Development of a computational tool to calculate the ST segment deviation area of digitalized

electrocardiographic signs

Laíse Oliveira Resende* Federal University of Uberlandia, School of Electrical

Engineering / Biomedical Engineering Laboratory Uberlandia, Brazil

laiseresende@yahoo.com.br

Éder Alves de Moura** Federal University of Roraima, Department of Electrical

Engineering / Center for Science and Technology Boa Vista, Brazil

edermoura@gmail.com

Abstract - The purpose of this article is to present the

development of a software to calculate the area of ST -segment

deviation of electrocardiographic signals. The software was

developed in Matlab®, generating an interface for quick and

easy use by health professionals. This development was

motivated by the possible to correlate the ST deviation area

with its elevation and to calculate a score of necrosis risk in

myocardial tissue. The possibility of necrosis extent

measurement achieved by electrocardiographic analysis enable

the reduction of cost and time, and the consequent improvement

in the prognosis of patients affected by acute myocardial

infarction.

Keywords – deviation area, ST segment, myocardial necrosis,

electrocardiography.

I. INTRODUCTION

The current development of electronics and computing led

to the development of digital equipment and software that are enabling the advancement of several areas of knowledge. The clinical area has also benefited from this technological change, therefore, it has fostered the development of equipment with better quality and permit the acquisition of digitized signals, which allow a better way of storing information, the application of signal processing techniques to eliminate distortions resulting from the acquisition process and favor the development of computational tools to aid the analysis of various biological signals. This scenario facilitates the work of health professionals and patients to get results more quickly and accurately.

______________________________

* Address: Av. Joao Naves de Avila, 2121, Santa Monica, Laboratorio de Engenharia Biomedica, 38400-902, Uberlandia, Minas Gerais, Brasil. Telephone and fax number: (34) 3239-4771.

** Address: Campus Paricarana, Av. Cap. Ene Garcez, 2413, Aeroporto, Campus Paricarana, 69304-000, Boa Vista, Roraima, Brasil. Telephone and fax number: (95) 3621-3137.

The electrocardiogram (ECG) is taken with a wide application in clinical practice, and it has great informative capacity and helpful in diagnosing heart disease, by analyzing the waveforms produced by cardiac activity [1]. There are possibilities of its analysis, an example is the Aldrich score [2].

It was observed in the literature that there are several studies that compare this score with reliable measures of infarct size, validating it clinically. However, the statistical correlations obtained for Acute Myocardial Infarction (AMI) are fairly low in the order of r = 0.4 to 0.7. Note also that the lowest values for AMI are lower, around 0.3 to 0.5 [3, 4]. Alternatively the common way of assessing this index, we assessed the area of ST-segment deviation, rather than the height of the J, so researchers with the aim of obtaining a higher correlation with the comparative measures of the Aldrich score. The interest in the ST-segment deviation was due to this segment be more frequent in ECGs of patients suffering from AMI [5, 6, 7].

Regarding the researches on the estimation of myocardial area at risk of necrosis [8, 9, 10], there is a new method to estimate the area at risk of myocardial necrosis based on information from the derivatives of the ST segment, with the assistance of a biomathematics model, called DECARTO [11]. The authors use a spherical surface as surface reference to approximate the ventricular wall and thus make the estimation of area at risk of necrosis.

This paper will report the development of a computational tool for the treatment of signals obtained from electrocardiograms, which is necessary to evaluate the area of ST-segment elevation, whose purpose is to assess the correlation between the area of this segment and myocardial infarction. This method is an innovation, whereas don’t exist a method of ECG analysis by the use of the area under the ST-segment to calculate the area at risk of necrosis, since the existing method calculate only the amplitude of the J point, not the area of the ST segment deviation.

II. METHODOLOGY

The study was previously approved by the Ethics

Committee of the Federal University of Uberlandia.

The first step in the investigation was the choice of a software to scan printed electrocardiograms. In this case, it was necessary the processing of scanned images of the available data in order to obtain a digital version of the printed signal. This process was carried out by ECGScan ® software.

Each ECG signal, scanned from the printed version, was processed by the software ECGscan®. As a result of this process, the software provides a file with the amplitude in microvolts, point to point, where each row represented one of the 12 electrocardiographic leads. The file was provided in plain text ASCII. The image conversion for the representation of amplitude, point to point, was directly affected by the quality of the scanned image, it has been recommended between 300 and 600 dpi. This information also implies the maximum "sampling rate" of the digitized signal (the maximum number of points sampled per second), it would also be affected by the quality of the image. For this reason, the images were scanned within the standard recommended by the software.

It was chosen MatLab ®, version 7.8.0.347 (R2009a), as a programming platform, to development of the area calculation, by presenting the mathematical tools necessary for developing and enabling the creation of an easy user interface. An example of the final interface is shown in Figure 1.

Figure 1. User Interface (UI), developed for the calculation of the ST-segment deviation area. The UI displays the time (x-axis) in milliseconds

and amplitude (y axis) in millivolts.

Data were obtained from collections performed in the

Clinical Hospital, Federal University of Uberlandia (HCU / UFU), it was selected 20 ECGs of patients suffering from AMI, with the following characteristics:

- Admission diagnosis: inferior acute myocardial

infarction; - Variations present in the ST segment of ECG, it was

selected only patients whose ECG contained significant ST-segment deviation (greater than 1 mm) in leads that indicate inferior infarction.

