U.S. patent application number 16/011535 was filed with the patent office on 2019-01-31 for humidity controller.
The applicant listed for this patent is FISHER & PAYKEL HEALTHCARE LIMITED. Invention is credited to Peter John Hunt, Stephen William McPhee, Mohammad Thudor, David Fraser Wixey.
Application Number | 20190030276 16/011535 |
Document ID | / |
Family ID | 65236500 |
Filed Date | 2019-01-31 |
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United States Patent
Application |
20190030276 |
Kind Code |
A1 |
Hunt; Peter John ; et
al. |
January 31, 2019 |
HUMIDITY CONTROLLER
Abstract
A breathing assistance apparatus adapted to deliver humidified
gases at a desired level of humidity to a patient including a
humidifier and a heated conduit is disclosed. The humidifier
includes a controller which determines the flow rate of the gases
and then determines the required power input to the humidifier to
deliver the gases to the patient at the required patient humidity.
This means the need for external sensors is dispensed with and thus
the apparatus is simple and less bulky.
Inventors: |
Hunt; Peter John; (Auckland,
NZ) ; Thudor; Mohammad; (Auckland, NZ) ;
Wixey; David Fraser; (Auckland, NZ) ; McPhee; Stephen
William; (Auckland, NZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FISHER & PAYKEL HEALTHCARE LIMITED |
AUCKLAND |
|
NZ |
|
|
Family ID: |
65236500 |
Appl. No.: |
16/011535 |
Filed: |
June 18, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14942040 |
Nov 16, 2015 |
10130787 |
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16011535 |
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13159345 |
Jun 13, 2011 |
9186477 |
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14942040 |
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12274020 |
Nov 19, 2008 |
7962018 |
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13159345 |
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10715324 |
Nov 17, 2003 |
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12274020 |
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10001596 |
Oct 19, 2001 |
7106955 |
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10715324 |
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09959226 |
Jan 23, 2002 |
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10001596 |
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09959226 |
Jan 23, 2002 |
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PCT/NZ2000/000156 |
Aug 9, 2000 |
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10001596 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61M 2205/3368 20130101;
A61M 16/1075 20130101; A61M 2016/0039 20130101; A61M 2205/50
20130101; A61M 16/1085 20140204; A61M 16/1095 20140204; G05D 22/02
20130101; A61M 16/162 20130101; G01F 1/684 20130101; A61M 2205/18
20130101; G01F 1/6888 20130101; A61M 16/0066 20130101; A61M 2205/33
20130101; A61M 16/0051 20130101; A61M 2205/3331 20130101; A61M
2205/3653 20130101; A61M 16/024 20170801; A61M 16/0875 20130101;
A61M 16/16 20130101; G01F 1/6842 20130101; A61M 16/109 20140204;
A61M 16/161 20140204 |
International
Class: |
A61M 16/16 20060101
A61M016/16; G05D 22/02 20060101 G05D022/02; A61M 16/10 20060101
A61M016/10; G01F 1/684 20060101 G01F001/684; G01F 1/688 20060101
G01F001/688; A61M 16/08 20060101 A61M016/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 1999 |
NZ |
337382 |
Oct 19, 2000 |
NZ |
507663 |
Claims
1-32. (canceled)
33. A breathing assistance apparatus comprising: a heater
configured to heat water to humidify gases, the heater further
configured to be in a fluid communication with a conduit configured
to deliver the gases to a patient; and a controller configured to:
cause an initial power level to be supplied to the heater to
maintain a heater temperature at an initial temperature level;
determine a flow rate of the gases based on an output of a flow
sensor; based on the flow rate of the gases, determine a power
level to heat the gases to a selected temperature or humidify the
gases to a selected humidity and cause the determined power level
to be supplied to the heater; monitor a change in the flow rate of
the gasses; in response to determining that the change in the flow
rate of the gases satisfies a first threshold but does not satisfy
a second threshold, redetermine the power level to heat the gases
to the selected temperature or to humidify the gases to the
selected humidity and cause the redetermined power level to be
supplied to the heater; and in response to determining that the
change in the flow rate satisfies the second threshold, cause the
initial power level to be supplied to the heater, the second
threshold being indicative of a larger change in the flow rate of
the gases than the first threshold.
