U.S. patent number 3,796,208 [Application Number 05/225,934] was granted by the patent office on 1974-03-12 for movement monitoring apparatus.
This patent grant is currently assigned to Memco (Electronics) Limited. Invention is credited to John Anthony Bloice.
United States Patent |
3,796,208 |
Bloice |
March 12, 1974 |
MOVEMENT MONITORING APPARATUS
Abstract
A system for monitoring movements of a patient and indicating
when the degree of movement is such as to require attention
includes a scanner providing a limited movement sensitive field
surrounding at least part of the patient. Circuitry monitors
variations in the field caused by the patients movements and
controls alarm circuitry for calling attention to the patient, for
example on cessation of normal breathing or at the onset of undue
restlessness in a patient to be kept quiet. The scanner is suitably
a microwave radar unit.
Inventors: |
Bloice; John Anthony
(Isleworth, EN) |
Assignee: |
Memco (Electronics) Limited
(Isleworth, EN)
|
Family
ID: |
10421054 |
Appl.
No.: |
05/225,934 |
Filed: |
February 14, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Sep 7, 1971 [GB] |
|
|
41720/71 |
|
Current U.S.
Class: |
600/534; 342/28;
342/61; 600/430; 600/595; 340/573.1 |
Current CPC
Class: |
A61B
5/113 (20130101); G01S 13/50 (20130101); G08B
13/2491 (20130101); A61B 5/0507 (20130101); G08B
13/24 (20130101) |
Current International
Class: |
A61B
5/11 (20060101); G08B 13/24 (20060101); G01S
13/00 (20060101); G01S 13/50 (20060101); A61b
005/08 () |
Field of
Search: |
;128/2R,2A,2S,2N,2.08
;340/279,258A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Howell; Kyle L.
Attorney, Agent or Firm: Dilts; Robert W. Weissenberger;
Harry G. Moore; Carlisle M.
Claims
I claim:
1. Movement monitoring apparatus comprising:
a. microwave radar means providing a limited movement-sensitive
field of microwave radiation;
b. monitor circuit means monitoring disturbances of said field and
providing at its output a pulsed signal indicative of the degree of
field disturbance; pg,22
c. aggregating circuit means coupled to the output of said monitor
circuit means to aggregate said pulsed signal to provide at its
output a signal voltage the level of which is indicative of said
degree of field disturbance;
d. threshold circuit means defining a predetermined threshold
voltage and coupled to the output of said aggregating circuit means
to receive said signal voltage;
e. trigger circuit means coupled to the output of said threshold
circuit means to be activated thereby when said signal voltage is
below said predetermined threshold voltage;
f. delay circuit means coupled to the output of said trigger
circuit means and providing a predetermined delay between the
appearance at its input of an input signal caused by activation of
said trigger circuit means and the appearance at its output of a
corresponding output signal; and
g. alarm circuit means connected to the output of said delay
circuit and activated by said output signal thereof.
2. Apparatus as set forth in claim 1, in which said microwave radar
means is a doppler radar unit and said pulsed signal is indicative
of the doppler frequency shift in the output signal of said
unit.
3. Apparatus as set forth in claim 1, in which said microwave radar
means is a microwave radar transmitter and said pulsed signal is
indicative of variations in the impedance loading of said
transmitter due to said field disturbances.
4. Apparatus as set forth in claim 1, in which said monitor circuit
means includes a filter which responds only to frequencies lower
than a predetermined upper limit, whereby said pulsed signal is not
indicative of field disturbances at frequencies above said upper
limit.
5. Apparatus as set forth in claim 4, in which said upper frequency
limit is 80 Hz.
6. Apparatus as set forth in claim 1, in which said trigger circuit
means includes terminals for connection to a recording instrument
to provide thereto said voltage signal, whereby a record of the
degree of field disturbance may be obtained.
7. Apparatus as set forth in claim 1 wherein said trigger circuit
means and said alarm circuit means include means for returning to
their de-activated state when said signal voltage is returned to
said threshold voltage.
8. Apparatus as set forth in claim 1 including means wherein said
alarm circuit means is activated by failure of said other means
thereof.
