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BACKGROUND OF THE INVENTION
There are an estimated 10,000-15,000 deaths in the United States each year
due to sudden infant death syndrome (SIDS), making it the most frequent
cause of death in the first year of life. Many hypotheses have been
formulated to explain its etiology. Most of the easily recognized
post-mortem abnormalities in the victims are in the lungs and they have
usually been interpreted as evidence of a sudden catastrophic event in an
otherwise normal infant. Years of speculation about the nature of the
catastrophic event have been unfruitful because the easily recognized
post-mortem abnormalities give few clues to the dynamic events surrounding
the deaths.
The term "apnoea" is often used in conjunction with SIDS as a symptom as
well as a cause. Apnoea may be defined as a pause in the infant's
breathing equal to or exceeding six seconds. Short periods of apnoea
during sleep are normal during infancy while prolonged periods are
abnormal. In 1972 Steinschneider (Pediatrics 50:646-654, 1972) reported
that several SIDS victims had prolonged periods of apnoea during sleep
before death.
In an attempt to combat SIDS, monitoring systems have been proposed in the
past which react to any period of apnoea in the sleeping infant. Although
some of the indications may be false alarms due to normal periods of
apnoea by the infant, in any event, the parents or attending nurses are
alerted by the monitors whenever the infant stops breathing, even for a
short period.
The prior art monitors, however, suffer from a disadvantage in that they
usually involve placing electrodes on the infant with leads extending to
the monitoring equipment. These electrodes and leads are a source of
discomfort to the infant and inhibit normal sleep. Moreover, the
electrodes themselves often cause skin irritation. Another problem is the
fact that such prior art systems exhibit failures with no detectable
electrical fault and, accordingly, are believed to be unreliable.
The use of infrared energy to detect if an infant has stopped breathing has
been suggested in the prior art. The exhalations of the infant include
large quantities of carbon dioxide. Carbon dioxide is absorbent to the
long wave infrared radiation. The detector detects the difference in the
infrared radiation due to the absorption incident to the exhalations of
the infant. The resulting signal is applied to a suitable alarm circuit to
indicate an interruption of the exhalation exceeding a predetermined time
interval.
A non-contacting apnoea detector is disclosed in U.S. Pat. No. 4,350,166.
However, the detector relies on infrared radiation from the infant itself,
which peaks at 9.6 microns; whereas carbon dioxide is absorbed at 15
microns. This militates substantially against the effectiveness of the
prior art device.
The monitoring system of the apnoea monitor of the present invention is
simple in its construction, yet it is extremely reliable, and it is
capable of detecting apnoea in the infant without any primary or secondary
electrical hook-ups to the child itself. As explained briefly above, the
apnoea monitoring system of the invention collects the exhaled breath of
the infant in an area in which infrared energy is emitted from an infrared
source and, through infrared absorption, measures the quantity of carbon
dioxide present in the breath. So long as the carbon dioxide is present,
the child is breathing. Should apnoea occur and the child stops breathing,
the carbon dioxide will disappear and an infrared detector will respond to
the resulting rise in the infrared intensity to cause an alarm to be
triggered. This permits the attending nurse or parents to take immediate
life saving action.
It is accordingly an objective of the present invention to provide an
improved simple and inexpensive apnoea monitoring system which detects and
continuously monitors actual infant breathing, without any physical
contact with the infant itself. The system of the invention is
advantageous in that it is highly reliable, it is not subject to failure,
and it does not involve complex and inconvenient wire hook-ups which are
uncomfortable and disturbing to the infant.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective representation of an infant's crib equipped with a
system for performing the apnoea monitoring function of the present
invention;
FIG. 2 is a schematic rendition of an apnoea monitoring system in
accordance with one embodiment of the invention;
FIG. 3 is a circuit diagram of an amplifier which may be included in the
system of FIG. 2;
FIG. 4 are curves showing the absorption of infrared energy by carbon
dioxide gas, and which are useful in explaining the operation of the
system of the invention;
FIG. 5 is a perspective representation of a second embodiment of the
invention;
FIG. 6 is a top sectional view of an alarm module included in the
embodiment of FIG. 5; and
FIG. 7 is a schematic diagram of a remote alarm system using the domestic
power lines.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
The crib shown in FIG. 1 is designated generally 6. The crib is equipped
with a hood 8 which may be composed of plastic, or other appropriate
material, and which fits over the crib 6 to collect the exhaled breath of
an infant sleeping in the crib. An infrared radiator 10 is mounted on one
of the headboards of crib 6, and it directs infrared energy to a filter 12
and infrared detector 14 mounted on the opposite headboard.
