ECG analyzer

Apparatus for analyzing ECG signals from a patient in the form of a data stream comprises a data stream monitor to locate for each heart beat a trigger point (26,37,25) having a constant temporal displacement with respect to the location of a portion of the ECG signal (31-34) which may include micropotentials. A processor (15) determines which are acceptable beats and, where a heart beat is acceptable, generates and stores an average from successive portions. The resolution at which the average is stored is such that micropotentials present are detectable. The processor is adapted to store a set of averages generated during a corresponding number of time intervals.

FIELD OF THE INVENTION 
This invention relates to a method and apparatus for analyzing ECG signals 
from a patient in the form of a data stream. 
DESCRIPTION OF THE PRIOR ART 
A number of systems exist for analysing ECG signals. Among these are 
bedside monitors which analyze signals from a patient over a period of 
time under the control of an operator. Micropotentials are found in ECG's 
of abnormal patients. For the analysis of micropotentials, a gated average 
of the heart beat data is stored which requires the rejection of any beats 
which do not conform to a standard pattern. The patient is required to 
remain still and quiet whilst being monitored. For micropotentials to be 
detectable a large amount of high resolution data is stored over a 
relatively short time period e.g. 15 minutes. 
Other systems provide real time analysis storing transient data for real 
time display, but do not have the facility to store or record high re! 
solution data obtained over a long period of time. 
A number of systems are known for recording electrocardiogram over a 
defined time period. IEEE Transactions of Biomedical Engineering, Vol. 
BME-33, No. 6, June 1986, pages 585-592, describes recording data for a 
patient during an exercise cycle. The data is recorded at 8 bit resolution 
and stored on tape for subsequent playback as well as providing a real 
time graphical output. In this system templates are generated from an 
initial signal and then used for subsequent processing. 
A disadvantage of this system is that it requires the patient to be 
connected to equipment which is inconvenient other than for short periods. 
Proceedings Computers in Cardiology, Jerusalem, Israel, Sep. 19-22, 1989, 
IEEE Computer Society Press, Los Alamitos, Calif. pages 497-500, describes 
a real time personal computer based system for analysis of 
electrocardiograms for use on hospital wards, during operations or during 
exercise test analysis. Again the system requires the patient to be 
connected to equipment which does not allow for longterm monitoring. 
IEEE Computers in Cardiology, Sep. 8-11, 1985, Linkoping, Sweden, pages 
35-38 describes a simplified software system for analysis of exercise ECGs 
which is adaptable for use on a number of systems which are concerned with 
short term monitoring. 
It is desirable to obtain data over a longer period e.g. 24 hours, but a 
system using a bed-side monitor which can be provided with sufficient 
memory space will be inconvenient to both the patient and the operator and 
therefore a portable system is desirable. 
The measurement of ECG micropotentials on ambulatory patients presents 
several problems. The two common methods of recording ECG on ambulatory 
=patients, tape based recorders and solid state recorders]have limitations 
when the measurement of micropotentials is required. 
Tape based recorders capable of long term recording on an ambulatory 
patient suffer from inherent distortion in the record/playback process, 
poor signal to noise ratio, insufficient bandwidth and track misalignment. 
Solid state recorders with adequate bandwidth and signal to noise ratio can 
be produced relatively easily. However, to measure micropotentials it is 
necessary to sample the ECG at a high rate with good resolution, the 
currently emerging standard being to sample three channels at a sampling 
rate of 1 kHz with 12 bit resolution resulting in 4500 bytes per second 
for each channel which would require 390 Mbyte of memory for 24 hours 
recording. There is then a problem in storing and processing the large 
amount of data produced. The .memory available and processing power in an 
ambulatory recorder are limited by cost, size and available battery power. 
Reducing the amount of data by compression algorithms can distort the 
signal. 