9. Apparatus as set forth in claim 4 constructed as respiration
monitoring apparatus including means wherein said field is adapted
to envelop at least the chest of a patient; said predetermined
threshold voltage is produced by normal respiration of said
patient, and said upper limit of frequencies corresponds to
movements of the chest wall of said patient characteristic of
normal respiration.
10. Apparatus as set forth in claim 1 wherein said alarm circuit
includes a switch means for deactivation thereof.
11. Apparatus as claimed in claim 1 including means wherein said
predetermined delay provided by said delay circuit means is
restored to its full value upon disappearance of said input signal
at said input of said delay circuit means.
Description
The present invention concerns improvements in the monitoring of
patients, and is more particularly concerned with apparatus
suitable for monitoring movements of a patient and indicating when
the degree of movement is such as to require attention.
Existing patient monitoring methods, as far as the applicants are
aware, all require some form of physical contact with the patient.
For example, some systems use thermistors attached to the nostrils
of the patient, photo electric systems attached to the ear lobes,
and electrodes fixed to the patient's chest to monitor cardiac
activity. One form of movement-monitoring system employs a
pressure-sensitive mattress on which the patient is placed,
movements of the patient disturbing the pattern of flow of a
pressure fluid through conduits in the mattress, such pressure
variations being monitored to provide an indication of the degree
of movement of the patient. In such systems, the patient has to be
disturbed in order to commence monitoring, and often the freedom of
movement of the patient is considerably reduced. In many situations
it may be distinctly unwise to disturb the patient, when his
physiological condition is already critical. For example, it may be
unwise to transfer a patient from an ordinary bed to one provided
with a pressure-sensitive mattress when his condition is such that
movement could prove damaging.
The present invention is intended to provide apparatus suitable for
monitoring patients in which no physical contact with the patient
is required, so that monitoring can be commenced at will without
any disturbance to the patient.
In accordance with the invention there is provided apparatus
suitable for monitoring movements of a patient and indicating when
the degree of movement is such as to require attention, comprising
a scanner arranged in use to provide a movement-sensitive field
enveloping at least part of the patient, circuitry for monitoring
variations in the field, and indicator circuitry controlled by the
monitoring circuitry to provide the indication that attention to
the patient is required.
The monitor system thus operates on a principle which may be called
the "field disturbance principle" in which a steady
electro-magnetic field, is set up in a region to be kept under
surveillance, and the disturbances in the field caused by movement
therein are monitored.
This principle is used in microwave intruder detection systems,
such as those described in U.S. Pat. No. 3,512,155 and U.S. Pat.
No. 3,691,556, both of which are assigned to the same assignee as
is this application and, the contents of which are hereby inserted
in this application by reference.
The scanner of the present monitoring system is suitably one of the
microwave radar scanners described in these two U.S. patents with
the monitor circuitry thereof modified in accordance with the
teaching of this invention.
In U.S. Pat. No. 3,512,155 the radar unit consists of a transmitter
and receiver and operates on the Doppler principle. A beam is
transmitted and reflections from objects in the beam path return to
the receiver. In the absence of movement in this field, there is no
frequency difference in the transmitted and reflected signals. When
movement occurs in the field, however a frequency change or Doppler
shift occurs, and this is monitored to provide an indication of
movements in the field of surveillance. In U.S. Pat. No. 3,691,556
the radar unit is a transmitter only and movements in the field of
surveillance dusturb the pattern radiated and cause load impedance
variations at the transmitter. These are monitored by an auxiliary
oscillator circuit to provide the movement indicator.
The apparatus of this invention is particularly, but not
exclusively, suitable for use as a respiration monitor or apnea
detector, registering the patient's respiration by means of the
movements of the chest wall. Apnea is the cessation of respiration
and is a common condition in premature babies (Apnea Neonatorum).
Provided apnea is detected quickly enough, it is generally possible
to restimulate respiration.
The detection of apnea is achieved by reversing the role of the
intruder detector, in that an alarm is given in the absence of
movement as opposed to the appearance of movement in the field of
surveillance. In addition, by suitably modifying the circuit values
of the intruder detector circuitry, sensitivity to people in the
near vicinity of the patient being monitored can be greatly
reduced.
