 |
Description  |
|
|
This invention relates to electrocardiography (EKG) signal
transmission-reception method and apparatus, and more particularly to
transmission from a patient's location via common carrier wire lines,
preferably telephone lines, to a central diagnostic office. In this text,
the terms "central," "central office", "central diagnostic office," are
used interchangeably; the terms are not limited to a "central" in the
usual telephony sense, but are applicable also to a hospital, a clinic, or
even a single cardiologist's office or even a general practitioner's
office, provided required equipment is present at that location.
EKG signal transmission schemes of the general character as contemplated by
the present invention, have been previously proposed, but have not taken
into account the peculiar problems which arise out of the use of the
Pacemaker. The present invention has among its objects, the provision of
EKG signal transmission-reception for patients with Pacemaker.
Beginning in the late nineteen-sixties, the Pacemaker has come into
widespread use by cardiac patients. The Pacemaker is a heart-beat
stimulator implanted in the patient's body. The most commonly used
Pacemaker are designed to be inactive for so long as the patient's natural
heart-beat rate is equal to or greater than a minimum rate acceptable for
the particular patient. Whenever the patient's heart-beat rate slips below
the minimum rate, the Pacemaker becomes active or "paces" to stimulate the
heart beat. Above this rate Pacemaker stimuli are suppressed.
The pacing action is discernible in the electrocardiogram as a signal or
"spike" of distinctive waveshape, herein also referred to as "artifact".
The just described mode of Pacemaker operation is known as the "demand"
mode in the sense that the Pacemaker operates only when needed or
"demanded", and is otherwise inactive.
The Pacemaker is battery-powered; the life of the battery, although quite
long is nevertheless finite; reduction in performance of the battery, and
certainly erratic performance or total failure of the battery can be quite
dangerous or even fatal to the patient. It therefore behooves to test the
battery periodically to determine remaining useful battery life, and
possible necessity for surgical replacement of the implanted Pacemaker
including its battery.
In order to test the battery, it had been heretofore the practice to switch
the Pacemaker from the demand mode to a "fixed" mode, in which the
Pacemaker operates at a fixed frequency independent of the patient's
heart-beat rate. Switching to the fixed mode is accomplished by placing a
permanent magnet over the Pacemaker, externally to the patient's body. For
the fixed mode, and in rather great contrast to the demand-mode action,
the Pacemaker artifact is detectable at regular precise intervals. As a
result, the Pacemaker induced heart rhythm may compete with the natural
and independent rhythm of the heart. Under such conditions it is possible
for the heart to stop beating normally and enter a state of fibrillation
with possible fatal results.
Heretofore it had been the practice, for purposes of periodically checking
battery life, to have the Pacemaker operate in the fixed mode, and measure
the fixed-mode operating frequency. Change in operating frequency is an
indication of future reduction in performance, or even total failure of
the battery.
There exist no technological obstacles to measuring the Pacemaker fixed
rate at a remotely located central, and it is within the purview of the
present invention to do just that. However, there exist psychological
obstacles to such measurements. The present invention contemplates that
the patient transmit EKG signals to the central, even without supervision
of a physician or of anyone else; that is, completely on his own. Many
patients refuse to perform the things necessary to transfer to the fixed
mode, namely, placing the magnet over the Pacemaker; this stems from
unwillingness or fear to disturb something which appears to be functioning
well and to which the patient owes his continuing life.
Not only the patients but also quite a few doctors are reluctant to operate
the Pacemaker fixed mode, for fear of inducing fibrillation, as noted
above; this is certainly a theoretical possibility, but has not happened
even once as far as is known.
In arriving at the present invention, it was discovered that periodic
measurement of the Pacemaker artifact frequency, even in the demand mode,
is a good and reliable indication of residual battery life. This assumes,
of course, that the Pacemaker artifacts have not been suppressed, as is
generally the case. For this purpose, it is desirable to derive at the
central, from the composite received EKG plus artifact signal train, a
train consisting solely of artifact-corresponding signals, and measure
their repetition rate. The present invention contemplates
electrocardiography transmissions of the order of one minute; if during
such a time interval there occurred as few as just two sequential
artifacts in the demand mode, their time spacing would be a good
indicator, considered in a history of similar periodic measurements, of
the remaining useful battery life. In addition, measurements may be made
in fixed mode Pacemaker operation since the artifacts are present on a
regular basis. Timed marker signals permit precise measurement of the
artifact frequency.
