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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of medical electronics and
specifically relates to apparatus and method for the detection, definition
and classification of ectopic heartbeats using two-channel vector ECG
signals.
2. The Prior Art
It has long been known that medically-significant vector electrocardiograms
can be produced through the use of a three-lead system. Previous studies
have already indicated merit in the vector cardiographic analysis of
anomalous and ectopic beats for identifying the site of origin of ectopic
beats. Such anomalous beats not only commonly result in alteration of
readily apparent direction and magnitude of QRS and T force vectors, but
also affect the direction of rotation QRS vector forces, often accompanied
by abnormal delays of QRS vector inscription. The latter characteristics
are not readily apparent in analog electrocardiographic signals, and thus
the vectorcardiogram gives additional discriminative data. The adjunctive
vectorcardiographic data complements the analog cardiographic signal data
by providing a visual integrated picture of the electrical activity.
The preponderance of the diagnostic vector electrocardiographic studies
have been carried out using the Frank lead system or a modified McFee lead
system, which lead systems were designed to measure horizontal,
longitudinal and saggital plane forces.
In U.S. Pat. application Ser. No. 786,252, filed Apr. 11, 1977 for
VECTORCARDIOGRAPHIC METHOD FOR AMBULATORY PATIENTS, Dr. Harold L. Kennedy
discloses a method for obtaining vectorcardiographs using a two-lead
system. This simplified lead system using the V.sub.1 and V.sub.5
electrode sites was developed specifically to facilitate diagnosis and
identification of cardiac dysrhythmias, particularly anomalous
extrasystols, as seen in the lead V.sub.1, and to best detect left
ventricular myocardial ischemia by employing a commonly used
exercise-sensitive lead, chest bipolar lead V.sub.5.
The simplified two-lead system disclosed in the Kennedy application
identified above has proven to be medically useful for producing
vectorcardiograms for the diagnosis of arrhythmias, and consequently, with
the widening use of the method, increasing amounts of two-channel
vectorcardiographic signals can be expected to be produced. Increasing
amounts of time and manpower will be devoted to the analysis of such data,
and the need for apparatus for automatically analyzing the data has become
clear.
SUMMARY OF THE INVENTION
The present invention is a first step towards automating the analysis of
two-channel vectorcardiographic data. Because vector data lends itself to
many different types of analysis, it is recognized that different types of
circuits will be required to implement the various types of analyses to be
performed.
The present application discloses apparatus and method for improved ECG
arrhythmia detection utilizing information from two-channel signals.
Although this apparatus can, in a particular case, use two of the three
channels normally employed for vector cardiography, it is particularly
suited, but not limited, to the high-speed analysis of long-term,
two-channel ECG magnetic tape recordings (commonly referred to as Holter
recordings) of ambulatory and bed-ridden subjects.
In vector cardiography, the tip of the vector which represents the
electrocardial potentials typically traces an oval or cardioid trajectory
during the course of each ventricular depolarization. Previous clinical
studies, using data from three-lead vector cardiograph systems, have
indicated the diagnostic value of the maximal QRS and T vectors which are
the vectors drawn from the starting point of the loop to the farthest
points of the QRS and T loops. The apparatus of the present invention
permits measurement and analysis of these maximal vectors.
The maximal vector should not be confused with the mean direction which is
the vector equal to the sum of all of the instantaneous vectors. The
present invention includes apparatus for measuring the angle between the
QRS peak vector and the T-wave peak vector. The apparatus can also be used
to measure either of these peak vectors separately.
The absolute values of the QRS peak vector, the T-wave peak vector, and
their difference are not of prime importance for diagnostic purposes,
since the absolute values vary from patient to patient as well as with
variations in the positioning of the electrodes on the patient. Instead,
in each instance, it is departures from the angles normally observed in a
given patient that are diagnostically significant.
