U.S. patent number 3,768,017 [Application Number 05/207,859] was granted by the patent office on 1973-10-23 for electrocardiograph telemetry system having circuitry for indicating inoperative conditions.
This patent grant is currently assigned to Hewlett-Packard Company. Invention is credited to Richard F. Dillman, James L. Larsen, Alfred M. Nardizzi.
United States Patent |
3,768,017 |
Dillman , et al. |
October 23, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
ELECTROCARDIOGRAPH TELEMETRY SYSTEM HAVING CIRCUITRY FOR INDICATING
INOPERATIVE CONDITIONS
Abstract
Special circuitry in the transmitter of an electrocardiograph
(ECG) telemetry system detects various malfunctions of the system
and changes the transmitted signal to indicate their presence to
the system receiver. When the transmitter detects that an input
electrode has become detached from the patient, it changes the
frequency of the subcarrier signal to indicate this problem to the
receiver. The receiver monitors the subcarrier frequency and
flashes an alarm light when the frequency corresponds to the
electrode inoperative condition. When the voltage output from an
aging battery becomes too low to adequately energize the
transmitter, special circuitry stops the transmission of signals
from the transmitter. When the receiver cannot detect a transmitted
signal, it indicates that either the battery needs replacement or
the transmitter is out of range.
Inventors: |
Dillman; Richard F. (Lexington,
MA), Larsen; James L. (Needham Heights, MA), Nardizzi;
Alfred M. (Dedham, MA) |
Assignee: |
Hewlett-Packard Company (Palo
Alto, CA)
|
Family
ID: |
22772270 |
Appl.
No.: |
05/207,859 |
Filed: |
December 14, 1971 |
Current U.S.
Class: |
455/45; 128/903;
455/212; 455/208; 455/214 |
Current CPC
Class: |
A61B
5/276 (20210101); A61B 5/316 (20210101); A61B
5/0006 (20130101); A61B 5/318 (20210101); Y10S
128/903 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 5/0408 (20060101); A61B
5/04 (20060101); A61B 5/0402 (20060101); A61B
5/0424 (20060101); H04b 001/00 () |
Field of
Search: |
;128/2R,2.6R.2.1A
;325/48,45,344,348,349,478,346,417,418-420,423 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mayer; Albert J.
Claims
We claim:
1. A system for wireless transmission of electrical signals,
comprising:
an input circuit having an electrode for receiving an input signal
applied thereto;
transmitter means including frequency-control means connected to
said input circuit for radiating an output signal that is modulated
by a subcarrier signal means which is modulated within a
preselected band of frequencies by an input signal applied to said
electrode, and means for radiating said output signal modulated by
a said subcarrier signal having a signal frequency outside the
limits of said band in response to an open-circuit electrode of the
input circuit;
reception means including a demodulator adapted to receive the
radiated output signal for producing said subcarrier signal
therefrom; and
detector means connected to said reception means for producing a
first output indicative of reception of the radiated output signal
modulated by said signal frequency.
2. A system as in claim 1 wherein:
said reception means includes first and second detectors, each
connected to receive the demodulated subcarrier signal, the first
detector producing therefrom a first signal representative of the
peak amplitudes of said subcarrier signal over a given first time
interval and said second detector producing therefrom a second
signal representative of the peak amplitudes of said subcarrier
signal over a given second time interval which is shorter than said
first time interval; and comprising
comparator means operable for comparing applied signals with a pair
of signal levels, said comparator means being connected to said
first and second detectors for producing a second output indicative
of an inoperative condition in response to the first signal from
the first detector attaining a value greater than a first one of
said pair of signal levels and the second signal from the second
detector attaining a value less than a second one of said pair of
signal levels.
3. A system as in claim 2 wherein:
the transmitter means includes an FM-FM radio wave transmitter
having an electrode of said input circuit adapted to be attached to
the body of a patient for receiving an input signal therefrom;
said frequency-control means sets the signal frequency of said
subcarrier signal to a frequency lower than the frequencies of said
band in response to an electrode detaching from the body of a
patient; and
logic means connected to said comparator means and to said detector
means for inhibiting the first output indicative of reception of
the radiated output signal modulated by said signal frequency in
response to said second output produced by said comparator
means.