The present invention thus enables measuring at the diagnostic central, of
the artifact frequency to the accuracy required to forecast battery
failures. In addition, the described system not only checks the condition
of the Pacemaker battery but also permits evaluation of possible failures
in the mechanisms of interaction between the Pacemaker and the heart (such
as failure to sense a conducted beat, or failure to properly stimulate the
heart because of a damaged or inoperative catheter) which result in "loss
of capture."
Previously proposed EKG signal transmission schemes had a major
shortcoming, the overcoming of which is an object of this invention,
namely, the transmitting apparatus could be provided only as "fixed
station", that is at a "permanent" location and as a "permanent"
installation. In contrast, by the present invention it is possible to
attach the transmitting apparatus quickly at well-nigh any location
equipped with a telephone, for only so long as the transmission is
required, and thereafter remove it just as quickly, and without the need,
at the transmitter or the telephone, of permanent and expensive coupling
devices. In this manner, the transmission may be effected by the doctor,
or even by the patient himself, from the patient's home, or wherever the
patient may be located, either under emergency conditions, or as part of a
periodic electrocardiography program.
Other objects, features and advantages of the invention will be apparent
from the following more detailed description, of which the appended claims
form a part, considered together with the accompanying drawings, in which:
FIG. 1A is a block diagram of the preferred method of precisely measuring
Pacemaker artifact frequency.
FIG. 1B is a block diagram of the equipment at the patient's or
transmitting location;
FIG. 2 is a block diagram of the diagnostic central, or receiving location,
both in accordance with a preferred embodiment of the invention;
FIG. 3 is a waveshape diagram useful in the interpretation of FIGS. 1B and
2; and
Fig. 4 is a "histogram", that is, a graphical statistical synopsis
machine-prepared in response to the EKG signals received at the central
illustrated in FIG. 2.
The following description is presented as applied to a patient with
implanted Pacemaker. Of the several signals or waveshapes obtainable by
EKG methods, which waveshapes are known as "complexes", "segments" and
"waves," the single one which by itself is most prominent is the QRS
complex illustrated in waveform A of FIG. 3. The QRS complex results from
depolarization of the ventricles prior to contraction. It is substantially
coincident with the actual contraction of the cardiac muscle, which
produces the pumping action. The QRS complex has in the case of a healthy
heart, a relatively rapid rise and fall; the interval between half
amplitude points may be typically about 0.04 seconds, and this width may
increase when the heart muscle is damaged.
For purposes of FIG. 3, it is assumed that the patient is equipped with a
Pacemaker operating in the demand mode. Illustrated are a number of QRS
complexes QRS1, QRS2 .sub.- - - QRSx (observe the "break" in the time-axis
following QRS3), but only some, and more specifically, complexes QRS1,
QRS2 and QRSx, are preceded by Pacemaker "spikes" or artifacts, PM1A, PM2A
and PMXA, respectively. In the fixed mode, each QRS complex would be
preceded by a Pacemaker artifact. And in the use of a patient without
Pacemaker, no artifacts would appear at all.
For a patient equipped with a Pacemaker, the QRS complex will be preceded
by a Pacemaker "spike" or "artifact", designated in FIG. 3 as PM. Whereas
the upper limit of the significant spectral components of the QRS complex
is of the order of 80 Hz, those of the Pacemaker artifacts are somewhat
higher, approaching 200 Hz; advantage of this fact is taken in separating
out Pacemaker spikes, as described below with reference to FIG. 2.
In the composite Pacemaker artifact-QRS complex wave train A, an artifact
PM precedes the QRS signal by a variable amount, here arbitrarily taken as
about 0.06 seconds; the period for complete repetition of a composite
PM-QRS signal for the fixed mode is arbitrarily taken as 0.8 seconds.
Referring to FIG. 1B, before transmission to the diagnostic central of FIG.