The scalar representation of an abnormal supraventricular complex,
particularly if nodal-originating, may appear as a bizarre waveform
closely resembling a ventricular-originating arrhythmia. However, the
relationship between the depolarization potentials represented by the QRS
vector forces and the repolarization potential represented by the T vector
forces has been proven to be nearly identical for all supraventricular
originating complexes, both normal and abnormal. As a result of this fact,
a first condition that can be distinguished is whether the ectopic complex
is truly of supraventricular origin, the categorization of which includes
the normal pacemaker (S-A node) complexes in addition to abnormal atrial
and nodal ectopic beats. Thus, it is of utmost importance and utility that
the differential vector angle can initially aid in the diagnoses and
categorization of supraventricular ectopics, whereas a single (scalar)
lead system cannot reliably be used to do so.
Additionally, ventricular ectopic complexes of significantly different
points of origin (foci) within the ventricles also display significantly
different vector angles. Therefore, further categories can be set up for
the purpose of identifying the relative foci of the ectopic events, and to
some extent (when the lead configuration and heart position are known),
the location of the foci within the heart muscle itself.
Ventricular ectopi rarely originate from more than five significantly
separate foci, and typically originate from one to three foci. Therefore,
considerable simplification can ultimately be achieved in the overall
circuit mechanization.
In the present invention, two-channel, approximately orthogonal ECG signals
are applied to a rectangular-to-polar coordinate converter, which produces
two output signals showing respectively the instantaneous magnitude and
angle of the vector. In the present invention, not all of the
instantaneous values of the vector angles are of interest, but primarily
the vector angles at the instants when the ventricular depolarization
(QRS) and repolarization (T) complexes reach their peaks. These angles are
then subtracted to determine the angular difference between the QRS and T
vectors which henceforth will be termed "QRS-T angle" or "QRS-T vector
angle".
The first vector measurement is performed on the QRS complex, since it
occurs first, is the most predominant and is always present, regardless of
the ectopic origin. In one embodiment, this QRS angle is determined as the
instant when the magnitude of the QRS vector reaches its maximum value. It
is recognized, however, that if the loop traced by the tip of the vector
is nearly circular, it will be difficult to determine with precision the
instant at which the maximum magnitude of the vector is reached. When
loops of this kind are encountered, it is convenient to switch to a
different mode of operation in which the apex of the ventricular complex
is determined from one of the channels of ECG signals as being the instant
when the R peak occurs.
In either event, when the QRS peak has been detected, the instantaneous
value of the vector angle is sensed by a first sample-and-hold amplifier
which temporarily stores the sensed value of QRS angle.
After each QRS angle has been measured, the peak detecting circuit is reset
to permit measuring the maximal angle of the T vector, if present. The
maximal T Vector angle will always be determined from the magnitude of the
T vector. When the peak of the T vector magnitude is detected, the
instantaneous value of the vector angle is sensed by a second
sample-and-hold amplifier which temporarily stores the sensed value of the
T angle. In a preferred embodiment, the QRS angle and T angle are measured
alternately and stored temporarily.
The stored voltage representing the maximal QRS vector angle is applied to
the non-inverting input of an operational amplifier, and the stored
voltage representing the maximal T vector angle is applied to the
inverting input of the amplifier to obtain a difference output voltage
which represents the differential QRS-T vector angle, as desired. (In the
absence of a T vector, the output of the second sample-and-hold amplifier
is zero volts, which represents a T vector angle of zero degrees. As a
result, ventricular ectopi which lack the T vector are automatically
measured and categorized for absolute QRS vector angle.)
The temporarily-stored sampled values of the QRS-T vector angle could be
printed out or plotted at the end of each cycle of operation. This would
result in a lengthy chain of data when a Holter recording of, for example,
24 hours' duration is analyzed. Such a lengthy chain of data would still
require an unnecessarily large amount of time to analyze and study.
Accordingly, a classifying system is included in the present invention. In
the classifying system, each successively stored value of QRS-T angle is
tallied into one of a number of angular ranges. The cumulative tally in
each of the pre-selected angular ranges is preserved on a counter specific
to the range. The continually-updated counts on the counters can be
continually displayed, printed out, or plotted.
Through the use of the present invention, in association with appropriate
play-back apparatus, a 24-hour recording of two-channel ambulatory ECG
signals can be quickly analyzed, and the arrhythmias present in the
vectorcardiograph can be detected and classified with minimal operator
intervention.