34. The apparatus of claim 33, wherein the controller is further
configured to: in response to determining that 1) the change in the
flow rate of the gases satisfies the first threshold but does not
satisfy the second threshold and 2) the change in the flow rate of
the gases indicates an increase in the flow rate of the gases,
delay redetermination of the power level to heat the gases to the
selected temperature or to humidify the gases to the selected
humidity for a first period of time.
35. The apparatus of claim 34, wherein the controller is further
configured to: in response to determining that 1) the change in the
flow rate of the gases satisfies the first threshold but does not
satisfy the second threshold and 2) the change in the flow rate of
the gases indicates a decrease in the flow rate of gases, delay
redetermination of the power level to heat the gases to the
selected temperature or to humidify the gases to the selected
humidity for a second period of time shorter than the first period
of time.
36. The apparatus of claim 33, further comprising a temperature
sensor configured to measure the heater temperature, wherein the
controller is further configured to: determine an expected heater
temperature based on the flow rate of the gasses; and in response
to determining that the expected heater temperature deviates from
the measured heater temperature by a threshold temperature,
redetermine the power level to heat the gases to the selected
temperature or to humidify the gases to the selected humidity and
cause the redetermined power level to be supplied to the
heater.
37. The apparatus of claim 33, wherein the flow rate of the gases
depends at least in part on one or more restrictions in a flow path
configured to accommodate a flow of the gases.
38. The apparatus of claim 33, wherein the controller is further
configured to determine a conduit heater power level to heat the
gases flowing through the conduit and cause the conduit heater
power level to be supplied to the conduit heater.
39. The apparatus of claim 38, wherein the controller is further
configured to increase the conduit heater power level in response
to at least one of: increase in the flow rate of the gases,
decrease in ambient temperature, or increase in a difference
between the ambient temperature and a temperature of the gases.
40. A breathing assistance apparatus comprising: a heater
configured to heat water to humidify gases, the heater further
configured to be in a fluid communication with a conduit configured
to deliver the gases to a patient; and a controller configured to:
cause an initial power level to be supplied to the heater to
maintain a heater temperature at an initial temperature level;
estimate a flow rate of the gases; based on the estimated flow rate
of the gases, determine a power level to heat the gases to a
selected temperature or humidify the gases to a selected humidity
and cause the determined power level to be supplied to the heater;
monitor a change in the flow rate of the gasses; in response to
determining that the change in the flow rate of the gases satisfies
a first threshold but does not satisfy a second threshold,
redetermine the power level to heat the gases to the selected
temperature or to humidify the gases to the selected humidity and
cause the redetermined power level to be supplied to the heater;
and in response to determining that the change in the flow rate
satisfies the second threshold, cause the initial power level to be
supplied to the heater, the second threshold being indicative of a
larger change in the flow rate of the gases than the first
threshold.
41. The apparatus of claim 40, further comprising a fan configured
to blow the gases through a flow path, wherein the controller is
configured to estimate the flow rate of the gases based on loading
of the fan.
42. The apparatus of claim 41, wherein the flow rate of the gases
depends at least in part on one or more restrictions in the flow
path.
43. The apparatus of claim 40, further comprising a temperature
sensor configured to measure ambient temperature, wherein the
controller is configured to estimate the flow rate of the gases
based on the power level to heat the gases to the selected
temperature or humidify the gases to the selected humidity and the
ambient temperature.
44. The apparatus of claim 40, further comprising a temperature
sensor configured to measure the heater temperature, wherein the
controller is configured to estimate the flow rate of the gases
based on a ratio of the determined power level to heat the gases to
the selected temperature or to humidify the gases to the selected
humidity and the heater temperature measured by the temperature
sensor.
45. The apparatus of claim 40, wherein the controller is further
configured to: in response to determining that 1) the change in the
flow rate of the gases satisfies the first threshold but does not
satisfy the second threshold and 2) the change in the flow rate of
the gases indicates an increase in the flow rate of the gases,
delay redetermination of the power level to heat the gases to the
selected temperature or to humidify the gases to the selected
humidity for a first period of time; and in response to determining
that 1) the change in the flow rate of the gases satisfies the
first threshold but does not satisfy the second threshold and 2)
the change in the flow rate of the gases indicates a decrease in
the flow rate of gases, delay redetermination of the power level to
heat the gases to the selected temperature or to humidify the gases
to the selected humidity for a second period of time shorter than
the first period of time.
