; Diagnostic temperature sensing, e.g. for malignant or inflamed tissue clinical contact thermometers G01K13/20
• CPC Class Codes: A61B 5/0261 using optical means, e.g. infrared light
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0538 invasively, e.g. using a catheter

Abstract

An apparatus for continuous cardiac output monitoring ascertains cardiac output by measuring the cross-sectional area of the vessel and the flow rate of fluid flowing through the vessel. The cross-sectional area is derived from the measured resistance within the vessel whereby a pair of signal electrodes injects a known electrical signal into the vessel and the resistance is derived from the known signal and the differential voltage between first and second measuring pairs of electrodes. Resistivity of the fluid is a component of the cross-sectional area derivation, and a temperature sensor is provided to allow for compensating for variations in resistivity with temperature. A velocity sensor is preferably of an optic fiber, Doppler shift type, and the accuracy of the velocity measurement is improved by focusing light emissions from the optic fiber(s) by either providing a Fresnel plate on the terminal end of the fiber or by forming the terminal end of the fiber in a generally conical shape.

Claims

1. An apparatus for continuously monitoring the volume flow rate of a fluid flowing through a vessel based on the relationship, Q=V×

A, where Q is the volume flow rate, V is the velocity of the fluid flowing through the vessel, and A is the cross sectional area of the vessel proximate a location in the vessel where the flow velocity is measured, said apparatus comprising;

a cross-sectional area sensor for deriving the cross-sectional area of the vessel based on an electrical impedance within the vessel, said cross-sectional area sensor comprising an impedance measuring instrument constructed and arranged to continuously ascertain the electrical impedance of the vessel, the derived cross-sectional area being directly proportional to the resistivity, ρ

, of the fluid flowing in the vessel and inversely proportional to the resistance, R, of the vessel;

a velocity sensor for measuring the velocity of the fluid flowing in the vessel; and

a temperature sensor for continuously measuring the temperature of the fluid flowing in the vessel so as to allow continuous compensation in the cross-sectional area derivation for variations in the resistivity ρ

with temperature.

2. The apparatus of claim 1 wherein said temperature sensor comprises a thermistor.

3. The apparatus of claim 1 wherein said velocity sensor comprises:

a flexible tube having an optical opening formed in a sidewall thereof;

an optical fiber disposed within said tube, said optical fiber having a longitudinal fiber axis and a terminal end disposed proximate said optical opening; and

a reflective surface disposed within said tube adjacent said terminal end of said optical fiber, said reflective surface being oriented such that light emitted from said terminal end of said optical fiber is reflected by said reflective surface in a direction having a component normal to the fiber axis through said optical opening into a measurement volume of the flow located outside and alongside said sensor.

4. The apparatus of claim 3 wherein said optical fiber includes an end face at said terminal end, said end face having a plurality of concentric Fresnel rings formed thereon to focus light passing through said Fresnel rings in front of said end face.

5. The apparatus of claim 3 wherein said terminal end is formed in a generally conical shape to focus light passing through said optical fiber in front of said terminal end.

6. The apparatus of claim 1 wherein said impedance measuring instrument of said cross-sectional area sensor comprises:

a pair of signal-emitting electrodes for injecting a known electrical signal into the vessel in which said instrument is disposed;

a first pair of measurement electrodes across which a voltage can be measured; and

a second pair of measurement electrodes across which a voltage can be measured,

wherein said signal-emitting electrodes inject the known signal into the vessel, a first voltage is measured across said first pair of measurement electrodes, and a second voltage is measured across said second pair of measurement electrodes, and the impedance of the vessel, from which the cross-sectional area can be derived, is determined from the known signal and the difference between the first and second voltages.

9. The sensor of claim 1 further comprising:

a second optical fiber disposed within said tube, said second optical fiber having a longitudinal fiber axis, a terminal end disposed adjacent said reflective surface, and an end face at said terminal end, said end face of said second optical fiber having a plurality of concentric Fresnel rings formed thereon to focus light passing through said Fresnel rings into said end face of said second optical fiber to define, along with said reflective surface, a focused field of view of the second optical fiber into the measurement volume,

wherein said reflective surface is oriented such that a portion of the focused light from said first mentioned optical fiber which is reflected into the measurement volume of the flow and scattered within the measurement volume is within the focused field of view of said second optical fiber.

