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Analysis of Abnormal Intra-QRS Potentials Associated with Chagasic Myocarditis using the High-Resolution Electrocardiogram

Gomis Pedro; García Irene; Passariello Gianfranco; Mora Fernando

Universidad Simón Bolívar, GBBA
La Guaira, Venezuela

Abstract
Introduction
Objectives
Material and Methods
Results
Discussion
Conclusions
References

Abstract
Introduction: Chagasic myocarditis may lead to bundle branch block (BBB), hypertrophy and sudden death as a result of ventricular tachycardia (VT). High-resolution ECG (HRECG) indexes, as QRS duration (QRSD), have been widely used as noninvasive marker of VT. However, QRSD is not suitable to assess patients with BBB. We have recently proposed the concept of abnormal intra-QRS potentials (AIQP), defined as abnormal signals than can occur anywhere within the QRS period.
Objectives: The aim of this research is to assess whether HRECG indexes of AIQP may identify different stages in the evolution of Chagasic myocarditis.
Material and Methods: The study was performed on the HRECG of 81 patients classified in 4 groups: I) 11 healthy subjects. II) 33 chagasic patients with sero-positive test, but no echocardiographic or standard ECG evidence of cardiac damage. III) 26 chagasic patients with evidence of cardiac involvement; include 19 BBB patients. IV) 11 chagasic subjects with evidence of cardiac involvement (4 with BBB) and episodes of VT. Abnormal intra-QRS potentials were quantified for each individual lead using a modeling technique, removing the smooth, predictable part of the QRS. The QRS duration was also measured, excluding BBB subjects.
Results: For groups I to IV, AIQP (lead X) values were: 1.79±0.92 mV, 1.80±0.96 mV, 3.07±1.77 mV and 4.03±1.61 mV. AIQP are significantly higher in group III and IV than in group I (p=0.007 and p=0.001, respectively). QRSD values (excluding BBB) were significant different in group IV (p<0.001).
Discussion: Patients from group IV are at risk of sudden death (or has died) due to ventricular instability contraction or VT. This study has found significant differences among the chagasic groups assessed by AIQP indexes, even in subjects from group III with conduction abnormalities.
Conclusions: AIQP shows promise as an index for predicting different stages of chagasic myocarditis.

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Introduction: Chagas disease is caused by a parasite, Trypanosoma cruzi, transmitted to humans by triatomine bugs or by blood transfusion. The disease exists only on the American continent, with 16–18 million people infected and some 100 million of the population is at risk [1]. There are three stages of the disease. The acute stage, which appears shortly after the infection, commonly without manifestations. Infected people may remain in a silent stage many years. In about one third of cases a chronic stages develops some 10-20 years later, causing chagasic myocarditis, a progressive inflammation of cardiac muscle produced by lymphocytic inflitration of T. cruzi. Patients with chronic cardiopathy may develop bundle branch block (BBB), ventricular arrhythmia, congestive heart failure, hypertrophy and atrioventricular block. A common endpoint is sudden death due to electrical instability in myocardial contraction after ventricular tachycardia (VT).

Late potentials (LP) measurements in high resolution ECG (HRECG) have been widely used as noninvasive marker of VT. Late potentials reflect the electrical activity originated from the border zones of scarred or diseased tissue, and indicate slow or delayed conduction in the myocardium. They are defined as abnormal signals that outlast the normal QRS period during sinus rhythm. Late potentials have been most commonly characterized in the time domain by measurement of total filtered QRS duration (QRSd). However, this index is not suitable to assess patients with extended QRS due to conduction abnormalities as BBB, which is a frequent first manifestation of chagasic myocarditis. We have recently proposed the concept of abnormal intra-QRS potentials (AIQP) [2,3], defined as abnormal signals (low amplitude notches and slurs) than can occur anywhere within the high resolution QRS period. AIQP extend the concept of LP from the terminal portion to the entire QRS complex.

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Objectives: Our objective is to determine whether HRECG indexes of abnormal intra-QRS potentials may correlate with different stages in the evolution of chagasic myocarditis.

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Material and Methods: A high-resolution electrocardiogram database of  81 subjects was used in this investigation: 70 chagasic patients and 11 healthy subjects. The dataset has been acquired at Institute of Tropical Medicine, Caracas, Venezuela. Ten minutes of continuous ECG was recorded in each subject, using orthogonal XYZ leads, sampled at 1000 Hz. The HRECG was obtained with averaging terminated at a noise-endpoint of 0.3 mV RMS. Signal averaging was performed with a Predictor system (Corazonix Corp., Oklahoma City). An example of a patient report is shown in figure 1. Subjects were classified into 4 groups. Group I composed of 11 healthy subjects, blood sero-negative. Group II consisted of 33 chagasic subjects, blood sero-positive, but no echocardiographic or standard ECG evidence of cardiac damage. Group III included 26 chagasic patients with evidence of cardiac involvement, of which 19 patients have BBB and others have ventricular extrasystoles seen in 24 hours Holter monitoring. Group IV were formed by 11 chagasic subjects with evidence of cardiac involvement as group 3 (4 with BBB) and documented episodes of VT.