2 begins, the patient will have attached to standard points on his body,
EKG electrodes designated in FIG. 1 as 9 to 13. For satisfactory detection
of the EKG and PM signals, a minimum of two electrodes is required; these
are usually the right and left arm electrodes 9 and 10. When additional
information is required three additional electrodes 11, 12, 13 are
utilized; these are, in order, a left leg electrode; a right leg electrode
(also known as indifferent or ground electrode); and a chest electrode
(also known as precordial electrode). These will have been connected, for
example by plug-in connector (not shown) into an amplifier 14, which
accordingly delivers at its output an amplified version of the waveshape
A. The amplifier may be of the type conventional for EKG purposes, but
owing to the higher frequency components of the Pacemaker artifact PM,
will have a higher upper cut-off frequency. In a working embodiment, the
amplifier 14 was given lower and upper down-three-decibel frequencies of
0.25 and 80 Hz respectively. Higher frequency Pacemaker components are
also transmitted since their amplitude is considerably greater than the
EKG signals. To suppress the conventional power line frequency, 60 Hz in
the United States, the amplifier 14 is designed for "balanced" dual input
with consequent common-mode suppression, that is self-cancellation of
in-phase power-line and other pickup noise signals at the electrodes 9 and
10. For this purpose, advantageously the right leg electrode 12 is placed
on the patient's body and is grounded as shown. The amplifier 14 is
battery-powered in order to maximize portability of the unit.
The amplified output signal A is applied to a frequency modulator or
voltage-controlled oscillator 15, which operates at a carrier frequency in
the middle audio frequency range, typically 1800 Hz, with maximum
frequency deviation of about .+-. 25% or .+-. 450 Hz. The instant
frequency deviation from carrier frequency is proportional to the instant
amplitude of the signal in wavetrain A, thus giving rise to the term
"voltage-controlled oscillator." For purposes of this text frequency
modulation and phase modulation shall be considered synonymous.
The output of the modulator 15 drives a loudspeaker 16 which accordingly
produces sound output in a frequency range from 1350 Hz to 2250 Hz.
The description so far given is preparatory to transmission of the EKG
signals. Transmission is initiated by telephone dialing the diagnostic
central, utilizing the telephone set 18 at the patient's station, via
outgoing telephone lines indicated symbolically as 19; the double ended
arrow signifies facility for two-way communication. When voice
communication with the central has been established, and when readiness at
both ends has been acknowledged, the telephone set 18 is placed with the
mouthpiece 17 in proximity to or even placed directly over, the
loudspeaker 16. Such direct acoustic coupling is in line with the
objective of utilizing the equipment well-nigh anywhere. Transmission of
the modulated carrier is thus commenced, and continued usually for a
preagreed time of the order of 1 minute; this to afford the possibility of
further voice communication.
A suitable EKG transmitter is disclosed in U.S. Pat. No. 3,872,251 filed
Feb. 20, 1973 for Electrocardiography Transmitter and Transmission Method
and issued Mar. 18, 1975 to the same assignee.
Referring to FIG. 2, the line 19 is shown symbolically as connected to a
switchboard 20 at the central, along with numerous other lines the last of
which is designated as 19x. The showing of plural lines may be considered
symbolic, since in principle the invention may be practiced with the
single telephone subscriber's line assigned to the central. However the
plural lines 19, etc. may include plural public lines, to accommodate
plural incoming calls simultaneously, and may even include private lines
to cardiologist's offices. The switchboard 20 may be Model 507
manufactured by Western Electric Co., and as such affords facility for
receiving incoming calls, dialing outgoing calls, voice communication,
connections to various extensions, facility for multi-party conference
connections, etc.
The receiving station equipment proper may be considered to be the units
shown to the right of the switchboard 20. When voice acknowledgement has
been made between patient's station with the attendant at the switchboard
20, as adverted to in the description of FIG. 1B, the attendant
interconnects at the switchboard 20, the incoming call-line 19, now
transmitting the modulated carrier, with the receiving equipment, and more
particularly with a coupler or set of couplers 21, which may be of the RDY
type, manufactured by the Western Electric Company. The couplers 21 are
provided primarily for the purpose of isolating the telephone lines, in
both directions, but they are not absolutely essential.
From the coupler 21, the received modulated carrier is inserted into a
well-known frequency discriminator 23, which reproduces the original
wavetrain A at its output. The thus reproduced or recovered wavetrain A is
applied to inputs of a high-pass filter 24 (which in turn feeds a signal
shaper 25, which is followed by a beats-per-minute counter digital display
and printer 26), an arrhythmia analyzer 28, and to an EKG pen-recording
mechanism 30 which accordingly plots the electrocardiograph on standard
EKG paper; note the reproduced trace A within unit 30.
The pen-recording mechanism 30 may be considered a minimum requirement for
a diagnostic central; the EKG trace is of itself sufficient to provide a
basis for determining all the other data provided by the units 26 and 28,
however with lesser accuracy, for the following reasons.