For the analysis procedure, the user must select a file

through a window that opens by clicking “open file”, which represents the available files on the computer.

After the file is open, it is necessary to select the

derivation, which should be marked according to the type of AMI, and the affected leads. It can be set the sampling rate of the signal, because it depends on the conversion process and it is used as the basis for the creation of the time axis, which is not provided by the scanning software ECGScan. For the data analyzed were used two values: 250 and 500 Hz. Soon after, it should be selected the option "show" that features the outline of the desired derivation, as in the example in Figure 1.

With the derivation selected, it is possible to carry out marking the desired points, which should represent the beginning of the ST-segment deviation (point 1 in Figure 2), marked by the J point and the end of it (point 2 in Figure 2). The J point marks the junction between the end of the QRS deflection and the beginning of the ST segment (Figure 3).

Figure 2. Delimiters marking the points of the desired segment of the ECG

signal.

This marking delimits the area to be considered. The horizontal limits are the orthogonal projection of the points in the x-axes. The amplitude limits are defined by the outline of the derivation in the upper case and by a horizontal line in the same position of lowest y-axes value of the marked points. This is illustrated in Figure 2, the shaded region. The trapezoidal method implemented by the Matlab function trapz was used to perform the integral and obtain the area under the curve.

In summary, the area is delimited inferiorly by a rectangle whose sides are parallel to the x and y axes and pass over the two points marked by the user and, above, the curve of the ECG signal, corresponding to the ST-segment deviation and T wave (ST-T), as shown in Figure 2.

Figure 3. The normal ST segment. The arrow indicates the J point [12]. In addition to the area, the software provides the height of

the ST-segment deviation and duration of the event calculated.

Furthermore, the analysis protocol required the selection of patient's clinical record, which was made to collect patient data, by completing a form containing the following patient information: age, gender, peak values of troponin T (TnT) and CK-MB (creatine kinase - MB fraction) [13]. These data were grouped into a table. This study included also the estimation of the mean age for patients with inferior AMI was 56.3 years

old, consisting of 12 men and 3 women. It has been calculated manually the height of the J point for the 20 patients. After calculating the area of software developed by ST, held on for the Spearman correlation statistical analysis due to non-normal distribution of data, verified by the Shapiro-Wilk test (W) and Kolmogorov-Smirnov performed in Statistica software, besides the sample is n < 30.

III. RESULTS AND DISCUSSION

From this software, measurements were carried out in the

areas of ST-segment elevation for the 20 selected patients, where the results presented by the program supplied the initial need to calculate the area under the ST-segment deviation on the ECG signal, therefore it is possible to correlate this area with the values of the molecular markers of myocardial injury (TnT and CK-MB).

TABLE I. CORRELATION BETWEEN THE AVERAGE, SUM AND MAXIMUM OF THE HEIGHTS OF ST-SEGMENT DEVIATION AND TNT AND CK-MB (INFERIOR

AMI).

Average of

height of the

J point

Sum of

heights of

the J point

Maximum

height of the

J point

Rsp 0,63625 0,6033 0,62035

TnT n 13 13 13

p-value 0,00969 0,01451 0,01184

Rsp 0,55117 0,54073 0,57067

CK-MB N 15 15 15

p-value 0,0166 0,0187 0,01315 * Rsp: Spearman correlation coefficient.

Table I shows the correlation between the average and sum of the heights and maximum height, calculated from the J point, which marks the junction between the end of the QRS deflection and the beginning of the ST segment, and molecular markers. Since Table II shows the correlation between the average and sum of the areas and the maximum area of ST-segment deviation.

TABLE II. CORRELATION BETWEEN THE AVERAGE, SUM AND MAXIMUM OF THE AREAS OF ST-SEGMENT ELEVATION AND TNT AND CK-MB (INFERIOR

AMI).

Average of

areas of the J

point

Sum of

areas of the

J point

Maximum

area of the J

point

Rsp 0,9921 0,9812 0,9804

TnT N 13 13 13

p-value <0,0001 <0,0001 <0,0001

Rsp 0,8805 0,9625 0,9723

CK-MB N 15 15 15

p-value <0,0001 <0,0001 <0,0001 * Rsp: Spearman correlation coefficient.

As noted in the tables, the results obtained for the correlation of the area of the ST-segment deviation was higher than the height of the J point, emphasizing the highest value of Spearman correlation to average for the heights of the J point and troponin T, equal to 0.63. However, there was a correlation of 0.99 for the same correlation relating the deviation area with the same marker.

IV. CONCLUSION

Since the high correlation between the area of ST-

segment deviation and the markers used in clinical practice, there is the extensive use of this measure in future studies, mainly related to the development of a prediction model of the area at risk of myocardial necrosis.

The prospects of this paper are to generate a prediction equation for this area at risk of necrosis after acute myocardial infarction in order to improve the diagnosis of heart disease, allowing better patients prognosis, with early revascularization.

This analysis using the software and digital system allows the process be done automatically posteriorly, simply and quickly by health professionals. This procedure can be expanded and adapted to the existing software for electrocardiographic analysis, which could reduce the cost and time to obtain a diagnosis with more consistent and accurate information.

ACKNOWLEDGMENT

To CAPES (Coordenação de Aperfeiçoamento Pessoal

em Nível Superior) for providing financial support to this reasearch.

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