Standards for analysis of ventricular late potentials using high-resolution or signal-averaged electrocardiography: a statement by a task force committee of the European Society of Cardiology, the American Heart Association, and the American College of Cardiology

Removal of power line interference in signal-averaged electrocardiography systems

A method for line interference reduction to be used in signal-averaged electrocardiography (SAECG) systems is proposed and its performance is analyzed. This new method is an adaptation of a previously reported technique for removal of line interference from conventional electrocardiograms. It involves the recording of a line reference signal simultaneous with the lead signals, so that a shifted and scaled version of it can be used to subtract line interference from the leads. It is shown that this line interference subtraction method can reduce line interference effectively and without introducing any additional noise into the ECG signal. It is also shown that Late Potential diagnostic decisions are not altered when this filter is applied. It is recommended that this technique be used in SAECG when line interference is unavoidable.

Usefulness of an abnormal signal-averaged electrocardiogram for predicting cardiac death in elderly persons without heart disease

We found an abnormal signal-averaged ECG in 7 of 51 elderly patients (14%) (mean age 83 years) without clinical evidence of heart disease, and in 1 of 25 volunteers (4%) (mean age 31 years range [17 to 47]) without clinical evidence of heart disease. The data from our prospective study indicate that at 43-month mean follow-up, elderly patients with an abnormal signal-averaged ECG but no clinical evidence of heart disease do not have an increased incidence of sudden cardiac death, total cardiac death, or total death.

Clinical significance of ventricular late potentials in patients with obstructive sleep apnoea

Patients with obstructive sleep apnoea (OSA) have an increased cardiovascular mortality and probably also an increased incidence of sudden cardiac death. Thus the question arises whether ventricular late potentials can constitute markers for an increased electric vulnerability in these patients. Signal-averaged electrocardiograms were recorded in 64 patients (6 female, 58 male; mean age 53.2 y) with OSA (mean apnoea-hypopnoea index (AHI) 41.7 h-1 +/- 24.3 h-1). Furthermore, a continuous ambulatory electrocardiogram and gated radionuclide ventriculography were performed. Ventricular late potentials were recorded in 5 men out of 64 patients. Two of them had coronary artery disease (1 patient post-myocardial infarction), 2 hypertension, and 1 nocturnal hypertension. No correlation could be traced between left ventricular ejection fraction, severity and extent of ventricular premature beats, or severity of OSA and occurrence of ventricular late potentials. It was noticeable, however, that the patients with ventricular late potentials had severe OSA (mean AHI 50.2/h vs. 40.9/h). Although OSA may lead to structural myocardial changes that could be the basis for re-entrant circuits, ventricular late potentials were found in only 7.8% of these patients. The results of this study demonstrate that at present ventricular late potentials and signal-averaged electrocardiograms do not prove useful as screening methods for risk stratification of patients with OSA.

Wavelet analysis of high-resolution ECGs in post-infarction patients: role of the basic wavelet and of the analyzed lead

Wavelet analysis provides a fruitful alternative to standard techniques for the detection of fractionated potentials in signal averaged high-resolution (SA-HR) ECGs. In this study, an attempt is made to optimize the discrimination of post infarction patients prone to ventricular tachycardia (VT), using wavelet analysis. Optimization is based on the choice of the ECG leads or lead combinations to be analyzed, and on the analyzing wavelet to be computed. A set of 40 post-infarction patients (20 patients with VT and 20 patients without any arrhythmia) is analyzed. Individual leads and lead combinations of the SA-HR ECGs are processed using a multiparametric algorithm, based on coherent detection of aligned local maxima of the wavelet transform. Seven basic wavelets are tested: the Morlet's wavelet, and the six first derivatives of a Gaussian function. The first derivative of a Gaussian function provides poor results, and is discarded. All other wavelets prove to perform equivalent classification. A vector magnitude computed from the wavelet transforms of the three SA-HR ECGs achieves better results than individual leads. An optimized risk stratification algorithm leads to 90% sensitivity and 100% specificity in the 40 patients learning set.

Performance of the signal-averaged electrocardiogram: relation to baseline QRS duration

Analysis of the duration and terminal components of the filtered QRS complex on the signal-averaged electrocardiogram (ECG) has been widely used for the detection of late potentials. Although filtered QRS duration is strongly related to 12-lead QRS duration, the relation of performance of the signal-averaged ECG to baseline QRS duration has not been critically examined. To examine the relation of test performance of the signal-averaged ECG to 12-lead QRS duration and to test the hypothesis that the difference between filtered and baseline 12-lead QRS duration would reflect more accurately the presence of late potentials than would analysis of the filtered QRS alone, we evaluated signal-averaged and 12-lead ECGs in 144 normal subjects and in 132 patients who were examined by electrophysiologic study and of whom 45 had inducible sustained monomorphic ventricular tachycardia. The signal-averaged ECG was considered positive by standard late potential criteria when the filtered vector QRS duration was > 114 msec and either the root-mean-square voltage of the terminal 40 msec of the filtered QRS was < 20 microV or the low-amplitude signal of the terminal filtered QRS was > 38 msec. A new signal-averaged ECG criterion for the presence of late potentials was developed in the 144 normal subjects on the basis of the difference between the longest filtered QRS duration in any of the orthogonal leads and QRS duration on the baseline 12-lead ECG ("the QRS difference"), which was adjusted by regression analysis for the decreasing QRS difference found with increasing baseline QRS duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Wavelets as alternative to short-time Fourier transform in signal-averaged electrocardiography

The paper reports experience of using the wavelet transform to build time-frequency distributions of the terminal portion of the QRS-complex. We used wavelets of Morlet at 12 scales, grouped in three sets, to analyse the frequency range 33-404 Hz. On the same patient data we applied the short-time Fourier transform and compared the results. Both representations reflected the time-frequency contents and detected irregular structures in the terminal portion of the QRS complex. The wavelet transform revealed more adequately QRS prolongations characteristic of patients prone to ventricular tachycardia. We may conclude that the wavelet transform can be a flexible alternative to short-time Fourier transform.

Impaired left ventricular filling in hypertensive left ventricular hypertrophy as a marker of the presence of an arrhythmogenic substrate

OBJECTIVE

To assess the prevalence of ventricular late potentials and ventricular tachycardia in hypertensive subjects with left ventricular hypertrophy and to study their relation to clinical characteristics.

SETTING

Teaching and general hospital in Padua.

METHODS

107 hypertensive subjects with echocardiographic signs of left ventricular hypertrophy were studied with signal averaged electrocardiography and 24 hour Holter monitoring. Signal averaged electrocardiogram analysis was performed with high pass filters of 25 Hz, 40 Hz, and 80 Hz. Ventricular late potentials were considered to be present if at least two determinants of the signal averaged electrocardiogram were abnormal in one of the three filters. 70 normotensive subjects served as age matched controls.

