U.S. patent application number 10/746730 was filed with the patent office on 2004-08-05 for humidified gases delivery apparatus.
This patent application is currently assigned to Fisher & Paykel Healthcare Limited. Invention is credited to Kadhum, Hussein, Smith, Daniel John, Smith, Malcolm David.
Application Number | 20040149284 10/746730 |
Document ID | / |
Family ID | 46278191 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149284 |
Kind Code |
A1 |
Smith, Daniel John ; et
al. |
August 5, 2004 |
Humidified gases delivery apparatus
Abstract
A gases transportation pathway for use in supplying a humidified
gases stream to a patient includes regulated conduit heating. The
regulated conduit heating may include a section of positive
temperature coefficient material wherein the localised electrical
resistance of the material is positively related to the localised
temperature. The regulated conduit heating may be a layer of
positive temperature coefficient material within the wall of the
gases transportation pathway with at least a pair of conductors
running the length of the pathway and in electrically conductive
contact with the positive temperature coefficient material.
Inventors: |
Smith, Daniel John;
(Auckland, NZ) ; Kadhum, Hussein; (Auckland,
NZ) ; Smith, Malcolm David; (Auckland, NZ) |
Correspondence
Address: |
TREXLER, BUSHNELL, GIANGIORGI,
BLACKSTONE & MARR, LTD.
105 WEST ADAMS STREET
SUITE 3600
CHICAGO
IL
60603
US
|
Assignee: |
Fisher & Paykel Healthcare
Limited
East Tamaki
NZ
|
Family ID: |
46278191 |
Appl. No.: |
10/746730 |
Filed: |
December 26, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10746730 |
Dec 26, 2003 |
|
|
|
09956723 |
Sep 20, 2001 |
|
|
|
09956723 |
Sep 20, 2001 |
|
|
|
09808567 |
Mar 14, 2001 |
|
|
|
Current U.S.
Class: |
128/203.16 |
Current CPC
Class: |
A61M 16/161 20140204;
A61M 16/08 20130101; A61M 16/16 20130101; A61M 16/1095 20140204;
A61M 16/109 20140204; A61M 16/1075 20130101 |
Class at
Publication: |
128/203.16 |
International
Class: |
A61M 015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2000 |
NZ |
503495 |
Claims
1. A gases transportation pathway for transporting humidified gases
to a patient, said pathway comprising: an enclosing wall including
at least a layer of positive temperature coefficient material
wherein the localised electrical resistance of said material is
positively related to the localised temperature, and at least two
electrical conductors running the length of said pathway and
disposed in electrical contact with said positive temperature
coefficient material.
2. A gases transportation pathway as claimed in claim 1 wherein
said wall includes at least one helically arranged ribbon of
positive temperature coefficient material with adjacent turns of
ribbon overlapping and fused to one another.
3. A gases transportation pathway as claimed in claim 2 wherein
said conductors are arranged helically along said gases
transportation pathway at least substantially constantly spaced
from one another along the length of said gases transportation
pathway.
4. A gases transportation pathway as claimed in claim 3 wherein at
least a pair of said conductors are disposed within a said ribbon
of positive temperature coefficient material, in constant position
in relation to said ribbon.
5. A gases transportation pathway as claimed in any one of claims I
to 4 wherein said positive temperature coefficient material has a
phase transformation temperature between 35.degree. C. and
40.degree. C.
Description
BACKGROUND TO THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to apparatus for the delivery
of humidified gases and in particular to conduits for humidified
breathing circuits.
[0003] 2. Summary of the Prior Art
[0004] A number of methods are known in the art for supplying
humidified gases to a patient requiring breathing assistance. Such
prior art humidifiers generally comprise a source of pressurised
air (or other mixture of gases), a humidification chamber including
a source of water and a heating means to vaporise the water, and a
conduit to convey the humidified gases to the patient or user.
