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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to apparatus for warming fluids for medical
applications, and more particularly to apparatus for immersion in a fluid
to be warmed that is selectively powered and sensed to assure fail-safe
operation in and out of the fluid to be warmed.
Many medical procedures require warming fluids in a sterile environment at
elevated temperatures that are closely regulated within a narrow range of
temperatures. In addition, inexpensive, pre-sterilized, disposable
equipment greatly facilitates preparation and clean-up procedures
associates with a surgical procedure, and promotes the preservation of
sterile conditions from patient to patient.
Electrically-controlled heaters for warming liquids or gases associated
with surgical procedures have attained wide acceptance but commonly must
be sterilized prior to re-use. Disposable heater units obviate the need
for re-sterilization between uses, but inexpensive, single-application
heaters to date have not included sufficiently high quality workmanship,
materials and designs to assure reliable operation throughout an entire
surgical operation. For example, safety features such as thermal cutoff
switches typically do not operate satisfactorily if only a portion of an
immersed heater remains in contact with a liquid to be warmed. Also, mass
production techniques commonly associated with inexpensive, disposable
heaters typically are incapable of maintaining close tolerances of
electrical parameters to assure repeatable performance from a population
of heaters operated in a given surgical application. Such variations in
electrical parameters usually contribute to difficulties in controlling
the operating temperature within close tolerances.
SUMMARY OF THE INVENTION
In accordance with the present invention, inexpensive, immersible
electrical heaters are mass produced with relatively low-tolerance
resistance parameters including heater conductors formed of materials that
exhibit positive or negative temperature coefficients of resistance. Added
costs and complexities of using thermistors as heat sensors in a power
controller are eliminated by using the heater conductors to sense the
operating temperature. The resistance of the heater conductors is sensed
during intervals of no applied power as an indication of the operating
temperature in a manner that is suitable for servo controlling the
supplied power. The heaters may be individually tested to determine
resistance at a selected testing temperature, and may be coded with the
test value of resistance in a manner that can be sensed by an electrical
power controller to accurately and repeatably power each heater in a
population of heaters to selected operating temperatures within close
tolerances despite wide variations in resistance values from heater to
heater in the population of heaters. In addition, the coding scheme on
each heater facilitates automatic programming of numerous selected
operating parameters of the heaters and controller. Alternatively, a
thermal sensor such as a thermistor may be incorporated into the
immersible electrical heater to provide electrical indication of the
operating temperature of the heater, and a thermally-sensitive fuse
element may be distributed about the heater to disconnect the heater
element from a source of applied electrical power in response to a portion
of the heater element not submerged in a liquid tending to operate at
excessive temperatures.
DESCRIPTION OF THE DRAWINGS
FIG. 1(a) is a plan view of the heater apparatus according to one
embodiment of the present invention;
FIG. 1(b) is a plan view of the heater apparatus according to another
embodiment of the present invention;
FIG. 2 is a perspective drawing of the heater apparatus according to FIG. 1
positioned within protective frames;
FIG. 3(a) is a schematic diagram of one embodiment of a power controller
for the heater apparatus of FIGS. 1 or 2;
FIG. 3(b) is a graph illustrating the operation of the circuit of FIG. 3(a)
during half cycles of power line signal;
FIG. 4 is a perspective view of one embodiment of connector apparatus for
encoding information about the heater apparatus and for detecting the
encoded information to affect operation of the controller in powering the
heater apparatus of FIGS. 1 or 2;
FIG. 5 is a sectional view of one embodiment of a gas heating unit
according to the present invention;
FIG. 6 is a sectional view of one embodiment of a liquid heating unit
according to the present invention;
FIG. 7 is a sectional view of a heater packet for applying heat, for
example, to a selected region of a patient.
FIGS. 8(a) and 8(b) are sectional views, respectively, of single-sided and
double-sided heater cassettes according to another embodiment of the
present invention;
FIG. 9 is a plan view of unfolded upper and lower flow channels of a
double-sided heater cassette according to the embodiment of FIGS. 8(a) and
8(b);
FIG. 10 is a partial perspective view of the heater cassette according to
the illustrated embodiment of FIGS. 8(a) and 8(b); and
FIG. 11 is a plan view of a typical heater foil for disposition between
upper and lower flow channels in the illustrated embodiment FIGS. 8(a) and
8(b).