RESULTS

25% (27) of the hypertensive subjects and 6% (four) of the controls showed late potentials on signal averaged electrocardiography (P < 0.0001). The hypertensive subjects with late potentials had a higher prevalence of ventricular tachycardia (33%, 9/27) than those without late potentials (13%, 10/80; P = 0.035). Twenty nine per cent (31/107) of the hypertensive subjects had an inversion of the early to atrial filling velocity (E/A ratio < 1) on Doppler analysis of transmitral flow. Within this group the percentage of subjects with late potentials (55%, 17/31) and ventricular tachycardia (42%, 13/31) was much greater than that within the group of subjects without an inverted E/A ratio (13%, 10/76 (P < 0.0001) and 12%, 9/76 (P = 0.001) respectively). In a multivariate analysis only the E/A ratio was related to the presence or absence of either late potentials (P = 0.0001) or ventricular tachycardia (P = 0.0008). Both late potentials and ventricular tachycardia were unrelated to left ventricular mass, geometry, and systolic performance.

CONCLUSIONS

A relation was found between the occurrence of ventricular tachycardia and the presence of late potentials in hypertensive subjects with left ventricular hypertrophy. Impaired left ventricular filling was the main marker for the arrhythmogenic substrate present in this disease.

Prognostic significance of spectral turbulence analysis of the signal-averaged electrocardiogram in patients with idiopathic dilated cardiomyopathy

The aim of this study was to assess whether spectral turbulence analysis (STA) of the signal-averaged electrocardiogram (SAECG) is of prognostic use in patients with idiopathic dilated cardiomyopathy. SAECGs were recorded at presentation in 84 patients with idiopathic dilated cardiomyopathy and STA was performed using 183 Del Mar software. STA was abnormal (> or = 3 of the 4 standard parameters beyond the normal range) in 31 patients (37%). Patients were followed for a mean duration of 24 +/- 18 months (range 1 to 59) during which time 24 (29%) developed progressive heart failure (14 underwent cardiac transplantation), 4 died suddenly or had aborted sudden death, and the others remained clinically stable. Progressive heart failure occurred more often in patients who had an abnormal versus a normal STA result (15 [48%] vs 9 [17%]; p < 0.002). Actuarial survival revealed a 1-year survival of 90% in patients with a normal STA result, and 63% in patients with an abnormal STA result (p < 0.01). The predictive ability of STA to identify patients with progressive heart failure was sensitivity 63%, specificity 77%, positive predictive value 54%, and negative predictive value 83%. Univariate analysis identified peak oxygen consumption as having the largest relative risk for the development of progressive heart failure (9.55, 95% confidence interval [CI] 2.1 to 43.9). Left ventricular end-diastolic dimension (relative risk 4.18, 95% CI 1.5 to 11.4) and STA (relative risk 3.81, 95% CI 1.7 to 8.8) were also significantly associated with the development of progressive heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of athletes with life-threatening ventricular arrhythmias with two groups of healthy athletes and a group of normal control subjects

Sudden cardiac death in well-trained athletes is most often superimposed on the presence of structural heart disease. However, some athletes die suddenly in the absence of overt heart disease. To improve identification of athletes at high risk for ventricular tachycardia (VT), ventricular repolarization, the signal-averaged electrocardiogram (ECG), and the echocardiogram from 13 male athletes with symptomatic VT and without evidence of manifest cardiac disease were compared with data obtained in 3 matched control groups (15 apparently healthy professional road cyclists, 10 professional basketball players, and 15 normal control subjects without any sports activity). All patients had apparently normal QRS duration on the routine ECG, and none were taking antiarrhythmic drugs. Echocardiography and signal-averaged electrocardiography were useful in distinguishing the group of athletes with tachyarrhythmias from the group of normal nonsporting controls, but not from both groups of normal athletes. The QT interval (V4) and the QT interval corrected with the cubic root were shorter for the nonsporting controls. Three parameters for QT dispersion showed significant differences (p < 0.003) between athletes with disease and all other groups. It is concluded that although significant differences were detected between normal subjects and the 3 groups of athletes by routine ECG, the signal-averaged ECG, and echocardiography, only an increased QT dispersion from the 12-lead ECG was helpful in distinguishing athletes with VT from other athletes.

High resolution ECG and left ventricular volume after acute myocardial infarction

The presence of ventricular late potentials (LPs) early after acute myocardial infarction (AMI) was recently reported to correlate with left ventricular dilatation subsequent to AMI. We assessed prospectively the relationship between LP (time domain) in the late phase of AMI and left ventricular end-diastolic volume (EDV) measured by equilibrium radionuclide angiocardiography 4 weeks and 12 months after AMI. In 80 consecutive patients 4 weeks and 12 months after thrombolytic therapy for AMI, LP and EDV were determined (EDV1, EDV2). There was no significant correlation between QRS duration (r = 0.18), RMS40 (r = 0.08), or LAS40 (r = 0.1) and EDV1 or EDV2 in patients with or without LP at baseline. In both groups (patients with [n = 15] and without LP [n = 65]), EDV1 and EDV2 were comparable (128 +/- 32 mL vs 126 +/- 35 mL; 114 +/- 40 mL vs 117 +/- 36 mL; P = NS). In addition, there was no significant difference between EDV1 and EDV2 in patients who developed new LP (n = 6) or lost LP (n = 9) 12 months after AMI. In contrast to LP in the very early phase after AMI, there seems to be no significant correlation between the high resolution ECG in the late phase after thrombolytic therapy for AMI and left ventricular EDV.

Open infarct artery, late potentials, and other prognostic factors in patients after acute myocardial infarction in the thrombolytic era. A prospective trial

BACKGROUND

Successful reperfusion of the infarct-related artery in patients with acute myocardial infarction has been shown to reduce in-hospital as well as 1-year mortality. Besides the thrombolysis-induced myocardial salvage, there is increasing evidence that an open infarct-related artery results in increased electrical stability of the heart and that this effect is at least in part responsible for the favorable long-term outcome of these patients. The exact incidence of arrhythmic events during the first year after myocardial infarction and the predictive value of different risk factors for these complications, however, have not been determined in patients in the thrombolytic era.

METHODS AND RESULTS

A total of 173 patients with acute myocardial infarction, 51% treated with thrombolysis, were prospectively entered into the study. At the time of hospital discharge, signal-averaged ECG, Holter monitoring, radionuclide angiography, coronary angiography, and levocardiography were performed in all patients. An open infarct-related artery was documented in 136 patients. The overall incidence of late potentials was 24% (41 patients). By multivariate analysis, an occluded infarct-related artery (P = .04) and the presence of regional wall motion abnormalities (P = .02) were the strongest independent predictors for the development of a late potential. Residual ischemia was treated by either percutaneous transluminal coronary angioplasty or surgery in 86 of 173 patients (50%). Seventy percent of the patients received beta-blocker therapy. During a mean follow-up of 12 +/- 5 months, 7 patients died suddenly or had ventricular fibrillation documented, while only 2 developed sustained monomorphic ventricular tachycardia. Overall 1-year mortality was 4.1%. Multivariate analysis revealed only an occluded infarct-related artery as an independent predictor of arrhythmic complications (P = .017).

CONCLUSIONS

In patients with acute myocardial infarction treated according to contemporary therapeutic guidelines, with a large proportion of individuals undergoing coronary artery revascularization, a low incidence of arrhythmic events, particularly of ventricular tachycardia, was observed in the first year after the index infarction. The presence or absence of an open infarct-related artery was the strongest independent predictor of these events, whereas other traditional risk factors, such as late potentials, were less helpful in identifying patients prone to sudden death. These findings emphasize the importance of the open artery hypothesis in patients recovering from acute myocardial infarction.