4. A system as in claim 1 wherein said reception means also
produces an information signal from said subcarrier signal which is
representative of said input signal; and comprising
inhibit means connected to said reception means for inhibiting said
information signal from the reception means in response to said
first output from said detector means.
5. A receiver comprising:
reception means for detecting radiated electrical signals;
first means connected to said reception means for demodulating a
detected input signal to produce a subcarrier signal;
second means connected to said first means for demodulating the
subcarrier signal to produce an information signal;
output means connected to said second means for deriving the
information signal from the receiver;
a first detector connected to said first means for converting the
peak amplitude of the subcarrier signal over a given first time
interval to a representative first D.C. signal;
a second detector connected to said first means that converts the
peak amplitude of the subcarrier signal over a given second time
interval to a representative second D.C. signal, said first time
interval being longer than said second time interval; and
comparator means operable for comparing applied signals with a pair
of signal levels;
said comparator means being connected to said first and second
detectors for producing an output indication of an inoperative
condition in response to the first D.C. signal of the first
detector attaining a value greater than a first one of said pair of
signal levels or in response to the second D.C. signal of the
second detector attaining a value less than a second one of said
pair of signal levels.
6. A receiver as in claim 5 wherein said first means includes a
demodulator and a local oscillator signal generator connected
thereto; and comprises circuit means connected to said comparator
means and to said signal generator for varying the frequency of the
signal produced thereby in response to the output indication of an
inoperative condition produced by said comparator means.
7. A receiver as in claim 6 comprising logic means connected to
said output means and to said comparator means for inhibiting the
information signal from the output means in response to the output
indication of an inoperative condition produced by said comparator
means.
Description
BACKGROUND OF THE INVENTION
A patient recovering from heart surgery or suffering a myocardial
infarction must be kept under constant observation until his heart
condition improves. Monitoring the electrocardiac signals,
sometimes called ECG signals, produced by the expansions and
contractions of the patient's heart is a common method of
observation during this time. These electrocardiac signals are
present on the skin and throughout the body. They are a valuable
medical indicator because their shape and repetition rate can
indicate to a trained observer whether the heart is operating
properly or nearing a dangerous condition.
During the initial phase of a heart patient's recovery, he is
bedridden and directly connected to a bedside monitor, such as an
electrocardiograph. The monitor is usually wired to electrodes that
are attached to the skin near the heart. The electrodes detect the
electrocardiac signals that are circulating on the skin, and the
wires transmit them to the monitor.
When a patient's condition improves, it is often desirable to let
him move about. This is difficult if he remains connected to the
bedside monitor because the wires restrict his movement. To remedy
this problem, a telemetry system is sometimes used to replace the
direct wired connection.
The telemetry system includes a portable transmitter carried by the
ambulatory patient and a stationary receiver connected to the
monitor. Electrodes still sense the electrocardiac signals, but now
the signals are transmitted by radio waves to the receiver. At the
receiver, the transmitted signal is demodulated and the resultant
electrocardiac signal is conveyed to the monitor. With such a
telemetry system, a heart patient can move about while his
electrocardiac signals are kept under constant surveillance.
If a heart monitoring system becomes inoperative, a special
indication should be given to the monitor operator so that the
fault can be quickly corrected and so that the inoperative
condition will not result in confusion and create a false heart
rate alarm. Because of the increased movement of an ambulatory
patient, there are more problems involved with a telemetry
monitoring system than with a stationary monitor. Patient movement
may disconnect an electrode, stopping detection of the
electrocardiac signals, or it may shift the position of an
electrode, weakening the detected ECG signals. An ambulatory
patient may also move out of the range of the receiver and ruin the
reception.
Since the transmitter must be portable, it usually contains a
battery for a power source. When the voltage output of the battery
decreases with age, the power supply may become unregulated and
cause the transmitter circuitry to drift with the unregulated
supply voltage. This will cause erroneous information to be
transmitted to the receiver.
SUMMARY OF THE INVENTION
The present invention detects various inoperative conditions that
are common to ECG telemetry systems, and it indicates to the system
operator that these conditions exist. It also inhibits the receiver
output when an inoperative condition is detected to prevent the
output of erroneous ECG signals to the monitor.