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1(a), there is shown a plan view of an immersible
heater according to one embodiment of the present invention. The heater
comprises a supporting layer 9 of a biocompatible, electrically-insulating
material such as polyvinyl chloride or silicone rubber, generally shaped
as a flat disk with a round or other suitable peripheral shape which may
include an integrally formed elongated tab 11. Heater element 13 is formed
theron of a material having positive (or negative) temperature coefficient
of resistance such as, for example, nickel-chromium alloy, or the like, in
generally serpentine, continuous pattern between connections or terminals
15, 17. The terminals 15, 17 are connected via a thermal fuse 19 and the
conductors 21, 23 which form a cable, or which may be disposed along the
elongated tabs 11, to a connector 25 for detachable connection to a
controlled source 12 of electrical power that regulates the power supplied
to the heater 13 in the manner as later described herein. A
thermally-sensitive detector 29 such as a thermistor or thermocouple is
also disposed on the disk 9 to provide an electrical indication of the
operating temperature of the heater 13. An encapsulating layer 35 of
similar biocompatible material is then formed over the supporting layer 9
and fuse 19 and detector 29 to seal in the heater 13 and all associated
conductors and terminals against contact with the ambient environment
during sterilization or use in warming fluids. The entire sealed structure
is ideally relatively flexible and may include one or more apertures 37
therethrough to promote liquid circulation around and through the
structure. The detector 29 may be located in proximity to one of the
apertures 37 to provide response that is more accurately representative of
liquid passing through aperture 37. Flexibility of the sealed structure
facilitates conforming of the heater to the shape of a confining vessel,
and may facilitate extension of the elongated tab 11 (or connecting cable
11, as illustrated in FIG. 2) out over the rim of a vessel that contains a
volume of liquid which is to be warmed by electrical power dissipated in
the heater 13.
Referring now to FIG. 1(b), there is shown a plan view of another
embodiment of the heater apparatus of the present invention. In this
embodiment, the heater includes a supporting layer 10 of a biocompatible,
electrically-insulating layer of material such as polyvinyl chloride or
silicone rubber, or the like, generally shaped as a flat disk with a
hexagonal or other suitable peripheral shape. Hexagonal or similar
non-round peripheral shape promotes circulation of warmed liquid from
below the apparatus within a typically round confirming vessel. Heater
element 14 is formed on the layer 10 of a material having a positive or
negative temperature coefficient of resistance such as, for example,
nickel-chronium alloy, aluminum, or the like, in generally serpentine,
continuous pattern within half sectors between terminals 16, 18. A
thermally-sensitive trace 26 of the heater element 14 may be disposed
substantially about the periphery of layer 10 and may be serially
connected with the heater conductors 14 in the half sectors to form a
continuous conductor between terminals 16, 18. The fuse formed by trace 26
may thus respond to excessive heating under operating conditions
associated with a portion of the apparatus, usually including a peripheral
portion, not being immersed in liquid being warmed. In these operating
conditions, electrical power supplied to the heater element 14 would heat
the portions of the element 14 not being cooled by liquid to sufficiently
elevated temperatures to decompose the materials of the present apparatus
and thereby contaminate the liquid being heated, and increase the risk of
fire in close proximity with surgical drapes and other flammable
materials. The peripheral trace 26 may, for example, be fabricated of
aluminum that is conventionally sputtered onto the layer 10, or that is
etched from aluminum foil attached to the layer 10, or the like, to
provide a fuse element that is distributed over an area of the heater
apparatus that preferably includes the areas adjacent the periphery. The
heater element 14 may be formed, for example, at the same time of the same
material as the peripheral trace 26, with the peripheral trace 26 having a
smaller cross sectional area, for example, by narrower line width and same
thickness to promote higher current density of electrical signal conducted
by the peripheral trace 26 than by the heater conductors 14 disposed
elsewhere over the surface of the layer 10. In this manner, higher
probability of conductor failure, or burn-out, is predicted in the
peripheral trace 26 than in the heater conductor 14 elsewhere on the
surface of the layer 10, particularly under operating conditions of a
portion of the heater apparatus, usually including some peripheral
portion, not being immersed in liquid to be heated. Thus, the peripheral
trace 26 forms a distributed fuse serially connected with the heater
conductor 14 between terminals 16, 18. The central region of the layer 10
may be removed to provide an aperture therethrough to facilitate passage
of tubes through the heater apparatus, or to provide the circulation
therethrough of liquid to be heated. A thermal detector 22 such as a
thermistor or thermocouple may be mounted on the layer 10 near the central
region thereof to provide an electrical indication of the operating
temperature for power controlling purposes, and a cable of multiple
conductors is electrically connected to the detector 22 and terminals 14,
16 for controlling and powering the heater apparatus from a source 12 of
electrical power. A second layer of biocompatible material (not shown to
preserve clarity) is then disposed over the heater element 14 and
distributed fuse 26 and thermal detector 22 and sealed to the first layer
and to the cable in conventional manner to encapsulate the heater
apparatus and associated terminals and connections against contact with
the ambient environment during sterilization or use in warming fluids.