The monitoring system will now be described in more detail, by way
of example only, and with reference to the accompanying
diagrammatic drawings, in which:
FIG. 1 is a plan view of the scanner of the monitoring system as
used for monitoring the respiration of babies in an infant
ward;
FIG. 2 is an elevation of the scanner;
FIG. 3 is a plan view of the scanner in use in an adult ward,
showing two possible positions of the scanner;
FIG. 4 shows the two possible positions of the scanner in
elevation;
FIG. 5 covers two sheets of drawing designated FIG. 5a and FIG. 5b,
respectively, and shows the monitor circuitry of the system;
and
FIG. 6 shows the system power supply and indicator circuitry in the
form of an alarm.
Referring to FIG. 1 and 2, the scanner 1 is mounted for pivotal
movement about a vertical axis at the free end of a horizontal arm
2 pivoted to a wall as at 3. The scanner is downwardly directed.
The mounting at 3 provides for adjustment of the height of the
scanner 1 above the floor.
As shown in FIG. 1, as many scanners 1 may be provided in an infant
ward as is appropriate to the capacity of the ward. It will be
appreciated that it is unlikely that all babies in the infant ward
will require to be monitored at any particular time, unless the
ward is specialized to the care of such infants.
The infants 4 to be monitored are placed in cots or incubators 5
beneath the respective scanners 1. The scanners are arranged to
provide a radiation pattern whose shape is as shown by the dotted
outline 6 in FIG. 1. It will be seen that this outline is generally
elliptical, and the scanner 1 is preferably arranged so that the
longer axis of the ellipse lies along the length of the infant's
body.
It will be seen from FIG. 1 that the mounting of the scanners 1 on
the horizontal arms 2 allows for the cots or incubators 5 to be
situated in various positions, and any particular scanner can be
hinged away against the wall when not in use, as shown in dotted
outline in the central sketch of FIG. 1, so as to take up less
room.
The scanner 1 may be as described in our U.S. Pat. No. 3,512,155.
This particular form of scanner is a two-aerial doppler device
employing interaerial coupling to feed microwave energy from the
transmitter horn directly to the receiver horn.
Microwave radiation from the transmitter horn is reflected from,
for example, the moving chest wall of the patient to the receiver
horn. The frequency of the reflected radiation is varied because of
the movement of the chest wall, and the difference in frequency
between the transmitted and received waves is seen after
amplification as a signal voltage of one cycle for every half
wavelength of relative movement. The particular scanner under
discussion operates at a transmission wavelength of 3cm, so that
the signal voltage is of one cycle for every 1.5cm of chest
movement.
If the respiratory excursion of the chest wall is considerably
greater than 1.5cm, several cycles of doppler signal will be
generated. According to the teaching of this invention the signal
is rectified and smoothed in such a way that a resulting signal is
produced representing the instantaneous velocity of the chest wall
at any point of the respiratory cycle. If this signal voltage falls
below a certain level and this condition persists for longer than a
preselected interval, an alarm sounds to indicate cessation of
breathing. During this interval, an adequate movement on the part
of the patient restores the instrument to its normal operating
mode, cancelling the alarm. Movement by the patient once the alarm
is sounding also has the effect of cancelling the alarm. These
processes will shortly be described in more detail.
If the chest wall movement is less than 1.5cm, a useful output
signal will still be generated as a result of the phase shift of
the reflected signal.
It has been found that this type of scanner will operate
effectively without the inter-aerial coupling. At the short ranges
involved in the patient-monitoring application, the intensity of
the reflected signal varies with the chest position to a sufficient
extent to provide efficient monitoring.
The maximum microwave energy level at the transmitter antenna
aperture is 50 .mu.W per cm.sup.2. Legs 5cm in length have been
added to the front of the scanner so that it cannot be brought
closer than 5cm to a patient, and at this distance from the antenna
aperture the maximum energy level drops to 10 .mu.W per cm.sup.2. A
minimum working distance of one foot is recommended for
infant-monitoring applications, at which distance the energy level
is only 1 .mu.W per cm.sup.2. This is below the upper permissible
level for this frequency range by a factor of thousands. With
adults, because of the large amplitude of chest movement,
satisfactory operation is obtained with the unit some 3 or 4 feet
from the chest wall.
FIG. 3 shows two positions of the scanner for monitoring an adult
patient.
