Flow estimation and compensation of flow-induced pressure swings in CPAP treatment and assisted respiration

• US 6,332,463 B1
• Filed: 11/17/1997
• Issued: 12/25/2001
• Est. Priority Date: 09/15/1995
• Status: Expired due to Term

• Current Assignee: Resmed Limited (ResMed Incorporated)
• Original Assignee: Resmed Limited (ResMed Incorporated)
• Inventors: Farrugia, Steven Paul, Finn, Shane Darren
• Primary Examiner(s): Lewis, Aaron J.
• Application Number:
  US08/894,304
• Time in Patent Office: 1,499 Days
• Field of Search: 128/204.18, 128/204.21, 128/204.23, 128/205.18
• US Class Current: 128/204.18
• CPC Class Codes: A61M 16/00 Devices for influencing the respiratory system of patients by gas treatment, e.g. mouth-to-mouth respiration; Tracheal tubes stimulating the respiratory movement by mechanical, pneumatic or electrical means, iron lungs combined with gas breathing means A61H31/00
• CPC Class Codes: A61M 16/0069 the speed thereof being controlled by respiratory parameters, e.g. by inhalation
• CPC Class Codes: A61M 16/026 specially adapted for predicting, e.g. for determining an information representative of a flow limitation during a ventilation cycle by using a root square technique or a regression analysis
• CPC Class Codes: A61M 16/0858 Pressure sampling ports
• CPC Class Codes: A61M 2016/0039 in the inspiratory circuit
• CPC Class Codes: A61M 2205/3365 Rotational speed
• CPC Class Codes: F04D 27/00 Control, e.g. regulation, of pumps, pumping installations or pumping systems specially adapted for elastic fluids
• CPC Class Codes: G01F 1/34 by measuring pressure or differential pressure
• CPC Class Codes: G01F 1/42 Orifices or nozzles
• CPC Class Codes: G01F 1/50 Correcting or compensating means
• CPC Class Codes: G01F 15/00 Details of, or accessories for, apparatus of groups G01F1/00 - G01F13/00 insofar as such details or appliances are not adapted to particular types of such apparatus
• CPC Class Codes: G01F 15/022 using electrical means

Abstract

A method of flow estimation during CPAP treatment is disclosed and involves measuring turbine (32) speed by means of the motor'"'"'s Hall-effect sensors (80). The equivalent pressure, p_est, that would be developed by the turbine (32) for the measured speed if the flow were zero is estimated using a predetermined mathematical function. Coincidentally with the speed measurement and pressure estimation, the delivery tube pressure, p_act, is measured by the pressure transducer (42), and from p_act together with p_est, the difference between the two pressures is calculated, representing an estimation of flow, f_est. The flow estimation value can be linearised by the square root device (86), and then filtered by the combination of low pass and high pass filters (90, 92) to derive a filtered flow estimate value, that has the non-respiratory components removed. A method of flow compensation of flow-induced pressure swings also is disclosed.

Claims

1. A method to estimate the flow of breathable gas to a patient from a flow generator having a turbine in the administration of continuous positive airway pressure (CPAP) treatment or assisted respiration, the method comprising:

measuring a pressure of gas (P_act) delivered by the flow generator;

measuring a rotational speed (ω

) of the turbine;

estimating a pressure of gas (P_est) delivered by the flow generator at the measured rotational speed (ω

) if there were no flow to the patient; and

. calculating the difference between P_est and P_act to give an estimate of the flow to the patient (f_est).

2. A method as claimed in claim 1, further comprising taking the square root of the flow estimate to provide a linearized flow estimate.

3. A method as claimed in claim 2, further comprising filtering said linearized flow estimate to remove non-respiratory components.

4. A method as claimed in claim 1, whereby said estimating comprises performing the calculation:

5. A method as claimed in claim 1, whereby the step of estimating comprises reviewing tabulated incremental rotational speeds with predetermined values of pressure for no flow to identify the relevant value of p_est.

6. An apparatus for estimating the flow of gas from a flow generator having a turbine, and supplying breathable gas in the administration of CPAP treatment or assisted respiration, the apparatus comprising:

pressure measurement device for measuring the pressure of gas from the flow generator when operating at a known turbine rotational speed;

pressure estimation device for estimating the pressure of gas that would be delivered by said flow generator at the known rotation speed if there was no flow; and

a processor for calculating the difference between the measured pressure and the estimated pressure to give an estimation of flow.

7. An apparatus to provide CPAP treatment or assisted respiration by the supply of a positive treatment pressure of breathable gas to a patient'"'"'s airway, the apparatus comprising:

a flow generator having a turbine;

a gas delivery tube connected at one end thereof with the flow generator;

a patient mask adapted to be worn by a patent connected to the other end of the delivery tube;

a pressure sensor sensing delivery pressure from either the flow generator or the delivery tube remote from the mask; and

a processor for controlling the flow generator, and further programmed to estimate the flow of breathable gas to the patient by estimating the pressure of gas that would be delivered by the flow generator at the actual turbine rotational speed for zero flow and calculating the difference between the measured pressure and said estimated pressure to give said estimate of flow, said estimate of flow being utilized by the processor to dynamically adjust a control signal provided to said flow generator to reduce flow-induced pressure swings at the mask.

8. A method to reduce flow-induced pressure swings at a patient mask during the administration of CPAP treatment or assisted respiration, said mask being coupled by a gas delivery tube to a flow generator having a turbine, and the pressure of delivered gas being measured at a point remote from said mask, said measured pressure used to derive a treatment pressure control signal supplied to said flow generator to control said desired treatment pressure, the method comprising:

determining a flow of gas to the patient including measuring the rotational speed (ω

) of the turbine, estimating the pressure of gas (P_est) that would be delivered by the flow generator at the measured rotational speed (ω

) if there were no flow to the patient, and calculating the difference between P_est and P_act to give the estimate of the flow to the patient (F_est);

scaling said determined flow; and

adjusting said treatment pressure control signal by said scaled flow.

9. A method as claimed in claim 8, further comprising filtering said scaled flow to pass only the scaled flow corresponding to increases in flow that correspond to a reduction in treatment pressure.

10. A method as claimed in claim 8, said estimating comprises performing the calculation:

11. A method as claimed in claim 10, estimating comprises obtaining the incremental rotational speed which corresponds to a pressure for no flow to identify the relevant value of P_est.