This invention monitors the input signal to the transmitter to
detect when an electrode detaches from the patient's skin or when
the input wires develop an open circuit. When such a condition is
detected, the subcarrier frequency of the transmitter signal is
changed by special circuitry in the transmitter. The receiver is
designed to detect this frequency change and light an alarm light
to signal the problem to the operator.
This invention also monitors the voltage output of the transmitter
battery. When the battery voltage begins to inhibit regulation of
the power supply, signal transmission from the transmitter is
prevented. The receiver examines the received signal for noise or
interference conditions. When it detects an input signal containing
only noise or interference and not the ECG signal, it lights an
alarm light to indicate that the transmitter is either inoperative
or out of range.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the preferred embodiment of the
inoperative circuitry in a conventional transmitter.
FIG. 2 and 2(a-h) are block and schematic circuit diagrams showing
the circuit configuration of the transmitter including the input
circuitry that detects an open circuit at the input terminals.
FIG. 3 is a block diagram showing the preferred embodiment of the
inoperative circuitry in the receiver.
FIG. 4 is a graphical diagram showing the transfer characteristics
of a frequency discriminator.
FIG. 5 is a graphical diagram showing the noise output voltage of
the frequency discriminator.
FIG. 6 is a graphical diagram showing the operation of comparators
on the output of a frequency discriminator.
FIG. 7 is a graphical diagram showing the charging characteristics
of the detector circuits.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 1 and 2, two input terminals 1 are connected
to sensing electrodes that are attached to a patient to pick up his
ECG signals. The input terminals are connected to circuitry 2 that
protects the transmitter from damage by the high voltages used in
defibrillation. After passing through an amplifier, the ECG signals
are used to frequency modulate a subcarrier signal generated by the
voltage controlled multivibrator 3. After the modulated subcarrier
is filtered, it is used to frequency modulate a carrier signal
generated by the voltage controlled crystal oscillator 5. The
resulting FM-FM signal is multiplied in frequency by a factor of
four, is filtered, and is then transmitted from the antenna 10. A
power supply 7 and a regulator 9 provide the energy to operate the
transmitter. Since the transmitter must be portable, the power
supply is usually a battery.
If an input electrode becomes detached from the patient, or if one
of the inputs develops an open circuit, the input inoperative
circuitry 12 detects the condition and reduces the subcarrier
frequency to signal this inoperative condition to the receiver. The
circuitry detects the open condition by sensing the unusually low
input current associated with an open circuit.
Referring now to the schematic diagram of FIG. 2 which shows the
input inoperative circuitry, a detached electrode causes the signal
at one of the inputs 1 to float towards the voltage level of the
supply 20. This will shut off the corresponding transistor of the
transistor pair 16. This, in turn, will saturate the corresponding
transistor of the transistor pair 17. The saturated transistor will
draw an increased current through resistor 18 and increase the
emitter-base voltage of transistor 19. This will turn on transistor
19 which is normally off. The collector of transistor 19 is
connected to a voltage divider that controls the output frequency
of the voltage controlled multivibrator.
The output frequency of a voltage controlled multivibrator, a
device well known in the art, is dependent on the input voltage.
Transistor 19 and its output voltage divider are constructed to
decrease the frequency of the multivibrator to approximately one
half or less of its normal frequency. Since this is the subcarrier
signal, the input inoperative signal is transmitted to the receiver
by the decrease in the subcarrier frequency. Circuitry in the
receiver will detect this frequency change and indicate the input
inoperative condition.
The carrier oscillator includes a single bipolar transistor
Q.sub.21 connected in a common base configuration. The collector of
transistor Q.sub.21 is tuned by the resonant circuit of capacitor
C.sub.25 and inductor L.sub.3 to maximize the power gain of the
stage at the desired frequency. Positive feedback to sustain
oscillations is provided by the capacitive divider formed of
capacitors C.sub.23 and C.sub.24, and by the feedback path
including varactor diode CR.sub.1, inductor L.sub.1, capacitor
C.sub.21, and crystal resonator Y.sub.1 connected to the emitter of
transistor Q.sub.21. Capacitor C.sub.22 is a signal bypass and the
frequency of oscillation is determined primarily by the crystal
Y.sub.1.
