FLOW RATE COMPENSATION FOR TRANSIENT THERMAL RESPONSE OF HOT-WIRE ANEMOMETERS

• US 20100218765A1
• Filed: 02/27/2009
• Published: 09/02/2010
• Est. Priority Date: 02/27/2009
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett LLC (Medtronic Plc)
• Inventors: McCoy, Edward R., Jafari, Mehdi M., Jimenez, Rhomere S., Aviano, Jeffrey K.
• Granted Patent: US 8,434,479 B2
• Time in Patent Office: Days
• US Class Current: 128/204.210
• CPC Class Codes: A61M 16/0057 Pumps therefor
• CPC Class Codes: A61M 16/06 Respiratory or anaesthetic masks
• CPC Class Codes: A61M 16/0816 Joints or connectors
• CPC Class Codes: A61M 16/0875 Connecting tubes
• CPC Class Codes: A61M 16/12 by mixing different gases
• CPC Class Codes: A61M 2016/0042 in the expiratory circuit
• CPC Class Codes: G01F 1/68 by using thermal effects
• CPC Class Codes: G01F 1/6965 comprising means to store calibration data for flow signal calculation or correction

Abstract

Systems and methods are described for application of a transitory corrective modification to a hot-wire anemometer flow voltage and/or calculated flow rate to compensate for transient thermal response of the anemometer during a change in mixture of a mixed gas being measured. According to one embodiment a method of applying the transitory corrective modification is provided. An output signal of an exhalation flow sensor of a medical ventilator is received. The flow sensor includes a hot-wire anemometer. The output signal is indicative of a rate of flow of expired gas by a patient. Transient thermal response of the hot-wire anemometer is compensated for by applying a corrective modification to the output signal or a value based thereon. The corrective modification is based at least in part on a fraction of inspired oxygen (FiO2) being supplied by the medical ventilator to the patient.

Claims

1. A method comprising:

one or more flow sensor processing modules of a medical ventilator receiving an output signal of an exhalation flow sensor including a hot-wire anemometer, the output signal indicative of a rate of flow of expired gas by a patient;

the one or more flow sensor processing modules compensating for transient thermal response of the hot-wire anemometer resulting from a change in concentration of gas types included within the expired gas by applying a corrective modification to the output signal or a value based thereon that is based at least in part on a fraction of inspired oxygen (FiO2) being supplied to the patient by the medical ventilator; and

wherein the one or more flow sensor processing modules are implemented in one or more microprocessors and one or more computer-readable media, the one or more computer-readable media having instructions tangibly embodied therein that are executable by the one or more microprocessors.

2. The method of claim 1, wherein the corrective modification involves a constant and an exponentially decaying factor.

3. The method of claim 2, wherein the corrective modification is a function of a magnitude of a setting change of the FiO2, a magnitude of average flow rates through the flow sensor and dynamics of temperature control circuitry of the hot-wire anemometer.

4. The method of claim 2, wherein the corrective modification comprises multiplying the output signal or the value based thereon by a mix transition multiplier generally represented as:

M=1.0+K*Δ

O2Conc*e<sup>−

t/T</sup>

where,

K=0.00125

T=15 seconds

t=time elapsed since FiO2 setting change

Δ

O2Conc=Current FiO2 setting−

Previous FiO2 setting.

5. The method of claim 1, further comprising determining an exhaled tidal volume based at least in part on the corrected output signal.

6. The method of claim 5, wherein the method further comprises transitioning a ventilatory cycle of the medical ventilator from an expiratory phase to an inspiratory phase based at least in part on the corrected output signal.

7. A medical ventilator comprising:

a breathing gas delivery system having an inspiratory port through which breathing gas is delivered to a patient and an expiratory port through which expired gas is received from the patient;

a flow sensor, including a hot-wire anemometer, operable to measure a rate of flow of expired gas; and

a controller coupled to the breathing gas delivery system and the flow sensor, wherein the controller is operable to receive an output signal from the flow sensor indicative of the rate of flow of the expired gas and responsive to receipt of the output signal the controller is further operable to compensate for transient thermal response of the hot-wire anemometer resulting from a change in concentration of gas types included within the expired gas by applying a corrective modification to the output signal or a value based thereon that is based at least in part on a fraction of inspired oxygen (FiO2) being supplied to the patient by the breathing gas delivery system.

8. The medical ventilator of claim 7, wherein the corrective modification involves a constant and an exponentially decaying factor.

9. The medical ventilator of claim 8, wherein the corrective modification is a function of a magnitude of a setting change of the FiO2, a magnitude of average flow rates through the flow sensor and dynamics of temperature control circuitry of the hot-wire anemometer.

10. The medical ventilator of claim 8, wherein the corrective modification comprises multiplying the output signal or the value based thereon by a mix transition multiplier generally represented as:

M=1.0+K*Δ

O2Conc*e<sup>−

t/T</sup>

where,

K=0.00125

T=15 seconds

t=time elapsed since FiO2 setting change

Δ

O2Conc=Current FiO2 setting−

Previous FiO2 setting.

11. The medical ventilator of claim 7, wherein the controller is further operable to determine an exhaled tidal volume based at least in part on the corrected output signal.

12. The medical ventilator of claim 11, wherein the controller is further operable to cause a transition a ventilatory cycle of the medical ventilator from an expiratory phase to an inspiratory phase based at least in part on the corrected output signal.

13. A program storage device readable by one or more microprocessors of a medical ventilator, tangibly embodying a program of instructions executable by the one or more microprocessors to perform method steps for compensating for transient thermal response of a hot-wire anemometer resulting from a change in concentration of gas types being measured, said method steps comprising:

receiving an output signal of a flow sensor including a hot-wire anemometer that is associated with an expiratory port of the medical ventilator, the output signal indicative of a rate of flow of expired gas by a patient; and

compensating for transient thermal response of the hot-wire anemometer resulting from a change in concentration of gas types included within the expired gas by applying a corrective modification to the output signal or a value based thereon that is based at least in part on a fraction of inspired oxygen (FiO2) being supplied to the patient by the medical ventilator.

14. The program storage device of claim 13, wherein the corrective modification involves a constant and an exponentially decaying factor.

15. The program storage device of claim 14, wherein the corrective modification is a function of a magnitude of a setting change of the FiO2, a magnitude of average flow rates through the flow sensor and dynamics of temperature control circuitry of the hot-wire anemometer.

16. The program storage device of claim 13, wherein the corrective modification comprises multiplying the output signal or the value based thereon by a mix transition multiplier generally represented as:

M=1.0+K*Δ

O2Conc*e<sup>−

t/T</sup>

where,

K=0.00125

T=15 seconds

t=time elapsed since FiO2 setting change

Δ

O2Conc=Current FiO2 setting−

Previous FiO2 setting.

17. The program storage device of claim 13, wherein said method steps further comprise determining an exhaled tidal volume based at least in part on the corrected output signal.

18. The program storage device of claim 17, wherein said method steps further comprise transitioning a ventilatory cycle of the medical ventilator from an expiratory phase to an inspiratory phase based at least in part on the corrected output signal.