In accordance with standard practice the paper moves at a speed of 25
millimeters per second, and is accurately ruled every millimeter. A
frequency measurement is usually made by the human interpreter over a 12
second interval. The number of Pacemaker spike-to-spike intervals
(including fractional intervals) is measured within a convenient 30
centimeter space, and the result, multiplied by five, is the Pacemaker
spike frequency in beats per minute. This result, however, may be
inaccurate by several percent because the paper speed varies with several
uncontrolled factors, such as power line frequency, variable paper drag,
etc.
In accordance with the invention, correction for the inaccuracy is made by
having a timer 32 actuate a second pen-recorder typically every 3 seconds.
This produces a second, parallel trace (see unit 30) of time markers. The
number of millimeters separating 11 consecutive markers made by this pen
should be 750, if the speed is precisely 2.5 cm per second. The actual
number of millimeters separating 11 such markers is counted. If this
actual number is "x", the factor (x/750) is a required correction on the
frequency measured above, i.e., the true Pacemaker spike frequency will be
the measured frequency multiplied by the factor "x/750". Since the time
markers can be measured to better than 0.2 millimeters, the paper speed is
thus calibrated to better than 1 part in 3750, which allows the Pacemaker
spike frequency to be measured to better than 0.1 beats per minute (in
contrast, the accuracy of the unit 26 is about 1 part in 5000). Timer 32
may be a crystal oscillator with an output frequency of 3 pulses per
second, of the type manufactured by Fork Standard Corporation, West
Chicago, Illinois. The accuracy of unit 30 is desirable for Pacemaker
patients and is considerably higher than is customarily accepted in
electrocardiography as applied to a patient without Pacemaker. Thus the
just described feature of the invention may of itself be sufficient to
provide, at lesser accuracy to be sure, an approximation of the data
available from the units 26 and 28, or may serve as a redundancy device to
safeguard against failures or malfunctions of these units. In concluding
part of the description there are given further examples of interpretation
of the data produced by units 26, 28 and 30.
FIG. 1A is a flow chart of the overall method of precisely measuring the
Pacemaker artifact frequency when EKG data is transmitted from a first
location to a receiving location. The EKG data includes information
relating to the heartbeat of a patient and the Pacemaker implanted in the
patient's body. The purpose of the method is to precisely measure the
frequency of the artifact signals to accurately assess the residual life
of the battery or other power source of the Pacemaker, or any artificial
heart stimulator. The method comprises the following steps:
a. sensing and generating a train of repetitive composite signals which
include a plurality of QRS complex signals attributable to the heartbeat
of the patient and a plurality of artifact signals attributable to the
operation of the heart stimulator and wherein the artifact signals are
interspersed with the QRS complex signals;
b. frequency-modulating the train of signals onto a carrier signal having a
carrier frequency in the audio frequency range;
c. transmitting the frequency-modulated carrier to the receiving location
via ordinary communication carrier lines;
d. demodulating, at the receiving location, the incoming
frequency-modulated carrier signal to reproduce a signal train which has a
waveform corresponding to the original signal train;
e. recording the reproduced signal train on a moving recording medium
having equally-spaced calibration lines and designed to move at a desired
speed;
f. generating and recording on the recording medium, at substantially the
same time as the recording of the reproduced signal train, and completely
independent from the recording medium moving means, periodically-timed
marker signals spaced in time so that during that time the moving
recording medium will move a desired space relative to the marker signals
when at the desired speed;
g. measuring the linear distance between sequential artifact signals on the
recording medium as indicated by the calibration lines to determine the
measured artifact signal frequency, assuming that the recording medium is
moving at the desired speed;
h. measuring the space between selected marker signals to determine if the
moving recording medium was moving at the desired speed during the
recording, thereby indicating that the measured artifact signal frequency
is the correct artifact signal frequency;
i. and obtaining the precise artifact signal frequency if the moving
recording medium was not-moving at the desired speed by correcting the
measured artifact signal frequency in accordance with a correction factor
determined by the ratio between the measured space between the selected
marker signals and the desired space when the moving recording medium is
moving at the desired speed.
The high-pass filter 24 serves to filter out those lower-frequency
components in wavetrain A, which are due to the QRS complex and other
components of the EKG, and thus serves to provide via signal shaper 25 a
signal train B (see FIG. 3) consisting of reshaped pulses derived from the
Pacemaker artifacts PM. The signal train B is applied to a counter 26,
which is preferably of the type manufactured by Monsanto Company, Type
107A, which has the capability of measuring and digitally displaying time
intervals between successive input signals (here, the pulses in waveshape
B) and also their frequency expressed in beats per minute, to an accuracy
of 1 part in 5000 as previously stated.
