Note: Descriptions are shown in the official language in which they were submitted.
~10~64 File 913,698
EVAPORATIVE HUMIDIFIER
B~ und of the Invention
It is well known in the art that humidifiers
are used with respiratory equipment to both warm and
humidify the breathing gas provided to a patient. The
breathing gas can be oxygen, air, anesthetic gas or
mixtures thereof.
Frequently, the breathing gas is composed of a
mixture of air and augmenting oxygen. It has been found
to be much simpler to have the breathing apparatus control
the volume of only one of the gases and have the correct
ratio of the other gas provided by aspiration through a
venturi aspirator. In order for this mixing technique
to operate correctly, the back pressure in the system
must be minimized. Many humidifiers provide excessive
back pressure so that their use in conjunction with a
venturi-type oxygen mixing aspirator is impractical.
Another area of concern in humidifier design is
to provide equipment which can be sterilized and/or
incorporate elements which are replaced after usage to
prevent transfer of contagion between patients. Conditions
within a respiratory circuit are conducive to very rapid
growth of pathogenic microorganisms which can be carried
into the circuit during the patient's exhalation cycle
and thrive and multiply within the warm, moist atmosphere.
If the assembly is not thoroughly sterilized before reuse,
contagion can be easily blown directly into the next
patient who may be in weakened condition and thus ill
equipped to cope with same.
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Still another area of concern in the design
of respiratory humidifiers is to provide a unit
which will provide adequate heating and humidifica-
tion for a wide range of gas flows from the very low
flow required for infants to the highest flow required
by adults. The humidifier unit should also be capable
of supplying almost instant heating and humidification
so that it is ready for instant use in an emergency
without the need of a warm-up period and so that
steady state can be reached quickly minimizing
temperature variations resulting from an extended
warm-up period which can vary from 10 minutes to as
long as S0 minutes in presently available humidifiers.
Respiratory systems frequently incorporate an
artificial breathing apparatus which generates pulses
of breathing gas to the patient through flexible
connective tubing. The breathing apparatus can
provide either pulses of a given pressure or of a
given volume. The apparatus can be set to provide
pulses which are adjusted to fit the particular
patient, whether juvenile with small lung capacity
or adult with large lung capacity. Normally the
breathing apparatus is located some distance away
from the patient so that the interconnecting tubing
comprises a considerable volume which makes accurate
control of the pulses difficult. The humidifier is
normally located intermediate along the inter-
connecting delivery tubing so that the internal volume
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of the humidifier also contributes more or less
significantly to the interconnection volume. Accord-
ingly, it would be advantageous to make the internal
volume of the humidifier as small as possible.
When the humidifier is used in conjunction with
an artificial breathing apparatus it will obviously
have pulses of breathing gas passing through it. These
pulses of gas have an instantaneous flow rate which
starts at zero at the beginning of the pulse, increases
to a maximum value, and decreases again to zero at the
end of the pulse. This maximum flow rate is termed the
"peak inspiratory flow rate."
~ humidifier can typically accommodate peak
inspiratory flow rates that exceed its maximum continuous
flow rate since the humidifier pre-heats the volume of
gas residing within the humidification chamber between
pulses in addition to heating the pulsed gas as it
passes through the humidifier. The "dead" or resident
gas will thus be heated to a higher temperature than
the "pulsed" gas and will be mixed in the humidification
chamber, the mixed humidified gases at the humidifier
outlet being generally of uniform temperature and
pulsatile.
Medical humidifiers generally fall into three
types of classifications: (1) Nebulizers or droplet
spray types, (2) Bubbler types, and (3) Evaporative or
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steam types. Other types or variations have been proposed
from time to time but the three categories which will be
discussed arethose which are most widely used and
accepted at the present time.
(1~ NEBULIZERS
Nebulizers are devices which are used fDr two
different but sometimes combined purposes. In its
simplest form, liquid is drawn from a reservoir through
a tube terminating in a nozzle at whose exit port the
stream is broken into fine droplets by a passing gas flow
in much the same manner as the well-known atomizers used
to spray many common household liquid products.
