Apparatus and method for electrocardiogram data compression

Abstract: An improved electrocardiac monitoring device is disclosed in which the sampling period utilized to sample the ECG signal is varied dynamically.Specifically, during rapidly varying portions of the ECG signal, the sampling rate is high. As the rate of change of the ECG signal decreases, the sampling rate is slowed so that only the minimum number of samples are taken to accurately reconstruct the analog ECG signal. The sampling period to be used is determined from the relative positions in time between a predetermined portion of the ECG signal and the portion of the ECG signal being sampled.

Description: BACKGROUND OF THE INVENTION This invention relates to an improved signal storage and processing system for compressing and reproducing electrocardiogram (ECG) signals. DESCRIPTION OF THE PRIOR ART Recent advances in implantable pacemakers/defibrillators and Holter monitors allow for digital storage and processing of ECG signals. Typically, an analog ECG signal is sampled and a plurality of digital samples are stored in an internal memory device. The stored samples can be retrieved later and the analog ECG signal can be reconstructed therefrom. Thus, the physician can program the device to sample the ECG output from the heart during certain times and store the samples for later analysis of the ECG signal by the physician. It is advantageous to minimize the number of samples of data required to be stored in order to reconstruct the analog ECG signal. One reason is that each time the analog signal is sampled and the sample stored, power is consumed, thereby shortening battery life. Furthermore, many prior art ECG analysis techniques require power consuming digital signal processing algorithms. As the number of samples increases, so does the power consumption of the signal processing algorithm. Finally, since the samples must be stored in memory for later analysis and/or reconstruction of the analog ECG signal, minimizing the number of stored samples minimizes the amount of memory required. U.S. Patent No. 4,503,510 to Weaver is directed to a method and apparatus for digital data compression. Compression of the data represents one possible solution to the above problem. Digital signals to be stored are first compressed using a finite-impulse response digital compression filter which generates estimated signal values which are then subtracted from actual signal values to provide a sequence of difference signals. The difference signals are then encoded using a Huffman type encoding technique in order to minimize the number of bits required to store the information. The Weaver arrangement, when compared to a conventional pulse code modulation sampling system, significantly reduces the number of bits of information which must be stored to reconstruct and/or process the ECG signal. U.S. Patent No. 4,499,548 to Beebe describes a data compression apparatus which selects for display a data sample of each non-overlapping and successive group of data samples which differs the most in amplitude from the average amplitude of the previous group of data samples, and also selects and displays the data sample of each group which differs the most in amplitude from the data sample selected for display from the previous group. The arrangement of Beebe also significantly reduces the number of data samples which must be stored to reconstruct an analog ECG signal. U.S. Patent No. 4,633,884 to Imai, et al., discloses a digital processing system utilizing several digital filters to minimize the number of samples being stored. U.S. Patent No. 4,947,858 to Smith discloses an apparatus for data compression of input ECG beats in an electrocardiogram system. The data compression includes the steps of converting the analog ECG beats into digital signals and compressing the digital signals. The step of compressing a beat includes selectively sub-sampling the beat. During sub-sampling, the QRS region is divided into two subregions centered about the QRS peak, and each subregion is sampled at a different rate. As indicated further in this reference, prior compression scheme included both distortion and non-distortion compression schemes. Distortion schemes such as Huffman coding and the Lemple-Ziv method require large memories and therefore are unsuitable for pacemakers which do not have sufficient memory to store up to 24 hours worth of compressed data using these schemes. Distortion compression schemes such as the AZTEC logarithm, and the SAPA compression method result in a loss of the original ECG data and in addition provide either insufficient compression rates to be of any real value, or distort the compressed signals too much and hence the recovered signals are not suitable for visual presentation. All of the known prior systems require extensive data processing in order to compress the data to be stored. Thus, in an attempt to minimize the number of samples to be later processed in reconstructing the ECG signal, the ECG samples must be extensively processed before they are stored in the first place. Accordingly, the prior art techniques are, in some sense, self-defeating. SUMMARY OF THE INVENTION The above and other problems of the prior art are overcome in accordance with the present invention which relates to a novel digital signal storage and processing technique for storing a digital representation of an ECG signal and for reconstructing the ECG signal at a later time. The present invention utilizes the fact that the ECG signal varies rapidly during certain portions thereof and varies quite slowly during