Non-invasive cardiac output and pulmonary function monitoring using respired gas analysis techniques and physiological modeling

• US 6,402,697 B1
• Filed: 01/21/2000
• Issued: 06/11/2002
• Est. Priority Date: 01/21/1999
• Status: Expired due to Fees

• Current Assignee: Metasensors, Inc.
• Original Assignee: Metasensors, Inc.
• Inventors: Calkins, Jerry M., Drzewiecki, Tadeusz M.
• Primary Examiner(s): Shaver, Kevin
• Assistant Examiner(s): Natnithithadha, Navin
• Application Number:
  US09/488,763
• Time in Patent Office: 872 Days
• Field of Search: 600/529, 600/531, 600/532, 600/533, 600/538, 600/537
• US Class Current: 600/532
• 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 5/0836 Measuring rate of CO2 production

Abstract

A cardiac output monitoring system (10) includes a respiratory flowmeter (14) and a gas analyzer (20, 22) capable of determining cardiac output on a breath-by-breath basis by non-invasively measuring properties of respiratory gasses and applying the Fick principle. The gas analyzer has the capability to simultaneously quantify multiple gas concentrations, including inhaled and end-tidal concentrations of any constituent of respiratory gas mixtures of a known number of possible constituents, in real time on a breath-by-breath basis, by measuring independent properties of the mixture. The respiratory flowmeter determines the volumetric and mass flow rates of any gas/gasses as calculated from the product of measured total respiratory flow and the measured volumetric concentration in real time on a breath-by-breath basis. From these measurements, cardiac output can be determined on a breath-by-breath basis by applying appropriate numerical algorithms based on the Fick principal, including corrections for physiological conditions such as shunts and deadspace.

Claims

1. A cardiac output monitoring system for non-invasively determining cardiac output on a breath-by-breath basis from respiratory gasses of a subject, comprising:

a flowmeter configured to receive a respiratory gas stream and determine respired flow rates at selected time intervals during a respiratory cycle, wherein the respiratory cycle includes inhalation of a gas stream into the subject'"'"'s lungs and exhalation of the gas stream out of the subject'"'"'s lungs, so as to provide flow rate data as a function of time over the respiratory cycle;

a gas analyzer configured to simultaneously determine individual concentrations of a plurality of constituents in the gas stream in real time at selected time intervals during the respiratory cycle so as to provide concentration data for each constituent as a function of time over the respiratory cycle; and

a processor coupled to the flowmeter and the gas analyzer and configured to determine cardiac output during the respiratory cycle utilizing flow rate data obtained from the flowmeter and the concentration data obtained from the gas analyzer.

2. The cardiac output monitoring system of claim 1, wherein the processor determines cardiac output by applying the Fick principle.

3. The cardiac output monitoring system of claim 1, wherein the processor is configured to continuously monitor the cardiac output of the subject by utilizing flow rate data obtained from the flowmeter and concentration data obtained from the gas analyzer during a plurality of respiratory cycles.

4. The cardiac output monitoring system of claim 2, wherein the processor determines the exchange rate between alveolar gas and pulmonary blood flow of the subject for at least one constituent utilizing the flow rate data obtained from the flowmeter and the concentration data obtained from the gas analyzer for the at least one constituent.

5. The cardiac output monitoring system of claim 2, wherein the gas analyzer is further configured to simultaneously determine individual partial pressures of the plurality of constituents in the gas stream so as to provide partial pressure data for each constituent as a function of time over the respiratory cycle.

6. The cardiac output monitoring system of claim 5, wherein the processor utilizes an algorithm incorporating the effects of deadspace and shunts in the subject'"'"'s lungs to determine cardiac output.

7. The cardiac output monitoring system of claim 5, wherein the algorithm iteratively applies a physiological model to determine cardiac output.

8. The cardiac output monitoring system of claim 7, wherein the processor is further configured to determine at least one of shunt amount and deadspace for the subject utilizing the algorithm.

9. A method of determining a subject'"'"'s cardiac output on a breath-by-breath basis, the method comprising:

(a) measuring a flow rate of a respiratory gas stream of the subject at a plurality of selected time intervals during a respiratory cycle, wherein the respiratory cycle includes inhalation of the gas stream into the subject'"'"'s lungs and exhalation of the gas stream out of the subject'"'"'s lungs, so as to obtain flow rate data as a function of time over the respiratory cycle;

(b) measuring a concentration of at least one constituent within the gas stream in real time at the plurality of selected time intervals during the respiratory cycle so as to obtain concentration data of the at least one constituent in the gas stream as a function of time over the respiratory cycle; and

(c) calculating a cardiac output for the patient based upon the measured flow rate data of the gas stream and the measured concentration data of the at least one constituent.