46. The apparatus of claim 40, further comprising a temperature
sensor configured to measure the heater temperature, wherein the
controller is further configured to: determine an expected heater
temperature based on the flow rate of the gasses; and in response
to determining that the expected heater temperature deviates from
the measured heater temperature by a threshold temperature,
redetermine the power level to heat the gases to the selected
temperature or to humidify the gases to the selected humidity and
cause the redetermined power level to be supplied to the
heater.
47. The apparatus of claim 40, wherein the controller is further
configured to determine a conduit heater power level to heat the
gases flowing through the conduit and cause the conduit heater
power level to be supplied to the conduit heater.
48. The apparatus of claim 47, wherein the controller is further
configured to increase the conduit heater power level in response
to at least one of: increase in the flow rate of the gases,
decrease in ambient temperature, or increase in a difference
between the ambient temperature and a temperature of the gases.
49. A method of operating a breathing assistance apparatus, the
method comprising, by a controller of the breathing assistance
apparatus: causing an initial power level to be supplied to a
heater of the breathing assistance apparatus to maintain a heater
temperature at an initial temperature level, the heater configured
to heat water to humidify gases; estimating a flow rate of the
gases; based on the estimated flow rate of the gases, determining a
power level to heat the gases to a selected temperature or humidify
the gases to a selected humidity and causing the determined power
level to be supplied to the heater; monitoring a change in the flow
rate of the gasses; in response to determining that the change in
the flow rate of the gases satisfies a first threshold but does not
satisfy a second threshold, redetermining the power level to heat
the gases to the selected temperature or to humidify the gases to
the selected humidity and causing the redetermined power level to
be supplied to the heater; and in response to determining that the
change in the flow rate satisfies the second threshold, causing the
initial power level to be supplied to the heater, the second
threshold being indicative of a larger change in the flow rate of
the gases than the first threshold.
50. The method of claim 49, wherein estimating the flow rate of the
gases comprises determining loading of a fan of the breathing
assistance apparatus.
51. The method of claim 49, wherein estimating the flow rate of the
gases is based on the power level to heat the gases to the selected
temperature or humidify the gases to the selected humidity and
ambient temperature.
52. The method of claim 49, wherein estimating the flow rate of the
gases comprises determining a ratio of the determined power level
to heat the gases to the selected temperature or to humidify the
gases to the selected humidity and the heater temperature measured
by a temperature sensor of the breathing assistance apparatus.
53. The method of claim 49, wherein estimating the flow rate of the
gases comprises determining the flow rate of the gases based on an
output of a flow sensor of the breathing assistance apparatus.
Description
TECHNICAL FIELD
[0001] This invention relates to breathing assistance apparatus,
particularly but not solely, for supplying optimal humidity
temperature of gases to a patient to assist the patient's
breathing.
BACKGROUND ART
[0002] A number of methods are known in the art for assisting a
patient's breathing. Continuous Positive Airway pressure or CPAP
involves the administration of air under pressure to a patient,
usually by a nasal mask. It is used in the treatment of snoring and
Obstructive Sleep Apnea (OSA), a condition characterised by
repetitive collapse of the upper airway during inspiration.
Positive pressure splints the upper airway open, preventing its
collapse. Treatment of OSA with nasal CPAP has proven to be both
effective and safe, but CPAP is difficult to use and the majority
of patients experience significant side effects, particularly in
the early stages of treatment.
[0003] Upper airway symptoms adversely affect treatment with CPAP.
Mucosal drying is uncomfortable and may awaken patients during the
night. Rebound nasal congestion commonly occurs during the
following day, simulating a viral infection. If untreated, upper
airway symptoms adversely affect rates of CPAP use.
[0004] Increases in nasal resistance may affect the level of CPAP
treatment delivered to the pharynx, and reduce the effectiveness of
treatment. An individual pressure is determined for each patient
using CPAP and this pressure is set at the mask. Changes in nasal
resistance affect pressure delivered to the pharynx and if the
changes are of sufficient magnitude there may be recurrence of
snoring or airway collapse.
[0005] Such symptoms can also occur in a hospital environment where
a patient is on a respirator. Typically in such situations the
patient is intubated. Therefore the throat tissue may become
irritated and inflamed causing both distress to the patient and
possible further respiratory problems.
[0006] A number of methods may be employed to treat such upper
airway symptoms, including pharmacologic agents to reduce nasal
disease, or heating the bedroom. One most commonly employed method
is humidification of the inspired air using an in line humidifier.