Figure 1

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Abnormal intra-QRS potentials were calculated by the residual of a parametric modeling process [2]. Each individual-lead HRECG QRS complex is presented unfiltered to be mathematically modeled. The time domain HRECG signal is pre-processed with the discrete cosine transform and then it is modeled as the impulse response of an the autoregressive model with an exogenous input (ARX). The modeling signal is subtracted from the original complex. The difference is the AIQP waveform. AIQP’s indexes were quantified by computation of the RMS amplitude between the QRS limits. Only an approximate knowledge of the QRS onset and offset is needed for the modeling procedure. A conventional analysis of the filtered vector magnitude was also performed, obtaining the QRSd index, but patients with BBB were exclude for this measurement.

Mean HRECG values among the four groups were compared by 1-way analysis of variance (ANOVA). Differences between each group were performed by Tukeys’s honestly significant difference (HSD) test. A value of p < 0.05 was deemed statistically significant.

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Results: Figure 2 shows an example of abnormal intra-QRS potential waveforms from the HRECG. Panel (a) shows the original QRS (solid line) in the Z lead from a chagasic patient belonging to group IV. The RMS amplitude of the AIQP signal in the QRS period is AIQPZ = 5.93 mV. Following the same format panel (b) displays the Z lead from a non-chagasic subject. The AIQP waveform is un-eventful and its RMS value in the QRS period is 2.40 mV.

Figure 2  Examples in the Z lead of the AIQP modeling process. (a) A Gr. IV chagasic patient. (b) A Gr. I normal subject.

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Table I presents the mean AIQP and QRSd values in the four groups of patients. AIQP indexes in lead X and Z (AIQPX and AIQPZ) provides a highly significant difference among groups (p < 0.0001). Each one of these indexes is significantly higher in groups III and IV compared with I and II, respectively (p< 0.05). AIQP values in lead Y are statistically different among the four groups. Mean values of AIQPY are significantly higher in group IV than in groups I and II, respectively. The QRSd index was only obtained from 7 patients of each one of the groups III and IV (BBB patients were excluded). QRSd was significantly different among the groups (p < 0.001). Group IV has QRSd mean values significantly higher than the other groups.

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Discussion: This study has found significant differences among the chagasic groups assessed by AIQP indexes, even in subjects from group III with conduction abnormalities. Subjects from group III typically have conduction abnormalities, which are difficult to evaluate with standard HRECG indexes. Patients from group IV are at risk of sudden death (or have died) due to ventricular instability contraction or VT and most of them had BBB. Chagasic patients belonging to groups III and IV presented higher AIQP values than those of group II and the control group I. About 25% of patients from group III have AIQP indexes similar or higher than those from group IV. This result may suggest risk of VT and these patients can be evaluated with other more invasive technique. AIQP are related with disruption of ventricular conduction and can improve the clinical predictive value of the HRECG for arrhythmic events in patients with Chagas’ disease.

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Conclusions:  AIQP indexes are related with disruption of ventricular conduction and can improve the clinical predictive value of the HRECG for detecting arrhythmic events of patients with Chagas’ disease. Although further experimental work and follow-up of patients is required, AIQP show promise as an HRECG index for predicting different stages of chagasic myocarditis.

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References:

[1] World Health Organization. Division of Control of Tropical Diseases. (WHO/CTD). Chagas Disease Elimination. http://www.who.int/html/chag.html. 1998.
[2] Gomis P, Jones D, Caminal P, Berbari EJ, Lander P. Analysis of abnormal signals within the QRS complex of the high resolution ECG. IEEE Trans Biomed Eng 1997; 44:681-693.
[3] Lander P, Gomis P, Goyal RM, Berbari EJ, Caminal P, Lazzara R, Steinberg JS. Analysis of abnormal intra-QRS potentials: Improved predictive value for arrhythmic events of the signal-averaged electrocardiogram. Circulation. 1997; 95:1386-1397.

 

Questions, contributions and commentaries to the Authors: send an e-mail message (up to 15 lines, without attachments) to ARRITMIAS@listserv.rediris.es , written either in English, Spanish, or Portuguese.

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Bioengineering
UNER
Update
Oct/31/1999