Late potentials, QTc prolongation, and prediction of arrhythmic events after myocardial infarction

In a series of 171 consecutive survivors of acute myocardial infarction, the predictive value of late potentials and QTc prolongation was prospectively assessed. QT intervals were measured in lead V2, corrected QT (QTc) was calculated using Bazett's equation (cut-off value 440 ms). Late potentials were considered to be present when all of the three signal-averaged electrocardiographic variables were abnormal (i.e. QRS > 114 ms, D40 > 38 ms, and V40 < 20 microV). Complete follow-up was obtained (mean 13 +/- 6 months, range 6-24 months). Six percent of the patients had an arrhythmic event (i.e. sustained ventricular tachycardia or sudden death). The relative risk of late potentials for arrhythmic events was 7.7 (P < 0.02). The relative risk of QTc > 440 ms was 1.1 (NS). In a multivariate analysis, the addition of QTc prolongation did not significantly improve the prognostic value of late potentials alone. It is concluded that late potentials are predictive of arrhythmic events after myocardial infarction, but the presence of concomitant QTc prolongation does not worsen the prognosis.

Factors influencing changes in the signal-averaged electrocardiogram within the first year after a first myocardial infarction

One hundred twenty-nine patients were prospectively studied after a first myocardial infarction. A first signal-averaged electrocardiogram (SAECG-1) was performed in the acute phase (within 48 hours after onset of symptoms) and a second one (SAECG-2) in the late phase (6 to 18 months after hospital discharge). We studied the influence of nine parameters on the evolution of the signal-averaged electrocardiogram: age, gender, myocardial infarction location, number of diseased coronary vessels, infarct-related coronary artery patency, use of thrombolytic therapy or percutaneous transluminal coronary angioplasty in the acute phase, left ventricular ejection fraction, and recurrence of ischemic events. No follow-up data were available in 15 patients. Of the remaining 114 patients, an ischemic event occurred in 25 (22%). The signal-averaged electrocardiogram remained unchanged in 97 (85%) (remaining normal in 78 and abnormal in 19). It became abnormal in 13 (11.5%) and became normal in 4 (3.5%). In patients with a normal SAECG-1, two factors were associated with the change to an abnormal SAECG-2: (1) an ischemic event occurred in 11 (85%) of 13 patients whose SAECG-2 was abnormal compared with only 13 (17%) of 78 patients whose SAECG-2 remained normal (p < 0.0001), and (2) 100% of patients with an abnormal SAECG-2 had an inferior myocardial infarction compared with 54% of patients with a normal SAECG-2 (p = 0.004).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of spectral temporal mapping to the time domain signal-averaged electrocardiogram in normal subjects and in patients with coronary artery disease and sustained ventricular tachycardia

Spectral temporal mapping is a new form of analysis for signal-averaged electrocardiography, which has the goal of improving the sensitivity and specificity of traditional time domain analysis. Our objective in this study was to determine the effectiveness of one form of spectral temporal mapping, in the face of conflicting results that have so far been reported with this approach. We prospectively performed both spectral temporal mapping and time domain analysis on 50 patients with a history of coronary artery disease and inducible sustained monomorphic ventricular tachycardia (Group 1) and on 25 normal subjects with normal electrocardiograms and no history of heart disease (Group 2). We found that for the 40 Group 1 patients without bundle branch block (Group 1A), the sensitivity of spectral temporal mapping was lower than that for time domain analysis (45% vs 80%, P < 0.005). The results of spectral temporal mapping for Group 1A patients were similar to that for all of Group 1. The sensitivity of spectral temporal mapping was 60% (n = 10) for patients with bundle branch block (Group 1B). The specificity noted in Group 2 was 88% by each means of analysis; however, no one in Group 2 had an abnormal finding by time domain and spectral temporal mapping. Attempts to optimize the criteria for an abnormal spectral analysis did not identify criteria that were superior to those currently in use. We conclude that spectral temporal mapping using Haberl's method is inferior to time domain analysis in identifying patients with sustained ventricular tachycardia, but may be of value in conjunction with the traditional approach in identifying normal subjects.

Signal-averaged ECG parameters in cardiac normals using Frank lead system and Fourier transform filter and gender specific differences: a multicenter study

There is only limited data on normal reference values for signal-averaged electrocardiograms (SAECGs) using Frank leads and fast Fourier transform filter (FFT). Furthermore, the influence of gender on reference values and their relation to body characteristics was only the subject of a few studies on small series of normals. One hundred eighty-five cardiac normals (85 women and 100 men) were examined in this multicenter study. The obtained SAECG values (mean +/- standard deviation) are as follows: filtered QRS duration (FQRSD) = 108.6 +/- 7.5 msec; low amplitude signal duration < 40 microV (LASD) = 30.4 +/- 8.4 msec; and root mean square voltage in the terminal 40 msec (RMSV) = 43.5 +/- 20.6 microV. Between men and women, significant differences were found in FQRSD (111.7 +/- 6.5 vs 105.0 +/- 7.0 msec, P < 0.001) and in RMSV (38.6 +/- 17.4 vs 49.4 +/- 22.7 microV, P < 0.001). No difference was observed for LASD. After normalizing the three SAECG parameters for body characteristics, FQRSD normalized for height was the only variable where gender differences were eliminated. For FQRSD and LASD the 90th percentile and for RMSV the 10th percentile are proposed as cut-off values. Only for the 90th percentile of FQRSD a clear difference between men and women was observed. The following gender specific normal values for SAECG, at 40-Hz high pass filtering, using Frank leads and an FFT filter are proposed: for males, FQRSD < 122 msec; for females, FQRSD < 115 msec; for both genders, LASD < 41 msec and RMSV > 20 microV.

Signal-averaged electrocardiography in elderly subjects with and without heart disease

Prevalence of abnormal signal-averaged electrocardiography in normal populations appears to be low, but has not been studied previously in an asymptomatic elderly population. To study the prevalence of abnormal ventricular late potentials in an elderly population, a group of 51 subjects with no evidence of cardiac disease and ranging in age from 62 to 102 years underwent signal-averaged electrocardiography. Results were compared to a group of 179 patients similar in age but with complex ventricular arrhythmias, and to a group of 25 asymptomatic volunteers under the age of 50. The prevalence of an abnormal signal-averaged ECG was 14% in the normal elderly subjects, and 31% in the patients (P = 0.01), and 4% in the young subjects (P = NS). We conclude that the prevalence of abnormal ventricular late potentials in elderly patients without heart disease is similar to levels reported in other populations of normal controls, but elderly patients with cardiac disease have a significantly higher prevalence of abnormal signal-averaged ECG studies than the normals.