Nebulizers are useful for delivering a fine fog of liquid
medicament to a patient along with the respiratory gas
stream. The droplet particle size somewhat determines
whether the fog will be deposited within the upper or
lower respiratory tract. When used for humidification
rather than for inhalation therapy, some nebulizer
designs incorporate baffles to separate larger droplets,
incorporate heating, or use other means in an attempt to
remove or evaporate the droplets of water. None of the
methods used to date has been entirely satisfactory in
completely removing water droplets from the gas stream.
Not only can excessive droplets have a traumatic effect
upon a weakened patient, but the droplets also provide a
vehicle for transport of microorganisms into the patient's
respiratory system. Such microorganisms are not readily
transported by humid gas in the absence of water droplets.
It is also difficult to provide warming of respiratory
air to a physiologically acceptable temperature in an
atomization process.
~2) BUBBLER HUMIDIFIERS
These humidifiers usually function by blowing
the air through a tube and allowing it to exit near the
bottom of a water reservoir and then bubble up through
; the water. Since the normal ascension of bubbles through
- a short water distance would not provide sufficient
humidification and/or heating of the gas bubbles, these
units are usually equipped ~lith baffles, porous packing
or the like to break up the bubbles into smaller sizes
and to slow their travel. The water can be heated in a
bubbler humidifier. Probably the greatest drawback of
bubbler humidifiers is the pressure required to force
the air down to the bottom of the liquid reservoir, such
pressure ~rop effectively precluding the use of oxygen/
gas Venturi mixers.
(3) EVAPORATIVE HUMIDIFIER_
This type of humidifier is the most recent in
design and is rapidly gaining in acceptance because of
its advantages over the two previously mentioned types
of humidifiers.
In their simplest form, evaporative humidifiers
allow the gas to pick up humidity through passage of the
gas over a wetted surface. Ef~iciency, as measured by
humidity increase, can be increased by increasing the area
of wetted contact either in terms of two-dimensional area
or by increasing surface porDsity or texture. It can be
further enhanced by heating the water, directly or
indirectly, or by heating the evaporative surface, or by
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heating the air. Further increase in evaporation can be
obtained through increasing the air velocity or by other
means of increasiny turbulence at the interface. Heating
of the air by the evaporative surface can be accomplished
by super-saturation of the air thereby warming the air
but which, however, also can result in fog formation.
Evaporative humidifiers provide advantages over the
other types of humidifiers by providing higher humidity
per unit volurne without producing water droplets.
Summary of the Invention
The present invention comprises an evaporative
humidifier with a humidification chamber of about 200 cc.
internal volume having an inlet connection port and an
exit connection port for gases, a liquid reservoir and
a removable porous evaporative element of open-ended
cylindrical design which fits loosely within the humidi-
fication chamber when dry but which swells into good
thermal contact with the side walls of the chamber when
wet and which extends into the liquid reservoir. The
chamber is tightly surrounded by a heating element so
that heat is transferred directly to the chamber walls
adiacent to the porous removable evaporative element and
not to the liquid reservoir. The heating element is
preferentially equipped with suitable electronic controls
to monitor the heat transfer surface temperature, and
to shut the unit off when said temperature exceeds a
predetermined level. The humidifier is designed so that
the gases reaching the patient will be at 100% relative
humidity under most conditions of gas flow and heater
temperature settings.
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Brief Description of the Drawings
In the accompanying diagrammatic drawings which
illustrate the invention:
FIGURE 1 is a top plan view, partly in section,
of the humidifier of the present invention; and
: FIGURE 2 is a sectional view of the humidifier
of the present invention.
