Note: Descriptions are shown in the official language in which they were submitted.
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Temperature Measurement System
This invention relates to a temperature measurement system.
Every year, millions of patients suffer from the effects of unintended
hypothermia. Operating rooms are kept cold and operating table surfaces
mirror the room temperature prior to the patient's arrival. Patients are
uncovered, surgical sites are open and often cold fluids are infused. If
patients undergoing general and major regional anaesthesia are not warmed
during surgery they may suffer from unintended hypothermia. The problems
of unintended hypothermia are especially acute with major abdominal or
orthopaedic surgery as they tend to involve large incisions exposing the
internal organs and patient's tissue to the cold operating theatre.
Hypothermia can happen quickly. Often, the patient begins losing body
temperature during pre-operative preparations and arrives in the operating
room below normothennia. Anaesthetic agents themselves interfere with the
ability of the body to regulate temperature and certain patient types are more
prone to losing body heat than others. As a result, patients are frequently
transferred to recovery in a hypothermic state. In the post anaesthesia care
unit, coldness is one of the most frequent patient complaints. In a survey of
post operative nurses, wannth was cited as the chief comfort concern
followed by pain. Warm blankets are piled on the patient to relieve the
effects
of hypothermia, increasing the amount of time spent in recovery.
Hypothermia has a number of deleterious effects and a relatively small drop
in core temperature can result in major complications or even death. These
deleterious effects are well documented and include:
1. increases in surgical site infections;
2. increased blood loss;
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3. increases in the number of cardiac events (including myocardial ischemia
and ventricular tachycardia); and
4. a prolonged and altered effect of the drugs used during anesthesia.
The LMA PerfecTempTM patient warming system has been designed to aid in
the maintenance of patient nornothermia before, during and after surgery for
both paediatric and adult patients. It is lightweight, durable, and easy to
clean. Since the system does not use a blanket, there is no problem with
surgical access. Additionally it is radio translucent, so it won't interfere
with
X-rays, and it is silent, so it won't distract the physicians.
The LMA PerfecTempTM patient warming system comprises an electrical
heating element sandwiched between foam layers to form a heated mattress
pad. Heat is gently transferred to the patient conductively, the most
efficient
method of heat transfer, with conductive warning temperatures selected by
physicians. The type of foam used enables pressure reduction, which helps
prevent decubitus ulcers (the pressure reducing properties are effective for
patients up to 500 lbs / 225 kg in weight).
Clearly for the LMA PerfecTempTM patient warming system, and other heated
mattress pad patient warning systems (such as mattress pads using water or
gel based heating elements to heat the patient), to work efficiently the
temperature of the mattress pad needs to be monitored to ensure
normothermia is maintained.
The present invention offers a safe and accurate temperature monitoring
system suitable for use with a heated mattress pad.
In accordance with a first aspect of the present invention, there is provided
a
temperature measurement system for use with a heated mattress pad, which
heated mattress pad comprises a heating element, said system comprising at
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least three temperature sensors and at least one converter module, at least
one
of the temperature sensors is adapted to be fixed to the heating element, at
least one of the temperature sensors is adapted to be fixed to the pad and at
least one of the temperature sensors is adapted to be fixed to a patient
located
on the pad. This system allows the temperature of the patient and the pad to
be monitored simultaneously, and ensures the heating element does not
overheat.
All three temperature sensors may be connected to a single converter module,
or multiple converter modules may be used with some temperature sensors
connected to a first converter module and others connected to a second
converter module. For example, the at least one temperature sensor adapted
to be fixed to the heating element and the at least one temperature sensor
adapted to be fixed to the pad may be connected to a first converter module
and the at least one temperature sensor adapted to be fixed to a patient may
be
connected to a second converter module. In embodiments using this
arrangement of temperature sensors and converter modules, the first converter
module may be located within a heated mattress pad whilst the second
converter module may be separate from the heated mattress pad.
In use, the at least one temperature sensor fixed to the heating element acts
as
a safety sensor. In the absence of a patient the two temperature sensors
adapted to be fixed to the pad and adapted to be fixed to the patient do not
provide any temperature feedback and so if the heating element were switched
on it could overheat. However, because at least one temperature sensor is
fixed to the heating element, the temperature of the heating element itself is
monitored and the heating element can be prevented from reaching
excessively high temperatures.