U.S. Pat. No. 4,883,065 describes a conventional Holter system to record 
long term data on tape. The tape is then played back at high speed and 
digitized at sufficient resolution and sampling rate to permit analysis of 
micropotentials. This system suffers from the high noise level and limited 
bandwidth of tape recording which makes accurate analysis of 
micropotentials difficult. Also, as micropotential analysis involves 
forming the vector sum of 3 channels any temporal misalignment, e.g. due 
to differences in alignment of the replay and recording heads, can cause 
errors. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, apparatus for analyzing ECG 
signals from a patient in the form of a data stream comprises means for 
monitoring the data stream to locate for each heart beat a trigger point 
having a constant temporal displacement with respect to the location of a 
portion of the ECG which may include micropotentials; processing means for 
determining which are acceptable heart beats and, where a beat is 
acceptable, for generating and storing an average from successive portions 
at a resolution such that micropotentials present are detectable, wherein 
the processing means is adapted to store a set of averages generated 
during a corresponding number of time intervals. 
By storing only relevant portions of the input signal and analyzing the 
input signal to determine which portions are acceptable and should be used 
to produce an average and which should be rejected it is possible to 
obtain accurate readings of micropotentials and permit data to be stored 
over an extended period without a large memory and power supply 
requirement. The average produced is permanently stored, if it is 
acceptable, for accessing at the end of the period for which the ECG 
signal is analyzed. The averages are uniquely generated for each time 
interval. 
The time lag between a portion of the data stream being defined and 
analysis of the beat contained within that portion to determine whether or 
not the portion should be added to the average is limited only by 
available memory. If there is sufficient memory and processor speed in the 
device the input data stream may be analyzed at the same resolution as 
that of the portions of the input data stream, but preferably the 
monitoring means comprises means to produce a parallel signal from the 
input data stream at a lower resolution, to monitor the lower resolution 
data stream to determine the approximate position of a trigger point, and 
thereafter to monitor the higher resolution data to determine trigger 
point location. 
The apparatus described may be used to monitor patients at the bed-side, 
but preferably the apparatus is suitable for ambulatory use, and can be 
carried by the patient in a manner similar to Holter or other ambulatory 
equipment. 
Preferably the apparatus further comprises means to set a fixed time 
interval for generating and storing am average from successive portions. 
Preferably the apparatus further comprises means to set a monitoring 
period. Preferably display means display averages for a number of fixed 
time intervals within e monitoring period sequentially. This enables the 
changing shape of a heart beat to be monitored over a monitoring period. 
Trends of data parameters may be obtained from the data within each 
average over the monitoring period. 
In accordance with a second aspect of the present invention, a method of 
analyzing an ECG signal from a patient in the form of a data stream 
comprises monitoring the data stream to locate for each heart beat a 
trigger point having a constant temporal displacement with respect to the 
location of a portion of the ECG which may include micropotentials; 
determining which are acceptable hear beats and for an acceptable heart 
beat, generating an storing an average from successive portions at a 
resolution such that micropotentials present are detectable; an storing a 
set of averages generated during a corresponding number of time intervals. 
Preferably the time intervals are spaced apart. Generation and storage of 
an average from successive portions may take place over any time interval 
within the limitations of the memory and power supply available, but 
preferably, it takes place over a fixed time interval. This allows 
waveforms to be produced which can be compared with one another directly. 
It also allows changes which occur during the monitoring period to be 
observed. 
Typically the fixed time interval is between 5 and 15 minutes. The patient 
may be monitored for one fixed time interval, to produce one waveform or 
for several hours, but preferably the patient is monitored for a period of 
24 hours. 
The method of analysing an ECG signal may be carried out at only one 
resolution but preferably further comprises producing a parallel signal 
from the input data stream at a lower resolution; monitoring the lower 
resolution data stream to determine the approximate position of a trigger 
point, and thereafter monitoring the higher resolution data to determine 
trigger point location. 
Acceptability of a heart beat is typically determined by comparison of a 
waveform representing the beat with one or more templates. The comparison 
may be with templates at the same resolution at which acceptable portions 
will be stored but preferably comparison is between the parallel signal at 
the lower resolution and one or more templates at the lower resolution. 
Once a waveform has been determined to be acceptable it can be used to 
generate an average for the time interval. This average is stored without 
further reference to the stored templates. It provides a more accurate 
representation of the high resolution signals recorded than prior art 
systems.