Detailed Description of the Invention
Referring now more particularly to the drawings,
humidifier 10 comprises a generally cylindrical housing
11 including a central humidification chamber 20 having
an internal volume of about 200 cc., a removable air flow
cap assembly 15 to which appropriate tubing (not shown)
would be assembled, and a removable bottom cap assembly
24. Air flow cap assembly 15 is frictionally and
mechanically assembled to the top of housing 11 by a
bayonet fastener with a 0-ring 18 providing a leak-proof
seal. Air inflow connector 16 and air outflow connector
17, which are sized so that the customarily used
corrugated respiration tubing can be quickly and easily
connected thereon, are integrally formed as part of cap
assembly 15 as shown in Figure 1. Depending from cap
assembly 15 is an air flow directing tube 19, which
serves to direct the inflowing air from air inflow
~5 connector 16 into the interior of humidifier 10 such
that the inflowing air must flow upwardly along most
of the length of the humidification chamber 20 and
the evaporative eiemen~ 35. Water inlet connector 21
is also integrally formed as a part of cap assembly 15
and a water supply tube 22 depends from water inlet
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connector 21. Air flow cap assembly 15 may be provided
with an exit gas temperature sensing element (not shown)
such as a thermistor which would monitor the temperature
of the exiting gas and shut off the heating element when
the temperature exceeds a pre-set level. Air flow cap
assembly 15 which is removable from the housing ll of the
humidifier 10 facilitates cleaning and/or sterilization
of the assembly. Alternatively, airflow cap assembly 15
could be formed as an integral part of the humidifier
housing ll, with only the bottom cap 24 being removable.
Cap assembly 15 is preferably molded of 20% glass filled
polypropylene, although it could be fabricated from
die-cast metal or other polymeric materials such as
Nylon, Epoxy, etc. However, it is preferred that the
material utilized be capable of withstanding repeated
; steam sterilization cycles. Less durable materials should
be gas sterilizable.
The air flow directing tube l9 has a cross-
sectional area comparable in size to that of the air
inflow connector 16 so that excessive pressure drop is
not experienced. Pressure drop in the humidifier of the
present invention is very small being on the order of
about 0.1 cm. of water at a continuous flow rate of 30
liters per minute. For similar reasons, the space be-
tween said air flow directing tube l9 and the surrounding
evaporative element 35 should be of comparable cross-
sectional area. The air flow directing tube 19 is
shown extending close to the bottom of the humidification
chamber ~0 so that the air must travel upwardly along
most of the length of the evaporative element 35.
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Efficiency would be reduced only slightly if the tube
were shortened and compensatory means could be accom-
plished through shaping the tube exit into a slightly
restricted nozzle or into a shape which would provide
a circular or cyclonic gas discharge with only a slight
additional pressure drop. Alternatively, the gas flow
could be reversed, i.e., flowing in from the right of
Figure 1, downwardly around the air flow directing tube
19, up through the air flow directing tube 19, and exit
to the left. Still other air flow paths are considered
within the scope of the present invention such as air
inflow or air outflow at the top or bottom of the
humidifier 10.
Water inlet connector 21, shown at the top
of humidifier 10 in Figure 1, is adapted for connec-
tion to a supply tube (not shown) from a source of
distilled water supported at a higher location for
gravity flow. Water supply tube 22 is constructed
of 0.125 inch O.D., 0.069 inch I.D. type "316"
stainless steel tubing. For better flow restriction,
a valve tip 23 is provided at the bottom of the water
supply tube 22 with a restricted opening of 0.020 inch
I.D. It is also to be considered within the scope
of the invention to provide an enlarged bottom water
reservoir which can allow the unit to function for a
prolonged single usage period without the need for an
external water supply connection, provided that such
water reservoir is so located that the water in the
reservoir will not be in contact with heater tube 31.
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In the preferred embodiment shown,bottom cap 24
functions as the liquid reservoir A central upstanding
guide stem 25 integrally formed in bottom cap 24 serves
to position the float 26~ while allowing the float to
freely move upward or downward with the level of the
liquid in the reservoir. The float 26 has a valve seat
29 which shuts off the flow of water from the valve tip
23 when the water level in the reservoir reaches a pre-
determined level.
