Estimation of flow and detection of breathing CPAP treatment

• US 6,237,593 B1
• Filed: 03/31/1998
• Issued: 05/29/2001
• Est. Priority Date: 12/03/1993
• Status: Expired due to Term

• Current Assignee: Resmed Limited (ResMed Incorporated)
• Original Assignee: Resmed Limited (ResMed Incorporated)
• Inventors: Brydon, John William Ernest
• Primary Examiner(s): Ruhl, Dennis
• Assistant Examiner(s): SRIVASTAVA, VIRENDRA KUMAR
• Application Number:
  US09/052,891
• Time in Patent Office: 1,155 Days
• Field of Search: 128/204.22, 128/204.21, 128/204.23, 128/204.24, 128/204.18, 128/205.24, 128/205.25, 600/529, 600/532, 600/536, 607/20
• US Class Current: 128/204.22
• CPC Class Codes: A61M 16/0069 the speed thereof being controlled by respiratory parameters, e.g. by inhalation
• CPC Class Codes: A61M 16/024 including calculation means, e.g. using a processor
• CPC Class Codes: A61M 2016/0021 with a proportional output signal, e.g. from a thermistor

Abstract

Apparatus and methods for the estimation of flow and the detection of breathing (respiration) in continuous positive airway pressure (CPAP) treatment are disclosed. CPAP apparatus typically includes a flow generator for supplying air to a mask via a gas delivery tube. With changing air flow, the flow generator'"'"'s speed and/or driving electrical current will alter in a manner defined by the controlling circuitry. Signals can be derived from measurements of motor speed and current, and these signals vary cyclically with patient respiration. By filtering to reject non-respiratory components, the resultant signal can be utilized to determine the instants in time at which the patient starts to inhale and exhale. The filtered signal also can be linearized using a predetermined knowledge of the pressure/flow/speed characteristics of the flow generator, and thus to derive a volumetric measure of airflow.

Claims

1. A method for determination volumetric respiratory flow during the administration of continuous positive airway pressure (CPAP) treatment by CPAP apparatus, said method comprising:

providing a flow generator for supplying breathable gas to a mask via a gas delivery tube;

measuring an electrical parameter indicative of power consumed by said flow generator to derive a power signal;

filtering said power signal to remove non-respiratory components; and

linearising said filtered signal with respect to a predetermined characteristic of said gas delivery tube and said mask relating flow to consumed power to provide a volumetric measure of respiratory flow.

2. The method of claim 1, whereby said predetermined characteristic is obtained from a pressure/flow/speed characteristic of a turbine system providing said CPAP treatment.

3. The method of claim 2, further comprising:

detecting a change in said filtered signal to give a signal indicative of respiration;

comparing said change signal with a first threshold value to detect start of inhalation phase if said first threshold value is exceeded; and

comparing said change signal with a second threshold value to detect start of exhalation phase if said second threshold value is exceeded.

4. The method of claim 3, whereby said detecting step is performed by differentiation.

5. The method of claim 4, wherein said first threshold value is positive and said second threshold value is negative so that when said differentiated signal is compared with said positive threshold value and is greater than said first threshold value that is determinative of start of inhalation phase, and when said differentiated signal is compared with said negative threshold value and is less than said negative threshold value that is determinative of the start of the exhalation phase.

6. The method of claim 5, whereby said filtering step comprises high-pass filtering said power signal.

13. The method of claim 1 wherein said supplying step includes supplying breathable gas at at least two different pressures.

7. An apparatus for the detection of respiration during the administratior of CPAP treatment by a CPAP system, said apparatus comprising:

a flow generator for supplying breathable gas to a mask via a gas delivery tube;

means for measuring an electrical parameter indicative of power consumed by said flow generator and deriving a power signal thereof;

means for filtering said power signal to remove non-respiratory components; and

means for linearising said filtered signal with respect to a signal of a predetermined characteristic of said gas delivery tube relating flow to consumed power to provide a measure of respiratory flow.

8. The apparatus of claim 7, wherein said predetermined characteristic is obtained from a pressure/flow/speed characteristic of said flow generator, gas delivery tube and mask.

9. The apparatus of claim 8, further comprising:

means for detecting a change in said filtered signal to give a signal indicative of respiration;

means for comparing said change signal with a first threshold value to detect start of inhalation phase if said first threshold value is exceeded; and

means for comparing said change signal with a second threshold value to detect start of exhalation phase if said second threshold value is exceeded.

10. The apparatus of claim 9, wherein said means for detecting a change in said filtered signal is a differentiator.

11. The apparatus of claim 10, further wherein said first threshold value is positive and said second threshold value is negative so that when said differentiated signal is compared with said positive threshold value and is greater than said first threshold value that is determinative of start of inhalation phase, and when said differentiated signal is compared with said negative threshold value and is less than said second threshold value that is determinative of start of exhalation phase.

12. The apparatus of claim 11, wherein said filter means is a high-pass filter.

14. The apparatus of claim 7 wherein said apparatus further includes:

means to deliver pressure to the mask at at least two different pressure values.