Time-frequency mapping of the QRS complex in normal subjects and in postmyocardial infarction patients

The effect of myocardial infarction upon the frequency content of the QRS complex was analyzed. Three bipolar signal-averaged surface electrograms, recorded during the early (10-15 days) and late (6 months) chronic phases of myocardial infarction, were analyzed in 61 patients and 11 healthy subjects. All patients were free of ventricular arrhythmia during 6 months of follow-up examinations. Time-frequency analysis of the QRS complex was based on the modified Wigner distribution, which is well suited to examine nonstationary character of data. Standard time-domain analysis for the presence of late potentials was used for comparison. High-frequency (> or = 90 Hz) components, separable from the dominant low-frequency components (< 90 Hz), were found in all groups. They were present throughout the QRS complex and were peaking in its middle portion. The high-frequency components were found significantly higher in postinfarction patients in both early (P < .007) and late chronic stage myocardial infarction (P < .05) compared to healthy subjects. Patients who tested positive for late potentials (24%) also had elevated high-frequency components; however, a comparable increase was also observed in late potential negative patients. Furthermore, the high-frequency component increase occurred in all patients earlier in the QRS than in its terminal 40 ms, where late potentials are traditionally evaluated. It is concluded that high-frequency components are an integral part of the QRS complex under physiologic conditions and persist in variable amount throughout its duration. The high-frequency components are increased in patients after myocardial infarction not associated with ventricular arrhythmia, and their elevation is not limited to the terminal QRS complex.

Identifying the end of ventricular activation: body surface late potentials versus electrogram measurements in a canine infarction model

INTRODUCTION

Identification of the end of the QRS is perhaps the single most important feature obtained from the high resolution signal-averaged electrocardiogram (SAECG). This point relies on computer algorithms to select a point above the noise levels. Prior studies to substantiate this approach using electrograms for comparison have demonstrated many examples of the body surface recordings failing to detect the full extent of the late potentials.

METHODS AND RESULTS

An animal model that generates late potentials was used in conjunction with epicardial cardiac mapping system to systematically examine the reasons for these failures. In 11 of 13 dogs we found a concordance between the signal-averaged recordings and the epicardial recordings within 5 msec. The two discordant studies were attributed to a failure of epicardial mapping to record all late potential sources. Also, a means of accurately comparing measurements from the two recording technologies was required in this study as well as a new definition for identifying the end of activation currents in epicardial electrograms.

CONCLUSION

To achieve these results required approaches different from those used in the clinical setting to record the SAECG. These include: (1) the analysis of individual XYZ leads as opposed to the vector magnitude derived from these leads; (2) visual identification of very low level signals, as automatic algorithms often fail to detect low level signals; and (3) the use of finite impulse response digital filters instead of the bidirectional Butterworth filter.

Spectral turbulence analysis versus time-domain analysis of the signal-averaged ECG in survivors of acute myocardial infarction

This study compared the time-domain and spectral turbulence analyses of signal-averaged electrocardiogram (ECG) for the prediction of risk after acute myocardial infarction. Signal-averaged ECGs were recorded in 553 survivors of acute myocardial infarction before hospital discharge. The study excluded cases with bundle branch block and other conduction abnormalities, and patients were followed for at least 1 year. During the first year of the follow-up period, 30 patients died and 20 presented with ventricular tachycardia/fibrillation. The signal-averaged ECG recordings were analyzed using conventional time domain at 40-250 Hz and spectral turbulence analyses. The indices provided by both types of analysis were compared in patients with and without endpoints. The optimum positive predictive characteristics were calculated for the prediction of all cause mortality and of ventricular tachycardia based on the time domain and on the spectral turbulence indices. Spectral turbulence analysis provided significantly lower positive predictive accuracy (14.5% at 40% sensitivity) than the time-domain analysis (26.7% at 40% sensitivity) for prediction of ventricular tachycardia/fibrillation during 1 year after infarction (P < .01). However, spectral turbulence analysis provided significantly higher positive predictive accuracy (27.2% at 30% sensitivity) than the time-domain analysis (16.9% at 30% sensitivity) for the prediction of 1-year all-cause mortality (P < .01). Thus, spectral turbulence analysis was inferior to the time-domain analysis in predicting ventricular tachycardia/fibrillation during the first year after myocardial infarction, but it was more powerful in predicting 1-year mortality.

Wavelet analysis of high-resolution signal-averaged ECGs in postinfarction patients

The authors present an original method for the discrimination of patients prone to ventricular tachycardia. The wavelet transform, which is a new time-scale technique suitable for transient signal detection, was applied to bipolar unfiltered X, Y, Z signal-averaged electrocardiograms in 20 postinfarction patients with sustained ventricular tachycardia, in 20 myocardial infarction patients without ventricular tachycardia, and in 10 healthy subjects. An improved automated algorithm for the detection and localization of sharp variations of the signal, based on coherent detection of the local maxima of the wavelet transform, was developed. A risk stratification method, based on the detection of at least one singularity at or after a point defined with reference to the QRS onset, was assessed. The optimum cutoff point, found 98 ms after the onset of QRS, provides a specificity of 90% and a sensitivity of 85%. The authors conclude that wavelet analysis makes it possible, in this group of patients, to discriminate those with ventricular tachycardia. It yields better results than those obtained from the conventional time-domain approach.

Effects of streptokinase during acute myocardial infarction on the signal-averaged electrocardiogram and on the frequency of late arrhythmias

Although a number of studies have shown that the incidence of late potentials is lower after thrombolytic therapy, it is not known whether this is paralleled by fewer arrhythmic events during long-term follow-up. In patients with first acute myocardial infarction, filtered QRS duration was significantly shorter when treated with streptokinase (95 +/- 11 ms, n = 53) than when treated with conventional therapy (99 +/- 12 ms, n = 77, p < 0.05). The low-amplitude signal (D40) was shorter after thrombolysis (28 +/- 11 vs 33 +/- 12 ms, p < 0.02). Terminal root-mean-square voltage did not differ significantly (41 +/- 24 vs 35 +/- 23 microV). Irrespective of treatment, late potentials were predictive in the complete group (n = 171) for arrhythmic events during follow-up (13 +/- 6 months, range 6 to 24) (hazard ratio 7.7, p < 0.02, Cox proportional-hazards survival analysis), but treatment (streptokinase vs conventional) did not significantly affect outcome when added to the model. It is concluded that thrombolysis prevents the development of late potentials. However, this study does not confirm the hypothesis that prevention of late potentials leads to a decrease in arrhythmic events.

Influence of infarct-related artery patency on the indexes of parasympathetic activity and prevalence of late potentials in survivors of acute myocardial infarction

OBJECTIVES

The purpose of this study was to determine whether infarct-related coronary artery patency influences myocardial electrical stability as measured by the prevalence of late potentials or heart rate variability.

BACKGROUND

Several studies have suggested that loss of vagal activity is associated with an increased incidence of arrhythmic death after myocardial infarction.

METHODS

A short-duration, high resolution electrocardiogram (ECG) was performed before hospital discharge in 175 patients with a first myocardial infarction. Seventy-three patients received thrombolytic therapy. All patients underwent coronary angiography. Coronary occlusion was defined as minimal or no anterograde flow. Eighty-eight patients (50.3%) had an occluded infarct-related artery. Sixty-two healthy subjects served as control subjects to determine the normal range of heart rate variability.