The bottom cap 24 is preferably molded of clear,
unfilled polycarbonate so that the presence or absence
of water in the reservoir can be visually observed. The
cap is threaded into engagement against a sealing 0-ring
3Q so that it can he easily removed or replaced without
the need for tools and so that it will provide leak-proof
engagement. Of course, the bottom cap 24 can be molded
as an integral part of the housing 11 in which event the
air flow cap 15 would be removable for cleaning of the
humidifier 10 and replacement of the evaporative element
35. This would be the preferred construction in the
enlarged water reservoir single use embodiment.
The float assembly 26 is normally made up of
two separate clear polycarbonate elements, a top 27 and
a body 28, bonded together in leak-proof fashion by, for
example, ultrasonic welding Of course, an adhesive may
be used to bond the two elements together. The float
top 27 includes a valve seat 29 of 0.06 inch thick
silicone rubber typically retained in place by a
suitable adhesive.
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Housing 11 is also formed of 20% glass filled
polypropylene and carries a removable heater tube 31
constructed of 1.875 inch O.D. aluminum tubing. Wrapped
around the exterior of the heater tube 31 and preferably
vulcanized thereto is a water-proof 200 watt, 120 volt
electric heater element 32 in the form of a blanket
embedded in silicone rubber. Covering the heater blanket
32 is a thermal insulating layer 33 which is preferably
a ltl6 inch thick silicone sponge rubber layer. Also
attached to the heater blanket 32 is at least one
temperature sensing element such as a thermistor 34~
Housing 11 is constructed in two parts which are
fitted together as by a plurality of latching fingers
spaced around and depending from the periphery of annular
top section 12. 0-ring 14 provides a seal between said
annular top section 12 and body member 13. Heater
blanket 32 and heater tube 31 are permanently mounted
and "sealed" in housing 11. An 0-ring 14a provides a
substantially leak-proof seal around the bottom periphery
of tube 31. Alternatively, the entire outer cavity could
be filled with thermal insulation, e.g. foamed in placed
insulation.
Humidifier 10 of the present invention is
intended to be connected to a separate unit containing
appropriate electronic controls and electrical power
supply connections. It is, of course, contemplated that
the electronic controls could be sealed within the
housing 11, in waterproof fashion so that the only
external connection accessory necessary would be the
electrical supply cord. The electronic controls, of
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conventional design, perform the necessary functions to
turn the humidifier "on" and "off", monitor and control
the temperature of the heat transfer surface of heater
tube 31, and the temperature of the gases reaching the
patient. The heat transfer surface temperature of heater
tube 31 is sensed by the thermistor 34 embedded in the
heating blanket 32. This information is fed into a
thermostatic control circuit in the electronic controls
which maintains the temperature constant at the selected
setting. The range of heat transfer surface temperatures
which may be selected is from about 40 to 100C.
In the event that the heat transfer surface
temperature exceeds a certain pre-determined temperature
(say 127C. or 139C.) a "thermal cut-out" device (not
shown) provided in the heater blanket 32 will melt and
open the heater circuit to shut off the heating element
32. The primary (and possibly the only) situation where
this would occur would be failure of the thermostatic
control circuit in such a way that the heating element
would be turned fully on. The thermal cut-out device
will thus protect the humidifier housing 11 from melting
or burning.
It has been empirically determined that the
evaporative element or wick 35 should be disposable so
that it can be removed, discarded and replaced after
each usage thereby removing a potential source of
contamination. Since cost of a disposable item is an
important commercial consideration, wick 35 should be
fabricated from relatively inexpensive materials.
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With the present evaporative concept whereby the
evaporativeelement or wick 35 is in a direct short-path
contact with the heat transfer surface of heater tube 31,
the evaporation rate is much enhanced and the evaporation
rate per unit area is quite high allowing use of a very
small evaporative surface. With this high rate of
evaporation per unit area, it is necessary that the
wicking rate be likewise high so that the rapidly
evaporated water is quickly replaced. Unfortunately
materials having good wet-strength characteristics
usually have reduced wicking rates because the water-
resistant reinforcing materials impart reduced hydro-
philicity to the reinforced structure.