Two types of humidifier are currently used. Cold passover
humidifiers rely on humidifying the air through exposure to a large
surface area of water. While they are cheap, the humidity output is
low at high flows, typically 2 to 4 mg\L absolute humidity at flows
above 25 L/min. The output is insufficient to prevent mucosal
drying. Heated water bath humidifiers are more efficient, and
produce high levels of humidity even at high flow rates. They are
effective at preventing upper airway mucosal drying, prevent
increases in nasal resistance, and are the most reliable means of
treating upper airway symptoms.
[0007] Any of these active systems will have, to some degree or
other, condensation (or rain out) in the tubing connecting the
humidifier to the patient. The degree of condensation is strongly
dependent on the ambient temperature, being much greater for
greater differences between the ambient temperature and the gas
temperature. The formation of large quantities of water in the
breathing tubing causes considerable inconvenience to the patient,
may accelerate cooling of the gas, may eventually occlude the
tubing, or may be expelled into the patient. Also, the patient may
experience discomfort, when breathing gases are delivered at
temperatures widely divergent from that of the ambient temperature.
Excessive condensation also results in inefficient usage of the
water in the humidifying chamber.
[0008] In a hospital environment, where the ambient temperature of
the atmosphere within the hospital environment is controlled by air
conditioning for example, the required temperature for the
humidified gases supplied by the apparatus may be controlled within
set temperature parameters that are sufficiently close to the
ambient temperature to prevent condensation within the conduit.
However it is still necessary to have good control over the
temperature and humidity of gases as they are actually supplied to
the patient.
[0009] In the home care environment in which a user requires to use
humidifying apparatus at home, the range of ambient and gas
temperatures may well exceed that of the hospital environment. In
the home care environment, the user will usually wear a face mask
which is connected to end of the conduit and such a humidifier may
be used in the home environment for the treatment of breathing and
sleep apnea disorders and/or in conjunction with ventilators or
CPAP devices. In addition, non active humidifiers are commonly
employed utilising the known pass over humidification
technique.
[0010] In U.S. Pat. No. 5,640,951 issued to Fisher and Paykel a
heated conduit for a humidified breathing assistance apparatus is
disclosed which includes a temperature probe at the end of a heated
conduit. By heating the conduit the problems relating to
condensation in the conduit may be overcome. However in order to
implement closed loop control over the temperature of the supplied
gases (and therefore the power input to the conduit heater
element), it is necessary to measure the temperature as close to
the point at which it is supplied as possible. The temperature
probe and its associated wiring included for this purpose make the
attachment to the face mask or intubated patient bulky and
therefore more uncomfortable for the patient. Therefore it would be
advantageous if a heated conduit for a humidified breathing
assistance apparatus could be implemented without the need for a
temperature probe at the end of the conduit. It would also be
advantageous to have some indication, when the conduit heater is
energised, that it is operating correctly.
DISCLOSURE OF THE INVENTION
[0011] It is an object of the present invention to provide a
breathing assistance apparatus which goes some way to overcoming
the abovementioned disadvantages or which at least provides the
public or industry with a useful choice.
[0012] Accordingly in a first aspect the invention consists in a
breathing assistance apparatus adapted to deliver humidified gases
at a desired level of humidity or at a desired temperature to a
patient using open loop control comprising:
[0013] a humidifier having an electrical input power and capable of
humidifying said gases up to a level of humidity prior to delivery
to said patient, said level of humidity depending on said input
power to said,
[0014] and
[0015] a controller or processor configured or programmed to:
[0016] (a) determine a parameter relating to the flow rate of said
gases through said apparatus;
[0017] (b) determine based on at least said parameter the required
electrical power input to said humidifier to deliver said gases to
said patient at a level of humidity or at a temperature
substantially similar to said desired level of humidity or said
desired temperature;
[0018] (c) supply as said input power to said humidifier a level of
power substantially similar to said determined power input to said
humidifier.