So long as the infant in the crib is breathing, the carbon dioxide in his
exhaled breath absorbs a portion of the infrared energy traveling from the
radiator 10 to the detector 14. However, should the infant stop breathing,
the carbon dioxide disappears, and no longer absorbs any portion of the
infrared energy. Accordingly, the output of detector 14 increases in
amplitude and activates an alarm 16. The system is turned on and off by a
switch SW1. An indicating light 17 adjacent to the switch is illuminated
when the system is operated.
In the schematic diagram of FIG. 2, the infrared radiator 10 is shown as
connected through switch SW1 to a source of regulated current. This
source, for example, may be activated by the usual alternating current
mains, and may also be equipped with a back-up battery which would take
over in the event of power failure.
The infrared radiator may be any appropriate black body device which is
heated, for example, to 680.degree. K. (407.degree. C.) for maximum
infrared radiation at wavelengths in the vicinity of 4 microns. For
example, the infrared radiator 10 is heated to provide infrared radiation
approximately in the wavelength range of 3-6 microns. This infrared energy
is present in the area under hood 8 of FIG. 1, and it takes the form shown
in FIG. 4. So long as the infant is breathing, the carbon dioxide in the
area under the hood absorbs the infrared energy in the region extending
approximately from 4.255-4.344 microns, as shown in FIG. 4.
The infrared energy is passed through a filter 12 to an infrared detector
14. Filter 12 may be an infrared thin film filter of approximately 4.1-4.4
microns in bandwidth, as shown in FIG. 4. The filter material itself,
designated 12A in FIG. 2 is mounted on a substrate 12B which exhibits high
transmission characteristics to infrared energy.
The output of the infrared detector 14 is amplified by amplifier 15 which
is shown in circuit detail in FIG. 3. The output of the amplifier 15 is
connected to an appropriate alarm 16. Amplifier 15 may be formed integral
with the infrared detector 14.
As shown in FIG. 3, amplifier 15 is made up of five operational amplifiers
A1, A2, A3, A4 and A5. Amplifiers A1 and A2 may be of the type designated
LM741, and the remaining amplifiers may be on a common integrated circuit
chip of the type designated LM324.
The output from the infrared detector 14 is passed through a 0.01
microfarad coupling capacitor C1 to the non-inverting input terminal of
amplifier A1. This terminal is connected to a 100 kilo-ohm resistor R1
which in turn is connected to a grounded 100 kilo-ohm resistor R2. A 0.001
microfarad capacitor C2 is connected across resistor R2. The output of
amplifier A1 is connected back to the inverting input terminal, and also
through a 5 microfarad capacitor C3 to the unction of resistors R1 and R2.
The output of amplifier A1 is also connected through a 1 kilo-ohm resistor
R3 to the inverting input terminal of amplifier A2. The non-inverting
input terminal of amplifier A2 is connected to a 1 kilo-ohm grounded
resistor R4. The output terminal of the amplifier A2 is connected back to
the inverting input terminal through a 100 kilo-ohm resistor R5, and
through a 1 kilo-ohm resistor R6 to the non-inverting input terminal of
amplifier A3. The output terminal of amplifier A3 is connected through a
10 kilo-ohm resistor R7 to the inverting input terminal, and through a 100
kilo-ohm resistor R8 to the inverting input terminal of amplifier A4. The
non-inverting input terminal of amplifier A4 is grounded.
The output terminal of amplifier A4 is connected back to the non-inverting
input terminal of amplifier A3 through an 18 kilo-ohm resistor R9, and is
coupled back to its inverting input terminal through a 0.2 microfarad
capacitor C4. The output terminal of amplifier A4 is also connected
through a 100 kilo-ohm resistor R10 to the inverting input terminal of
amplifier A5. The non-inverting input terminal of amplifier A5 is
grounded. The output terminal of amplifier A5 is coupled back to its
inverting input terminal through a 0.2 microfarad capacitor C5, and the
output terminal is connected back to the inverting input terminal of
amplifier A3 through a 10 milo-ohm resistor R11. The output of amplifier
A5 is applied to alarm 16.