RESULTS

Comparison between the control group and patients without patency of the infarct-related artery in the time domain and spectral analyses revealed in the latter patients a reduced heart rate variability (p < 0.0001) and a lower power spectrum density in both the 0.05- to 0.15-Hz band (p < 0.0001) and the 0.15- to 0.35-Hz band (p < 0.0001). The heart rate variability in patients with late potentials was lower than in those with a normal signal-averaged ECG. Those patients with spontaneous or thrombolysis-induced reperfusion have less occurrence of late potentials and higher parasympathetic activity than do patients with a closed artery.

CONCLUSIONS

This study suggests that the patency of the infarct-related artery determines both the absence of late potentials and the preservation of vagal tone and may explain the reduction in mortality induced by thrombolytic therapy in myocardial infarction.

New approach to the estimation of the extent of myocardial fibrosis in patients with dilated cardiomyopathy: use of signal-averaged electrocardiography

To determine whether the extent of myocardial fibrosis in dilated cardiomyopathy could be estimated noninvasively, signal-averaged electrocardiograms were recorded in 32 patients with dilated cardiomyopathy, followed by left ventricular endomyocardial biopsy. The root mean square voltage for the last 40 msec (V40), the duration of the filtered QRS complex (fQRSd) and the duration of low amplitude signals < 40 microV (LAS) were obtained by signal-averaged electrocardiography. The extent of fibrosis in all biopsy samples was measured by the point-counting method. The extent of myocardial fibrosis closely correlated with fQRSd (r = 0.623, p < 0.001), LAS (r = 0.570, p < 0.001), and V40 (r = -0.355, p < 0.05). When fibrosis was classified into intercellular and interfascicular types, the extent of intercellular fibrosis more closely correlated with fQRSd (r = 0.695, p < 0.0001), LAS (r = 0.640, p < 0.0001), and V40 (r = -0.533, p < 0.005). These results suggest that signal-averaged electrocardiograms might be useful for estimation of the extent of myocardial fibrosis, especially intercellular fibrosis in patients with dilated cardiomyopathy.

Detection of late potentials in the signal-averaged ECG combining time and frequency domain analysis

The paper reports experience of flexible, computer-based instrumentation for high-resolution ECG, designed for laboratory research and clinical applications. It is an attempt to improve methods of detection of late potentials when identifying patients at increased risk of ventricular tachycardia (VT) and sudden cardiac death. Several factors influencing the efficiency of a high-resolution ECG system are examined. Of primary importance are signal, gain, type of filter, length and position of the time interval taken for spectral analysis, type of window used etc. Some possible error sources are analysed. Time-domain analysis is illustrated by representative XYZ-vector magnitude tracings of a non-VT and a VT patient. In an effort to further refine the diagnostic strategy, an approach to data analysis is presented, combining time-domain with frequency-domain measurements. A spectral ratio is proposed that helps recognise patients prone to VT. Two-dimensional plots of this ratio against time-domain parameters reveal clearly distinguishable normal and pathologic groups. It can be concluded that an appropriate frequency analysis can add significant diagnostic power to existing instrumentation for signal-averaged ECG to predict VT risk.

Electrophysiologic variables characterizing the induction of ventricular tachycardia versus ventricular fibrillation after myocardial infarction: relation between ventricular late potentials and coupling intervals for the induction of sustained ventricular tachyarrhythmias

OBJECTIVES

The aim of this study was to analyze the relations between the presence of ventricular conduction delay and the necessary coupling intervals for the induction of sustained ventricular tachyarrhythmias.

METHODS

The electrophysiologic and signal-averaged electrocardiographic (ECG) data from 83 patients with previous myocardial infarction and inducible sustained monomorphic ventricular tachycardia (n = 71) and ventricular fibrillation (n = 12) were analyzed.

RESULTS

The sum of the coupling intervals needed for inducing ventricular tachycardia and ventricular fibrillation was 485 +/- 59 ms and 387 +/- 36 ms, respectively (p < 0.001). The mean difference between the effective refractory period and the second coupling interval for the induction of ventricular tachycardia and ventricular fibrillation was -3 +/- 40 ms and 24 +/- 29 ms, respectively (p < 0.02). QRS duration and duration of terminal low amplitude signals of the QRS complex (p < 0.004) were longer in patients with inducible ventricular tachycardia than in patients with inducible ventricular fibrillation. The root mean square of the voltage during the last 40 ms of QRS complex was lower in patients with inducible ventricular tachycardia than in patients with inducible ventricular fibrillation (p < 0.007). Patients with inducible ventricular tachycardia presented with a greater prevalence of ventricular late potentials than that of patients with inducible ventricular fibrillation (p < 0.007). For arrhythmia induction, significantly shorter coupling intervals were necessary in patients without than in patients with ventricular late potentials. A positive correlation was found between the cycle length of the induced ventricular tachycardia and the filtered QRS duration as well as with the sum of the coupling intervals.

CONCLUSIONS

Induction of ventricular fibrillation requires shorter coupling intervals than does induction of ventricular tachycardia. The presence of ventricular conduction delay seems to be a marker of facilitated induction of sustained monomorphic ventricular tachycardia rather than of ventricular fibrillation. The coupling intervals required to induce ventricular tachycardia or fibrillation are longer in patients with than in those without an abnormal signal-averaged ECG.

Effects of procainamide on the signal-averaged electrocardiogram in relation to the results of programmed ventricular stimulation in patients with sustained monomorphic ventricular tachycardia

OBJECTIVES

The aim of this study was to assess the ability of the signal-averaged electrocardiogram (ECG) to predict the efficacy of procainamide.

BACKGROUND

The main role of the signal-averaged ECG has been the identification of postinfarction patients at risk of sudden death. Prediction of the efficacy of antiarrhythmic drugs represents another potential clinical application of this technique.

METHODS

The study examined the effects of procainamide on the time domain and spectral temporal analysis of the signal-averaged ECG in relation to the results of programmed ventricular stimulation studies in 31 patients with inducible sustained monomorphic ventricular tachycardia.

RESULTS

Procainamide significantly prolonged the total and the initial QRS complex and low amplitude signal durations (mean +/- SD 135 +/- 30 vs. 161 +/- 46 ms, p < 0.0001; 87 +/- 16 vs. 98 +/- 20 ms, p < 0.0001, and 48 +/- 23 vs. 63 +/- 36 ms, p < 0.001, respectively) whereas the root-mean-square voltage of the total QRS complex and of the last 40 ms of the QRS complex was significantly reduced (mean +/- SD 112 +/- 36 vs. 87 +/- 36 microV, p < 0.0001; 21 +/- 19 vs. 13 +/- 12 microV, p < 0.002, respectively). The results of spectral temporal mapping of the signal-averaged ECG were similar before and after procainamide administration. Procainamide prevented the inducibility of sustained ventricular tachycardia or prolonged the cycle length of ventricular tachycardia by > or = 100 ms in 16 patients (52%) (responders). The fractional prolongation of the total QRS duration was significantly greater in responders (26 +/- 15%) than in nonresponders (10 +/- 10%) (p < 0.002) and, when this prolongation was > or = 15%, identified responders with a sensitivity of 94%, a specificity of 87% and an overall predictive accuracy of 90%.

