Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
1998-04-20
2001-02-06
Weiss, John G. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204180, C128S204210, C128S204230, C128S204260, C137S102000
Reexamination Certificate
active
06182657
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus and methods for the control of pressure in the administration 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. It has been postulated that 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 4-25 cm H
2
O are commonly encountered.
Common to all forms of CPAP apparatus is a mask worn by a patient having connection via a flexible air delivery tube to a flow generator. Most often, the flow generator is driven by an electric motor that is under the control of a motor controller. In this specification reference to a “mask” is to be understood as including a nose mask, a mouth mask, a nose and mouth mask in combination, nasal prongs or nasal pillows, or a full face mask.
CPAP treatment can be in a number of forms, including (i) the maintenance of a constant treatment pressure level, (ii) alternating between two constant levels in synchronism with the inspiratory and expiratory phases of respiration (“bi-level CPAP”), and (iii) having an autosetting level in accordance with a patient's therapeutic needs. In all of these cases there must be control over the pressure of air or breathable gas supplied to the patient's airway.
In one form in the prior art, control over the treatment pressure is achieved by speed control of the electric motor driving the turbine (or fan) that together constitute the flow generator. In the case of bi-level CPAP, the motor must be able to accelerate (or decelerate) respectively to double (or half) its operational speed within about 100 ms. For typical CPAP treatment, this equates to the need to supply (or sink) approximately twice the steady state electrical power within the noted time interval. Disadvantages in motor performance associated with the rapid transitions in speed are, for example, noise due to magnetostrictive effects and bearing vibration, and increased thermal dissipation requirements. Lower noise will increase patient compliance with the treatment.
FIG. 1
shows, as a cross-sectional view, a conventional flow generator
10
comprising a chamber
12
that is segregated from the casing
14
of the CPAP apparatus. The casing
14
houses the control circuitry (not shown) associated with the flow generator
10
. The flow generator further is comprised by a motor
16
driving an induced flow centrifugal turbine (impeller)
18
, which induces the flow of air or breathable gas by an air inlet
20
to pass the air or breathable gas under pressure by an air outlet
22
to the air delivery tube (not shown) and so to the mask (also not shown). The turbine
18
has radially directed impeller blades
24
. The alternate use of axial fans is known also in CPAP apparatus.
Another form of controllable flow generator involves operation of the driving motor at a constant speed, and venting or bleeding-off excess air from the output side of the turbine. As shown in
FIG. 2
, the turbine
18
is connected to a plenum chamber
30
by a supply pipe
32
. The plenum chamber has a controllable spill valve
34
operable to indexingly open and close an opening
36
in the chamber wall to allow the venting of air to atmosphere so as to achieve the desired output pressure at the air outlet
38
.
Such an arrangement also has disadvantages. Firstly there is excessive noise due to the venting of air when the treatment pressure is adjusted. This is particularly the case for each expiratory event during bi-level CPAP treatment when the treatment pressure typically is reduced from 16 cm H
2
O to 6 cm H
2
O, and thus over one half of the pressure head of the air within the plenum chamber
30
must be vented by the spill valve
34
. It is also difficult to maintain precise treatment pressure regulation, since small variations of the spill valve position give relatively large variations in the pressure at the air outlet
38
. This configuration also leads to an inherently low maximum flow rate which can compromise the efficacy of CPAP treatment. In particular, the spill valve
34
works by increasing outlet flow from the plenum chamber
30
, thereby increasing the pressure drop in the supply pipe
32
and the turbine
18
, thus dropping the pressure in the plenum chamber. The combined pneumatic impedance of the supply pipe
32
and the turbine
18
limit the maximum achievable flow rate into the plenum chamber
30
, and so to the patient, on subsequent closure of the spill valve
34
.
An example of another prior art arrangement that operates on the output of the flow generator can be obtained from International Publication No. WO 90/14121 (PCT/US90/02800), in the name Puritan-Bennett Corp.
As is noted, the invention also has application to apparatus for the provision of assisted respiration. Use of the term “assisted respiration” is to be understood as embracing both ventilators and respirators. Ventilators can broadly be characterised as providing for patient ventilation in a volume cycled mode, and do the work of breathing for the patient. Respirators, on the other hand, may or may not do the complete work of breathing for a patient, and are characterised by their bi-level operation, with a large treatment pressure differential between inspiration and expiration and a high inspiratory treatment pressure, which may reach 30-40 cm H
2
O.
DISCLOSURE OF THE INVENTION
It is an objective of the present invention to overcome or at least ameliorate one or more of the problems associated in the prior art. The gist of the invention is to provide control of output pressure by controlling the efficiency of a flow generator or its component turbine.
Therefore, the invention broadly discloses a controllable flow generator for the supply of breathable gas in the administration of CPAP treatment or assisted respiration, the flow generator comprising: a motor coupled to drive a turbine, an inlet for breathable gas in communication with the turbine, an outlet for the supply of said breathable gas at a pressure elevated above atmospheric pressure, and means to control the efficiency of the flow generator and thus the pressure of breathable gas exiting the flow generator.
In this specification the term “efficiency” in relation to a flow generator or to the component turbine is to be understood as the ability to pressurise a mass of air at a given flow rate and a given pressure.
In one preferred form the control means controls the efficiency of the turbine, and most preferably comprises adjustable pitch turbine blades or turbine louvres. In another preferred form, the control means controls the breathable gas available to the turbine. Alternatively, it controls the gas available to the inlet. Further, the control means can control the impedance of the outlet. The flow generator can further comprise pressure sensor means for sensing the pressure of air or breathable gas exiting the flow generator by the outlet, said sensed pressure being provided to said control means, and said control means further operable to compare said sensed pressure with a set pressure to maintain said exiting pressure substantially the same as said set pressure by controlling the efficiency of the turbine in accordance with the result of said comparison.
The invention further discloses a controllable flow generator for the supply of pressurised breathable gas in the administration of CPAP treatment or assisted respiration, the flow generator comprising a motor coupled to drive a turbine, an inlet for breathable gas in communication with the turbine, an outlet for the supply of said breathable gas at a pressure elevated above atmospheric pressure, and control means for controlling the breathable gas available to the turbine and thus the pressure of breathable gas at the o
Bachak Miroslav
Brydon John William Ernest
Hollis Shane Douglas
Wickham Peter John Deacon
Pillsbury Madison & Sutro LLP
ResMed Limited
Weiss John G.
Weiss, Jr. Joseph F.
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