EMBODIMENT 
Three channels of data in X, Y, and Z directions respectively are obtained 
by positioning electrode leads as shown in FIG. 3. The X channel positive 
and negative electrodes are positioned at points 5 and 1, the Y channel 
electrodes at points 4 and 6, and the Z channel electrodes at point 3 and 
an equivalent position on the back of the patient. A ground connection is 
provided at position 2. Each channel of data is processed separately. 
FIG. 1 shows a block diagram of apparatus incorporating one example of the 
present invention. A single channel of data from the patient is input to 
an amplifier 10 and after amplification converted from an analogue signal 
to a digital signal by an analogue to digital converter (ADC) 11 at a high 
resolution and sampling rate, e.g. 500 Hz/12 bits. Output from the ADC 
passes in parallel directly to a high resolution buffer 14 and via a data 
reduction circuit 12, which averages groups of samples from the high 
resolution data to produce a low resolution data stream, to a low 
resolution buffer 13. Data in the low resolution buffer 13 is then 
examined by a microcomputer (CPU) 15 using a trigger algorithm. 
An example of a set of low resolution data 20 stored in the buffer 13 is 
illustrated in FIG. 2. The trigger algorithm scans the data 20 to detect 
the occurrence of heart beats 21 to 24 and produces a trigger point for 
each beat detected. The trigger point may be a positive or negative peak 
of the signal, zero crossing or position of maximum slope, or preferably 
the mid point between minimum and maximum slopes. For each trigger point 
the corresponding region 31-34 of the high resolution data buffer 14 is 
examined to accurately determine a position of the beat. Thus the high 
resolution data need only be analyzed in the region of a known trigger 
point 26, 37, 25 thereby reducing the time required to accurately 
determine the trigger point from the high resolution data. Using a low 
resolution data stream for analysis of the trigger point allows a low 
power microprocessor to be used. 
A short sequence of data around each trigger point 26, 37, 25 is then 
copied from the high resolution data buffer 14 under control of a CPU 15 
to a temporary store 19 which can be embodied in a FIFO. The corresponding 
data in the high resolution buffer 14 is then discarded. By continuously 
carrying out this process a series of short portions of high resolution 
data 29,35,36 are produced representing each beat on each channel. 
The low resolution data buffer 13 is analyzed to determine which beats 
should be used for the measurement of micropotentials. This enables noisy 
and ectopic beats to be rejected. The analysis may make use of a series of 
templates, in this example 40 templates although other numbers may be 
used, to which the sample is matched. The templates are generated from 
patient data during analysis. For each qualified or acceptable beat the 
corresponding high resolution data, in this example, portions 29,36 in the 
temporary store 19 is used to generate an average in a current store 16 by 
summing the data for each beat over a fixed time period and dividing by 
the number of beats stored. At the beginning of the fixed time period no 
data is present in the current store 16 and the waveform of the first 
portion deemed to be acceptable will be stored directly. 
Alternatively, averages of a fixed number of beats may be taken or 
averaging continued until a particular signal to noise ratio is achieved. 
For a beat 35 which does not qualify as a good beat, the corresponding high 
resolution data is discarded. When data has been transferred from the 
temporary store 19 to the current store 16 it is then discarded from the 
temporary store. After a predetermined time period, typically between 5 
and 15 minutes the data in the current store 16 is transferred to a long 
term store 17 and the current store begins to store new data. The long 
term store 17 stores a series of sequentially addressable data 27 each of 
which is a waveform of the average of all the acceptable portions input to 
the current store in each fixed time period. The current store contains a 
gated average of all the acceptable data stored over the predefined time 
period. 
The gated averages 27 are free of distortion caused by recording the ECG 
signals onto tape or using compression algorithms to store the data in 
solid state memory. The series of representations enables trends of 
parameters, for example, beat width or micropotential amplitude to be 
produced for the whole of the recording period when the data in the store 
17 is replayed, for example displayed on a monitor. The number of beats 
used to produce each representation is also recorded. When replayed, 
weighted averages of 2 or more representations may be produced to obtain 
the required compromise between temporal resolution and signal to noise 
ratio. The three channels X, Y, and Z can be combined to produce a vector 
signal for display using the root mean square (rms) of the signal in each 
channel. 
No operator intervention is required to produce a record of micropotentials 
over an extended period e.g. 24 hours, by using the method and apparatus 
of the present invention.