other portions. Moreover, there is a large part of the ECG signal which is nearly constant over a long period of time. The present invention utilizes multiple sampling rates which sample the signal at low, medium and high sampling rates, depending upon whether the signal is not varying, varying slowly, or varying rapidly, respectively. The determination of how fast the signal is varying is based upon the relative positions, in time, between the portion being sampled and the R-wave. Unlike conventional variable rate sampling systems, the sampling rate is not determined by continuously calculating estimates of the derivative of the analog signal being sampled. Rather, the derivative, and thus the required sampling rate, are assumed from the relative time positions of the R-wave and the portion of the signal being sampled. Thus, the complex power consuming operation of constantly estimating a derivative is unnecessary. The arrangement ensures that the sampling rate is always great enough to reconstruct the signal, but that the sampling rate is never faster than it needs to be during slowly varying portions of the ECG signal being sampled. Thus, the number of required samples is minimized. It should also be noted that other encoding techniques may be further utilized to minimize the number of stored samples even more. Accordingly, the invention provides a method for compressing an electrocardiac (ECG) signal composed of successive R-waves as set out in Claim 1, and apparatus for performing such a method as set out in Claim 5. Preferred features of the invention are set out in Claims 2 to 4 and 6 to 8. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts in block diagram form the major components to be installed in an implantable pacemaker/defibrillator for implementation of the present invention; and FIG. 2 shows an exemplary ECG signal and a digital representation of the ECG signal which is stored in accordance with the technique of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the basic components of an exemplary system for implementation of the present invention, an analog to digital converter (A/D) (10), sample controller (11), R-wave detector (12) and memory device (24). As is well known in the art, A/D converter (10) must sample the incoming EGG signal 21 at a minimum rate as required by the well-known Nyquist sampling theory. The Nyquist rate is proportional to the rate at which the incoming ECG signal varies, i.e., the first order derivative. The faster the incoming ECG signal varies, the more frequently it must be sampled in order to ensure accurate reconstruction. The R-Wave detector transmits R-Wave detect 22 to sample controller 11. The sample controller 11 controls A/D converter 10 via sample control signal 16. The digitized samples 23 and R-Wave detect 18, as well as a time signal 19, are all transmitted to memory device 24 as indicated. In operation, R-wave detector (12) monitors the incoming ECG signal for the presence of R-waves. Once R-wave detection occurs, sampling and A/D conversion for the associated cardiac cycle begins. The sampling interval is adjusted at predetermined times after R-wave detection since the rate of change of the ECG signal slows considerably at predetermined times after the R-wave is detected. An exemplary system with the preferred sample rate is described below with reference to FIG. 2. FIG. 2 shows an ECG signal (21) including R-wave (25) and T-wave (30). Shown in conceptual form beneath the R-wave are ECG samples (23). The samples are arranged into 4 groups (26-29), each of which is taken at a different sampling rate. Memory (24) is also shown in FIG. 2 in conceptual form for purposes of explaining the types and quantities of information which must be stored therein for accurately reconstructing the ECG signal. In operation, an R-wave (25) is detected and R-wave detector (12) generates the R-wave detect (22) signal. R-wave detect (22) is input into sample controller (11) which causes sample controller (11) to generate samples at 12 ms. intervals. FIG. 2 shows five such samples immediately prior to the R-wave detect. These first five samples require an analog delay line or any other system capable of storing the ECG prior to the R-wave (e.g., an additional digital register arranged to store past samples) so that when the R-wave is detected, the analog signal just prior to the R-wave detect will still be available. As shown in FIG. 2, five additional closely spaced samples, immediately subsequent to the R-wave detect, are then taken for a total of ten closely spaced samples (26). Following the ten samples at 12 ms. intervals, the sampling interval is increased to 48 ms. FIG. 2 shows six samples (27) 48 ms. apart. Thereafter, three samples (28) are taken at 96 ms. intervals. Thereafter, all samples have a period of 192 ms. until the last sample prior to the next R-wave detect. It can be appreciated from comparing ECG signal (21) to the samples (23) shown below it that during the flat portion of the ECG signal (21) following T-wave (30), samples are taken at large intervals, while during the rapidly varying portions surrounding R-wave (25), samples are taken much more frequently. The sampling rules ensure that the ECG signal is sampled at a fixed and nominally short interval (12 ms.) around the R-wave detect point (22), and that the sample period is increased to 48 ms., then to 96 ms., and finally to 192 ms. as the required Nyquist rate for the ECG signal being sampled decreases. The sequence of sampling periods for an ECG cycle is denoted herein as a "sample profile." Since the stored samples will be