The apparatus of the present invention can also be used to measure and
classify the vector angle at the peak of either the QRS complex or the T
complex as desired, rather than to operate on their difference. This can
be accomplished by a very simple modification to the preferred embodiment,
which, when so modified, may be viewed as an alternative embodiment.
The novel features which are believed to characterize the invention, both
as to organization and method of operation, together with further objects
and advantages thereof, will be better understood from the following
description considered in connection with the accompanying drawings in
which a preferred embodiment of the invention is illustrated by way of
example. It is to be expressly understood, however, that the drawings are
for the purpose of illustration and description only and not intended as a
definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of the present
invention;
FIG. 2a is a schematic drawing of a practical digital one-of-N select logic
used illustratively in a preferred embodiment of the present invention;
FIG. 2b is a table showing the states of the output lines for various
states of the input lines of the digital one-of-N select logic circuit;
FIG. 3a is a block diagram showing the generation of the QRS Detected Pulse
and the Valid QRS Pulse which are applied as inputs to the preferred
embodiment of the present invention; and,
FIG. 3b is a graph showing the ECG complex, the QRS Detected Pulse, and the
Valid QRS Pulse versus time.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to the drawings, FIG. 1 is a block diagram of a preferred
embodiment of the present invention, which also shows the best mode of
practicing the invention.
The inputs to the apparatus include two approximately orthogonal channels
of ECG signals shown at the left side of FIG. 1 and an optional scalar
complex validation input shown at the upper right hand portion of the
diagram of FIG. 1. The latter input will be discussed below. In the
application entitled VECTORCARDIOGRAPHIC METHOD FOR AMBULATORY PATIENTS by
Harold L. Kennedy, Ser. No. 786,252 filed Apr. 11, 1977, it was disclosed
that medically significant vectorcardiograms can be produced using only
two leads (4 electrodes) preferably a modified V.sub.5 lead and a modified
V.sub.1 lead, as shown in that application. These leads can also be used
as the channel 1 and channel 2 inputs to the apparatus of FIG. 1. In such
event, the V.sub.5 lead would preferably be used as the channel 1 input,
because the peak of the R wave is more sharply defined in that lead
generally. Leads other than V.sub.5 and V.sub.1 can also be used in the
present invention, although the leads used would preferably be
approximately orthogonal.
The channel 1 and channel 2 signals are the inputs to
commercially-available rectangular-to-polar coordinate converter 12 of
FIG. 1. The inputs are treated as the rectangular coordinates or
components of the vector, and the device produces output signals
representing the magnitude and the angle of the vector. These outputs are
produced continually as long as the vector has any detectable magnitude,
and the outputs are continually varying in time. A peak-determining
circuit 14 is provided for use in first determining the apex of the QRS
ventricular depolarization waveform and subsequently the apex of the T
wave repolarization waveform. At the instant each apex occurs, the
instantaneous values of each vector angle are sampled and held by the
sample-and-hold amplifiers 16 and 17 of FIG. 1, the outputs of which are
applied to differential amplifier 19 to obtain a voltage representing the
differential QRS-T vector angle.
The peak-determining circuit may use either of two alternative inputs for
determining the instant of occurrence of the apex of the ventricular
depolarization (QRS) complex. A manual switch 21 is provided to permit
enabling the selection of the alternative via electronic switch 18 at the
proper time in the analysis sequence. In the preferred embodiment, the
peak-determining circuit 14 monitors the magnitude of the vector present
on line 20. The vectorcardiogram has different normal shapes for different
people, and for the same person at difference times. In some cases, the
magnitude M of the vector remains nearly constant over a moderate range of
vector angle. In such cases, any errors made in determing the instant of
occurrence of the peak magnitude will result in a seemingly
disporportionately large error in the QRS vector angle. In this event, it
is desirable to alter the position of the electronic switch 18 to the
alternative position shown by the dashed lines, and in this alternative
configuration, the apex of the ventricular depolarization waveform is
determined from the peak of the R-wave of the signal on channel 1 and line
22, preferably the V.sub.5 ECG lead.