Unexpectedly, it has been found that when
certain types of paper are used, they tend to swell
greatly when wetted thereby increasing their outside
diameter. Paper is, of course, a desirable material due
to its low cost and because its natural hydrophilicity
contributes to good wicking action. In order to take
full advantage of this phenomenon, it is imperative that
both the wick 35 and heat transfer surface of heater tube
31 be either cylindrical or frustroconical and that the
wick fit inside of the heat transfer surface so that
wick 35 can be undersized when dry and swell into good
intimate thermal contact when wet.
The preferred form of wick 35 comprises a
laminate of two layers of absorbent materials, the inner
layer having good stiffness and dimensional integrity,
especially when wet~ and the outermost layer having good
wicking and swelling properties. In this embodiment,
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wick 35 comprises an outer layer of Scott Hi-Loft 3055
non-w~ven paper, 55 pounds basis weight, purchased from
Scott Paper Company, Chester, Pennsylvania, and an inner
layer of Jamer River - Pepperell, Inc., P 2212, Grade
06101A, Bleached Absorbent Wet Strength Kraft paper
having a basis weight of about 95 pounds. Lamination
can be accomplished by adhesive bonding, heat sealing,
needle-looming, etc., but must not result in a complete
moisture barrier between the two layers of the laminate
since the outer high1y absorbent layer must be able to
transfer water hori~ontally to the inner layer so that
the relatively slower wicking inner layer need not depend
wholly on its own vertical wicking ability to remain
wet as water is evaporated from it. The horizontal
transfer of water is one of the primary purposes of the
highly absorbent outer layer. To prevent formation of a
complete moisture barrier between layers, any adhesive
or heat sealing material is applied in the form of thin
strips or dots.
After the materials are bonded together, the
resultant laminate is formed on a cylindrical mandrel or
folded, with the edges slightly overlapped or butted
together and sealed by taping, heat sealing, sewing, or
other appropriate means. Wicks 35 are then cut to the
appropriate length and flattened (if formed on the
cylindrical mandrel) for packaging, shipping and storage.
Providing the wick 35 in a flat configuration rather than
as a cylinder reduces the required shipping and storage
space by as much as about 80%.
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The cylindrical shape reguired for normal
operation of the wick 35 is quickly restored by pressing
the wick in opposite directions along the opposing creases
and inserting it into the heater tube 31. The stiff
nature of the inner layer of material allows the wick 35
to be easily "popped" open into a generally cylindrical
shape.
When the wick 35 manufactured by the above
techniques is restored to cylindrical shape, the outside
diameter thereof will vary somewhat from wick to wick.
This variation has not presented a problem because the
wet swelling property of the wick provides for good
thermal contact in spite of this outside diameter size
variation. The absorbency of this laminate is such as
to provide wicking of water in excess of 2 inches in
height within 60 seconds when the bottom of the wick is
placed in a shallow supply of water. The degree of
. swell when wetted is enough to provide a 1-3% increase
in outside diameter.
Another form of wick 35 comprises a laminate
of at least three layers of absorbent material, the
innermost layer having good wet-strength properties and
one of the outer layers having good wicking and swelling
properties. In this embodiment, wick 35 comprises a
composite outer layer of two sheets of 55 pounds basis
weight Scott-Hi-Loft 3055 nonwoven paper and the two
inner layers of Monadnock 1309-030 paper, a high wet
strength paper having a basis weight of about 30 pounds.
The three separate layers are laminated and lightly
adhesively bonded together by wrapping each in turn at
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an angle around a forming cylinder to create a tightly
formed "core" and cut to the desired lengths, similar
to the process used in producing cores for toilet-tissue
rolls. It will be appreciated that this method of
manufacture produces a cylinder having an accurately
dimensioned inside diameter but with variable outside
diameters. Again, this does not present a problem because
the wet swelling property of the wick provides for good
thermal contact in spite of the outside diameter size
variation. The absorbency of this laminate is such as
to provide wicking in excess of 3 inches in height
within 60 seconds when the bottom of the wick is p~aced
in a shallow supply of water. The degree of swell of
this wick when wetted is sufficient to provide 2-3%
increase in outside diameter.