[0005] For example U.S. Pat. No. 4,038,980 describes a "flash
vaporisation" humidifier where water drips onto a low thermal mass
heater to create respiratory humidity. It mentions "control means
may be provided automatically to regulate the water supply rate in
response to means sensing the relative humidity", however they
prefer a manual control of water flow rate. Thus it incorporates a
humidity sensor and controls the water rate, as opposed to
controlling the amount of electrical heating.
[0006] U.S. Pat. No. 5,092,326 also describes the use of a humidity
sensor in a humidifier. It describes a high frequency ventilation
system that incorporates a heated humidifier and a humidity sensor,
where these are linked to a central microprocessor. Apparatus is
disclosed to moisten a gas mixture supplied to the airway, and a
microprocessor controls the amount of moisture supplied to the gas
mixture.
[0007] U.S. Pat. No. 5,769,071 describes a humidifier incorporating
a heat and moisture exchanger (HME), supply of water to the HME,
heater element and humidity sensor. The humidity sensor can control
humidity via water supply rate or temperature (via the heater
element). The humidity sensor is described as being at the patient
airway.
[0008] U.S. Pat. No. 5,988,164 describes a heated breathing tube
system for use with a humidifier. This uses a relative humidity
sensor (located near the patient) to control the amount of heating
provided by the heated broathing circuit so that the gas is at a
constant level of relative humidity. The heated breathing circuit
may use either electrical heating, or heating via warm
recirculating water in a tube. Also described is a method of
control of the electric heater wire or heated water tube based on
the output of relative humidity sensor.
[0009] The previously mentioned U.S. Pat. Nos. 4,038,980 and
5,769,071 both describe humidifiers where the humidification
chamber is located close (proximal) to the patient. These have the
disadvantage of introducing weight, heat and complexity near the
patient which is inconvenient and could be painful to the patient.
Of the cited prior art only U.S. Pat. No. 5,988,164 specifically
describes the humidification chamber as being located remotely from
the patient.
[0010] There are several disadvantages of the prior art systems
using a humidification chamber located remotely from the patient.
It is normally assumed that gases leaving such prior art
humidifiers are saturated with water vapour (100% relative
humidity). However there is no guarantee that the gases leaving
such humidifiers are in fact saturated with water vapour. In
certain circumstances (e.g. with the incoming air already warm),
the gases leaving such humidifiers can be significantly less than
100% relative humidity. This is because the humidifiers are
typically controlled to achieve a desired outlet gas temperature,
which in some cases may not be much more than the incoming air.
[0011] Another drawback of the prior art systems is that
condensation can occur in the (sometimes heated) conduits
connecting the patient to the respiratory assistance equipment.
This may occur if the temperature profile along such conduits is
not even and allows some parts of the conduit to be colder than the
gas at these points.
[0012] A third disadvantage of such prior art systems is that where
the gas leaving the humidifier is at 100% relative humidity it must
be heated immediately by some form of conduit heater or it may lose
heat through the walls of the conduit otherwise condensation and
therefore a drop in the amount of absolute humidity contained in
the gas will result.
[0013] Another fourth disadvantage of the prior art systems is the
need for a sensor very near to the patient, which adds to the
weight and bulk of equipment at the patient's airway.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of the present invention to
provide apparatus for the delivery of humidified gases which goes
some way to overcoming the above mentioned disadvantages.
[0015] Accordingly in a first aspect the present invention consists
in a gases transportation pathway for transporting humidified gases
to a patient, said pathway comprising:
[0016] an enclosing wall including at least a layer of positive
temperature co efficient material wherein the localised electrical
resistance of said material is positively related to the localised
temperature, and
[0017] at least two electrical conductors running the length of
said pathway and disposed in electrical contact with said positive
temperature co efficient material.
[0018] 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.
[0019] The invention consists in the foregoing and also envisages
constructions of which the following gives examples.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] One preferred form of the present invention will now be
described with reference to the accompanying drawings.