[0019] In a second aspect the invention consists in a breathing
assistance apparatus adapted to deliver humidified gases at a
desired level of humidity or at a desired temperature to a patient
comprising:
[0020] humidifier having an electrical input power capable of
humidifying said gases up to a level of humidity prior to delivery
to said patient, said level of humidity depending on said input
power to said humidifier,
[0021] conduit for conveying said humidified gases from said
humidifier to said patient, and
[0022] conduit heater having an electrical input power, and being
associated with said conduit wherein the gases flowing through said
conduit are heated either directly or indirectly by said conduit
heater whereby the level of heating depending on said input power
to said conduit heater;
[0023] controller or processor which supply said input power to
said humidifier and said conduit heater, and providing a control
output indicative of said conduit heater being correctly connected
to said controller or processor and capable of operating in
according within predefined limits; and
[0024] a connector means to electrically connect said controller or
processor and said conduit heater and including an indicator in use
connected to said control output, wherein when said said conduit
heater being correctly connected to said controller or processor
and capable of operating in according within predefined limits said
controller or processor energising said indicator.
[0025] In a third aspect the invention consists in a method of
delivering humidified gas at a desired level of humidity or at a
desired temperature to a patient using an open loop controlled
humidifier comprising the steps of:
[0026] (a) determining a parameter relating to the flow rate of
said gas through said humidifier;
[0027] (b) determining based on at least said parameter the
required electrical power to said humidifier to deliver said gas to
said patient at a level of humidity or at a temperature
substantially similar to said desired level of humidity or said
desired temperature; and
[0028] (c) supplying a level of power to said humidifier
substantially similar to said determined power.
[0029] In a fourth aspect the invention consists in a method of
connecting a conduit heater within a conduit to a humidifier
comprising the steps:
[0030] providing an electrical connection between said conduit
heater and said humidifier; and
[0031] indicating whether conduit heater being correctly connected
and capable of operating in according within predefined limits.
[0032] In a fifth aspect the invention costs in a breathing
assistance apparatus adapted to deliver humidified gas at a desired
level of humidity or at a desired temperature to a patient using
open loop control comprising:
[0033] humidifier having an electrical input power and capable of
humidifying said gas up to a level of humidity prior to delivery to
said patient, said level of humidity depending on said input power
to said humidifier,
[0034] means for determining a parameter relating to the flow rate
of said gas through said apparatus;
[0035] means for determining based on at least said parameter the
required electrical power input to said humidifier to deliver said
gas to said patient at a level of humidity or at a temperature
substantially similar to said desired level of humidity or said
desired temperature;
[0036] means for supplying as said input power to said humidifier a
level of power substantially similar to said determined power input
to said humidifier.
[0037] To those skilled in the art to which the invention relates,
many changes in construction and widely differing embodiments and
applications of the invention will suggest themselves without
departing from the scope of the invention as defined in the
appended claims. The disclosures and the descriptions herein are
purely illustrative and are not intended to be in any sense
limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] One preferred form of the present invention will now be
described with reference to the accompanying drawings in which;
[0039] FIG. 1 is a illustration of a respiratory humidifier
system,
[0040] FIG. 2 is a illustration of the humidifier base of the
respiratory humidifier system of FIG. 1,
[0041] FIG. 3 is a block diagram of the control system which
controls the humidifier in the preferred embodiment of the present
invention,
[0042] FIG. 4 is a flow diagram of the algorithm used to control
the heater wire within the respiratory conduit,
[0043] FIG. 5 is an example of how the heater plate temperature
varies over time, when the pressure is controlled constant,
[0044] FIG. 6 is a graph of heater plate power against flow rate,
and
[0045] FIG. 7 is a graph of conduit heater element power and flow
rate.
DETAILED DESCRIPTION OF THE INVENTION
[0046] Whether used in a hospital environment or in a home care
environment, the present invention will generally have associated
two main pieces of apparatus. Firstly an active humidifier which
controls the temperature of a heater plate heating a body of water
to achieve a desired temperature and humidity of the gases being
humidified. Secondly a transport conduit from the humidifier to the
patient is also required, which is preferably heated to reduce
condensation, or "rain out".
[0047] Referring to FIG. 1 a humidifying apparatus as might be used
in a hospital generally referenced 1 is shown. The apparatus
comprises a body 2 containing heating means comprising a heating
plate 20 having an electric heating element therein or in thermal
contact therewith and control means for example electronic
circuitry which may include a microprocessor for controlling the
supply of energy to the heating element. The body 2 is removably
engageable with a. humidifying chamber 3 which contains water for
humidifying gases. Referring to FIG. 2 which show the humidifier
apparatus in more detail, the humidifying chamber 3 has edges which
engage with collar 24 on the humidifier apparatus. The gases to be
humidified may be a mixture of air, oxygen and anaesthetic for
example which are supplied to the chamber through a gases inlet 4.
