IEEE TRANSACTIONS ON BIO-MEDICAL ENGINEERING, APRIL 1968

Short Communications

AZTEC, a Preprocessing Program forReal-Time ECG Rhythm Analysis

Abstract-A preprocessing program developed for real-timemonitoring of the electrocardiogram by digital computer has proveduseful for rhythm analysis. The program suppresses low amplitudesignals, reduces the data rate by a factor of about 10, and codes theresult in a form convenient for analysis.

Digital computers are able to provide the biologicalscientist witlh extensive comlputational capacity. How-ever, a common difficulty that biologists encounter isthe transformation of the observed analog signals intoa form that can be easily processed by a digital com-puter. This poses a significant problem for those whowish to use a small computer for on-line or real-timeinteraction with their experiments. Our problem is thereal-time analysis of electrocardiographic rhythm andwe have developed a scheme to assist this analysis calledAZTEC (amplitude-zone-timie-epoch-coding).The electrocardiogram is composed of low frequency

components (P and T waves, ST segments) and mediumfrequency components (QRS complexes). We havechosen to ignore higher frequency components in theQRS.1 The signal amplitude is 0.1 to 1 mV, and fre-quently contains undesired signals of similar amplituderanging from very lowA frequencies (respiration at about0.2 Hz) to high frequencies (muscle noise up to about200 Hz). Power line interference at 60 Hz is also corn-mon. For accurate quantification of the duration andamlplitude of the rapidly changing QRS we have chosento sample the electrocardiogram at a rate of 500 samplesper second.2 For anything but a trivial continuous real-time analysis, this data rate would overwxhelm mostcomputers. The memory size of the LINC3 employed inthis work, 2048 twelve-bit words, made the need fordata reduction even greater. In addition, some form ofpreprocessing was desirable to filter unavoidable noiserecorded with the ECG signal.Our AZTEC preprocessing program is activated by a

real-time interrupt that converts a sample of the inputvoltage to digital form every 2 milliseconds. The firstsample v0 (see Appendix for notation and a compact

Manuscript received September 25, 1967; revised November 5,1967. This work was supported by the Division of Research Facilitiesand Resources, National Institutes of Health, Bethesda, Md., underGrant FR-00161.

I P. H. Langer, Jr., D. B. Geselowitz, and F. T. Mansure, "High-frequency components in the electrocardiograms of normal subjectsand of patients with coronary heart disease," Am. Heart J., vol. 62,pp. 746-755, July-December 1961.

2 C. A. Steinberg and L. WV. Paine, "Methods and techniques ofdata conversion," Ann. N. Y. A cad. Sci., vol. 115, pp. 614-626, July31, 1967.

3 V. A. Clark and C. E. Molnar, "A description of the LINC," inComputers in Biomedical Research, vol. 2, Stacy and Waxman, Eds.New York: Academic Press, 1965, pp. 35-66.

description of the processing algorithm) sets initial con-ditions on tWo Iimits: Vmax=Vmin=VO. Samples obtainedat subsequent interrupts are compared to these limits.If exceeded, a limit is replaced by the voltage justsamlpled. As long as the difference betweein the limiiits,(V,m,ax -Viin), does not exceed an experimentally deter-mined threshold, the fluctuating voltage is consideredto be adequately represented by a constant voltage, or"line,'" midway between the limnits. When finally a sam-ple would necessitate separating the limits by more thanthe threshold, the preceding average of the two limits isstored in the miiemory of the computer and called thevalue of the line. The time since the limits were initial-ized is stored as the duration of the line. In order toavoid long delays in the presentation of data to theECG analysis program, a long line is terminated after126 nis. In our program, the duration of a line occupiessix bits of the first of a pair of twelve-bit wvords and itsvalue occupies the second word.When the difference between the voltage limits ex-

ceeds the threshold and a pair of data words is recorded,the process is restarted by setting Vrnax and Vmin equal tothe latest sample voltage. When a signal of higher fre-quency and amplitude such as the QRS begins, thevoltage samples will change rapidly, and lines of shortduration will be formed. A series of lines, each containingfour samples or less, is considered to be adequately rep-resented by a constant rate of voltage change, or slope,as long as the voltage difference between adjacent linesdoes not change sign. The slope is terminated by a linelonger than four samples or a change in signs. The slopeduration and the voltage betweein the lines boundingthe slope are then stored as the next pair of data words.In order that this pair may be recognized subsequentlyas a slope, the duration wvord is marked by setting itssign bit.

Fig. 1 illustrates an ECG signal and its resultingAZTEC representation. The data reduction is from a rateof 500 samples per second to an average of 25 word pairs(50 words) per second, or about 10 to 1. The high fre-quency but low amplitude noise is interpreted simply asa line as long as the peak-to-peak amplitude does notexceed the threshold. The QRS consists of seven AZTECword pairs, four of which are slopes.The AZTEC algorithm can be easily followed by addi-

tional digital processing, since the calculation time persample is considerably less than the time between sanm-ples for most digital nmachines. For example, we are cLr-rently using it as input to a program that evaluates thesignal noise level and, where possible, detects QRS comi1-plexes, measures their width, and codes their shape interms of their sloping segments. Our implementation ofthis detection algorithm is quite fast and can identify

128

SHORT COMMUNICATIONS

Fig. 1. ECG sampled at 500 samples per second (top) and resultingAZTEC representation (bottom) as reproduced by a digital plotter.The horizontal line at the upper left indicates 100 ms and thevertical line indicates one eighth of full scale, in this case 400 ,V.

QRS complexes of widely varying shape. The AZTECalgorithm itself could probably be implemented with acomiplex analog system but our plans call for a series ofdigital data reduction steps of which AZTEC iS the first.

Observation of the AZTEC representation of a widevariety of pathological ECG signals has shown thatAZTEC may be a useful preprocessing procedure for real-time ECG rhythm analysis, as required for continuousmonitoring of patients in intensive care units. TheAZTEC procedure offers the advantages of on-line smooth-ing, data reduction, and coding. In this monitoringapplication noise is relatively low in amplitude andis therefore smoothed, whereas signal elements are rela-tively larger in amiplitude and are retained. Since aftersuch processing the bulk of the ECG consists of steadyvoltages, considerable data reduction is achieved bystorage of onlyN a pair of numbers (duration and value)for a steady voltage up to 126 ms in duration. In addi-tion, the line-slope coding permits rapid searching of thestored data to locate the higher frequency QRS event.

APPENDIX

The AZTEC transformation may be described as fol-lows. Let v, be the ith sample following an initial samii-ple vO. Let Vriiax and v.in be the maximum and minimumvalues, respectively, of the set vi } o' where i ranges from0 to m. If Vin5ax-Vnin.K for the set (v}il" andVmax-Vmin>K for the set f v}i then the set {v,}In issaid to form an AZTEC "line" L. The line L is completelyspecified by its value, v(L) -(Vn,+Vrnmin), and its dura-tion, r(L) = n times the sampling interval. Let v,, now bethe new v0 and repeat the process. This much of thetransformation is called a zero-order linear interpolatorwith aperture K/2.4 It has the property that, if L con-tains the sample vi, then v(L) -v|