Experimental tests indicate that spurious oscillations are
generated by the saturation of the collector-base junction of
Q.sub.21 under normal operating conditions. This junction of the
transistor is shunted by a metal-semiconductor Schottky-barrier
type diode CR.sub.8 that has lower saturation voltage than that of
the collector-base junction to prevent saturation with concomitant
generation of spurious frequencies. This simplifies the transmitter
circuitry by reducing the filtering requirements and also greatly
facilitates the tuning-up procedures required to establish proper
operation on the assigned frequency.
Referring again to FIGS. 1 and 2, the oscillator bias and shut down
circuitry 11 detects a low battery supply 7. A weak battery causes
the regulator 9 to become ineffective, and signals generated during
this condition may be erroneous because of supply voltage drift.
The regulator includes a series-pass transistor, a device well
known in the art. As the battery output current decreases, the
voltage drop across the emitter-collector terminals of the
series-pass transistor decreases. This causes the transistor to
begin to saturate and draw more base current. When the transistor
saturates, the regulator loses control over the output of the
voltage supply.
To detect this problem, the oscillator shut down circuitry 11
monitors the base current in the series-pass transistor. When the
current exceeds a given value, the shut down circuitry prevents the
generation of the carrier signal and stops the radiation from the
transmitter. In the process of the transistor 6 turning on to
prevent the oscillator from operating, it draws more current than
is normally supplied to the oscillator, thus further reducing
battery voltage and assuring that the oscillator remains locked
off. Without this current drain to replace the oscillator current
drain, the reduced current drain on the battery would result in
increased battery voltage sufficient to reactivate the oscillator.
This would produce an unstable condition that would result in
intermittent transmission. The present circuit thus assures that
the transmission of erroneous signals due to an unregulated power
supply is prevented.
Referring now to FIG. 3, wherein is shown a functional diagram of
the receiver circuitry, the FM-FM signal transmitted from the
ambulatory patient is received at the antenna 25 of the stationary
receiver. After amplification, this signal is demodulated to an IF
signal in the conventional manner. It is frequency mixed at the
mixer 27 with a local oscillator signal which is generated by the
local oscillator 30. After passing through the discriminator, the
resultant signal at node 33 is the frequency modulated subcarrier
signal carrying the ECG information.
After passing through a buffer amplifier and a filter, the signal
is converted from a sinusoid to a square wave by the subcarrier
amplifier 35. The square wave then drives a monostable
multivibrator 36 that gives a pulse output for every positive or
negative transition of the square wave. The multivibrator output
carries the ECG signal in its frequency modulated pulse train. The
pulse train is time averaged and filtered by the ECG filter 38, and
the filter output is the original ECG signal detected by the
electrodes attached to the patient. This signal is amplified by the
output amplifier 39. The output terminal 40 can be connected to an
electrocardiograph or any other appropriate monitoring device.
Thus, the receiver performs two demodulations of the FM-FM input
signal to extract the original ECG signal.
The receiver detects a detached electrode by monitoring the
frequency of the subcarrier signal. Since the pulse output from the
multivibrator 36 is directly proportional to the ECG modulated
subcarrier, the receiver compares the period between the pulse to a
predetermined period. This is done by the period comparator 42 that
is connected to an output from the multivibrator. The pulse output
is used to discharge a capacitor. When there is no pulse, the
capacitor charges. Consequently, for lower frequencies, i.e.,
longer periods between pulses, the capacitor charges to higher
voltages. For a low enough frequency, the capacitor charges to a
voltage high enough to trigger the electrode inoperative
circuitry.
The period comparator is adjusted to trigger the turn off delay 44
when the pulse train frequency corresponds to a subcarrier
frequency indicative of the electrode inoperative condition. For
the transmitter shown in FIG. 1, the comparator would be set to
trigger when the subcarrier is at one-half its normal frequency
which is outside the normal operating band of frequencies. The
turn-off delay energizes the electrode inoperative indicator 50 to
signal to the operator that this problem exists. The turn-off delay
also shuts down the output from the receiver by energizing the
output hold off circuitry 41. This is done to prevent erroneous
output signals.
The receiver senses the signal conditions that indicate when the
patient is out of range or when the transmitter is inoperative.
This sensing circuitry includes two peak-to-peak detectors 52, 54
connected to the output of the buffer amplifier 34 and a window
comparator 56 that examines the output from the detectors. The
comparator drives circuitry 60, 61 that controls the local
oscillator frequency. It also controls the output hold off
circuitry 41 and the range/battery inoperative indicator 48. When
the window comparator detects an inoperative condition, it disables
the electrode inoperative circuitry 42, 44 to prevent erroneous
indications of detached electrodes.