In use, the at least one temperature sensor fixed to the pad will commonly be
located on the outer surface of the mattress pad or on the lower surface of
the
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mattress pad's surface covering, but beneath the mattress pad's surface
covering (which the patient lays on). In the context of the invention being
"adapted to be fixed to the pad" encompasses both being adapted to be fixed
to the pad itself and being adapted to be fixed to the pad's surface covering.
The important feature is that the location of this sensor allows for accurate
measurement of the temperature of the interface between the mattress pad and
the patient. The interface sensor will commonly be located in the area of
greatest weight placement, such as the shoulders area, the buttocks area
and/or
the upper thigh area.
In use, the at least one temperature sensor fixed to the patient will be fixed
to
the patient's skin and will enable the patient's body temperature to be
monitored. In the context of this invention the patient's body temperature
encompasses a patient's core temperature, a patient's skin temperature, the
temperature of a particular organ, and any other temperature that it may be
important to monitor during surgery.
Of course, more than one temperature sensor may be used in each of the
locations, for example two sensors may be fixed to the patient such that the
temperature measurement system comprises four temperature sensors. During
an invasive procedure one temperature sensor may be fixed to the patient's
skin and another temperature sensor may be fixed in an invasive location.
Due to the variability of the weight of a patient on the mattress pad (e.g. 5
kg
child - 200 kg adult) calibration of the "interface sensor" can be difficult.
Therefore, careful monitoring of the temperature of the heating element and
the temperature of the patient are also necessary. However, the "interface
sensor" is important as, in the event that the sensor fixed to the patient is
not
properly located and so is not providing accurate temperature feedback, the
"interface sensor" ensures the pad is not allowed to reach excessive
temperatures that could hann the patient. Hence it is clear that each of the
at
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least three temperature sensors of the temperature measurement system of the
present invention are key to enabling the safe and efficient operation of a
heated mattress pad.
5 Temperature sensors suitable for use in the present invention include
electrical temperature sensors such as thermocouples, thennistors or
resistance thermometers (also known as Resistive Temperature Detectors or
RTDs) which generate electrical signals which are then converted into
temperature readings.
It is also possible to measure temperature using non-electrical techniques,
e.g.
optical temperature measurement techniques. Non-electrical temperature
sensors such as fibre optic sensors may also be used with the present
invention.
Fibre optic sensors are capable of measuring the full temperature range
appropriate to a patient enviromnent (e.g. from 15 C to 50 C). Additionally
fibre optic sensors enable accurate temperature measurement (e.g. 0.5 C)
and sharp temperature resolution (e.g. 0.1 C). Suitable optical fibre
sensors include the Fluotemp range of sensors, available from Photon Control
Inc.
Arrays of optical fibres may be used to construct a single fibre optic sensor.
An array of optical fibres forning a single sensor is distinguished over
multiple sensors each with a single optical fibre by the manner in which the
sensors connect to the converter module. A single connection represents a
single sensor, even if multiple optical fibres' signals are merged prior to
connection with the converter module.
Fibre optic sensors suitable for use in the present invention may comprise at
least one optical fibre, and at least one phosphor material located at a
distal
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end of the at least one optical fibre said at least one phosphor material
being
characterised in that it emits, when excited, detectable optical radiation
that
varies as a known function of the temperature of the at least one phosphor
material. In use, the temperature measurement system may transmit light,
optionally pulses of light, from the proximal end of an optical fibre to the
distal end of the optical fibre. The light returning from the distal end of
the
optical fibre to the proximal end of the optical fibre would include light
emitted by the at least one phosphor material. Detection of the emitted light
and comparison against the known variation of the emitted light as a function
of the temperature of the at least one phosphor material enable the
temperature at the distal end of the optical fibre to be measured.
The at least one phosphor material should be selected such that the intensity
of a line within the emission spectrum of the at least one phosphor material
changes across the temperature range of interest. Preferably these changes
occur rapidly, to keep pace with any rapid temperature changes that might
occur, and occur with good signal strength, to enable accurate temperature
measurement. Therefore careful selection of the at least one phosphor
material allows temperature measurements across a temperature range of
interest to be made quickly and accurately. For example, an accuracy of
0.1 C is achievable in the range of 36 C to 41 C using a phosphor tipped
sensor.
The at least one optical fibre should be robust so that it does not easily
kink or
become bent. This ensures that the detectable optical radiation is transmitted
to the proximal end of the at least one optical fibre.