[0021] FIG. 1 shows an example of an humidification system, with
three basic parts
[0022] FIG. 2 is a plan view of a section of a ribbon of PTC
material with an electrode embedded along each edge.
[0023] FIG. 3 is a plan view of a spirally configured heater
element using the PTC ribbon of FIG. 2.
[0024] FIG. 4 is a plan view of a second form of spirally
configured PTC ribbon heater element.
[0025] FIG. 5 is perspective view of a tube formed with a spirally
wound PTC ribbon (without pre-embedded conductors) with
longitudinally oriented conductors in the tube.
[0026] FIG. 6 is a plan view of a section of a ribbon of PTC
material with a conductor embedded along one edge and second
conductor embedded near the centre.
[0027] FIG. 7 is a plan view of a spiral forming arrangement
performing a conduit using the ribbon of FIG. 6 (with the forming
mandrel not shown).
[0028] FIG. 8 shows construction of a tube incorporating flexible
PTC elements in a parallel wire configuration.
[0029] FIG. 9 shows a chamber combined with an unheated, well
insulated delivery tube,
[0030] FIG. 10 shows construction of a tube incorporating flexible
PTC elements in a parallel wire configuration,
[0031] FIG. 11 shows a humidifier configuration; using the tube in
any one of FIGS. 8 to 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] FIG. 1 illustrates a typical respiratory humidification
system, comprised of three parts:
[0033] 1) a humidification chamber located at a distance from the
patient, which heats and substantially saturates gases flowing
through it;
[0034] 2) a delivery system consisting of a flexible tube which
carries humidified gases from the humidification chamber 1 to the
gas outlet 5; and
[0035] 3) a heater base which heats the humidification chamber 1
and provides measurement and control functions.
[0036] The gas to be humidified flows into the chamber 1 from port
4 and leaves the delivery system 2 at gas exit port S. Gas from
exit port 5 flows to a patient via a face mask or similar (not
shown). Dry gases at the gas input 4 are heated and humidified by
passing over the surface of hot water 6 in the chamber 1 so that
they are substantially saturated with water vapour when they leave
chamber 1 at exit port 10. Hot water 6 is heated by heater plate 9
and the amount of heating is controlled so that the gas reaches a
predetermined temperature at exit port 10. Therefore the
humidification chamber 1 acts to heat and humidify the medical
gases so that they are substantially saturated at the output of
chamber 1, and are at a predetermined temperature.
[0037] The gas delivery system 2 (also known as a delivery tube or
breathing circuit) consists of a flexible tube 11 containing a
heater 12. The gas from the humidification chamber 1 passes through
the tube 11 and is heated by heater 12 to offset heat losses
through the walls of tube 11.
[0038] The system as described has gas entering gas inlet 4 from a
continuous flow gas source (not shown) and exiting the system
through gas outlet 5. However the system is equally applicable
where the gas source is a ventilator, which creates intermittent
flow patterns to provide breaths to a patient. In this case gas
outlet port 5 is connected directly to gas inlet port 16. The
patient is connected to port 17 via an endotracheal tube, mask,
mouthpiece or other patient interfaces (not shown). During patient
inspiration dry gases from the ventilator enter the system at inlet
port 4, pass through chamber 1, delivery system 2, pass through
wye-piece 13 and reach the patient through port 17. During patent
exhalation gases pass back through port 17, through wye-piece 13,
tube 14 and leave through gas outlet port 18. Tube 14 may also be
heated by heater 15 to prevent condensation.
[0039] One aspect of the present relates to removing the need for a
sensor at the patient airway To remove this sensor safely, we must
be certain that the gas entering the delivery tube has a safe level
of temperature and absolute humidity, and that the surfaces inside
the delivery tube do not exceed safe temperature levels. This
implies a delivery tube that has a constant internal wall
temperature.