Whichever signal is chosen, it is carried to the peak-determining circuit
14 on the line 24. The peak-determining circuit 14 determines the instant
of occurrence of the peak of the input on the line 24 as follows. The
input on the line 24 is applied to a voltage peak detector 26 which
produces a continuous output on the line 30 equal to the maximum input
voltage that has been applied to the line 24 within the current cycle of
operation. The instantaneous value of the input signal on the line 28 is
compared with the maximum signal on the line 30 in the voltage signal
comparator 32. As long as the input to the voltage signal comparator 32 on
the line 28 equals or exceeds the maximum signal on the line 30, there is
no output from the voltage signal comparator 32. However, when the peak
has been reached and the signal on the line 28 begins to decrease and
falls below its maximum value on the line 30, the voltage signal
comparator produces an output on the line 34. This output signal on the
line 34 is produced approximately simultaneously with the occurrence of
the peak of the signal on the line 24.
The peak-indicating signal on line 34 is applied to the logic AND gates 23
and 25 which select the proper sample-and-hold amplifier 16 or 17 that is
to receive the resulting signal, depending upon whether the QRS or the T
vector is being analyzed. When the QRS, or first portion of the signature
is to be measured, the QRS detected pulse, described in connection with
FIG. 3, is present, and the output AND gate 23 is enabled, which in turn
is used to trigger the operation of the sample-and-hold amplifier 16,
which stores the instantaneous signal on the line 36 at the time of
occurrence of the triggering signal on the line 34.
At the termination of the QRS detected pulse, which is typically 200 ms
after the commencement of the QRS complex, one-shot multivibrator 44 is
triggered to produce a reset pulse on the line 46 to reset the voltage
peak detector 26 back to an initial zeroed condition in preparation for
measuring the T-wave magnitude.
From the above description, it can be seen that the operation of the
peak-determining circuit 14 is the same, regardless whether the signal on
the line 20 or the signal on the line 22 is used as an input. It is
further pointed out that any variable could be selected for application to
the line 36, to be sampled at the instant of occurrence of the apex of the
ventricular depolarization complex; the angle of the vector is applied in
the preferred embodiment of the invention because of its diagnostic
significance.
The output of the sample-and-hold amplifier 16 is a constant signal on the
line 48 which represents the sampled value of the vector angle. That value
is maintained on the line 48 until replaced by the value obtained at the
peak of the next occurring complex.
After one-shot 44 has reset the voltage peak detector 26, the detector will
function as previously described to follow and hold the highest voltage
amplitude that exists after it was reset. This will now be in the portion
of the complex typically containing the T-wave, the peak magnitude of
which generally occurs in the region of 250 to 650 milliseconds after the
start of the QRS complex. Since the T-wave is typically the signal of
greatest magnitude in this region, the voltage stored in the peak detector
will be representative of it. At the instant the peak of the T-wave vector
is detected, voltage comparator 32 will generate the signal for a second
time. In this instance, however, AND gate 23 is inhibited by the absence
of the QRS detected pulse. AND gate 25 is now enabled to receive the
signal 34, because of the action of logic inverter 27. Sample-and-hold
amplifier 17 is now triggered by the output of AND gate 25 to store the
instantaneous signal on the line 36 at the time of occurrence of the
triggering signal on the line 34. The stored output of amplifier 17 is
applied to the inverting input of differential amplifier 19. Since line
48, containing the stored voltage representing the QRS vector angle, is
applied to the non-inverting input of differential amplifier 19, the
output voltage will represent the difference between the two angles, which
is the desired differential QRS-T vector angle.
The output of the differential amplifier 19 is an essentially constant
signal on the line 29 which represents the sampled value of the QRS-T
vector angle. That value is maintained on the line 29 until after the next
QRS detected pulse occurs.
The successive sampled values on the line 29 are applied to a plotter 50 to
produce a visual display of the successive values versus time. The signal
on the line 29 is the data input to the classifying circuit 52.
