Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1997-11-17
2001-12-25
Lewis, Aaron J. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204210, C128S204230
Reexamination Certificate
active
06332463
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and apparatus for the estimation of respiratory flow, and to the compensation of pressure swings at the entrance of a patient's airway induced by respiratory flow in the course of continuous positive airway pressure (CPAP) treatment or assisted respiration.
BACKGROUND OF THE INVENTION
The administration of CPAP is common in the treatment of Obstructive Sleep Apnea (OSA) syndrome and Upper Airway Resistance syndrome. CPAP treatment effectively acts as a pneumatic splint of a patient's upper airway by providing air or breathable gas at a pressure elevated above atmospheric pressure to the entrance of the patient's airway. Treatment pressures in the range 4-20 cm H
2
O are commonly encountered. More sophisticated forms of CPAP include bi-level CPAP in which different treatment pressures are applied in synchronism with the inspiratory and expiratory phases of respiration, and autosetting (controlled variable treatment pressure) CPAP, as described in U.S. Pat. No. 5,245,995. In all forms of CPAP treatment it is desired to maintain the treatment pressure, usually clinically determined by a physician, to be as constant as possible to maintain treatment efficacy without causing the patient undue discomfort by having to work against an unnecessarily high positive airway pressure.
Common to all forms of CPAP apparatus is a mask worn b a patient having connection via a flexible air delivery tube to a flow generator. The flow generator has a turbine driven by an electric motor that is under the control of a microprocessor-based controller.
In this specification a reference to a “mask” is to be understood as including a nose mask, a mouth mask, a nose and mouth mask in combination or a full face mask.
Simple CPAP machines estimate the mask pressure from the motor speed and operate under speed regulation. More sophisticated machines incorporate a pneumatic pressure transducer that provides a feedback signal representative of pressure at either the mask or a point within the flow generator itself. If the pressure feedback signal is from the mask, additional tubing or wires must extend between the mask and the flow generator which can give rise to sterilisation and/or safety problems. If pressure feedback is from a point within the flow generator, or at some other point removed from the mask, the impedance of the air delivery tube can result in flow-induced pressure swings at the mask. The pressure swings can be up to +5% of treatment pressure, and since it is desired to provide the patient with the minimum treatment pressure to provide treatment efficacy and yet avoid the patient doing unnecessary work during exhalation, these pressure swings are undesirable and should be eliminated as far as possible.
It is also desirable to be able to accurately determine dynamic air flow in the delivery tube and mask. “Flow” is to be understood as including both ventilation volume and a volumetric flow rate. The flow will have a component due to the flow generator that is modulated by patient respiration. The measurement of air flow in the air delivery tube can beneficially be used to measure the average volume breathed by the patient and to determine whether the patient is inhaling (inspiring) or exhaling (expiring), the latter of which is crucial in the implementation of bi-level CPAP. Currently this is done using an in-line sensor to directly measure flow, or by measuring the pressure drop across a restriction in the air delivery tube (or alternatively, the pressure drop alone the air delivery tube). These methods require the use of additional transducers, and in some cases additional wiring or tubing to connect the transducer to an appropriate point in the control circuitry of the CPAP apparatus.
It thus also is desirable to accurately measure or estimate air flow from the stand-point of enabling advanced control over the administration of CPAP treatment and ensuring efficacy of treatment and patient compliance without reducing patient comfort.
The present invention is directed to overcoming or at least ameliorating one or more of the foregoing problems.
SUMMARY OF THE INVENTION
In one broad form, the invention discloses a method to estimate gas flow from a turbine, the method comprising the steps of:
measuring the pressure of the gas flowing from the turbine when operating at a known rotational speed;
estimating the pressure of gas that would be delivered by said turbine at the known rotational speed if there was no flow; and
calculating the difference between the measured pressure and the estimated pressure to estimate the gas flow.
In another broad form, the invention discloses 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 the steps of:
measuring the pressure of gas (p
act
) from the flow generator;
measuring the rotational speed (&ohgr;) of the turbine;
estimating the pressure of gas (p
est
) that would be delivered by the flow generator at the measured rotational speed if there were no flow to the patient: and
calculating the difference between p
est
and p
act
to give the estimate of flow (f
est
).
The said difference further can be signal-processed to give a direct estimate of flow. The processing can include filtering to give a direct estimate of flow.
The invention yet further discloses 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 to a flow generator by a gas delivery tube and the pressure of delivered gas being measured at a point remote from said mask, said measured pressure being used to derive a treatment pressure control signal supplied to said flow generator to control said desired treatment pressure, the method comprising the steps of:
determining the flow of gas to the patient;
scaling said determined flow; and
adjusting said treatment pressure control signal by said scaled flow.
The scaled flow can be further filtered to pass only the scaled flow corresponding to increases in flow, corresponding to a reduction in the desired treatment pressure.
The invention yet further discloses apparatus to estimate the flow of gas from a flow generator turbine supplying breathable gas in CPAP treatment or assisted respiration, the apparatus comprising:
pressure measurement means for measuring the pressure of gas from the turbine when operating at a known rotational speed;
pressure estimation means for estimating the pressure of gas that would be delivered by said turbine at the known rotation speed if there was no flow; and
processor means for calculating the difference between the measured pressure and the estimated pressure to give an estimation of flow.
The invention yet further discloses apparatus to provide CPAP treatment or assisted respiration for supplying a positive treatment pressure of breathable gas at the entrance of 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 connected to the other end of the delivery tube to be worn by the patient;
pressure sensor means sensing delivery pressure from either the flow generator or the delivery tube remote from the mask; and
processor means for controlling the flow generator, and further operable to estimate the flow of breathable gas 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 an estimate of flow.
The apparatus can be further operable to scale the flow and compensate said pressure control signal by the scaled flow. The said difference further can be signal processed to give a direct estimate of flow. The processing can include filtering to remove non-respiratory components.
Farrugia Steven Paul
Finn Shane Darren
Lewis Aaron J.
Pillsbury & Winthrop LLP
ResMed Limited
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