The input to the peak-to-peak detectors 52, 54 is the ECG modulated
subcarrier signal. These detectors, well known in the art, convert
the peak-to-peak voltage of the FM subcarrier signal to a
representative D.C. voltage. One detector 52 holds the peak-to-peak
voltage for a relatively long time while the other detector 54
holds the voltage for a much shorter time. Each detector includes
two capacitors that charge respectively to the peak voltage of the
negative half cycle and the peak-to-peak voltage excursion. The
period of measurement of a detector is determined by the discharge
times of the capacitors.
The range/battery inoperative detection may be considered as
working on the amplitude of the demodulated subcarrier. The output
of an FM discriminator is a wave having an amplitude that is
related to the frequency deviation of the carrier, as shown in FIG.
4. The discriminator output for a noise input signal thus typically
has a higher peak amplitude and, for interference input signals,
may generally have either higher or lower peak amplitude than on
applied input signals, as shown in FIG. 5. As an example, consider
an AM signal as an interference signal applied to the frequency
discriminator. Since the carrier frequency does not deviate with
time, the discriminator output may be simply a static value that
can be readily analyzed.
By using the comparators 56 and 56', it is possible to set a narrow
"window" about the discriminator output voltage and require that
the peak output amplitude remain in the "window" selectively, as
shown in FIG. 6, to unlock the inoperative circuits. The time
constants of the detectors 52, 54 may be chosen such that for
transitions between noise and interference, there is no interim
period where the inoperative circuits unlock, i.e., one comparator
would be activated before the other comparator releases. The
detectors charge quickly on output signal and discharge at the
selected time-constant rate, as shown in FIG. 7.
Referring again to FIG. 3, the upper limit of comparator 56 of
conventional design is set to trigger on amplitudes above the ECG
modulated subcarrier amplitudes. These higher amplitude signals are
caused by noise received by the antenna when the transmitter is out
of range or no longer transmitting. The latter occurs when the low
battery circuit in the transmitter shuts down transmission. When
the output of the longer time constant detector 52 reaches a
voltage higher than a reference voltage that corresponds to the
upper trigger amplitude, the comparator energizes the range/battery
inoperative indicator 48.
The lower limit of the comparator 56' is set to trigger on
amplitudes below the ECG modulated subcarrier amplitude. These
lower amplitude signals result from an unmodulated or off channel
interfering frequency. When the output of the shorter time-constant
detector 54 decreases to a voltage below a reference voltage that
corresponds to the lower trigger voltage, the comparator 56'
energizes the range/battery inoperative indicator 48. Thus the
window comparator 56 is unresponsive to signals within its
"window," but for signals above or below set limits, it energizes
the range/battery inoperative circuitry.
Besides energizing the inoperative indicator 48, the comparator
triggers the output hold off circuitry 41 when it detects an
inoperative condition. The hold off grounds the output terminal 40
to prevent an erroneous output from the receiver. The comparator
also disables the electrode inoperative circuitry 42, 44, 50 to
prevent an erroneous indication of a detached electrode when there
is a range/battery inoperative condition.
The window comparator is connected to the local oscillator loop in
the demodulation circuitry. The comparator output controls the
automatic frequency control (AFC) 30 to regulate the local
oscillator frequency. When the comparator detects an inoperative
condition, it energizes the free running multivibrator 60. The
multivibrator varies the local oscillator frequency from near one
band edge and then releases it to the control of the AFC loop. If
the inoperative circuit does not clear, the multivibrator then sets
the local oscillator to a frequency near the other band edge and
releases it to the control of the AFC loop. This action continues
until the receiver locks on an appropriate received signal. When a
transmitted signal is received and detected, the range/battery
inoperative circuitry 41, 60, 61, 62 and the indicator 48 will turn
off. Then the receiver will operate normally.
As shown, this invention detects and indicates certain malfunctions
of an ECG telemetry system. Accurate detection and prompt
indication of malfunctions are invaluable to ECG monitoring systems
because they permit continual monitoring of the patient's actual
heart condition. Without them, it would be more difficult for the
system operator to determine the cause of an unusual ECG
signal.
* * * * *