In the left-hand sketch the scanner is near the foot of the bed and
directs a fan-shaped radiation curtain giving wide coverage of the
patient's head and shoulder region. In the right-hand sketch, the
scanner is placed directly above the patient's chest and directed
downwardly, to provide a radiation pattern similar to that used for
monitoring infants. The coverage provided is more narrow, but
monitoring is more sensitive with the scanner in the position shown
in the right-hand sketch.
FIG. 4 shows these two positions of the scanner in elevation,
indicating the height adjustment facility provided by the scanner's
support.
FIG. 5 (appearing on two sheets of drawing designated 5a and 5b)
shows the monitor circuitry.
The receiver diode D1 is located in the receiver aerial, and has
its cathode connected to the aerial which is grounded. Its anode is
connected to ground through a decoupling capacitance and through an
electrolytic capacitance C1. This is shunted by a pre-set
potentiometer VR1 with which the response range of the circuitry
may be adjusted and whose adjustable center tap is connected
through an electrolytic capacitance C3 to the input 1 of an
amplifier A1. A supply connection 2 of the amplifier A1 is
connected to ground through an electrolytic capacitance C2 and to a
supply terminal P13 through a resistance R2. The amplifier output 3
is connected to the terminal P13 through a resistance R5. It is
also connected to ground through the series-connected combination
of a resistance R6 and an electrolytic capacitance C4. The junction
of resistance R6 and capacitance C4 is connected to the amplifier
input 1 through a feedback resistance R4. The series-connected
combination of resistance R6 and capacitance C4 is shunted by an
electrolytic capacitance C5.
The amplifier's output 3 is connected through an electrolytic
capacitance C6 to the base of a first NPN transistor VT1. The base
of this transistor is also connected to the cathode of a diode D3
whose anode is connected to earth. The emitter of transistor VT1 is
connected to ground and its collector to a terminal P12 through a
resistance R7. The collector is also connected to ground through an
electrolytic capacitance C7, and to the anode of a diode D6. The
cathode of the diode D6 is connected to a terminal P11. The
components so far described, with the exception of the receiver
aerial, receiver diode and its decoupling capacitance, and the
pre-set potentiometer VR1 are mounted on a first printed circuit
board.
A terminal P2 on a second printed circuit board is linked to
terminal P11 on the first board. Terminal P2 is connected to the
base of a second NPN transistor VT2. The emitter of this transistor
is connected to ground. Its collector is connected to its base
through the series-connected combination of a resistance R9 and an
electrolytic capacitance C8. The junction of the resistance R9 and
the capacitance C8 is connected to one end of the energisation
winding of a relay RL1. The other end of the energisation winding
is connected to a terminal P7 to which is connected one end of a
resistance R3 whose other end is connected to the cathode of a
Zener diode ZD2. The anode of this Zener diode is connected to
earth. The cathode is also connected to a terminal P8, which is
linked to terminal P13 on the first printed circuit board.
Terminal P7 is connected to the cathode of a diode D5 whose anode
is connected to ground. The cathode is also connected to the
collector of a third NPN transistor VT3, this collector being
connected to the base through a resistance R10.
The base of transistor VT3 is connected to the cathode of a Zener
diode ZD4 whose anode is connected to ground. The cathode is also
connected through the series-connected combination of resistances
R11 and R12 to that end of the potentiometer VR1 which is not
grounded. The junction of those resistances R11 and R12 is
connected to ground through an electrolytic capacitance C11. The
Zener diode ZD4 is shunted by an electrolytic capacitance C9.
The emitter of transistor VT3 is connected to ground through a
capacitance C10 and to the cathode of the Gunn diode GD1 of the
transmitter aerial. The anode of the Gunn diode GD1 is connected to
the aerial and to ground. The diode is shunted by a capacitance
C12.
The contacts of relay RL1 are connected to terminals P9 and P10.
These are connected to ground through respective capacitances C13
and C14.
Terminal P7 is linked to terminal P12 on the first printed circuit
board and is connected to ground through a fuse FS1 and a
capacitance C15.
Terminal P9 is connected through a lamp shunted by a Zener diode
ZD7 to an output terminal labelled GRN.
Terminal P10 is connected to an output labelled BLUE, which is
connected to ground.