[0040] It would be desirable, therefore, to have a heated delivery
tube which self-regulates its temperature at a desired level. The
heater could either be embedded in the wall of the delivery tube
itself, or it could lie inside the lumen of the delivery tube, or
it could be wrapped around the outside of the delivery tube. Such a
heater could be made from positive temperature coefficient (PTC)
material (such as "Winterguard" from Raychem Corp., Menlo Park,
Calif. USA), so that the resistance of the heater increases if the
heater is hot, resulting in reduced power. However the delivery
tube may pass through more than one environment, or may have
localised drafts present on certain parts of the tube. If the PTC
elements are arranged in parallel, then the fill benefit of the PTC
heater can be envisaged. If the PTC elements are arranged in
parallel, then the cold portions of the tube will have a lower
resistance, which will result in more heat being dissipated. Thus
the tube will tend to regulate its own temperature.
[0041] FIG. 10 shows construction of a tube incorporating flexible
PTC elements in a parallel wire configuration. The tube 48 is made
of a flexible PTC material, which has two low resistive strip
connections, 46 and 47, on either side of it. This allows each
portion of the tube to consist of short conducting segments of tube
connected in parallel between conductors 46 and 47. These segments
are represented by dotted lines encircling the tube in FIG. 10. The
conductors 46 and 47 are connected to adjustable voltage source 49,
which may be AC or DC. The tube would have an outer layer (not
shown) which provides electrical insulation and thermal insulation
to the tube. Each longitudinal segment of the tube will be able to
regulate its own temperature independently of the rest of the
tube.
[0042] Although one specific PTC heated tube design has been
envisaged and described, other PTC tube designs could be used. Some
additional tube designs are d&scribed out below. It may also be
of advantage to create a PTC tube that has a differing temperature
profile along its length rather than a constant temperature
profile. The PTC design could also be extended to incorporate PTC
heaters in other parts of the patient breathing circuit, such as
the flexible extension tube which is usually connected between the
Y-piece (port 17 of FIG. 1) and the patient's endotracheal tube. A
faker extension of the PTC tube concept would be into a self-heated
and temperature controlled endotracheal tube.
[0043] The PTC tube described with reference to FIG. 10 allows us
to create a humidifier which doesn't use any sensor at the patient
airway. FIG. 11 shows a humidifier configuration using this tube.
Gas enters humidification chamber 52 via inlet port 51 and is
humidified by water 53, heated by heater plate 54. Absolute
humidity sensor 55 controls the heater plate so that the gas
passing sensor 55 is at a desired level of absolute humidity. PTC
tube 56 is heated by an external voltage (not shown) so that the
internal surface temperature is at a constant desired temperature,
which is selected to be above the dewpoint of the gas. The gas
which leaves tube 56 at outlet 57 will therefore be near the
temperature of the tube, and containing the desired level of
absolute humidity which was controlled by absolute humidity sensor
55.
[0044] A variation of the system shown in FIG. 11 would be to use a
temperature sensor at position 55. Another variation of a tube with
a constant internal wall temperature would a delivery tube with
heated water or other fluid pumped through smaller conduits in the
wall of the delivery tube. Since the heated fluid has a high
specific heat relative to air, the temperature of the fluid remains
fairly constant during passage through the delivery wall
conduits.
[0045] Referring to FIGS. 2 to 8 further preferred forms of the
present invention are described. These forms provide a heated
delivery tube which self-regulates its temperature at a desired
level. The heater may be embedded in the wall of the delivery tube
itself, form the fabric of the tube or lie inside the lumen of the
delivery tube. The heater of the present invention is formed from a
positive temperature coefficient (PTC) material.
[0046] The resistance of a PTC material increases markedly once it
reaches a threshold temperature, resulting in reduced power
consumption and subsequent cooling. The delivery tube may pass
through more than one environment, or may have localised drafts
present on certain parts of the tube.