10. The method of claim 9, wherein step (c) comprises:

(c1) determining an exchange rate of the at least one constituent between alveolar gas in the patient'"'"'s lungs and the subject'"'"'s pulmonary blood flow during the respiratory cycle;

(c2) determining an arteriovenous concentration difference of the at least one constituent during the respiratory cycle; and

(c3) calculating the cardiac output based upon a ratio of the exchange rate of the at least one constituent to the arteriovenous concentration difference of the at least one constituent.

11. The method of claim 10, wherein step (c1) comprises determining the exchange rate of the at least one constituent by multiplying the measured flow rate data by the measured concentration data of the at least one constituent and integrating the product over the plurality of selected time intervals.

12. The method of claim 10, wherein step (b) includes measuring a partial pressure of the at least one constituent in the gas stream at the plurality of selected time intervals during the respiratory cycle, and the arteriovenous concentration difference for the at least one constituent is correlated with a measured partial pressure value of the at least one constituent within the gas stream at an end tidal expiration of the respiratory cycle.

13. The method of claim 9, wherein the gas stream includes a bolus of an indicator gas injected into the gas stream prior to step (a), and the at least one constituent includes the indicator gas.

14. The method of claim 13, wherein the indicator gas is selected from the group consisting of nitrogen, nitrous oxide, sevoflurane, desflurane and helium.

15. The method of claim 9, wherein the at least one constituent includes oxygen and carbon dioxide, and each concentration of oxygen is measured simultaneously with each concentration of carbon dioxide during the respiratory cycle.

16. The method of claim 15, wherein step (b) includes measuring a partial pressure of oxygen and a partial pressure of carbon dioxide in the gas stream at the plurality of selected time intervals during the respiratory cycle so as to obtain partial pressure data for oxygen and carbon dioxide in the gas stream as a function of time over the respiratory cycle.

17. The method of claim 16, wherein step (c) includes utilizing an algorithm to determine at least one of an amount of deadspace and an amount of shunt associated with the subject'"'"'s lungs, and modifying the calculation of the cardiac output based upon the determination.

18. The method of claim 17, wherein the algorithm iteratively applies a physiological model in order to determine cardiac output.

19. The method of claim 17, wherein the algorithm comprises the following iterative steps:

(c1) assigning partial pressure values of the measured carbon dioxide partial pressure data, including a value representing the carbon dioxide alveolar partial pressure, to a converging data set;

(c2) estimating the deadspace in the subject'"'"'s lungs based upon the converging data set of carbon dioxide partial pressure values;

(c3) estimating a cardiac output value based upon the measured flow rate data, the measured concentration data for oxygen and carbon dioxide, the estimated deadspace value and an estimated shunt adjustment value;

(c4) estimating new values for the converging data set, including a new value for the carbon dioxide alveolar partial pressure, based upon a gas uptake simulation model for the patient, wherein the gas uptake simulation model incorporates a plurality of physiological factors including oxygen consumption, estimated deadspace and shunt values and estimated cardiac output; and

(c5) repeating steps (c2)-(c4) until the converging data set converges with the measured carbon dioxide partial pressure data within a selected range of values, such that a final estimated cardiac output calculated in step (c3) represents the cardiac output of the subject.

20. The method of claim 9, wherein the method further comprises:

(d) repeating steps (a)-(c) so as to continuously monitor the subject'"'"'s cardiac output during a plurality of respiratory cycles.

21. A method of determining a subject'"'"'s cardiac output on a breath-by-breath basis, the method comprising:

(a) measuring an entrance flow rate of a respiratory gas stream of the subject at a first plurality of selected time intervals so as to obtain entrance flow rate data as a function of time during an inhalation of the gas stream into the subject'"'"'s lungs;

(b) measuring an exit flow rate of the gas stream of the subject at a second plurality of selected time intervals so as to obtain exit flow rate data as a function of time during an exhalation of the gas stream out of the subject'"'"'s lungs;

(c) measuring a concentration of at least one constituent within the gas stream in real time at the first and second pluralities of selected time intervals so as to obtain concentration data of the at least one constituent in the gas stream as a function of time during the inhalation of the gas stream into the subject'"'"'s lungs and the exhalation of the gas stream out of the subject'"'"'s lungs; and

(d) calculating a cardiac output for the subject based upon the measured entrance and exit flow rate data of the gas stream and the measured concentration data of the at least one constituent.