The junction of fuse FS1 and capacitance C15 is connected to an
output labelled RED.
Referring to FIG. 6, a main transformer T1 has its primary winding
connected to the AC main supply. The primary winding is
center-tapped to provide for dual-voltage operation, a voltage
selector switch being provided as shown.
The transformer secondary winding provides 12 volts AC which is
rectified by a bridge rectifier comprising diodes D64 to D67. The
rectifier output is smoothed by a resistance R64 and capacitance
C65. The smoothed output across capacitance C65 is applied to
inputs labelled RED and BLUE, connected to the similarly labelled
outputs of the circuitry of FIG. 5.
A lamp L1 is connected between the input labelled RED and an input
labelled GREEN which is connected to the output labelled GRN of the
circuitry of FIG. 5. A 1mA drive is available from a pair of
terminals, one of which is connected to the input labelled RED and
the other which is connected through a resistance R1 to the input
labelled GREEN.
The input labelled GREEN is connected through an electrolytic
capacitance C61 to the cathode of a diode C61 whose anode is
connected to the input labelled BLUE. It will be recalled that the
similarly labelled output of the circuitry of FIG. 5 is connected
to ground. The cathode of the diode D61 is connected to the anode
of a diode D62 whose cathode is connected to earth through a
resistance R62. The resistance R62 is shunted by a selected one of
three electrolytic capacitances C62, C63 and C64.
The cathode of diode D62 is also connected through a resistance R63
to the base of an NPN transistor VT61 whose emitter is connected to
ground. Its collector is connected to one end of the energisation
winding of a relay RL61, the other end of this winding being
connected to the unsmoothed supply through a switch SW1. The
energisation winding is shunted by a diode D63 whose anode is
connected to the collector of transistor VT61.
That end of the energisation winding remote from transistor VT61 is
connected through a noise generator to one of the relay RL61
contacts, the other of which is connected to ground.
The operation of the circuitry will now be described with reference
to FIGS. 5 and 6.
It will be recalled that the Doppler frequency shift is seen, after
amplification, as a voltage. This voltage is the output voltage of
amplifier A1. The amplifier A1 is frequency selective, and is tuned
to an optimum frequency by means of the feedback resistance R4 and
associated component values. It has been found that the optimum
response frequency for monitoring of respiration is 1Hz.
The output voltage of the amplifier A1 is applied to the
capacitance C7, which acts as a store, through the pump circuit
provided by transistor VT1. The voltage across capacitance C7 is
applied to the base of transistor VT2 which acts as a relay
driver.
So long as the voltage across capacitance C7 exceeds a preselected
value, the relay RL1 is kept energised, and the normally closed
contacts connected between terminals P9 and P10 are open.
A circuit may be traced from one pole of the smoothed supply, that
is one plate of capacitance C65, through lamp L1, the lamp shunted
by Zener diode ZD7, the contacts of relay RL1, to the other pole of
the smoothed supply, that is the other plate of capacitance
C65.
Thus, so long as the Doppler output voltage exceeds a predetermined
value, and the relay contacts are held open, these two lamps are
not lit.
If this voltage falls below that level, the voltage at the base of
transistor VT2 drops to such an extent that the relay RL1 drops
out. The normally closed contacts therefore close to complete the
lamp energisation circuit, and both lamps therefore glow.
Lamp L1 is situated at an alarm station remote from the scanner and
the other lamp is situated on the scanner, but appropriately
situated so as to be invisible to the patient being monitored.
As the patient breathes, the voltage across capacitance C7
continually varies, passing above and below the limit value at the
respiration rate. The two lamps consequently blink on and off at
the same rate.
The sensitivity may be adjusted by means of potentiometer VR1 so
that if the degree of chest excursion falls below a particular
amount, more particularly if breathing stops altogether, the relay
RL1 is no longer energised, the normally closed contacts remaining
continuously closed. Consequently both lamps shine
continuously.
While the patient was breathing normally, the intermittent
operation of the relay RL1 was supplying pulses to the GREEN input
of the alarm circuitry. These passed through diode D62, negative
pulses being shunted to ground by diode D61, to be aggregated in
the selected one of the capacitances C62, C63 and C64. The selected
capacitance is maintained charged by these pulses, so that
transistor VT61 remains conducting. The relay RL61 driven by this
transistor therefore remains energised, and its contacts are held
open to inhibit operation of the noise generator.