CONCLUSIONS

The effects of procainamide on inducibility of ventricular tachycardia during programmed ventricular stimulation can be predicted by the degree of drug-induced prolongation of the signal-averaged QRS complex.

Effects on the signal-averaged electrocardiogram of opening the coronary artery by thrombolytic therapy or percutaneous transluminal coronary angioplasty during acute myocardial infarction

One hundred twenty-nine patients were retrospectively analyzed and divided into 3 groups according to (1) the presence of a patent artery obtained either spontaneously or after thrombolytic therapy but without percutaneous transluminal coronary angioplasty (PTCA) (group I, n = 83), (2) the presence of a patent artery after opening by PTCA (group II, n = 29), or (3) absence of reperfusion despite thrombolytic therapy or PTCA (group III, n = 17). Thrombolytic therapy was given within 4 hours after onset of symptoms (mean 2.5 +/- 1.0 hours) and PTCA was performed within 24 hours after the onset of symptoms (mean 6 +/- 6 hours). Signal averaging was performed within 24 hours after cardiac catheterization. An abnormal signal-averaged electrocardiogram was present in 10 of 83 (12%) group I, 9 of 29 (31%) group II and 7 of 17 (41%) group III patients (p < 0.05 group I vs II, p < 0.01 group I vs III, no statistical difference group II vs III). Therefore, in contrast to reperfusion by thrombolytic therapy the incidence of abnormalities on the signal-averaged electrocardiogram early after myocardial infarction is not reduced by an early opening of the culprit vessel by PTCA.

Arrhythmogenicity of catheter ablation in supraventricular tachycardia

To evaluate arrhythmogenicity in patients who receive a modified direct-current (DC) shock ablation (distal pair of electrodes connected in common as the cathode) or radiofrequency (RF) ablation of supraventricular tachycardia, a prospective study was performed with signal-averaged ECG, 24-hour Holter monitoring, electrophysiologic study (EPS) for ventricular tachycardia (VT), and treadmill exercise test. Sixty-nine consecutive patients with documented paroxysmal supraventricular tachycardia were included. Twenty-eight patients proved to have atrioventricular nodal reentrant tachycardia, and 41 patients had atrioventricular reciprocating tachycardia that involved accessory atrioventricular pathways. The first 34 patients received DC shock ablation and the other 35 patients received RF ablation. Signal-averaged ECG, Holter monitoring, and EPS for VT were performed before ablation, immediately after ablation, then 1 week, 2 weeks (Holter monitoring), 1 month (except EPS), and 3 months after ablation. Treadmill exercise testing was performed before ablation, and at 1 week and 3 months after ablation. The root mean square, low-amplitude signal and QRS duration of signal-averaged ECG disclosed no significant change after either DC or RF ablation up to 3 months. Late potential developed in only one patient in the DC shock group and it was considered to be innocuous because neither VT nor ventricular fibrillation was noted or induced. Increases in the number of ventricular premature contractions and in short-run VT were detected by Holter monitoring in the first week after either mode of ablation (p < 0.001 for the DC shock group; p < 0.05 for the RF group), which were greater (p < 0.05) and lasted longer in the DC shock group than in the RF group.(ABSTRACT TRUNCATED AT 250 WORDS)

Critical analysis of the signal-averaged electrocardiogram. Improved identification of late potentials

BACKGROUND

This study performed a critical analysis of signal-averaging methods. The objective was to optimize detection of late potentials.

METHODS AND RESULTS

We studied two patient populations: a low-arrhythmia-risk group with no evidence of heart disease and a group with clinically documented ventricular tachycardia (VT). Filtered QRS duration (QRSD) and terminal QRS amplitude (RMS40) were measured from the vector magnitude. A QRS duration based on the latest detectable ventricular activity in any of the three individual XYZ leads was also measured. Because of improved signal-to-noise ratio, both individual lead analysis and extended (600-versus 200-beat) averaging yielded significant changes in signal-averaged ECG parameters. Both approaches gave an increased sensitivity for VT identification. Sensitivity, specificity, and accuracy were evaluated as functions of critical values of QRSD and RMS40. RMS measurements in the terminal QRS, ranging from 20 to 100 msec and including RMS40, did not contribute to maximizing sensitivity and were highly correlated with QRSD. Our results from the low-arrhythmia-risk group suggest that age and sex should be considered in the definition of late potentials.

CONCLUSIONS

We propose a VT risk stratification scheme using signal-averaged ECG parameters obtained from both individual lead and vector magnitude analysis. This allows definition of four categories of VT risk derived statistically from the study data. This definition is based on combined measures of sensitivity, specificity, and negative and positive predictive value.

Effect of coronary angioplasty on late potentials one to two weeks after acute myocardial infarction

In survivors of acute myocardial infarction (AMI), the restoration of anterograde flow in the infarct artery, even if accomplished beyond the time for myocardial salvage, may reduce the frequency of subsequent arrhythmic events and sudden death. Twelve subjects (8 men and 4 women, aged 39 to 69 years) with a first AMI, signal-averaged electrocardiographic late potentials, and an occluded infarct artery were prospectively identified. Seven (group I) had successful coronary angioplasty 6 to 15 days after AMI, and 5 (group II) were managed conservatively. Follow-up signal-averaged electrocardiography was performed 3 to 7 months later. From baseline to follow-up, the 7 group I subjects had a significant change in QRS duration (117 +/- 13 [mean + SD] to 102 +/- 10 ms), root-mean-square voltage (10.4 +/- 4.7 to 31.0 +/- 7.6 microV), and low-amplitude signal duration (47.5 +/- 8.5 to 32.4 +/- 5.2 ms) (p < or = 0.05 for all 3 variables). No group I patient had a late potential at follow-up. In contrast, the 5 group II patients showed no change in QRS duration or low-amplitude signal duration from baseline to follow-up, and all 5 had a late potential at follow-up. At follow-up, the root-mean-square voltage was significantly greater and the low-amplitude signal and QRS durations significantly less in group I than in group II (p < 0.05 for all 3 variables). Thus, in our patients, the mechanical restoration of anterograde perfusion in an occluded infarct artery 1 to 2 weeks after AMI caused the resolution of signal-averaged electrocardiographic late potentials.

Frequency analysis of signal-averaged electrocardiogram in patients with right ventricular tachycardia

OBJECTIVES

The purpose of this study was to analyze the frequency content of signal-averaged electrocardiograms (ECGs) in patients with idiopathic ventricular tachycardia of right ventricular origin and in patients with arrhythmogenic right ventricular dysplasia.

BACKGROUND

The late potentials in the time domains are usually found in patients with arrhythmogenic right ventricular dysplasia. They are not usually found in patients with idiopathic ventricular tachycardia of right ventricular origin.

METHODS

Fast Fourier transform analysis of signal-averaged ECGs was performed with the use of a Blackman-Harris window in 43 subjects: 20 normal volunteers (group I), 12 patients with idiopathic ventricular tachycardia of right ventricular origin (group II) and 11 patients with arrhythmogenic right ventricular dysplasia (group III), and the frequency spectrum was displayed in a three-dimensional graph. Area ratio (ratio of the area under the spectral plot from 40 to 120 Hz to the area from 0 to 120 Hz) was calculated in all subjects.