Referring now to FIG. 2 there is shown a perspective view of the heater
according to FIG. 2 disposed between a pair of cages or frames 41 that are
oriented in spaced relationship to the upper and lower surfaces of a
heater 9 or 10. These cages or frames 41 are mounted in the spaced
relationship from the heater 9 or 10 by spacers 20 positioned near the
perimeter of the heater 9 or 10, and an additional set of `feet` or
spacers 43 may be oriented about the perimeter of the lower cage or frame
41 to support the assembly above a supporting surface (say, the bottom of
a basin). The connecting cable 11' of conductors 21 and 23 for the heater
element 13 or 14 may be sealed to the heater 9 or 10 and may be
sufficiently flexible to permit the convenient routing thereof over the
rim of a basin.
Referring now to FIG. 3(a), there is shown a schematic diagram of one
embodiment of a power controller according to the present invention which
measures the resistance of the heater 13 or 14 during intervals in which
power is not applied to the heater to assure fail safe operation within
changing operating conditions. The material that forms the heater has a
selected positive or negative temperature coefficient (TC) of resistance
and therefore has a certain range of resistances over an operating range
of temperatures, say, 15.degree. C. to 43.degree. C. for medical
applications, that can be sensed to provide indication (via correlation
through the TC) of the temperature of the heater 13 or 14. The resistance
thus sensed may be supplied to the power controller as temperature
feedback information for closed-loop servo control of applied power, or
for safety shutdown control against thermal runaway. The normal operating
temperatures are less than the damage thresholds of the heater 13 or 14
and surrounding materials, but injury or damage may occur at elevated
temperatures in excess of this temperature range due, for example, to a
portion of the heater apparatus being out of contact with a liquid being
warmed while the heater 13 or 14 is being powered. Thus, in FIG. 3(a),
amplifiers 28 and 30 are connected to operate as a differential amplifier.