13. The instrument of claim 5 comprising six electrically-conducting rings longitudinally spaced along a catheter tube wherein two outermost rings of said six rings constitute said pair of signal emitting electrodes, two innermost rings constitute said first pair of measurement electrodes, and two intermediate rings, each being disposed between a one of the innermost rings and a one of the outermost rings, constitutes said second pair of measurement electrodes.

7. A method for continuously monitoring the volume flow rate of a fluid flowing through a vessel based on the relationship, Q=V×

A, where Q is the volume flow rate, V is the velocity of the fluid flowing through the vessel, and A is the cross sectional area of the vessel proximate a location in the vessel where the flow velocity is measured, said method comprising;

ascertaining the electrical impedance of the vessel and deriving the cross-sectional area of the vessel based on the electrical impedance within the vessel, wherein the derived cross-sectional area is directly proportional to the resistivity, ρ

, of the fluid flowing in the vessel and inversely proportional to the resistance, R, of the vessel;

measuring the velocity of the fluid flowing in the vessel; and

measuring the temperature of the fluid flowing in the vessel and compensating in the cross-sectional area derivation for variations in the resistivity ρ

with temperature based upon the measured temperature.

8. A velocity sensor for remote fluid flow measurements, said sensor comprising:

a flexible tube having an optical opening formed in a sidewall thereof;

an optical fiber disposed within said tube, said optical fiber having a longitudinal fiber axis, a terminal end disposed proximate said optical opening, and an end face at said terminal end, said end face having a plurality of concentric Fresnel rings formed thereon to focus light passing through said Fresnel rings in front of said end face; and

a reflective surface disposed within said tube adjacent said terminal end of said optical fiber,

said reflective surface being oriented such that the focused light emitted from said terminal end of said optical fiber is reflected by said reflective surface in a direction having a component normal to the fiber axis through said optical opening into a measurement volume of the flow located outside and alongside said sensor.

10. A velocity sensor for remote fluid flow measurements, said sensor comprising:

a flexible tube having an optical opening formed in a sidewall thereof;

an optical fiber disposed within said tube, said optical fiber having a longitudinal fiber axis and a terminal end disposed proximate said optical opening, said terminal end being formed in a generally conical shape to focus light passing through said optical fiber in front of said terminal end; and

a reflective surface disposed within said tube adjacent said terminal end of said optical fiber,

said reflective surface being oriented such that the focused light emitted from said terminal end of said optical fiber is reflected by said reflective surface in a direction having a component normal to the fiber axis through said optical opening into a measurement volume of the flow located outside and alongside said sensor.

11. The sensor of claim 10 wherein comprising:

a second optical fiber disposed within said tube, said second optical fiber having a longitudinal fiber axis and a terminal end disposed adjacent said reflective surface, said terminal end of said second optical fiber being formed in a generally conical shape to focus light passing through said terminal end of said second optical fiber to define, along with said reflective surface, a focused field of view of the second optical fiber into the measurement volume,

wherein said reflective surface is oriented such that a portion of the focused light from said first mentioned optical fiber which is reflected into the measurement volume of the flow and scattered within the measurement volume is within the focused field of view of said second optical fiber.

12. An instrument for ascertaining the impedance of a vessel in which said instrument is disposed, the cross-sectional area of the vessel being derivable from the ascertained impedance, said instrument comprising:

a pair of signal-emitting electrodes for injecting a known electrical signal into the vessel in which said instrument is disposed;

a first pair of measurement electrodes across which a voltage can be measured; and

a second pair of measurement electrodes across which a voltage can be measured,

wherein said signal-emitting electrodes inject the known signal into the vessel, a first voltage is measured across said first pair of measurement electrodes, and a second voltage is measured across said second pair of measurement electrodes, and the impedance of the vessel, from which the cross-sectional area can be derived, is determined from the known signal and the difference between the first and second voltages.