This might be connected to a ventilator, or in the case of CPAP
therapy a CPAP blower. A gases outlet 5 is also provided and the
gases outlet 5 is connected to the conduit 6 (FIG. 1) which conveys
humidified gases to a remote destination such as an intubated
patient at the end 7 of the conduit. Alternatively, the end 7 of
the conduit may have a gas mask attached thereto, which mask is
used to cover a nose and/or mouth of a user so as to supply
humidified gases to the user for breathing, as in the delivery of
CPAP therapy. The humidifier heater plate 20 has a temperature
transducer 8 which is in electrical connection with the electronic
control circuitry in body 2 of the apparatus so that the control
means monitors the temperature of the heating plate.
[0048] A heating element 10 is provided within the conduit 6 to
help prevent condensation of the humidified gases within the
conduit. Such condensation is due to the temperature of the walls
of the conduit being close to the ambient temperature, (being the
temperature of the surrounding atmosphere) which is usually lower
than the temperature of the humidified gases within the conduit.
The heater element is effectively replaces the energy lost from the
gases through conduction and convection during transit through the
conduit. Thus the conduit heater element ensures the gases
delivered are at an optimal temperature and humidity.
[0049] The present invention provides a means of controlling at
least the heater plate and preferably also the conduit heater
element without the need for any sensors, either in the humidifier
chamber or positioned in the conduit. This is achieved by
estimating the rate of flow of gases through the humidifier using
parameters already available to the controller. For a given
humidifier an appropriate level of power can then be determined to
apply to the heater plate to achieve the desired temperature of
gases delivered to the patient. Additionally this may be used to
provide a more appropriate level of energisation at this conduit
heater element. This not only saves the cost of the extra sensors
but also allows the apparatus connected to the end of the conduit
to be simpler and lighter.
[0050] In the preferred embodiment of the present invention the
controller 100, shown in FIG. 3, uses a range of inputs to control
both the power 108 supplied to the heater plate 110 as well as the
power 114 supplied to the conduit heating element 116 (if present).
In certain applications it may also be used to provide control
instructions to auxiliary apparatus such as a blower fan. Using an
internal algorithm 106 the controller 100 estimates the power 108
to supply to the humidifier heater plate 110 to achieve a given
humidity and or temperature of gases at the top of the humidifier
chamber alternatively (or estimates the temperature to achieve a
given power). It then uses a second algorithm 102 to estimate the
required power 114 to supply to the conduit heater element 116 and
the humidifier heater plate 110 to achieve optimal temperature
and/or humidity of the gases delivered to the patient 118.
[0051] Referring to FIG. 4, when the humidifier starts up the
controller executes a supervisory algorithm, which controls the
heater plate and if present the conduit heater element. Initially
128 the heater plate is controlled to a temperature of 40.degree.
C. and the conduit heater element may be energised with a duty
cycle of for example 50%. The heater plate temperature (or
alternatively the power supplied to the heater plate) is then
monitored 130 until it settles to a stabilised level. Effectively a
window 132 is superimposed over the heater plate temperature
profile 134 of which an example is shown in FIG. 5. When the
profile 134 (over the entire period of the window 132) fits within
the bounds of the window 132, it is effectively considered to have
stabilised. Once this has occurred the controller enters a
calculation stage.
[0052] Firstly, it calculates the flow rate of the gases 136 using
any one of a number of methods which will be described later.
[0053] Secondly knowing the rate of flow of the gases the algorithm
then calculates the required heater plate power 138 (alternatively
heater plate temperature) to achieve a desired temperature/humidity
of gases (alternatively heater plate power). A relationship has
been empirically determined using a humidifier and a heated conduit
such as that as described in U.S. Pat. No. 5,640,951, the contents
of which are incorporated herein by reference. The actual
relationship for any other arrangement would either have to be
empirically determined by experimentation or theoretically
calculated. For a desired temperature of gases exiting the
humidifier of for example 37.degree. C. the relationship between
the power supplied to the heater plate (P.sub.HP), the rate of flow
of gases (F.sub.gas) and the ambient temperature (T.sub.amb) is
graphed in FIG. 6. From this an approximate general algebraic
equation has been extrapolated which the controller can use to
determine an approximate level of power to apply to the heater
plate:
P.sub.HP=(-0.1239.times.T.sub.amb+5.383).times.F.sub.gas+(-0.3112.times.-
T.sub.amb+10.738)
[0054] Thirdly the algorithm calculates the required power input to
the conduit heater wire 140 to deliver a desired temperature of the
gases to the patient. With gases flowing at a known rate of flow it
is possible to calculate the resultant temperature of the gases
once they have flowed through a conduit of known characteristics
surrounded by the atmosphere at a known or assumed ambient
temperature. Thermal characteristics of the conduit will either be
known or can be calculated by experimentation. This relationship is
based off empirical data using a humidifier and a heated conduit
such as that as described in U.S. Pat. No. 5,640,951. The actual
relationship for any other arrangement would either have to be
empirically determined by experimentation or theoretically
calculated. With a conduit entry gas temperature of 37.degree. C.