Amplifier 15 operates in known manner to amplify the output from the
infrared detector 14, and the amplifier is controlled so that an output
will be applied to alarm 16 only when the output from the infrared
radiator 10 rises above a predetermined threshold, to indicate that the
infant has stopped breathing. When that occurs, alarm 16 is activated.
Infrared radiation is produced principally by the emission of solid and
liquid materials as a result of thermal excitation and by the emission of
molecules of gases. Thermal emission from solids is contained in a
continuous spectrum, whose wavelength distribution is described by:
##EQU1##
where I.sub..lambda. =spectral radiant emittance of the solid into a
hemisphere in the wavelength range from .lambda. to (.lambda.+d.lambda.).
c=velocity of light
h=Planck's constant--6.62.times.10.sup.-27 erg second
.epsilon..lambda.=spectral emissivity
k=Boltzmann's constant=1.38.times.10.sup.-16 erg/K.
T=absolute temperature of the solid emitter, K.
The spectral emissivity, e.lambda., is defined as the ratio of the emission
at wavelength.lambda. of the object to that of an ideal blackbody at the
same temperature and wavelength. When .epsilon..lambda. is unity, the
foregoing equation becomes the Planck radiation equation for a black body.
Gaseous emission of infrared radiation differs in character from solid
emission in that the former consists of discrete spectrum lines or bands,
with significant discontinuities, while the latter shows a continuous
distribution of energy throughout the spectrum.
The propagation of infrared radiation through various media is, in general,
subject to absorption which varies with the wavelength of the radiation.
Molecular vibration and rotation in gases, which are related to the
emission of radiation, are also responsible for resonance absorption of
energy. The lesser gases in the atmosphere exhibit pronounced absorption
throughout the infrared spectrum. However, nitrogen and oxygen do not
absorb significantly in the infrared region. Water vapor, carbon dioxide,
and ozone are responsible for strong absorption in the infrared. The
absorption of radiation is so prevalent that those spectral bands in which
relatively little absorption occurs are identified as atmospheric windows.
Detection of the presence, distribution and/or quantity of infrared
radiation requires techniques which are, in part, unique to this spectral
region. The frequency of the radiation is such that essentially optical
methods may be used to collect, direct, and filter the radiation.
Transmitting optical elements, including lenses and windows, must be made
of suitable materials, which may or may not be transparent in the visible
spectrum, as shown in Table 1.
TABLE 1
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MATERIALS WHICH TRANSMIT IN THE INFRARED REGION
USEFUL TRANSMISSION
REGION
MATERIAL (Micrometers) CHARACTERISTICS
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Optical glasses
0.3-2.7 Best for near-infrared
Fused silica
0.2-3.5/4.5 Some types show absorption near 2.7
micrometers
Arsenic trisulfide
0.6-12.0 A glass; subject to striations
Calcium aluminate
0.3-5.5 A glass; subject to attack by water
Sapphire 0.17-6.0 Single crystal, hard, refractory
Silicon 1.1->20 Low density; opaque to visible
Germanium 1.8->20 Opaque to visible
Sodium chloride
0.2-15 Water soluble
Potassium bromide
0.21-27 Water soluble
Lithium fluoride
0.11-6 Low solubility in water
Calcium fluoride
0.13-9 Insoluble
Thallium bromide-iodide
0.5-40 Moderately soft; cold flow
Silver chloride
0.4-25 Soft; cold flow
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The detector for infrared is an important component of the detection
system. Photographic techniques can be used for part of the near-infrared
region. Photoemissive devices, comparable to the visible- and
ultraviolet-sensitive photocells, are available with sensitivity extending
to about 1.3 micrometers. The intermediate-infrared region is most
effectively detected by photoconductors. These elements, photosensitive
semiconductors, are essentially photon detectors, which respond in
proportion to the number of infrared photons in the spectral region of
wavelength. This wavelength corresponds to the minimum photon energy
necessary to overcome the forbidden gap of the semiconductor. All spectral
regions from ultraviolet through visible, infrared, and microwaves, can be
detected by an appropriately designed thermal element, which responds by
being heated by the absorption of the incident radiation. In the infrared
region, thermal detectors take the form of thermocouples, bolometers, and
pneumatic devices. The thermal devices, in general, are not so sensitive
or as rapidly responding as photoconductors. Some useful infrared
detectors are listed in Table 2.