The input to amplifier section 28 is the voltage drop across reference
resister 32, and the input to amplifier section 30 is the voltage drop
across the heater 13 or 14. The following amplifier section 34, 36 operate
on the output of amplifier section 28 as a precision limiter to assure
that the resultant output representing current through the heater 13, 14
(as a denominator in the following divider circuit 38) cannot become zero
(or, in practice, less than 0.7 volts positive). The voltage signal from
amplifier section 30 is applied to the divider circuit 38 as the
numerator, and the current signal from amplifier section 28 is applied to
the divider circuit 38 as the denominator. Of course, digital division
under control of a microprocessor according to conventional technology may
also be used in place of divider circuit 38. Voltage comparator 40 and
reference voltage source 42 senses a rise in the voltage applied across
the heater 13 and 14 to produce an output pulse 44 when the heater voltage
exceeds, say 37.5 volts. This output pulse triggers latch 46 that senses
whether the output. 48 of the divider 38 is representative of a heater
resistance above or below a selected value, say 9 ohms, for a heater of
positive TC and initial value that tested at room temperature to be, for
example, about 7.3 ohms. The resistance value represented at the output 48
of the divider is not constant over time, and provides only inaccurate
resistance indication during one-polarity half cycle of the power-line
sine wave and near the zero crossings of the sine wave, as illustrated in
the graph of FIG. 3(b). The output of amplifier 40 thus establishes time
marks of a period within the opposite polarity half of the sine wave
during which the resistance of the heater 13, 14 can be sampled accurately
for comparison with an upper-limit value to control the shut off of
applied power, or to provide servo feedback information for the
proportional or direct control of applied power. If the resistance of the
heater is sensed to be above the upper limit of, say, 9 ohms, then the Q
output of latch 46 goes high to set latch 51. Amplifier 53 senses the Q
output of latch 51 which turns on an alarm indicator 55 and turns off
power transistor 57 which, in turn, releases relay 58 that turns off power
supplied to the heater 13, 14. The upper limit of allowable heater
resistance may be exceeded when, for example, a portion of the heater
apparatus is out of contact with liquid being heated and thereby is
capable of heating to temperatures capable of damaging the materials of
the heater apparatus. After the operating conditions that caused the alarm
condition previously described are corrected, the heater 13 or 14 may
again be powered by pressing the ON button 59 which turns on transistor 61
that re-energizes relay 58 and recycles the power-on reset circuit
including amplifier 63 and latch 51. Suitable modifications or switched
reconfigurations of the described circuitry may be made to detect the
resistance of the heater 13 or 14 only after an extended period of no
applied power to assure that the heater conductor is in substantially
thermal equilibrium with the fluid being heated. Then, the sensed
resistance of the heater 13, 14 provides an indication of the temperature
of the heater fluid rather than of the heater 13, 14 for controlling a
conventional temperature readout, or for further controlling power
thereafter applied to the heater 13, 14. Actual thermal equilibrium may be
predicted promptly, rather than being achieved only after long delay, by
sensing the heater resistance at successive brief intervals to predict the
asymtotic final value (i.e. the equilibrium temperature) from the
exponential variations of resistance in successive intervals. Of course,
other conventional power controller circuits, for example, including
self-balancing bridge circuitry with the heater 13, 14 connected in one
circuit arm of the bridge, may also be used to assure fail safe operation
of the connected heater apparatus during changing operating conditions.
Alternatively, conventional control circuitry, for example, as described
in the aforecited U.S. Patent may be used to control applied electrical
power in response to the electrical indication provided by thermistor 22
or 29, or may be modified in conventional manner to operate on the sensed
resistance of the heater rather than on the thermistor signals, as
disclosed therein.
In accordance with another embodiment of the present invention, a coded
connector or plug, as illustrated in FIG. 4, is attached to the heater
apparatus for retaining coded information about the heater apparatus that
can be detected by a power controller which is capable of being programmed
to operate in accordance with the coded information contained in the plug.
In the illustrated embodiment, the elongated tab 11 or connecting cable
11' of the heater apparatus terminates in an attached plug 66 having a
pair of conductors 65, 67 for the heater 13 or 14, and having two or more
electrical contacts, and having a tab or `tongue` 69 attached to the plug
66 containing coded information. Specifically, the tab 69 includes a flat
region capable of retaining coded information, for example, as bar codes
or as rows and columns of holes 71 at selected locations that can be
sensed by a mating receptacle 73 as the tab 69 and conductors 65, 67 are
inserted into the receptacle. Several rows of 4-bit code may be assembled
on the tab, as shown, or more complex 8-bit coding may be compacted onto
the tab of any convenient length. Thus, the heater apparatus may be mass
produced with low tolerances for resistance values of the heater 13 or 14
that is formed as a wire, or deposited film, or etched foil, or the like,
and that is then tested at, say, room temperature to determine its initial
test-value resistance which is then encoded into the tab 69 via suitable
bar codes or patterns of holes 71, or the like. In addition, the
temperature coefficient of resistance of the heater may be coded into the
tab 69 along with data regarding other operating parameters such as
maximum allowable operating temperature, nominal allowable current and
voltage values to power the heater, and the like. Thus, according to the
method of the present invention, an inexpensive electrical heater for
medical applications of any description (e.g. immersible heaters,
hemostatic thermal scalpels, and the like) may be mass produced to
low-level tolerances, and then tested accurately to establish the
requisite coded information to be encoded on tab 69 regarding its
electrical and operational parameters.