and a temperature of gases delivered to the patient of 40.degree.
C., the relationship between the flow rate of the gases
(F.sub.gas), the power input to the conduit heater element
(P.sub.c), the ambient temperature (T.sub.amb) is graphed in FIG.
7. This is extrapolated to a general algebraic expression:
P.sub.c=(-0.0005*T.sub.amb+0.0169)
F.sub.gas.sup.2-[10.sup.-5*T.sub.amb.sup.3-0.0042*T.sub.amb.sup.2+0.2189*-
T.sub.amb-3.0075]F.sub.gas-1.0169*T.sub.amb+38.956
Practically this relationship can be simplified whereby P.sub.c is
dependent only on T.sub.amb. This is an acceptable approximation
for the conduit heater element, as it is not as crucial as the
heater plate.
[0055] Once the heater plate and conduit heater element have been
appropriately energised, the controller continues to monitor 142
the system for any changes in the variables. The main reason for
this is to avoid thermal overshoot i.e. where the flow drops
suddenly, the temperature of gases can become dangerously high.
[0056] In order to monitor effectively, two methods are used.
Firstly the flow rate is monitored and secondly the change in flow
rate (with respect to time) is also monitored. The first 144 is to
allow the system to respond to any changes in the system. The
second 146 is a fast response system in order to avoid thermal
overshoot. Effectively where either P.sub.HP or T.sub.HP is
controlled constant, monitoring the other variable gives an
indication of any change in flow, or any other variable which
requires a recalculation.
[0057] In order to monitor the flow a variable x (defined as
P.sub.HP/T.sub.HP), which is closely related to the flow rate, is
constantly calculated and monitored. If it goes up there is a 30
minute delay before the controller initiates a recalculation, to
avoid spurious readings and unnecessary calculations. If it goes
down there is a 30 second delay before the controller recalculates,
to avoid any possibility of the delivered gases being, even
transiently, too hot.
[0058] Where large step changes occur the controller needs to react
quickly. In such cases it will reset to initial conditions to wait
until the system stabilises again, as any calculations in the
interim would be pointless. To achieve this dx/dt is calculated and
monitored. While a negative value is more dangerous, any deviation
over a certain value will reset the controller.
[0059] In an alternative embodiment of the present invention the
expected heater plate temperature is calculated using
T.sub.HP=-7.3319*Ln(F.sub.gas)+63.655
and if the actual heater plate temperature deviates by more than
5.degree. C. then the program recalculates the required powers.
[0060] Thus in summary controller carries out the following steps:
[0061] 1) Estimates the rate of flow of gases keeping all variables
constant 136. [0062] 2) Estimate the required heater plate
power/temperature to achieve a specified temperature/humidity of
gases in the humidification chamber 138. [0063] 3) Calculate the
power input to the heater wire to achieve a desired output
temperature 140.
[0064] It will be appreciated that a greater level of power will be
supplied to the conduit heater element if: [0065] i) the rate of
flow of the gases reduces, [0066] ii) the ambient temperature
decreases, [0067] iii) the differential between ambient and gases
temperature increases.