used to reconstruct an analog ECG signal, memory (24) must not only store the values of the samples themselves, but must store information sufficient to permit the reconstruction algorithm and/or hardware to determine the proper sampling interval and the time of R-wave detect (22). As show in FIG. 2, memory (24) stores not only the values of the individual digital samples of the ECG signal, but also stores the R-wave detects. Additionally, a value T is stored after each R-wave detect (22), where T indicates the amount of time between the last sample (taken after a 192 ms. interval in the example herein) and the R-wave detect itself. The stored information from memory (24) may be utilized to reconstruct the analog ECG signal. The stored information is read from memory 24 via electrical path 20 as shown in FIG. 1. Specifically, with reference to memory (24) of FIG. 2, the individual memory locations have been labeled a - x. As can be seen from FIG. 2, the time T stored in location c, coupled with the R-wave detect from location b and the ten samples, d through m, can be used to reconstruct the analog ECG signal which immediately surrounds the R-wave detect (22). The remaining samples are at known sampling intervals and are simply utilized by a conventional D/A converter to reconstruct the ECG signal. The D/A converter may be programmed to specifically implement the technique. The purpose of storing the time T is that the sampling interval between any two successive samples is known for the entire ECG signal, except for the sampling interval between sample (29) and the first of the group of samples (26). The time T allows this one unknown sampling interval to be determined by the reconstruction algorithm/hardware. Once T is known, reconstruction of the analog ECG signal may be done utilizing conventional components in a straightforward manner which will not be described in detail herein. The number of samples at the slowest rate varies from cycle to cycle, but recognition of R in effect determines the last sample stored for a cycle. The above describes the preferred embodiment and sampling intervals, although it is obvious that other variations and sample profiles will be apparent to those of ordinary skill in the art. For example, while the embodiment described herein utilizes a ventricular ECG signal, an atrial ECG signal may also be monitored and stored in a similar manner. An atrial ECG signal would require replacing the R-wave detector described herein with a T-wave detector. Indeed, any portion of the ECG signal may be detected, and the derivative and required sampling rate assumed from the relative time positions of the portion of the signal being sampled and that detected. It is contemplated that the sample profile may be varied at the physician's discretion, and may even be programmable by a telemetric means which are well known in the art and are in wide-spread use for varying pacemaker/defibrillator parameters in known pacemaker systems.

Claims: A method for compressing an electrocardiac (ECG) signal composed of successive R-waves (25), by: detecting each R-wave; defining a first region adjacent to said detected R-wave; sampling said first region at a first sampling rate (26); defining a second region adjacent to said first region; sampling said second region at a second sampling rate (27) lower than said first sampling rate; and storing the samples from said sampling steps; characterized in determining a time (T) elapsed between one R-wave and the next R-wave; and storing said time (T) together with said samples to enable the reconstruction of the R-waves from said stored samples. The method of Claim 1 wherein said R-waves have peaks, further comprising delaying each R-wave; and defining said first region to extend over a portion of said delayed R-wave, wherein said first region includes samples den before and after said R-wave peak. The method of Claim 2 further comprising storing the occurrence of said peak. The method of any preceding claim wherein each said R-wave is followed by a T-wave (30), further comprising detecting said period (T) from an end of said T-wave to the next R-wave. Apparatus for compressing an electrocardiac (ECG) signal composed of successive R-waves (25), the said apparatus comprising: means (12) for detecting each R-wave; means (11) for defining a first region adjacent the said detected R-wave; means (11) for sampling the first region at a first sampling rate (26); means (11) for defining a second region adjacent the said first region; means (11) for sampling the said second region at a second sampling rate (27) lower than the first sampling rate; and means (24) for storing the samples from the sampling steps; characterised by: means for determining a time (T) elapsed between one R-wave and the next R-wave; and means for storing the said time (T) together with the said samples to enable the reconstruction of the R-waves from the said stored samples. Apparatus according to Claim 5 and comprising means for delaying each R-wave and defining the said first region to extend over a portion of the said delayed R-wave, wherein the said first region includes samples taken before and after a peak of the R-wave. Apparatus according to Claim 6 and comprising means for storing the occurrence of a peak in an R-wave. Apparatus according to any one of Claims 5 to 7, comprising means for detecting a period (T) between the end of a T-wave and a successive R-wave.

Inventors: JEFFREYS ANTHONY MARK, JEFFREYS, ANTHONY MARK
Applicants: TELECTRONICS NV, TELECTRONICS N.V.
Application year: 1992
Main classification: A61B5
IPC codes: A61B5