The repetition rate of the Pacemaker artifacts (see FIG. 3) remains fairly
constant throughout the life of the Pacemaker battery, but tends to change
by several percent near the end of the battery life, and this is true not
only for fixed mode operation, but also for demand mode operation; for the
demand mode the resolution accuracy of frequency measurement is
necessarily reduced, but is still sufficient for forewarning potential
failure of the Pacemaker battery life.
The arrhythmia analyzer 28 may be of the kind manufactured by Instruments
for Cardiac Research. When the "demand mode" EKG is transmitted and
recorded, the unit 28 measures R--R intervals between consecutive R-waves
(see FIG. 3) and groups these intervals into 20 millisecond classes. The
number of intervals in each class is counted, and the counts are displayed
as a function of the interval duration as shown in FIG. 4, by the
arrhythmia analyzer 28. From the displayed data, it is possible to compute
an average R--R interval from the succession of QRS complexes, and to
compute the coefficient of variation (CV) of the R--R intervals. The
various graphs or clusters shown in FIG. 4 are an indication of the
statistical distribution of R--R intervals of various durations. The
significance of these clusters will be explained in the concluding part of
the description. The description so far given has been for "real time"
operation of the diagnostic central of FIG. 2; however, it is desirable
for some purposes, to have available the facility of having the functions
of the units 26, 28, 30 or of units equivalent to them, at a later time.
Such purposes are, amongst others:
As a redundancy safeguard against possible malfunction or failure of the
equipment.
For re-constructing the final output records at a future time.
For generating the final output records in the first instance, later, when
the equipment is busy with an earlier-commenced transmission.
For re-transmitting the transmission (voice and EKG, if desired) to another
"central", for example a cardiologist's office.
Accordingly, the coupler 21 is also connected, and bidirectionally so, to a
magnetic tape recorder 34, which under control of a retransmission control
unit 36, may be switched in and out to record the incoming transmission,
for concurrent real time processing by the units 26, 28, 30, or upon
playback for subsequent processing; or for retransmission via switchboard
and an outgoing line, again on a concurrent or a subsequent basis.
The described method and system is highly versatile and advantageous;
amongst the many possible uses and advantages are the following:
The described system checks both the condition of the Pacemaker battery and
"loss of capture" of the heart beat by the Pacemaker.
Lack of capture (adverted to in the specification introduction), either due
to failure to sense the conducted beat or failure to stimulate the left
ventricle, may be determined both by direct observation of the total EKG
as well as by measurement of the coefficient of variation (CV) of the R--R
interval. Direct observation of the total EKG indicates that the Pacemaker
spike and the QRS complex no longer maintain an essentially constant phase
relationship when there is "loss of capture." On other occasions, the QRS
complex is missing altogether, despite the presence of the Pacemaker
spike. The variability of the QRS complex (or its absence) is apparent to
the trained cardiologist by direct observation of the total EKG record.
The normal variability of the QRS complex gives rise to a coefficient of
variation (CV) usually on the order of 3-5%. During loss of capture, the
variability exceeds 10%. This is recognized on the interval histogram (see
FIG. 4) as a cluster of premature beats VPB, a normal cluster NB somewhat
displaced in time (with a longer period), and a third cluster CP of
"compensatory pauses," with an abnormally long period. The numerical value
of the CV of such an interval histogram is consequently greater than
normal. An increase in this CV quantifies the visual observation made by
the cardiologist and thus helps define the specific Pacemaker defect.
Under normal conditions, solely the cluster NB would be present. With the
Pacemaker even in its normal demand mode, the EKG record itself, and the
R--R interval histogram, indicate whether the heart Pacemaker interaction
is normal. The data thus extracted from the transmitted signal is plotted
for each transmission (usually monthly, but more often than once per month
at the beginning and end of Pacemaker battery life). Changes in these
extracted parameters (Pacemaker spike frequency and coefficient of
variation), together with a cardiological interpretation of the
transmitted EKG, permit a determination of the need for surgical
replacement of the Pacemaker or of its catheter sensing and stimulating
electrode.
* * * * *
|
|
 |
|
 |
Description |
|