In the classifying circuit 52 of FIG. 1, a constant reference voltage is
applied to a resistor divider network 54 to produce a set of reference
signals on the lines 56 having various constant reference voltages in a
preferred embodiment. Each of the lines 56 is an input to a separate
voltage comparator circuit within the voltage comparator 58. The signal on
the line 29 is applied to each of the individual voltage comparators
comprising the voltage comparator circuit 58. These individual circuits
sense the polarity of the difference between the signal on the line 29 and
the reference voltage. The polarity sensed by each of the voltage
comparators is represented by a binary signal on one of the lines 60.
Assuming that the reference voltages on the lines 56 are an increasing
sequence of positive voltages, the polarity of the signals on the lines 60
will depend on the magnitude of the line 29. If the signal on line 29 is
zero, each of the lines 60 will have a negative polarity. On the other
hand, if the signal on the line 29 is extremely large--greater than any of
the reference voltages--all of the lines 60 will carry a positive
polarity. In the usual case, the signal on the line 29 will lie within the
preselected range of reference voltages and therefore, typically, a first
subset of the lines 60 will have a positive polarity, while the remaining
lines 60 will have a negative polarity. This situation is illustrated in
FIG. 2a where, by way of example, and not by way of limitation, six lines
60 are shown. FIG. 2a shows the digital one-of-N select logic circuit 62
of FIG. 1 in greater detail. The six AND gates 64 of FIG. 2a permit the
vector angle on the line 29 to be classified into exactly one of seven
angular ranges by way of illustration; in the preferred embodiment, 23 AND
gates are used for greater resolution to classify the angle into one of 24
angular ranges, each 15 degrees wide. Additional resolution can be had by
increasing the quantity of circuitry. Each of the AND gates 64 has three
inputs. The lines 60 may be thought of as an electrical representation of
the boundaries of the successive angular ranges. Based on the polarities
on the lines 60 shown in FIG. 2a, it is seen that the vector angle lies
within the range bounded by B and C. Activation of the AND gate W is based
on the recognition that lines C and D have positive polarity and line B
has negative polarity. The inverters 66 associated with each of the AND
gates 64 reverse the polarity of the line corresponding to the boundary of
the angular range which exceeds the vector angle.
FIG. 2b shows the state of each of the input lines A-F as the vector angle
assumes ever-increasing values. Corresponding to each of these input
states, the circuit 62 defines and produces a unique corresponding output
state, as shown in FIG. 2b.
Thus, for each ECG signature, a single value of the QRS-T vector angle is
sampled and is then classified into one of a number N of angular ranges,
as indicated by the presence of the logic 1 on one of the output lines 68
of the digital one-of-N select logic circuit 62.
Returning to FIG. 1, the output lines 68 of the circuit 62 are applied to
individual counters shown collectively as the counter circuit 70. After
the start of each QRS complex has occurred, only one of the counters is
incremented. The incrementing occurs when a clock pulse is applied to the
counters via the line 72 which is connected to the clock input of each of
the individual counters.
In the preferred embodiment, the clock pulse is not applied to the line 72
through the AND gate 42 unless a scalar complex validation input had
occurred immediately previous on the line 74.
Flip-flop 31 provides a means for storing the occurrence of a scalar
validation input pulse until it is needed after the T-wave vector angle
has been measured. In the event flip-flop 31 was set by a signal on line
74, line 33 will rise to a logic 1 level. Since line 35 has no signal at
this point, gate 42 will have no output. Upon the occurrence of another
QRS detected pulse, the output of gate 42 will now go to a logic 1. This
will always follow the occurrence of the T maximal vector. Thus, the
counters 70 will be incremented by the presence of a logic 1 signal on
line 72.
The use of the scalar complex validation input and flip-flop 31 of the
preferred embodiment is optional in alternative embodiments. If used, the
validation input is a signal derived from a scalar (as opposed to vector)
arrhythmia detection system, as shown in FIG. 3a, to permit further
rejection of artifacts such as are caused by muscle tension, electrode
movement, electrical interference, etc., which might produce channel 1 and
channel 2 inputs sufficiently large to be misinterpreted as ventricular
depolarization complexes. Thus, the validation input assists the apparatus
of the present invention in discriminating against false complexes. In the
preferred embodiment, the AND gate 42 is activated by the leading edge of
the pulse on the line 35 with the simultaneous presence of a logic 1 on
the line 33. The leading edge of the pulse on the line 35 is coincident
with the detection of the following QRS complex. However, this can
alternately be accomplished at any time after the maximal T-wave vector
was determined.