Then, in operation, the plug 66 is inserted into the receptacle to
establish connection between the respective sets of conductors 65-75 and
67-77 in the plug 66 and receptacle 73. In the process of establishing the
connections, the tab 69 enters the slot or receiving port 76 of the
receptacle in alignment with rows (or columns) of optical sensors 78.
These sensors include a light source or sources 79 and individual optical
detectors 81 of conventional design which sense light through apertures 71
(or reflections from bar codes) as the tab 69 is inserted through the slot
or receiving port 76. The coded segment of the tab 69 is preferably
positioned sufficiently forward of the conductors to assure detection of
the code prior to connection of the conductors. In additions, the plug 66
and receptacle may be made asymmetrical, for example, by off-setting the
tab from central orientation, or by dissimilar shapes of conductors 65,
67, or the like, to assure only one orientation of the plug 66 within the
receptacle 73. The detectors 81 are connected to a
microprocessor-controlled power source 74 of conventional design that can
sense the coded information from detectors 81 to set the operating
parameters within which the connected heater will be powered and operated.
Thus, the power source 74 may automatically accomodate a heater for
operation at not more than 22.degree. C. at nominally 1 ampere and 20
volts, and a different heater having a different thermal coefficient of
resistance for a different application may be operable up to 65.degree. C.
at nominally 3 amperes and 35 volts using the same
microprocessor-controlled power source 74. In addition, time of operation,
and numbers of operations, and other such operational data of a designated
heater may all be encoded in the tab 69 using serial number or model
number designation which are then compared with look-up table data in
storage in the memory 80 to assure correct automatic setting of operating
parameters for any resistance unit connected to the power source 74.
Referring now to FIG. 5, there is shown a sectional view of a water-bath
type of heat exchanger including a foil-type heater element 83, as
previously described, disposed within a volume of water 85 that is
confined within a vessel 87. A gas inlet tube 89 (e.g., for CO.sub.2 or
air) terminates below the surface level of the water and diffuses inlet
gas under pressure through the water 85 which has been warmed by the
heater element 83 to provide warmed, moistened gas at the outlet 91 that
exhausts gas above the surface of the water 85. Of course, the vessel 87
may be formed as a sealed, flexible container, for example, of polyvinyl
chloride that may be disposed of after a single use.
Similarly, with reference to FIG. 6, there is shown a cross sectional view
of a water-bath type of heat exchanger including a foil-type heater 91, as
previously described, disposed within a volume of water 93 that is
confined within a vessel 95 to surround a coil of tubing 97 that carries a
gas or liquid therethrough which is to be warmed. Of course, the vessel 95
may be formed as a sealed, flexible container, for example, of polyvinyl
chloride to form a heating unit that may be disposed of after a single
use.
Referring now to FIG. 7, there is shown a sectional view of a heater packet
including a foil-type heater, as illustrated in FIG. 1, immersed in a
thermally-conductive gel or liquid 100 such as silicone, or bentonite or
starch in water, or the like, encapsulated within a flexible container
102, for example, of polyvinyl chloride to form a convenient, controllable
heat pack that may be powered by the controller circuitry of FIG. 3(a) and
then disposes of after a single use. The gel or liquid 100 serves as a
thermal conductor that promotes more uniform heating over the surface of
the packet, and a foam-type insulating layer 103 may be provided on one
face of the packet to moderate the heat available for transfer from the
packet to a patient in a topical heating application, or to protect
attending personnel from contact with surfaces of elevated temperature
while handling the packet.
Referring now to FIGS. 8(a) and 8(b), there are shown sectional views,
respectively, of single-sided and double-sided heater cassettes in
accordance with other embodiments of the present invention. In these
embodiments, a foil-type heater 110 of the type previously described is
disposed adjacent or between fluid-directing flow controllers 112, 114,
116 that are sealed about the foil-type heater 110 to form a serpentine or
labyrinth-like pattern, as shown in FIG. 9, of fluid flow over one or more
surfaces of the heater 110. As illustrated in FIG. 9, the fluid path may
enter the upper segment 114 of the flow contr | | |