TABLE 2
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REPRESENTATIVE INFRARED RADIATION DETECTORS
OPERATING USEFUL TIME
TEMPERATURE
REGION CONSTANT
DETECTOR (K) (Micrometers)
(Seconds)
FEATURES
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Lead sulfide 295 Visible-2.8
2 .times. 10.sup.-4
Thin-film photoconductor
Lead selenide 195 Visible-5.6
2 .times. 10.sup.-3
Thin-film photoconductor
Indium/antimony
77 1-5.6 <2 .times. 10.sup.-7
Photovoltaic crystal
Germanium (mercury-doped)
25 1-16 <10.sup.-6
Photoconductor crystal
Germanium (copper-doped)
5 1-29 <10.sup.-6
Photoconductor crystal
Germanium (zinc-doped)
5 1-40 10.sup.-8
Photoconductor crystal
Thermistor bolometer
295 All 10.sup.-3 - 10.sup.-2
Flake of mixed oxides
Golay cell 255 All 1.5 .times. 10.sup.-2
Pneumatic
Thermocouple 295 All 1.5 .times. 10.sup.-2
Used in spectrometers
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The infrared radiator 10 is selected of appropriate material, and is heated
to a selected temperature, for example 680.degree. K., so that the
radiator will radiate infrared energy through a band shown in FIG. 4. The
infrared narrow band thin film filter 12A filters out the infrared
radiation shorter than 4.158 microns or longer than 4.356 microns. The
thin film layer 12A, as mentioned above, is deposited on a high
transmission substrate 12B which may be composed of selected quartz,
sapphire, or the like which is transparent to infrared energy in the
selected bandwidth.
After passing through the infrared filter 12, the infrared radiation will
have a much narrower bandwidth, approximately 4.1-4.4 microns. The
infrared detector 14 may be any appropriate commercially available type,
such as thermopile, thermocouple, photoconductive, photovoltaic, and the
like. Lead selenide or lead sulphite is appropriate for use in the
infrared detector 14.
In the embodiment of FIG. 5, a hood 8A is removably mounted on a
conventional crib 6A by hinged brackets 9. An alarm module 7 is mounted on
top of hood 8A, the module having holes in its top and bottom, and
including an appropriate circulating fan (not shown) which serves to draw
the carbon dioxide emitted by the baby in the crib through the alarm
module. The alarm speaker 16 is mounted on the alarm module, as well as a
pair of indicator lights 17A and 17B. Indicator light 17A may be green,
for example, to indicate that the system is operating, and the indicator
light 17B may be red, for example, to glow when the alarm is activated.
Appropriate circuitry may be incorporated into the alarm module to produce
an intermittent beep of the speaker 16, and an intermittent flashing of
the red indicator light 17B should the system become inoperative.
As shown in FIG. 5, the infrared radiator 10, as well as the infrared
detector and amplifier 14, 15 are mounted at one end of the alarm module
7. The radiator 10 radiates infrared energy within the alarm module in all
directions, and the infrared energy is incident on a spherical reflector
20 at the other end of the alarm module either directly from the radiator
10, or as a result of reflections from the sides of the alarm module.
The spherical reflector 20 reflects the infrared energy back to the
detector 14, which, together with filter 12 is positioned at a distance
from the spherical reflector such that all the energy reflected from the
spherical reflector is incident across the surface of filter 12 and
detector 14. The internal walls of the alarm unit are coated with
appropriate infrared reflective material.
By use of the alarm unit 7, a "folded" optical path is used for infrared
generation, absorption and detection, which greatly enhances the
sensitivity of the detector 14 due to longer path geometry and internal
wall reflection of the infrared rays.
In the system of FIG. 6, amplifier 15 is connected to an appropriate
encoder 22 which may, for example, be a binary decimal encoder. The output
signals from encoder 22 are coupled to the domestic alternating current
power line through capacitors C10, C12, and through a plug 24 and socket
26.
A remote alarm 32 may then be plugged into a socket 30 at any desired
location within the house. The unit 32 incorporates an appropriate decoder
which responds to the coded output of encoder 22 to activate an alarm
within the unit whenever the system detects that the infant has stopped
breathing.
The invention provides, therefore, an apnoea monitoring system which is
relatively simple and inexpensive to construct, and which is easy to
operate. The system is advantageous in that it responds directly to the
breathing of the infant, and does not involve any connections to the
infant itself. Moreover, the system of the invention is extremely reliable
in operation, and is not subject to malfunction.
While a particular embodiment of the invention has been shown and
described, modifications may be made. It is intended in the claims to
cover all modifications which come within the spirit and scope of the
invention.
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