[0047] In one embodiment of the present invention the PTC heater is
provided as an elongate structure laying within the lumen of the
delivery tube. The construction according to a preferred embodiment
is illustrated with respect to FIGS. 2 to 4. In particular the
heater structure is formed from a ribbon 20 of PTC plastic material
with conductors 21, 22 embedded in the plastic material adjacent
the opposite edges thereof. In use the conductors are attached to a
power supply to provide a voltage difference between the conductors
and cause a current to flow between them depending on the
resistance of the PTC material.
[0048] The ribbon may be provided in the tube as a single length of
ribbon blindly terminated at one end and terminated with a power
connector at the other end. This configuration is illustrated in
FIG. 3 where the ribbon 20 is wound into a generally helical
configuration and is terminated at one end with a blind connector
23. Termination of the other end at a power connector is not shown.
In a alternative configuration the ribbon may be provided as a loop
so that both ends terminate at the power connector with both ends
of the positive electrode terminating at the positive pin and both
ends of the negative or ground electrode terminating at the ground
and negative pin. This configuration is depicted in FIG. 4, in
which the ribbon 20 is provided in a generally double helical
configuration. The conductors 21 and 22 have both ends terminating
in the power connector 25 at one end of the heater structure. The
ribbon 20 loops back upon itself at the other end 24 of the heater
structure.
[0049] With the pair of conductors provided along opposite edges of
the ribbon the PTC material offers an amorphous array of parallel
current paths along the entire length of the ribbon. Where the
internal conduit temperature is lower the heater structure will
have a lower resistance and more current will flow producing a
greater heater effect. Where the internal temperature in the
conduit is higher the PTC material will have a higher resistance,
choking off current flow and reducing heating in that region of the
conduit.
[0050] In a fixer aspect of the invention the PTC material is
arranged in a parallel circuit over the length of the tube and
forming part of the wall itself the full benefit of using PTC
heater can be obtained. At the cold portions of the tube the
material will have a lower resistance, which will result in more
heat being dissipated in that area. Thus the tube will tend to
regulate its own temperature.
[0051] In particular if the PTC material is composed to provide a
threshold temperature at or just above the preferred gases
temperature (eg above the dew-point of the humidified gases) the
PTC material will maintain itself at that threshold temperature
(with some hysteresis fluctuation) and condensation on the conduit
surface will be at least substantially eliminated. This provides
effective condensation control then maintaining an elevated
temperature for the humidified gases where condensation may still
form on the cold wall surfaces.
[0052] PTC material behaviour is exhibited in a range of polymer
compositions with electrically conductive fillers. The behaviour
can be characterised by a general statement that "providing certain
other conditions are fulfilled, the composition becomes
electrically conductive when particles of electrically conductive
filler form a continuous chain, penetrating the material from the
point of entry of electric current to the place where it leaves the
polymer material". Polymer compositions containing electrically
conductive filler can exhibit PTC properties due to the formation
of a chain of filler particles that are close enough for current to
flow at a certain temperature, generating heat which increases the
temperature of the material until it reaches a phase transformation
temperature. At the phase transformation temperature the
crystalline polymer matrix changes to an amorphous structure. This
change is accompanied by a small thermal expansion, forcing filler
particles to move apart, breaking the conductive paths. Accordingly
resistance rises sharply at this phase transformation temperature.
As the material cools the small thermal conduction allows new
conductive paths to form and current flow to resume. The rise and
fall in temperature and the thermal contraction and expansion
provides an inherent hysteresis in the cycle.
[0053] In producing a PTC material a number of factors have a
bearing on the performance of the material. Particular factors
include the quantity, type and particle size of the carbon black
(or other conductive filler) used in the composite, the polymer
that the carbon black binds with during mixing of the base
materials and the process conditions such as temperature, pressure
and time of mixing. It is important that the conductive filler
particles are distributed evenly through the composite so that the
composite exhibits uniform PTC behaviour.
[0054] For the present invention a PTC material having a phase
transformation temperature not exceeding 40.degree. C. is desired.