When relay RL1 becomes continuously de-energised, activating the
contacts thereof, on cessation of breathing, no further pulses
reach the selected one of capacitances C62, C63 and C64, which
therefore discharges through resistance R62. The values of
capacitances C62, C63 and C64 are chosen to give respective delay
times of 10, 20 and 30 seconds. The result is that 10, 20 or 30
seconds after relay RL1 becomes continuously de-energised,
activating the contacts thereof, the voltage on the base of
transistor VT61 drops below a limit value, the relay RL61 is
de-energised and its contacts close to activate the noise generator
to sound an alarm.
It will be appreciated that for the alarm circuitry to operate the
switch SW1 must be closed. The function of the switch will be
described shortly.
The transistor VT3 is a series-current regulator for the Gunn diode
GD1. The capacitances C13, C14 and C15 decouple any RF present at
terminals P9, P10 or P7, due for example to X-ray equipment
operating in the vicinity of the monitor.
The Zener diode ZD7 shunting the second lamp is intended to
maintain circuit continuity in the event of lamp failure.
Should the lamp L1 fail the selected capacitance C62, C63 or C64
discharges to sound the alarm. In the event of failure of the radar
devices or the amplifier A1, both relays will open to sound the
alarm. The device will become inoperative if there is a failure of
the main supply, but provision could be made to keep the unit
operating for a few hours from a 12 volt battery.
The 1mA drive terminals may be connected to a recording instrument
such as a chart recorder, to provide a permanent record of the
patient's respiration. As well as providing a suitable pulse record
of the respiration rate, it is envisaged that the output of the
amplifier A1 must be brought out to an external connection from
which a record of the respiration waveform could be obtained.
Heart-beat waveforms are of great assistance in diagnosing various
forms of cardiac complaint, and it is thought that respiration
waveforms might provide similarly useful information regarding
respiratory complaints.
If the instrument has been disconnected from the main supply for
some time a "warm-up" period of about 3 minutes is required before
it will function. This is because of the long time constant
circuits employed in the apparatus. If the switch SW1 is closed
during this period, the detector will remain insensitive to
movement, the monitor lamps will glow continuously, and the alarm
will sound. As this will be rather inconvenient, the switch SW1 is
opened while the unit is allowed to warm up, so disabling the alarm
circuit. When the "warm-up" period is complete, the detector
becomes sensitive to movement, the monitor lamps begin to blink at
the respiration rate, and the switch may be closed to ready the
alarm. This "warm-up" period may be obviated by keeping the
instrument permanently connected to the main supply, switch SW1
being held open until the unit is required for use.
When the unit is to be used, the detector is positioned so as to
observe the patient's respiratory movements, the appropriate alarm
delay time of 10, 20 or 30 seconds is selected, and the detector is
adjusted for optimal sensitivity.
A delay time of 10 seconds has been found to be the most
appropriate for detecting apnea in infants, but longer periods may
be used for adults, depending on the illness or complaint
concerned.
Optimal sensitivity is obtained by gradually advancing the detector
sensitivity control from a minimum position, while observing the
monitor lamps. The optimal setting is considered to be that in
which the on and off phases of the monitor lamps are of
approximately equal duration. If sensitivity is too low, the lamps
will be on most, or all of the time, If the sensitivity is too
high, the lamps will be off most or all of the time, and with too
low sensitivity the instrument is liable to give an alarm even when
breathing is only shallower than normal. If sensitivity is too
high, however, there will be a great risk of extraneous movements
in the vicinity of the patient exciting the instrument.
The sensitivity setting required will depend on the distance
between the detector and the patient, the amplitude and frequency
of breathing, and the presence of an interposed material, such as
the roof of an incubator in the case of premature babies. In all
cases, sensitivity should be held low as possible whilst compatible
with detecting respiration.
The instrument has been made rather insensitive to movements at
frequencies higher than those likely to be encountered in
breathing, that is to say of the order of 80 cycles per minute.
This lessens the likelihood of the device being activated by
movements extraneous to the patient.