FIG. 3a is a block diagram showing how the QRS Detected Pulse input and the
Validation input are derived for application to the preferred embodiment
of the present invention shown in FIG. 1. These two inputs can be obtained
from a commercially available ECG arrhythmia analysis device such as the
Electrocardiographic Computer produced by Del Mar Avionics of Irvine,
Calif., and described in U.S. Pat. No. 4,006,737 issued Feb. 8, 1977.
As shown in FIG. 3a, a scalar ECG signal on the line 77 can selectively be
derived directly from electrodes 79 on a patient or from previously
recorded signals on a magnetic tape 81, as determined by the position of
the switch 83. The scalar ECG signal on the line 77 has the waveform
indicated in the figure, and this signal is applied as an input to the QRS
Detection circuit 85 which includes filtering, threshold and timing
circuits to distinguish the QRS complex from the remainder of the ECG
waveform and to produce on the line 75 the QRS Detected Pulse shown in the
figure and applied as an input on the line 75 of FIG. 1.
The block diagram of FIG. 3 further includes the Scalar Arrhythmia Computer
87 which measures several scalar parameters of each ECG waveform, such as
QRS width and amplitude, R-R prematurity, etc., and in so doing develops a
logic signal on the line 74 which signifies that a valid QRS complex was
received.
The QRS Detected Pulse on the line 75 is typically 200 milliseconds wide,
commencing at the beginning of the QRS complex, as shown in FIG. 3b. The
Valid QRS signal on the line 74 occurs at the end of the QRS Detected
Pulse, by which time sufficient analysis has been performed by the Scalar
Arrhythmia Computer 87 to determine whether the triggering signal was
truly a QRS complex or was an artifact. A Valid QRS pulse will be
developed only if a QRS complex is actually present.
The counters 70 are not reset at the end of each complex, and thus they
accumulate the respective numbers of angles determined on successive
signatures to have fallen into each of the angular ranges.
The accumulated data stored in the counters 70 is continually applied to
the digital display 76 for visual observation, and is also applied to the
printers 78 to produce a graphic record of the results of the analysis.
As pointed out above, the apparatus of the present invention is indifferent
to whether the input signals on channel 1 and channel 2 are "live" signals
obtained directly from ECG leads or are played back recorded signals.
Further, the present invention is indifferent to the duration of the input
signals, so long as the capacity of the counters 70 is not exceeded.
Further, with appropriate choice of circuit parameters, the circuit of FIG.
1 can be designed to operate with inputs obtained from the high speed
playback of a magnetic tape recorded at real time speeds. Operation with
high speed playback permits rapid analysis of recordings of relatively
long duration. Prior to the invention of the present apparatus, the
analysis could be performed only by observing the trace of the vector on
the face of a cathode ray tube or by photographing the face of the cathode
ray tube for each succeeding complex. If mere observation were used, the
vector angles could not be determined with any precision, and lengthy
samples could be tallied into various angular ranges only with great
difficulty. On the other hand, photographing the face of the cathode ray
tube for each successive vectorcardiogram and then measuring the vector
angle on each photograph was a time-consuming and expensive proposition.
In contrast, the apparatus of the present invention senses and classifies
the vector angles automatically, without intervention of an operator. The
present invention permits a 24-hour sample to be analyzed in minutes,
thereby multiplying the effectiveness of the medical staff.
It is recognized that if it is desired to measure and classify only one of
the peak angles separately, rather than the difference between the peak
angles, this can be accomplished simply by disconnecting one of the input
leads to the differential operational amplifies of FIG. 1.
Numerous variations of the method in apparatus described above will be
apparent to those skilled in the art. For example, the signals represented
by voltages in the preferred embodiment could be represented by current or
frequency in other embodiments. Such variations are included within the
scope of the present invention which is limited only by the following
claims.
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