One composition meeting these criteria has been developed and has
the following composition:
[0055] 20% by weight carbon black powder having a surface area of
254 m.sup.2/g and oil Di-Butyl- Phthalate absorption of 188
cm.sup.3/100 g. This powder is available as VULCAN XC-72 (powder)
from Cabot Corporation.
[0056] 64% Ethylene-Vinyl-Acetate. This material is available as
ELVAX (grade 40 w) from Dupont (E.I. du Pont de Nemours and
Company), with a density of 965 kg per m.sup.3, a melting point of
46.degree. C. and melting index of 52.
[0057] 13.5% Plastomer. An example plastomer is available as EXACT
2M055 from ExxonMobil Corp, having a density of 882 kg/m.sup.3, a
melting point of 70.degree. C. and a melting index of 3.
[0058] 2.5% Wax.
[0059] This material was uniformly mixed and extruded to form a PTC
ribbon with embedded conductors using a segmented screw extruder.
The composite performance showed an acceptable level of self
regulation without the temperature exceeding 40.degree. C.
[0060] There are many possible ways of producing a tube having a
PTC wall material with a pair of conductors running the length of
the tube to have all of the potential pathways through the PTC
material operating in parallel. A number of preferred embodiments
are now described.
[0061] With reference to FIG. 5 a smooth walled tube 140 is shown
by way of a first example. The smooth walled 140 tube has a PTC
plastic material extruded as a narrow and thin ribbon 141 and wound
helically with overlapping edges of adjacent turns. The edges of
adjacent turns bound firmly to one another, fusing together in
their moulten state. A pair of conductors run 142, 143
longitudinally in the tube wall. The conductors are diametrically
opposed; The conductors may be applied to either the internal or
external surfaces of the molten PTC material during forming of the
tube. To apply the conductors to the internal surface the
conductors are applied longitudinally to the forming mandrel prior
to laying the extruded PTC ribbon in place. Alternatively they may
be applied directly to the outside of the PTC material while the
material is still in a molten state. It would be appreciated that
these conductors may also be applied helically rather than in a
straight longitudinal direction, and that multiple conductors may
be used.
[0062] Design of a PTC tube of this type involves selection of a
wall thickness, a conductor gauge and a density of conductors in
the PTC tube wall. The total resistance R (.OMEGA.) of the tube
wall in its pre-threshold state will be a measure of the available
power output for a given voltage. The available power output must
be sufficient to offset the heat lose from the tube to its
surrounding environment and (if the gases are entering the tube in
a cooler state) to the humidified gases. The total resistance is
proportional to the pre-threshold volume resistivity X (.OMEGA. m)
of the material and to the average shortest path distance between
the conductors of opposite plurality. The total resistance is also
proportional to the inverse of the length L.sub.C (m) of the
conductors and to the inverse of the wall thickness t (m) of the
PTC material. Furthermore, typically there will be a pair of
opposite and alternate paths for current to flow from a conductor
of one polarity to the conductor of the other polarity, halving the
total resistance. Thus the total resistance can be found from the
formula: 1 R = X w _ 2 L c t
[0063] where {overscore (w)}(m) is the average shortest length path
between conductors.
[0064] Therefore for a given tube length and diameter the total
cold resistance may be varied by varying the density of conductors
(varying the average shortest path distance between conductors) or
by varying the wall thickness. The density of conductors may be
varied by adding additional conductors in parallel (eg: a second or
more pair of conductors) or by disposing the conductors in a
helical arrangement with decreasing pitch corresponding to an
increased density. For a given tube diameter D (m) and tube length
L.sub.t (m) then the average shortest path length can be found
using the total conductor path length for a single polarity (half
the total conductor length) by: 2 w _ = DL T 2 L c
[0065] The tube of FIG. 5 may be reinforced by applying a spiral
bead, or by applying circumferential ribs to the outside of the
tube, or by corrugating the tube, or by adding additional layers to
the tube, particularly of a spiral ribbed or corrugated
configuration, which would also provide additional external
insulation from the ambient conditions.