It should be noted that a large amplitude movement by the patient
will paralyse the circuitry for 1 or 2 seconds, rendering it
temporarily insensitive to respiratory movement. Consequently, the
instrument cannot be relied upon for an entirely accurate record of
the respiratory rate, as such record is liable to be frequently
interrupted by a restless patient. Of course, if a record of the
degree of restlessness of the patient is required, this factor is
of considerable usefulness.
The apparatus can moreover, be adapted to give the alarm if the
patient is unduly restless.
The alarm is given when there is insufficient movement in the field
when a signal voltage drops below a threshold (in FIG. 6 when
capacitance C62, C63 or C64 becomes sufficiently discharged). By
arranging that the alarm is given when the voltage exceeds a
further and higher threshold, the undue restlessness indication may
be given.
The system has been used to monitor respiration in several normal
newborn infants. In each case the scanner was arranged
approximately 1 foot above the baby, and tests were made with the
long axis both paralled to and perpendicular to the baby's height.
Better results were obtained in the latter case. The alarm signal
was duly given when one infant subject to periodic breathing
suffered apnea lasting more than 10 seconds. The alarm is also
given when, to test the instrument, the infant was removed from the
cot. The presence of persons standing within a foot or two of the
cot did not appear to interfere with the operation of the detector,
provided that its sensitivity was appropriately regulated.
Newborn infants in incubators have also been monitored with the
device, the detector being held approximately 5cm above the upper
surface of the incubator. The alarm signal was given reliably in
the case of two premature babies subject to frequent attacks of
apnea. A third infant was not subject to apnea but had shallow
breathing at a rate of approximately 60 cycles per minute. The
monitor lamps flashed at a rate closely corresponding to this. The
alarm duly sounded with the scanner situated over an empty
incubator, even with persons standing closeby.
It is considered that the detector should not be placed against the
side wall of an incubator, since the infant may come close to this
wall and thereby increase the energy level received, and the
detector is then liable to be activated by reflections from
individuals standing close to that side of the incubator.
Tests have been carried out on adult patients in a normally crowded
hospital ward, with the scanner situated in both positions
illustrated in FIGS. 3 and 4. The alarm was duly given when the
patient held his breath and otherwise remained motionless. The
presence of staff along the side of the bed and of patients in
adjacent beds caused no interference in operation.
The instrument conveniently consists of two units, the wall mounted
or free-standing scanner and the alarm unit. The scanner is
connected to the alarm unit by a cable carrying the 12 volts supply
and the alarm and monitor lamp signals. This cable need only have
three cores, linking the GREEN, RED and BLUE terminals of the
circuits of FIGS. 5 and 6.
The alarm unit may be placed adjacent or remote from the patient.
Where it is located adjacent the patient, it may be advantageous to
set up a slave alarm at a remote station.
The instrument is simple to operate, and can be arranged as a
permanent monitoring station to which patients to be monitored are
brought. Where the patients cannot be moved, portable scanners on
free-standing supports may be used.
No apparatus need be attached to the patient, and neither the
patient nor his bed need be especially prepared for monitoring to
begin. The inconveniences which may arise if monitoring apparatus
has to be attached to or otherwise placed in contact with the
patient are eliminated. These inconveniences include restriction of
breathing or other movement, skin irritation, accidental
disconnection of links, and interference with normal medical care.
As the apparatus is not in contact with the patient, it requires no
special cleaning or sterilisation. The scanner may be quickly moved
to one side of there is urgent need for access to a patient, for
example an infant in an incubator subject to apnea.
As has already been mentioned, the energy level at the patient is
reduced to an extremely small value which is below that regarded as
the upper permissible level by a factor of thousands.
It is not thought that the instrument will interfere with the
taking of ECG records, in view of the high frequency used.
The instrument may find extensive applications in labor wards, in
special neonatal units, and in children's wards. It may also be
useful in casualty cubicles, in adult medical and surgical wards,
in anaesthetic or recovery rooms. It will be used for monitoring
patients suffering from drug overdose, or head injuries, and for
monitoring patients on respirators to ensure that there is actually
movement of the chest. The instrument will be of particular
assistance in monitoring patients in side rooms or private wards
where close nursing supervision may be difficult.
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