Another exemplary wick was made from paper
manufactured with 100~ Nersanier XJ pulp. Flat sheets
of this paper, 5-3/8 inches x 4-1/2 inches x 0.040 inches
thickness, were rolled into open-ended cylinders 4-1/2
inches in length and approximately 1.75 inches in O.D.
The cylinders were closed by heat-sealing a tape of
polyester-polypropylene laminate film along the seam.
The wet strength of this construction was marginal so
that care had to be taken in removing a wet wick from the
humidifier without tearing the wick. The paper swelled
by about 85% in thickness when wetted resulting in an
increase of outside diameter of about 0.035 inch, which
provided good wet contact with the inner surface of
heater tube 31. The wicks had a 2 inch wicking speed of
about 2 minutes which was only fair but acceptable.
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Other wicks were made by spirally winding four
layers of N-2380 nonwoven webs, obtained from C. H.
Dexter Division of Dexter Corporation, onto a mandrel
to form a laminate. Portions of the web were lightly
coated with adhesive to bond the laminate together. Each
layer was about 0.10 inch thick. This wick swelled by
about 33Y in thickness when wetted resulting in an
outside diameter increase of about 0.015 inch. The
wetted wicks could be easily removed from the humidifier
without tearing. The finished wick had a wicking rate of
about 3 inches in 2 minutes which was more than adequate.
~ The following is a brief description of the
; operation of the humidifier of the present invention.
Before commencing use, the humidifier 10 is disassembled
and the various components thereof sterilized or otherwise
sanitized depending upon the hospital practice and/or
the state of health of the patient upon which it is to
be used. A new wick 35 is installed in heater tube 31
during reassembly of the humidifier, either before or after
sterilization, the electronic controls connected and a
water supply tubing from a source of sterile water con-
nected to water inlet connector 21. Finally, appropriate
respiratory tubing to the patient and to the respirator
apparatus is connected to the air outflow connector 17 and
the air inflow connector 16 of the humidifier 10, re-
spectively. With the electrical supply cord connected to
a suitable power source, the electronic controls are set
and turned "on" -- humidification begins almost
immediately and 100% relative humidity of the exit gas
is achieved within one or two minutes.
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The temperature sensing thermistor 34 attached
to the heating element 32 allows the electronic controller
to cycle the electrical input to the heating element 32
"off" and 'lon" over short time intervals to control the
temperature of said heater tube 31 within narrow limits
which in turn controls the temperature of the saturated
gases reaching the patient to within 1C. The control
setting is adjusted by the operator according to the
temperature of the saturated air reaching the patient.
The relationship between setting and air temperature at
the patient will depend upon the volumetric throughput of
the system, a variable which changes from patient to
patient.
The humidifier of the present invention normally
provides respiratory gases to the patient at 100%
relative humidity at a temperature of 25 to 40C. at a
continuous flow rate of from 1 to 60 liters per minute
and a peak inspiratory flow rate of up to 100 liters per
- minute. Normally, because of the high evaporative rate,
the surt`ace of heater tube 31 will be at a fairly low
temperature e.g., 160F. compared to the higher
temperatures found in other known humidifiers. However,
if for any reason the temperature selected by the
operator is too high or due to a failure in the thermo-
static control circuit, the temperature of the air
reaching the patient would rise, in which case the exit
gas sensing element or the remote temperature sensor
near the patient would shut the humidifier "off" and
activate an audible and visual alarm. Shut-off of the
humidifier normally occurs when the gas temperature
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reaching the patient exceeds 41C. As a further safe-
guard, excessive temperature rise of the heat transfer
surface of heater tube 31 causes the thermal cut-out
device to shut off the humidifier power supply.
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