RESULTS

Area ratio was significantly higher in group II than in group I (243 +/- 45 vs. 196 +/- 15, p < 0.01) and significantly higher in group III (396 +/- 51) than in group I or II (p < 0.001). The high frequency components in group II were confined within the QRS complex in the three-dimensional graph, whereas those in group III extended outside the QRS complex.

CONCLUSIONS

Frequency analysis of the signal-averaged ECG with fast Fourier transform analysis can detect the high frequency components in patients with right ventricular tachycardia, including idiopathic ventricular tachycardia and arrhythmogenic right ventricular dysplasia.

Late potentials in a porcine model of anterior wall myocardial infarction and their relation to inducible ventricular tachycardia

In this study, normal values for signal averaged electrocardiographic parameters were assessed in healthy pigs (n = 100) and the development of late potentials after myocardial infarction (n = 41) in relation to inducible ventricular tachycardia was investigated. Normal values are: filtered QRS duration (QRS) < or = 78 msec; root mean square voltage of the averaged QRS complex (V(tot)) > or = 51 microV, and duration of terminal activity below 30 microV (D30) < or = 37 msec. The distribution of the root mean square voltage in the last 30 msec (V30) was biphasic. Two weeks after myocardial infarction, QRS was prolonged from 55 +/- 10 to 66 +/- 19 msec (P < 0.002). D30 was prolonged from 19 +/- 6 msec to 28 +/- 13 (P < 0.002). V30 was decreased from 107 +/- 135 microV to 45 +/- 77 (P < 0.02). The total voltage (V(tot)) was decreased from 195 +/- 78 to 123 +/- 61 microV (P < 0.002). In four pigs (19%) late potentials developed. Sustained ventricular tachycardia was inducible in 11 pigs (52%), ventricular fibrillation in two pigs (10%) and eight pigs (38%) were noninducible. Three of 11 inducible pigs and one of the noninducible pigs had a late potential. The incidence of late potentials and their relation to inducible sustained ventricular tachycardia is comparable to the situation in man. Therefore, this pig model is an attractive alternative to the commonly used dog models.

Ambulatory (Holter) electrocardiography signal-averaging: a current perspective

Signal-averaging is an emerging new application of the ambulatory ECG. Whereas real-time signal-averaging of the high-resolution ECG has led to the measurement of abnormal QRS complex parameters and to the detection of ventricular late potentials in patients with a history of ventricular tachycardia or fibrillation at increased risk of an arrhythmic event, similar measurements can also be derived from ambulatory ECG tape recordings. This review describes the technical differences between real-time and ambulatory high-resolution ECG signal-averaging, and the early clinical studies that correlated measured QRS complex parameters and defined the agreement of late potential detection with the two technologies. Although there appears to be a promising cost-effective benefit from ambulatory ECG signal-averaging, limitations imposed by the technical differences must be recognized, and additional investigation is needed to define the appropriate clinical use and criteria for best diagnostic and prognostic value.

Electropharmacologic effects and pharmacokinetics of almokalant, a new class III antiarrhythmic, in patients with healed or healing myocardial infarcts and complex ventricular arrhythmias

The electropharmacologic effects and pharmacokinetics of almokalant, a new class III antiarrhythmic, were investigated in a randomized, placebo-controlled, double-blind study, and efficacy was evaluated. Ten post-myocardial infarction patients with complex ventricular arrhythmias were included and received, in randomized order on consecutive days, 4.5 mg (12.8 mumol) of almokalant or placebo intravenously over 10 minutes. One patient received infusion at a higher rate and developed self-terminating torsades de pointes. In the remaining 9 patients the corrected QT interval increased significantly: At the end of placebo infusion the corrected QT was 445 +/- 18 ms and after almokalant 548 +/- 53 ms (p = 0.0015). The signal-averaged electrocardiographic parameters did not change. The number of ventricular premature complexes decreased significantly during the first 15 minutes after almokalant infusion (p = 0.04). No additional proarrhythmic or other significant adverse events were noted. The almokalant plasma concentration showed a biphasic decrease with an elimination half-life of 2.4 +/- 0.1 hours. Almokalant was rapidly cleared from the body with a clearance of 11 +/- 1 ml/min/kg. When given with certain precautions almokalant appears safe and well-tolerated and may be antiarrhythmic by prolonging refractoriness.

Frequency of late potentials on signal-averaged electrocardiograms during thallium stress testing in coronary artery disease

Late potentials detected by signal-averaged electrocardiography (SAECG) are an important noninvasive indicator identifying patients with previous myocardial infarction at risk for developing ventricular tachycardia. The role of myocardial ischemia in the development of late potentials is undefined. This study attempts to determine if late potentials on SAECG can be produced during scintigraphically proven ischemia. A signal-averaged electrocardiogram was obtained before and immediately after single-photon emission computed tomography thallium exercise testing in 51 patients. Reversible ischemia was documented in 25 cases with no significant changes in the parameters of SAECG; patients with previous myocardial infarction (n = 10) also had no significant changes from baseline. Multivariate analysis with respect to reversible ischemia and previous myocardial infarction was unrevealing. Patients with late potentials at baseline (n = 10) who developed reversible ischemia (n = 5) had a shorter QRS duration than those with late potentials at baseline and no reversible ischemia. The data indicate that exercise-induced scintigraphically proven ischemia does not alter SAECG even in the presence of previous myocardial infarction. Patients with late potentials at baseline may actually have a shortened QRS duration during reversible ischemia as opposed to the expected lengthening of the QRS.

Establishment of signal-averaged electrocardiographic criteria with Frank XYZ leads and spectral filter used alone and in combination with ejection fraction to predict inducible ventricular tachycardia in coronary artery disease

Signal-averaged electrocardiographic criteria are reported for corrected Frank XYZ leads and a spectral filter. The new criteria were used alone and in combination with ejection fraction to predict inducibility of ventricular tachycardia (VT) at electrophysiologic testing. Signal-averaged electrocardiographic criteria were developed in 87 control subjects and validated in 182 patients (aged 63 +/- 10 years) with coronary artery disease and QRS duration less than 118 ms. Patients underwent electrophysiologic testing in which up to 3 extra-stimuli were used during 2 paced drives from 2 right ventricular sites. A positive finding was monomorphic VT lasting 30 seconds or needing intervention. An ejection fraction less than 40% was considered abnormal. Signal-averaged electrocardiographic variables that best characterized control subjects and separated patients with and without inducible VT were filtered QRS duration less than 120 ms, low-amplitude signal duration less than 38 ms and root-mean-square voltage greater than 20 muv. With these criteria, signal-averaged electrocardiographic and ejection fraction sensitivities were 87 and 45%, respectively, and specificities were 65 and 77%, respectively. Combining signal-averaged electrocardiography with ejection fraction improved the predictive accuracy. In conclusion, diagnostic criteria for signal-averaged electrocardiography with use of Frank XYZ leads and a spectral filter produced results similar to those reported for use of bipolar XYZ leads and a Butterworth filter. Signal-averaged electrocardiography was a better predictor of VT than was ejection fraction.