Radio translucent fibre optic sensors may be used with the claimed
temperature measurement system and a radio translucent heated mattress pad
to ensure that both the patient warming system and the claimed temperature
measurement system won't interfere with X-rays. Furthermore, the patient
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warming system and a temperature measurement system with fibre optic
sensors may be used with a radio translucent operating table, such as the
Wilson Plus radiolucent Wilson frame. Patient warning systems and the
claimed temperature measurement system may also be used with a variety of
other operating tables, such as the Jackson table, the Amsco table and the
Skytron table.
In one embodiment of the present invention, a combination of electrical
temperature sensors and optical temperature sensors may be used. In
particular, the at least one temperature sensor adapted to be fixed to a
patient
located on the pad may be an electrical temperature sensor, and the at least
one temperature sensor that is adapted to be fixed to the heating element and
the at least one temperature sensor that is adapted to be fixed to the pad may
be optical temperature sensors. If used with a radio translucent patient
warming system such as the LMA PerfecTempTM, the arrangement of the
temperature sensors described above ensures that the heated mattress pad
remains radio translucent.
In embodiments using fibre optic sensors, the converter module may be a
three channel opto-electronic converter module (or any other device capable
of converting optical signals, e.g. a beam splitter).
The at least one converter module may generate an output signal to be
transmitted to control means to control the heating of the mattress pad. The
control means and the at least one converter module may be located within the
same housing, or in embodiments comprising more than one converter
module, one of the converter modules may be located within the same
housing as the control means and one of the converter modules may be
located somewhere else.
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Commonly the mattress pad is heated electrically and the control means
controls the heating of the mattress pad by adjusting the current flowing
through the mattress pad in response to the output signal from the at least
one
converter module. However alternative means of heating the mattress pad
may be used, e.g. using a flow of heated water. Optionally the output signal
is
transmitted digitally to the control means, either via an electrical cable or
wirelessly.
Sensor monitoring means may be used to monitor whether one or more of the
sensors becomes disconnected from the converter module. If one or more of
the sensors become disconnected from the converter module, the sensor
monitoring means may initiate an audible or visible alarm and/or the control
means may switch off the heating element. For example, if the mattress pad
is heated electrically and the sensor monitoring means detects that one or
more of the sensors has become disconnected from the converter module the
control means may cut the supply of electrical current to the heating element.
The system may also comprise means suitable for recording and/or displaying
temperature information, which means may be integrated with the control
means or form a separate device.
The temperature sensors of the present invention may be disposable or
reusable or a combination of disposable and reusable temperature sensors may
be used. In particular, the at least one temperature sensor adapted to be
fixed
to a patient located on the pad may be disposable since this temperature
sensor makes direct contact with a patient whereas the at least one
temperature sensor that is adapted to be fixed to the heating element and the
at
least one temperature sensor that is adapted to be fixed to the pad will not
make direct contact with a patient.
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The use of disposable temperature sensors in medical applications allows
new, sterile sensors to be used with each patient. In this case, the
disposable
sensors should be inexpensive and able to be readily connected to and
disconnected from the converter module. Alternatively, reusable temperature
sensors may be used in medical applications provided the temperature sensors
are isolated such that they do not need to be sterilised or, if they cannot be
isolated, the reusable temperature sensors should be sterilised between uses.
In this case, the reusable sensors should be easy to sterilise and able to be
readily connected to and disconnected from the converter module.
The temperature sensors may comprise means for fixing the distal end of the
temperature sensor in a desired location. The means for fixing the distal end
of the temperature sensor may vary depending on the nature of the desired
location, i.e. the means for fixing the distal end of the temperature sensor
to
the heating element may differ from the means for fixing the distal end of the
temperature sensor to the pad and may vary from the means for fixing the
distal end of the temperature sensor to the patient. Nonetheless, the means
for
fixing the distal end of the temperature sensor in a desired location may
comprise adhesive, for example the means for fixing could be an adhesive
pad. The adhesive pads may be designed to ensure the adhesive does not
insulate the sensor and distort the sensor's signal. Adhesive pads comprising
hypoallergenic adhesive are particularly suited to medical applications.
The temperature sensors of the present invention are principally intended for
non-invasive use, however, the skilled person could readily devise means of
fixing the distal end of the temperature sensor in an invasive location. For
example, an invasive sterile probe could function as a channel for inserting
and fixing the distal end of the temperature sensor in an invasive location.
Any insulation effect associated with the invasive sterile probe could be
calculated and offset to ensure an accurate temperature reading at the
invasive
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location. Use of the temperature measurement system in an invasive location
could better enable a patient's core temperature to be monitored.