[0068] It will also be appreciated that the heater plate
temperature could be controlled to a set valve (using closed loop
control) as opposed to power. In this case the power supplied would
be monitored as a measure of system stability. Furthermore where
relationships are expressed algebraically they could equally be
stored in look-up tables. First preferred embodiment of flow
estimation
[0069] Generally when used in a hospital setting a humidifier such
as that described in the present invention will be used in
conjunction with a respirator to supply humidified gases to an
incubated patient, or possibly using a respiratory mask. As such
the humidifier will operate effective independently of the
respirator and therefore must make all of its control decisions
based on only the sensors contained therein. In the preferred
embodiment of the present invention the flow rate of the gases
passing through the humidification chamber can first be estimated
by comparing the power input required 108 for the humidifier heater
plate to the measured temperature 112 of the heater plate. In
effect the higher the rate of flow of gases the larger the amount
of power required by the heater plate in order to achieve a given
heater plate temperature. Thus for a given system the relationship
between power to heater plate and flow rate for a given heater
plate temperature can either be determined empirically or
theoretically calculated. Again using a humidifier and a heated
conduit such as that as described in U.S. Pat. No. 5,640,951 the
following empirically determined relationship applies:
F gas = - ( 0.831 - 0.0049 * T amb ) + abs ( 0.831 - 0.0049 * T amb
) 2 - ( 4 * ( 0.00004 * T amb - 0.0057 ) * ( ( 14.348 - 0.25 * T
amb ) - P HP ) ) 2 * ( 0.0004 * T amb - 0.0057 ) ##EQU00001##
where P.sub.HP is the power applied to the heater plate to achieve
a given heater plate temperature in steady state of 50.degree. C.,
T.sub.amb is the ambient temperature and F.sub.gas is the gas flow
rate.
[0070] It will be appreciated this method is more appropriate in
the hospital care environment where the ambient temperature can be
assured with a high degree of confidence.
Second Preferred Embodiment of Flow Estimation
[0071] In the homecare environment the present invention will often
be employed in conjunction with a continuous positive airway
pressure (CPAP) device or such other breathing apparatus which will
include a fan such as that described in U.S. Pat. No. 6,050,260,
the contents of which are incorporated herein by reference. It will
be appreciated that in such applications it may be possible to
connect the controllers of the various devices together in an
arrangement such that data may be readily exchanged. In such cases
the rate of flow of the gases may be estimated directly from
information available either from the fan or, where provided, a
flow sensor.
[0072] In this embodiment of the present invention the flow is
estimated based on the loading of the fan. Generally the fan will
be controlled to run at a specified speed and therefore deliver a
constant pressure output. The flow rate of the gases will depend on
the restrictions in the flow path. In turn in order to maintain the
specified speed a certain power input will be required for the fan.
Therefore an algebraic relationship between the actual gas flow
rate and the power input to the fan can be developed for a fan of
known characteristics. This relationship may either be determined
empirically by experimentation or theoretically calculated using
specified motor characteristics.
[0073] A number of methods are known in the art for determining the
loading on a motor from the supply it draws. The simplest such
method would be to firstly meter the current drawn 148 from the fan
150, as indicated in FIG. 3. The current 148 is the input to the
conduit heater element controller 102 where either an algebraic
relationship or a look up table is used to determine the flow rate
of the gases.
[0074] For example in U.S. Pat. No. 5,740,795, the contents of
which are hereby incorporated herein by reference, a method is
disclosed using both motor voltage and current to estimate the flow
rate. While this represents one method, as mentioned above, it will
be appreciated that other methods, such as based on just current,
will be equally applicable.
Third Preferred Embodiment of Flow Estimation
[0075] As mentioned in the second embodiment that in certain cases
a flow sensor may already be provided in the gas flow path. This
being the case, the gas flow rate 152 can be extracted directly
from the flow sensor 154 and used as an input to the humidifier
controller 100, as indicated in FIG. 3. This is then used directly
in the conduit heater element controller 102 to determine the power
to apply to the heater plate 110 and conduit heater element 116
according to the algorithm shown in FIG. 4 and described
earlier.
Heater Wire Adaptor
[0076] In order to connect the conduit heater element to the power
supply in the humidifier, an adaptor cable is required. In the
preferred embodiment of the present invention, the adaptor 200
includes an indicator 202 to indicate whether the conduit heater
element is operating correctly, when the adaptor is plugged in, as
shown in FIG. 1.
[0077] The humidifier controller continually detects for the
conduit heater element and determines whether it is operating
correctly. It does this by energising the conduit heater element
intermittently, and if the expected current results it energises
204 the indicator (e.g. an LED).
[0078] The present invention as described in the foregoing provides
a novel method and apparatus for controlling the heater plate
temperature in a humidifier for supplying humidified gases to a
patient under respiratory therapy. This has the advantage of
removing external sensors making the system simpler, cheaper and
lighter. Similarly it may also allow for effective control over
energisation of the conduit heater element, ensuring the system as
a whole operates correctly as well as being as efficient as
possible.
* * * * *