Frequency versus time domain analysis of signal-averaged electrocardiograms. III. Stratification of postinfarction patients for arrhythmic events

The predictive characteristics of spectral temporal analysis and time domain analysis of the signal-averaged electrocardiogram (ECG) for postinfarction arrhythmic events were compared in 257 patients. During a 6-month follow-up period, 7 patients (2.7%) died suddenly and 9 (3.5%) developed spontaneous sustained ventricular tachycardia. The mean numeric values of the standard time domain signal-averaged ECG variables in patients without arrhythmic events differed significantly from those in patients with arrhythmic events. The mean values of the spectral temporal signal-averaged ECG variables did not differ between the two patient groups. A strategy requiring positivity in any two time domain signal-averaged ECG variables provided the optimal receiver operating characteristic curves for predicting arrhythmic events. With spectral temporal analysis, a strategy using the Hanning window and diagnosing a positive signal-averaged ECG when two variables were abnormal provided the optimal curve for predicting arrhythmic events. Receiver operating characteristic curves showed that over a wide range of sensitivity, time domain variables had higher specificity for predicting arrhythmic events than did spectral temporal variables. Time domain analysis also provided significantly fewer false positive results than did spectral temporal analysis up to sensitivity values of 70%. It is concluded that time domain analysis of the signal-averaged ECG is superior to spectral temporal analysis for predicting arrhythmic events after myocardial infarction.

Late potentials in a bradycardia-dependent long QT syndrome associated with sudden death during sleep

The purpose of this study was to determine the incidence of late potentials and their relation to QT prolongation in a family with a high incidence of sudden death during sleep at a young age and bradycardia-dependent QT prolongation (n = 9) and to compare the findings with those in consanguineous family members without QT prolongation (n = 13). Six (67%) of the 9 family members with QT prolongation had late potentials on the signal-averaged electrocardiogram (ECG) compared with 1 of the 13 normal subjects (p less than 0.007). Positive predictive accuracy of the signal-averaged ECG for the detection of subjects with QT prolongation was 86%; negative predictive accuracy was 80%. During exercise testing, the QT interval normalized, whereas late potentials did not change significantly. Exercise testing did not reveal the presence of coronary artery disease as a possible cause of late potentials. It is concluded that 1) compared with family members with a normal QT interval, patients with this type of bradycardia-dependent QT prolongation have a high incidence of late potentials; 2) late potentials persist despite normalization of the QT interval at high heart rates, indicating that there is no direct relation between late potentials and QT prolongation; and 3) late potentials are not caused by coronary artery disease in these subjects. Therefore, the detection of late potentials might be a new aid in the detection and risk stratification of patients with the long QT syndrome. Late potentials possibly indicate a substrate for ventricular tachyarrhythmias in this type of bradycardia-dependent QT prolongation.

Detection of late potentials. Comparison of two commercial high-resolution ECG systems

Signal-averaged electrocardiogram (SAECG) is used for detection of ventricular late potentials (LPs) in cardiac patients. As many commercial SAECG systems become available, it is essential to determine if they provide equivalent diagnostic information. Two high-resolution (Hi-Res) ECG systems (MAC-12, Marquette Electronics, Inc (MEI), Milwaukee, WI and LVP101, Arrhythmia Research Technology (ART), Austin, TX) were tested on 143 subjects (13 controls and 130 cardiac patients, 21 of whom were tested for inducible ventricular tachycardia [VT]). Late potential measurements (total QRS duration, high-frequency low-amplitude signal duration, and root-mean-square voltage) obtained from the two systems were in good agreement in most of the controls and patients. Application of Multicenter criteria for the MEI system and Gomes criteria for the ART system yielded very good agreement in LP diagnosis (at least 2 parameters abnormal). The two Hi-Res systems predicted inducible VT with good accuracy. The MEI system gave slightly higher sensitivity (90% vs 70%) and specificity (91% vs 82%) than the ART system in patients tested for inducible VT. In controls, both systems gave the same specificity (92%) and the LP diagnosis agreed in all controls (100%). Although the number of patients was small, neither sensitivity nor specificity were significantly different between the two systems at p < 0.05. To conclude, MEI and ART Hi-Res systems gave very similar LP diagnoses when appropriate criteria were applied.

Signal-averaged electrocardiography: history, techniques, and clinical applications

Signal averaging principles have been utilized since around 1875. Their application in medicine to enhance biologic signals was first made in 1947 for improved detection of electroencephalographic signals. The year 1963 marked the first application of this technique to cardiac signals. The more prevalent use in adult cardiology is in the detection of ventricular late potentials (VLPs) which correspond to areas of delayed ventricular activation as slowed conduction velocity. These VLPs have value in predicting the likelihood of malignant ventricular arrhythmias after myocardial infarction (MI). Negative predictive values on the order of 96-99% are convincing as to their ability to predict no arrhythmic event post-MI. Positive predictive values, on the other hand, are poor (10-29%) but improve (35-62%) in combination with other parameters such as depressed left ventricular function and frequent ventricular ectopy. Additional accepted uses for the signal-averaged electrocardiogram (SAECG) include prediction of successful ablative surgery for intractable ventricular tachycardia and facilitating evaluation of patients presenting with syncope of an unclear etiology. Potential future applications of the SAECG are (1) in following patients for rejection after cardiac transplant, (2) as a screening tool for the need for electrophysiologic study, (3) as a predictor of vessel patency after thrombolysis or percutaneous transluminal coronary angiography, and (4) as a tool to assess proarrhythmia. As the technique of signal averaging continues to evolve, its applications may become more diverse and its clinical effectiveness may improve.

Importance of myocardial infarct artery patency on the prevalence of ventricular arrhythmia and late potentials after thrombolysis in acute myocardial infarction

Sustained infarct artery patency is an important determinant of survival in patients with acute myocardial infarction. We studied 61 patients with acute myocardial infarction who received intravenous recombinant tissue-type plasminogen activator, aspirin or heparin within 6 hours of symptom onset, to determine if infarct artery patency after intravenous thrombolytic therapy influences myocardial electrical stability as measured by the prevalence of spontaneous ventricular ectopy or late potential activity. Infarct artery patency was determined by angiographic evaluation 2.5 +/- 3 days after infarction. Forty-eight patients (79%) had a patent infarct-related artery and 13 (21%) patients had an occluded vessel. The mean number of ventricular premature complexes (VPCs)/hour (p less than 0.01) and the prevalence of late potentials (54 vs 19%; p less than 0.03) were significantly higher in patients with an occluded versus patent-infarct related vessel. Although VPC frequency and late potentials were not influenced by the time to thrombolytic treatment, patients with a patent infarct-related artery had a lower prevalence of late potentials regardless of whether treatment was initiated less than or equal to 2 hours (25% patent vs 50% occluded; p = not significant) or 2 to 6 hours (16% patent vs 55% occluded; p greater than 0.03) after symptom onset. Thus, successful thrombolysis decreases the frequency of ventricular ectopic activity and late potentials in the early postinfarction phase. The reduction in both markers of electrical instability may help explain why the prognosis after successful thrombolysis is improved after acute myocardial infarction.