[0066] A further construction is illustrated in FIGS. 6 and 7. FIG.
6 shows a pair of conductors 145, 146 extruded into a ribbon of PTC
material. The first conductor 145 is disposed adjacent one edge of
the PTC ribbon 147. The second conductor 146 is disposed adjacent
the centre of the PTC ribbon 147. The exact location of the
conductors within the PTC material is not critical, however the
spacing between the conductors should be half of the pitch of
winding the ribbon on to the former (eg: (width of ribbon--width of
overlap between turns).div.2). For additional conductor density,
additional pairs of conductors may be used. For lower conductor
density the width of ribbon may be increased or alternatively a
single conductor may be provided in the ribbon but two ribbons may
be extruded and wound on to the former as a double helix.
[0067] Referring to FIG. 7 a manufacturing configuration is shown
(without the rotating former, which may for example be a spiral
pipeline mandrel available from OLMAS SRL of Italy). In this
manufacturing configuration the PTC ribbon 147 is co-extruded with
the embedded pair of conductors 145, 146 by a first extruder head
148. It is extruded directly on the former at a angle corresponding
to the pitch of the former (the relationship between the advance
and rotation speeds of tubes formed on it). The ribbon 147 is laid
on the former so that the leading edge 149 of each new lap overlaps
the trailing edge 150 of the immediately preceding turn. A
reinforcing bead 162 is preferably extruded on to this overlap by
an additional extruder head 161. The reinforcing bead 162 assists
the bonding between overlapping turns of the ribbon as well as
providing reinforcing against crushing of the formed tube.
[0068] Alternatively a conduit may be formed on a spiral pipeline
mandrel with the reinforcing bead extruded to lie between the
overlap of turns of the ribbon. This is particularly suited to
where the ribbon is preformed and will not bond to itself without
assistance. In this case contact may be provided between adjacent
turns of the PTC ribbon along either side of the bead (for example
by extended overlap) or the ribbon used may be have a conductor
along each edge (as in FIG. 2).
[0069] FIG. 8 shows a further construction of a tube incorporating
a parallel wire configuration The tube 158 is a flexible PTC
material; which has two conductors built in,, it.
[0070] The tube 158 according to this construction may be a
directly extruded tube with the conductors co-extruded into the
tube wall, or alternatively the conductors may be added subsequent
to forming the tube by direct application to the exterior of the
tube as wires or as conductive ink.
[0071] The tube may have an outer layer (not shown) which provides
electrical insulation and thermal insulation to the tube.
[0072] The tube may be corrugated by passing through a set of
corrugating rollers, to provide flexibility and lateral reinforcing
against crushing.
[0073] The PTC design could also be extended to incorporate PTC
heaters in other parts of the patient breathing circuit, such as
the flexible extension tube which is usually connected between the
Y-piece (port 17 of FIG. 1) and the patient's endotracheal tube. A
further extension of the PTC tube concept would be into a
self-heated and temperature controlled endotracheal tube.
[0074] The tube with PTC wall material allows a humidifier to be
used without any sensor at the patient airway. FIG. 9 shows a
humidifier configuration using a PTC tube according to the
embodiments of FIGS. 5 to 8. Gas enters humidification chamber 152
via inlet port 151 and is humidified by water 153, heated by heater
plate 154. An absolute humidity sensor 155 controls the heater
plate so that the gas passing sensor 155 is at a desired level of
absolute humidity. PTC tube 156 is heated by an external voltage
(not shown) so that the internal surface temperature is at a
constant desired temperature, which is selected to be above the
dewpoint of the gas. The gas which leaves tube 156 at outlet 157
will therefore be near the temperature of the tube, and containing
the desired level of absolute humidity which was controlled by
absolute humidity sensor 155.
* * * * *